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Java Programming/Print version
The beautiful thing about learning is nobody can take it away from you —B.B. King (5 October 1997) Learning a computer programming language is akin to a toddler's first few steps towards an upright walk. You stumble and you fall but when you start walking, you take upon it as second nature. Learning to program is similar in many ways to learning to walk. However once you start programming, you never cease to evolve. Learn one programming language and you know them all. This book is a concentrated effort in trying to get you to take your baby steps towards programming in the Java™ programming language. Although this book is primarily meant to act as a learning aid for beginners, it can equally be helpful as a reference manual and guide for the intermediate and experienced programmers. The book is laid out in such a manner that with each successive chapter, the complexity of programming constructs increase. Thus, programmers are constantly challenged to develop their skills with a progressive learning curve.
Are you new to programming?
If you have chosen Java as your first programming language, be assured that Java happens to also be the first choice for educational programs in many universities and colleges. Its simple and intuitive syntax (that's grammar for programming languages) helps beginners feel at ease with complex programming constructs in no time.
This stunning image of the sunset on planet Mars wouldn't have been possible without Java.
But be aware, Java is not just any basic programming language. In fact, NASA used Java as the driving force (quite literally) behind its Mars Rover missions. Computers, mobile phones, set-top boxes, and even digital television sets can all be programmed using the Java programming language. Robots, air traffic control systems and the self-checkout barcode scanners in your favourite supermarkets are all being programmed in Java.
Programming with Java™
By now, you might truly be able to grasp the power of the Java programming language – there is still lots more you can do with Java. Not every programmer gets to program applications that take unmanned vehicles onto other planets. Software that we encounter in our daily life is somewhat humble in that respect. Software programs and applications written in Java however cover vast arena of the computing ecosphere. Here are just some examples of the ubiquitous nature of Java applications in real-life: • OpenOffice.org, a desktop office management suite that rivals the Microsoft Office suite has been written in Java. • Online browser-based games like Runescape, a 3D massively multi-player online role playing game (MMORPG), run on graphics routines, 3D rendering and networking capabilities powered by the Java programming language. • Two of the world's renowned digital video recorders, TiVo and BSkyB's Sky+ use built-in live television recording software to record, rewind and play your favourite television shows. These applications make extensive use of the Java programming language.
Java Programming/Print version The above mentioned application illustrates the reach and ubiquity of Java applications. Here's another fact – almost 80% of mobile phone vendors adopt Java as their primary platform for the development of applications. The second most widely used mobile-based operating system, Android, uses Java as one of its key application platforms – developers are encouraged to develop application for Android in the Java programming language.
What can Java not do?
Well, to be honest, there is nothing that Java can not do. You can program just about anything in Java. This book would however get you acquainted with the basics of the language in order for you to create that masterpiece of a software you dream of creating one day. People have written operating systems in Java, they have programmed robots in the language and managed power stations and conveyor belts in factories; thus, you can do anything and everything with Java. Your applications and code need not fear any bounds because with Java, possibilities are endless – all you need is creativity.
About This Book
The Java Programming Wikibook is a shared effort in amassing a comprehensive guide of the complete Java platform - from programming advice and tutorials for the desktop computer to programming on mobile phones. The information presented in this book has been conceptualised with the combined efforts of various authors and contributors, and anonymous editors. The primary purpose of this book is to teach the Java programming language to an audience of beginners, but its progressive layout of tutorials increasing in complexity, it can be just as helpful for intermediate and experienced programmers. Thus, this book is meant to be used as: • a collection of tutorials building upon one another in a progressive manner; • a guidebook for efficient programming with the Java programming language; and, • a comprehensive manual resource for the advanced programmer. This book is intended to be used in conjunction with various other online resources, such as: • the Java platform API documentation [1]; • the official Java website [2]; and, • active Java communities online, such as Java.net [3] and JavaRanch [4], etc.
Who should read this book?
Every thing you would need to know to write computer programs would be explained in this book. By the time you finish reading, you will find yourself proficient enough to tackle just about anything in Java and programs written using it. This book serves as the first few stepping stones of many you would need to cross the unfriendly waters of computer programming. We have put a lot of emphasis in structuring this book in a way that lets you start programming from scratch, with Java as your preferred language of choice. This book is designed for you if any one of the following is true. • • • • You are relatively new to programming and have heard how easy it is to learn Java. You had some BASIC or Pascal in school, and have a grasp of basic programming and logic. You already know and have been introduced to programming in earlier versions of Java. You are an experienced developer and know how to program in other languages like C++, Visual Basic, Python or Ruby. • You've heard that Java is great for web applications and web services programming. Although, this book is generally meant to be for programmers who are beginning to learn programming; it can be highly beneficial for intermediate and advanced programmers who may have missed up on some vital information.
Java Programming/Print version By the end of this book, you would be able to solve any complicated problem and tackle it using the best of your learnt skills in Java. Once you finish, you are also encouraged to take upon ambitious programming projects of your own as you certainly would be able to do that as well. This book assumes that the reader has no prior knowledge of programming in Java, or for that matter, any object-oriented programming language. The book makes it easier to understand development methodology as one reads through the book with practical examples and exercises at the end of each topic and module. Although, if you are a complete beginner, we would suggest that you move slowly through this book and practice each exercise at your pace.
How can you participate
Content is constantly being updated and enhanced in this book as is the nature of wiki-based content. This book is therefore in a constant state of evolution. Any Wikibooks users can participate in helping this book to a better standard as both a reader, or an author/contributor.
As a reader
If you are interested in reading the content present in this book, we encourage you to: • share comments about the technical accuracy, content, or organisation of this book by telling the authors in the Discussion section for each page. You can find the link Discussion on each page in this book leading you to appropriate sections for discussion. • leave a signature when providing feedback, writing comments, or giving suggestion on the Discussion pages. This can be achieved by appending -- ~~~~ to your messages. Do not add your signatures to the Book pages, they are only meant for the Discussion pages. • review pages – at the bottom of every page of this book, you are likely to find controls that help you review and give feedback for pages. Content on each page should be rated according to the four Wikibooks quality metrics: Reliability, Completeness, Neutrality and Presentation. • share news about the Java Programming Wikibook with your family and friends and let them know about this comprehensive Java guide online. • become a contributing author, if you think that you have information that could fill in some missing gaps in this book.
As an author or contributor
If you intent on writing content for this book, you need to do the following: • When writing content for this book, you can always pose as an anonymous author, however we recommend you sign-in into the Wikibooks website when doing so. It becomes easier to track and acknowledge changes to certain parts parts of the book. Furthermore, the opinions and views of logged-in users are given precedence over anonymous users. • Once you have started contributing content for this book, make sure that you add your name to the authors and contributors list. • Be bold and try to follow the conventions for this Wikibook. It is important that the conventions for this book be followed to the letter to make content consistent and reliable throughout.
which is needed to develop those Java applications and applets. which is needed to run Java applications and applets. . you would need to follow along each and every tutorial rather than simply reading through the book. the following table presents direct links to the latest JDK for the Windows operating system. it is highly recommended to restart your Windows operating system.7.0_01. you JDK should now be installed at C:\Program Files\Java\jdk1. To acquire the latest JDK (version 7). you can manually download the Java software [5] from the Oracle website. however for the purposes of writing Java code by following the tutorials in this book. Given below are installation instruction for the JDK for various operating systems: Installation instructions for Windows For Windows Download instructions Some Windows based systems come built-in with the JRE. thus it is divided into two essential software: • the Java Runtime Environment (JRE).7. see the chapter on the Java platform.0_01\bin Updating environment variables (Optional) In order for you to start using the JDK compiler utility with the Command Prompt. In order to make use of the content in this book. you would only require the JDK which comes equipped with a JRE as well. But to do so. However as a developer. you would require the JDK nevertheless. and. • the Java Development Kit (JDK). If you kept the default settings for the setup installer wizard. Once the setup is completed. Operating system Setup Installer License Windows x86 Windows x64 Download Download [6] [8] Oracle Binary Code License Agreement Oracle Binary Code License Agreement [7] [7] You must follow the instructions for the setup installer wizard step-by-step with the default settings to ensure that Java is properly installed on your system.Java Programming/Print version 4 Necessary prerequisites Installing the Java platform For in-depth information. Follow the steps below to permanently include your Java platform to your environment variables. For the convenience of our readers. You would require the location to your bin folder at a later time – this is located at C:\Program Files\Java\jdk1. you would need access to a computer with the Java platform installed on it – the Java platform is the basic prerequisite for running and developing Java code. you would need to set the environment variables that points to the bin folder of your recently installed JDK.
. click Start › Control Panel › System and Maintenance › System.). append the location of your JDK bin folder separated by a semi-colon (. Once the Notepad application has fired up. . Alternatively. type the following command at the prompt: rundll32 shell32. Nevertheless. For Windows XP. The problem with Notepad however is that it does not support developer-friendly features. Click OK on every opened dialog to save changes and get past to where you started. 4. However. Click Start › All Programs › Accessories › Notepad to invoke the application. To use the terminal to download and install the Java platform. To add a new path. click Start › Settings › Control Panel › System. you would need a decent text editor. Start writing code Once you have successfully installed the JDK on your system. Alternatively. you need to follow the steps below: 1. select the variable named Path and click Edit.cpl 2. a free and open-source fully integrated text editor that supports syntax highlighting and code completion. For Windows 2000. the most recommended editor is the Notepad++. follow the instructions below. there are a variety of different open-source editors available as alternatives to Notepad that support these features. type the following command at the prompt: notepad 2.Control_RunDLL sysdm. go the the Variable value field. you are ready to program code in the Java programming language. In order to use notepad to write code in Java. such as syntax highlighting and code completion. With the dialog open. 3.. You need to download the latest version of Notepad++ [9] in order to start writing code with the editor. you can also press ⊞ Win+R to open the Run dialog. click Start › Control Panel › System. In the Edit System Variable dialog. These features are a vital part of the exercise of writing code. 6.). and select Environment Variables. 5. Installation instructions for Linux For Linux Installation using Terminal Downloading and installing the Java platform on Linux machines (in particular Ubuntu Linux) is very easy and straight-forward.. to write code.Java Programming/Print version 5 1. you can use the editor to write code for the Java programming language.. Under System variables. Navigate to the Advanced tab on the top. For Window Vista or Windows 7. With the dialog open. For the purposes of the tutorials in this book. Windows comes with a default text editor by default – Notepad.dll. you can also press ⊞ Win+R to open the Run dialog. This field is a list of directory paths separated by semi-colons (.
one can find Kate. Installation instructions for Mac OS . For Ubuntu. All Java software should be installed and instantly available now. Alternatively. Follow the instructions below if you wish to install SciTE: 1. Download instructions Alternatively. For Ubuntu. write the following: $ sudo apt-get install scite 3. At the prompt. You should now be able to use the Scite editor for your programming needs. At the prompt. while on the KDE desktops. type xterm or gnome-terminal to open the Terminal window. At the prompt. type xterm or gnome-terminal to open the Terminal window. you can press Alt+F2 to open the Run Application window. Alternatively. the following table presents direct links to the latest JDK for the Linux operating system. if you require a robust and standalone text-editor like the Notepad++ editor on Windows. However. Operating system RPM Linux x86 Linux x64 Download Download [10] [12] Tarball Download Download [11] [13] License Oracle Binary Code License Agreement Oracle Binary Code License Agreement [7] [7] Start writing code The most widely available text editor on Gnome desktops is the Gedit application. 2. you can manually download the Java software [5] from the Oracle website. write the following: $ sudo apt-get install openjdk-7-jdk openjdk-7-jre openjdk-7-doc 3. you can press Alt+F2 to open the Run Application window. go to Application › Accessories › Terminal. go to Application › Accessories › Terminal. However unlike Notepad on Windows.Java Programming/Print version 6 1. 2. At the prompt. both these editors support syntax highlighting and code completion and therefore are sufficient for our purposes. you would require the use of the minimalistic editor loaded with features – SciTE. For the convenience of our readers.
John Gage. a faster version.[14] This team would shape the future in the next decade and no one had any idea as to what was in store. whose aim was to create a programming tool that could render obsolete the C and C++ programming languages. .[14] At the time the total number of people working on Java was less than 30. For the team. From being the mind of an unmanned vehicle on Mars to the operating environment on most of the consumer electronics.[14] Over the period of time that the Green Project teethed. had to be developed for an embedded system with limited resources.[14] Reshaping thought The team started thinking of replacing C++ with a better version. James Gosling. co-founder and executive vice president at Netscape announced to an audience of SunWorldTM that Java technology wasn't a myth and that it was a reality and that it was going to be incorporated into Netscape Navigator. This team would be called "Green Team" henceforth in time.g. And. Let's see how it all began. The team thus had to think in a different way to go about solving all these problems. Engineer Patrick Naughton had become extremely frustrated with the state of Sun's C++ and C APIs (application programming interfaces) and tools. No longer was its aim to create a new language far superior to the present ones. they began work in a small office on Sand Hill Road in Menlo Park. California. architect and designer of the compiler for the Java technology Staffed at 13 people. other than this. While he was considering to move towards NeXT. the language lacked some other important features as well. a secret nobody but he knew. toasters and also for personal digital assistants (PDAs). For such a system. This Stealth Project was later named the "Green Project" when James Gosling and Mike Sheridan joined Patrick. e. a responsive version. the director of the Science Office of the Sun Microsystems along with Marc Andreesen.Java Programming/Print version 7 For Mac OS Installation instructions for Solaris History On 23 May 1995. this meant at least one significant trend. that of the convergence of digitally controlled consumer devices and computers. The Green team Behind closed doors. But the one thing they hadn't thought of. was that the language they were aiming for. An embedded system is a computer system scaled to a minimalistic interface demanding only a few functions from its design. VCRs. but it aimed to target the language to devices other than the computer. cable set-top boxes. The project they underwent was chartered by Sun Microsystems to anticipate and plan for the "next-wave" in computing. the prospects of the project started becoming clearer to the engineers working on it.[15] Java has come a long way from its inception. a project was initiated in December of 1990. as of yet. he was offered a chance to work on new technology and the "Stealth Project" was started. C++ or any successor would seem too large as all the languages at the time demanded a larger footprint than what was desired.
The team now needed a proper identity and they decided on naming the new technology they created Java ushering a new generation of products for the internet boom.meant. Sun Microsystems Yes Lead engineer and key architect of the Java technology Co-founder and VP. President.[15] . the work on Oak had been significant but come the year 1993. National Center for Supercomputing Applications (NCSA) had just unveiled its new commercial web browser for the internet the previous year. putting in and taking out stuff. now diverted towards where they thought the "next-wave" of computing would be – the internet. people saw the demise of set-top boxes. The focus of the team. Sun's Java Software Division Yes FirstPerson product marketing Original director of public relations for Java technology (Burson-Marsteller) Yes FirstPerson President Former Sun Microsystems Chief Technology Officer Green Team member The demise of an idea. Andreesen unveiled the new technology to the masses. version of the UNIX® OS Jonni Kanerva Tim Lindholm Scott McNealy Patrick Naughton George Paolini Kim Polese Lisa Poulson Wayne Rosing Eric Schmidt Mike Sheridan Yes Yes Yes Yes Java Products Group employee. and CEO of Sun Microsystems Yes Green Team member. Bill Joy. Sun Microsystems. Riding along with the explosion of interest and publicity in the Internet. Principal designer of the UC Berkeley. The ++ and -. Java quickly received widespread recognition and expectations grew for it to become the dominant software for browser and consumer applications. FirstPerson co-founder Corporate Marketing (Director). author of The Java FAQ1 Yes FirstPerson employee and member Java Products Group Chairman. He soon abandoned the name and called it Oak[14] after the tree that stood outside his office. 8 Table 1: Who's who of the Java technology[14] Has worked for GT (Green Team). Gosling began work on enhancing C++ and named it "C++ ++ --".Java Programming/Print version Co-founder of Sun Microsystems. Only a miracle could make the project a success now. And such a miracle awaited anticipation. A by-product of the project was a cartoon named "Duke" created by Joe Parlang which became its identity then. interactive TV and the PDAs. envisioned a language combining the power of Mesa and C in a paper he wrote for the engineers at Sun named Further. birth of another By now. FP (FirstPerson) and JP (Java Products Group) Name Lisa Friendly John Gage James Gosling Bill Joy Yes Yes GT FP Yes JP Details Yes FirstPerson employee and member of the Java Products Group Science Office (Director). The team then divulged into the realms of creating the same embeddable technology to be used in the web browser space calling it an applet – a small application. Gathering ideas. a pun on the evolutionary structure of the language's name. Finally at the SunWorldTM conference. A failure that completely ushered the inventors' thoughts to be reinvented.
JDK 1.g.0 (JDK 1.0 and 1.4 is The Java(TM) 2 Platform.2 was the start of a new beginning for Java. not by Sun Microsystems. Java has displayed a steady growth. the Javac compiler). a free software license.0). and the development tools (e.. The JDKs of version 1. From version 1. Mac OS Classic and Linux. we'd quickly go through the development paths that Java took with age. Sun also provided a runtime-only package. Sun released the source code for its HotSpot Virtual Machine and compiler in November 2006. Standard Edition version 1. Java 2 (version 1.2) Introduced in 1998 as a quick fix to the former versions. This included the Java runtime (the virtual machine and the class libraries). The released parts were published under the terms of the GNU General Public License.2 and later versions are often called Java 2 as well. so usually people refer to a particular version of Java by its JDK version (e. Windows. For example. Later.Java Programming/Print version 9 Recent history Initially Java was owned by Sun Microsystems.4. Here. The first name stuck.. Versions Unlike C and C++. and the most of the source code of the class library in May 2007.[citation needed] the term Java was a trademark of Sun Microsystems. but later it was released to open source. Java was initially released as the Java Development Kit 1.0).0 to version 1. called the Java Runtime Environment (JRE).g. Some parts were missing because they were owned by third parties. version 1. Initial Release (versions 1. however.1) Introduced in 1996 for the Solaris. Development of Java over the years. Major changes include: • Rewrite the event handling (Add Event Listeners) • Change Thread synchronizations • Introduction of the JIT-Just in time compilers . Java's growth is pretty recent.7. the official name of JDK 1.
Really aimed at people who create tools for Java development and for frameworks like JavaServer Pages (JSP) or Personal Home Page construction kit (PHP) engines that need to generate a bunch of .No more need to find and configure your own JDBC database when developing a database application! Developers will also get the updated JDBC 4. • Annotation/Metadata . such as special support for XML as an SQL datatype and better integration of Binary Large OBjects (BLOBs) and Character Large OBjects (CLOBs) into the APIs. • Static imports . • JVM Improvements . Large portion of the runtime libraries will now be shared among multiple JVM instances. • Database . Java SE 5) Released in September 2004 Major changes include: • Generics .0.0.Shorten the for loop with Collections use.4) Java 1. More advanced developers can plug in their own scripting engines and mix their favorite scripting language in with Java code as they see fit. • Enhanced for . Java SE 6) Released on 11 December 2006. a well-used API with many important improvements.Provides compile-time type safety for collections :and eliminates the drudgery of casting. Also useful when you have teams with a variety of skill sets.GUI developers get a large number of new tricks to play like the ever popular yet newly incorporated SwingWorker utility to help you with threading in GUI apps. • Autoboxing/unboxing . • Monitoring and Management .You can now mix in JavaScript technology source code.The really big deal here is that you don't need do anything special to the startup to be able to attach on demand with any of the monitoring and management tools in the Java SE platform.6. • Compiler Access .Most of the run time library is now mapped into memory as a memory image. Annotations can be inspected through source parsing or by using the additional reflection APIs added in Java 5. JTable sorting and filtering.0. and a new facility for quick splash screens to quiet impatient users.Java Programming/Print version 10 Merlin (Java 1.Enabling tools to generate code and deployment descriptors from annotations in the source code. as opposed to being loaded from a series of class files. This leads to a "declarative" programming style where the programmer says what should be done and tools emit the code to do it.[17] What's New in Java SE 6: • Web Services . (from [16]) Mustang (version 1.4 has improved programmer productivity by expanding language features and available APIs • • • • • • Assertion Regular Expression XML processing Cryptography and Secure Socket Layer (SSL) Non-blocking I/O(NIO) Logging Tiger (version 1.Eliminates the drudgery of manual conversion between primitive types (such as int) and wrapper types (such as Integer). • More Desktop APIs .Lets you import all the static part of a class.First-class support for writing XML web service client applications.5. • Scripting . useful for prototyping.
. and access to LDAP servers for authenticating users. com/ otn-pub/ java/ jdk/ 7u1-b08/ jdk-7u1-linux-i586. Clark S. se/ ~lindsey/ JavaCourse/ Book/ Part1/ Java/ Chapter01/ history. com/ aboutsun/ media/ presskits/ 2006-1211/ ). you could write an annotation processor that verifies whether your program obeys naming conventions. Java Generic Security Services (Java GSS) and Kerberos services for authentication. • Security . com/ otn-pub/ java/ jdk/ 7u1-b08/ jdk-7u1-linux-x64. such as native Public Key Infrastructure (PKI) and cryptographic services on Microsoft Windows for secure authentication and communication.At long last. oracle. html [2] http:/ / www. gz [12] http:/ / download. oracle. [15] "History of Java" (http:/ / www.. com/ otn-pub/ java/ jdk/ 7u1-b08/ jdk-7u1-windows-i586. html). exe [9] http:/ / notepad-plus-plus. . which just makes sense. html [8] http:/ / download. exe [7] http:/ / www. com/ javase/ 7/ docs/ api/ overview-summary. sun.Java SE 6 has simplified the job of its security administrators by providing various new ways to access platform-native security services. Sun Microsystems. The platform does not just enable a user to run and develop Java application. javaranch. com/ otn-pub/ java/ jdk/ 7u1-b08/ jdk-7u1-linux-x64. kth. Java SE 6 unifies the Java Plug-in technology and Java WebStart engines.0. . com/ features/ 2003/ 05/ bloch_qa. Lindsey. com/ technetwork/ java/ javase/ downloads/ index. particle. com/ features/ 1998/ 05/ birthday. com/ us/ technologies/ java/ index. html [6] http:/ / download. . gz [14] "Java Technology: The Early Years" (http:/ / java. Retrieved 9 May 2008. com [5] http:/ / www. Java SE 7) Anticipated for 2010 Citations [1] http:/ / download. com/ otn-pub/ java/ jdk/ 7u1-b08/ jdk-7u1-linux-i586.Java Programming/Print version classes on demand. [16] http:/ / java. but also features a wide variety of tools that can help developers work efficiently with the Java programming language. oracle. sun. 11 Dolphin (version 1. The platform consists of two essential software: • the Java Runtime Environment (JRE).. the compiler API opens up programmatic access to javac for in-process compilation of dynamically generated Java code. tar. Installation of the Java WebStart application got a much needed makeover. com/ technetwork/ java/ javasebusiness/ documentation/ java-se-bcl-license-430205. which is needed to run Java applications and applets. html [3] http:/ / home. net [4] http:/ / www. but for those of you deafened by your screaming inner geek. html). java. Compatibility. org [10] http:/ / download. we asked you download and install the Java platform. com/ otn-pub/ java/ jdk/ 7u1-b08/ jdk-7u1-windows-x64. oracle. rpm [13] http:/ / download. oracle. oracle.7. and. And the rest of us will happily benefit from the tools and the improved Java frameworks that use this. rpm [11] http:/ / download. sun.. The Java platform is the name given to the computing platform from Oracle that helps users to run and develop Java applications. The Java Platform As one of the necessary prerequisites for using this book to program in the Java programming language. oracle. • The -lities: Quality. html [17] "Java Platform Standard Edition 6" (http:/ / www. tar. Retrieved 9 May 2008. Sun Microsystems.Bug fixes . • Desktop Deployment . • Pluggable Annotations allows programmer to write annotation processor so that it can analyse your code semantically before javac compiles. oracle. oracle. oracle. roll up your sleeves and give it a try. For example. Stability . The compiler API is not directly intended for the everyday developer. Retrieved 7 May 2008.
it looks and behaves like a native application. If you have installed the JDK. If a software was written for a Unix-based system. platform or architecture – in fact. it was impossible to run the same application on a Windows system – in this case. platform and architecture. the JRE acts as a liaison between the underlying platform and that application. So. you would only require the JDK. we would explore in further detail what these two software components of the Java platform do. Before Java.Java Programming/Print version • the Java Development Kit (JDK). . This feature of the Java platform is commonly abbreviated to WORA in formal Java texts. platform or architecture. 12 Java Runtime Environment (JRE) Any piece of code written in the Java programming language can be run on any operating system. It interprets the Java application to run in accordance with the underlying platform. A major milestone in the development of the Java programming language was to develop a special runtime environment that would execute any Java application independent of the computer's operating system. for all the purposes of this book. We will discuss the JVM in detail later. The part of the JRE that accomplishes this complex liaison agreement is called the Java Virtual Machine (JVM). such that upon running the application. you should know that it comes equipped with a JRE as well. the application was native only to Unix-based systems. which is needed to develop those Java applications and applets. this amount of ubiquity was very hard to achieve. In this section. Figure 1: Java applications can be written once and run anywhere. it can be run on any device that supports the Java platform. The Java Runtime Environment (JRE) sits on top of the machine's operating system. If and when a Java application is run.
Java Programming/Print version 13 Executing native Java code (or byte-code) Native Java applications are preserved in a special format called the byte-code. operating system.jar). You can always check what version of the JRE is installed on the computer by writing the following command in Command Prompt (on Windows) and Terminal (on Linux or Mac OS): java -version Java Virtual Machine (JVM) Quite possibly. Byte-code remains the same. Java byte-code resides in files that have the . Suppose your byte-code is called SomeApplication. SomeApplication. Even though the process of interpretation brought the WORA principle to diverse machines. it had a drawback – it consumed a lot of time and clocked the system processor intensively to load an application. If your computer doesn't have a JRE.class.jar (also known as a Java archive) extension. the JRE comes with a special tool (appropriately named java). On a file-system. To run byte-code. The olden Java architecture used this process of interpretation to execute Java byte-code. then the above commands would not work. you would need to use the following command in Command Prompt (on Windows) and Terminal (on Linux or Mac OS): java -jar SomeApplication. The JVM acts like a virtual processor enabling Java applications to be run on the local system. Figure 2: A JVM interpreter translates the byte-code line-by-line to make it appear as if a native application is being executed. It's main purpose is to interpret (read translate) the received byte-code and make it appear as native code. .jar extension (say. If you want to execute this Java byte-code.jar Do you have a JRE? Most computers come with pre-installed copy of the JRE. no matter what hardware architecture.class If you want to execute a Java byte-code with a .class (also known as a class file) or the . you would need to use the following command in Command Prompt (on Windows) and Terminal (on Linux or Mac OS): java SomeApplication. the most important part of the JRE is the Java Virtual Machine (JVM). or software platform it is running under.
allowing applications to execute much faster at the cost of a small delay on first execution. though it is by far the most successful and well-known. the JVM merely uses the already compiled native code to optimize performance. This process only occurs when the byte-code is executed for the first time. we do not need to write Java byte-code. Figure 3: A just-in-time compiler only compiles the byte-code to equivalent native code at first execution. the JVM uses the compiled version of the byte-code on every successive execution. In order to convert the Java code in these source files to Java byte-code.java. a developer ensures that their system has the appropriate JRE and additional tools to help with the development of applications in the Java programming language.2) features a more robust JVM. the JVM would optimize it to run even better. Upon downloading the JDK.Java Programming/Print version Just-in-Time Compilation The new versions of the JRE (since version 1. Previous uses for virtual machine technology primarily involved emulators to aid development for not-yet-developed hardware or operating systems. Upon every successive execution. This process of conversion is called just-in-time compilation or JIT-compilation. Instead of interpreting byte-code. however as developers. It has the ability to identify areas within the Java code itself that can be optimized for faster and better performance. Based on every successive run of your Java applications. we are interested in writing pure code in Java which can then be converted into Java byte-code for mass deployment. Unless the byte-code itself is changed. As developers. but the JVM was designed to be implemented entirely in software. Doing so saves a lot of time and processor effort. These files are called Java source files. Java Development Kit (JDK) The JRE takes care of running the Java code on multiple platforms. while making it easy to efficiently port an implementation to hardware of all kinds. Java code can be found in files with the extension . Was JVM the first virtual machine? Java was not the first virtual-machine-based platform. rather we write the code in the Java programming language (which is quite similar to writing C or C++ code). 14 Native optimization The JVM is an intelligent virtual processor. you need to use the Java compiler tool installed with your JDK. it down-right converts the code straight into equivalent native code for the local system. .
sun. The most difficult thing with apt is that Sun decided to use a whole set of new terminology. For the reader's convenience. and a web server serving the applet.html) In Java 1. com/ j2se/ 1. Typically.java source file into its equivalent Java byte-code. However. and annotation processors are typically just code generators.5 (alias Java 5. The most common task is to generate some particular source code. This process of converting Java source files into their equivalent byte-codes is known as compilation. SomeApplication. during development and testing of an applet it might be more convenient to start an applet without the need to fiddle with a browser and a web server.0) Sun added a mechanism called annotations. Annotation processing For more about annotation processing. apt can simply be seen as a source code preprocessor framework. this environment is provided by a browser with a Java plug-in. you would need to use the following command in Command Prompt (on Windows) and Terminal (on Linux or Mac OS): javac SomeApplication. and even provide mechanisms to carry that meta-data forth into a compiled class files. Figure 4: The basic Java compilation process A list of other JDK tools There are a huge array of tools available with the JDK that will all be explained in due time as you progress with the book. Sun's appletviewer from the JDK can be used to run an applet.Java Programming/Print version 15 The Java compiler The Java compiler tool (named javac in the JDK) is the most important utility found with the JDK. • apt – An annotation processing tool which digs through source code. In such a case. The actions apt performs when finding annotations in the source code are not hard-coded into apt. 5.java) to its respective Java byte-code. Annotations allow the addition of meta-data to Java source code. finds annotation statements in the source code and executes actions if it finds known annotations. These handlers are called annotation processors. these tools are listed below in order of their usage: Applet development • appletviewer – Java applets require a particular environment to execute. . one has to code particular annotation handlers (in Java).class. In order to compile a Java source file (say. read this (http:/ / java. The resultant byte-code would exist in a newly created file named SomeApplication. 0/ docs/ guide/ apt/ GettingStarted.java This command would convert the SomeApplication. Instead.
programmers need ways to track changes and to understand the code better at each step of its evolution. configuration files. The JDK tool javah helps to write the necessary C code. It can be used prior to the installation of a Java extension into the JDK or JRE environment. The details and procedures are specified in the JNI (Java Native Interface). native code is written in C (or C++). the javap tool prints information regarding members (constructors. Internally. methods and variables) in a class. It checks if a particular Jar file conflicts with an already installed extension.regions that help declare user definitions for a code snippet within the source code. A Java archive is a collection of compiled Java classes and other resources which those classes may require (such as text files.java source files and generates HTML documents that are easier to read and understand without looking at the code itself. Class library conflicts • extcheck – This tool appeared first with Java 1.Java Programming/Print version 16 Integration of non-Java and Java code • javah – A Java class can call native. programmer have been employing the use of special programming constructs called comments . javadoc reads source file comments from the . The javadoc tool generates documentation about the application programming interface (API) of a set of user-created Java classes.jar as the extension. Usage of these tools first requires study of the particular security mechanisms. In other words. code that has been prepared to be called from Java. IDE's normally provide easier to use debugging environments.zip file. Although you can use this command line console.a file with . But comments are prone to be verbose and incomprehensible. Commonly. let alone be difficult to read in applications having hundreds of lines of code. Software security and cryptography tools The JDK comes with a large number of tools related to the security features of Java. For decades. Documenting code with Java As programs grow large and complex. a jar file is really a .5. • javadoc – Java provides the user with a way to easily publish documentation about the code using a special commenting system and the javadoc tool. . • javap – Where Javadoc provide a detailed view into the API and documentation of a Java class. javap is a formatting disassembler for Java bytecode. The tools are: • • • • keytool – To manage keys and certificates jarsigner – To generate and verify digital signatures of JARs (Java ARchives) policytool – To edit policy files kinit – To obtain Kerberos v5 tickets • klist – To manage Kerberos credential cache and key table • ktab – To manage entries in a key table The Java archiver • jar – (short for Java archiver) is a tool for creating Java archives or jar files . by generating C header files and C stub code. The Java debugger • jdb – (short for Java debugger) is a command-line console that provides a debugging environment for Java programs. images) at runtime. it lists the class' API and/or the compiled instructions of the class. or non-Java.
native2ascii can also convert from ASCII to native. (Interestingly. though underappreciated. Java class libraries (JCL) In most modern operating systems.files containing configuration data -. tool for writing properties files -. containing much of the same reusable functions commonly found in modern operating systems. some underlying platforms may not support all of the features a Java application expects.io libraries implement the required native code internally. The Java class libraries serve three purposes within the Java platform. This code is typically provided as a set of dynamically loadable libraries that applications can call at runtime. The monitoring and management tools are intended for monitoring and managing the virtual machine and the execution environment. Despite the name. they provide the programmer with a well-known set of functions to perform common tasks. the class libraries can either emulate those features using whatever is available. or provide a consistent way to check for the presence of a specific feature. The Java java. then use native2ascii to generate the necessary ASCII text with embedded Unicode escape sequences. . Because the Java platform is not dependent on any specific operating system.5 a set of monitoring and management tools have been added to the JDK. Finally. In these cases. Instead. The troubleshooting tools provide rather esoteric insight into aspects of the virtual machine. a large body of reusable code is provided to simplify the programmer's job. the class libraries provide an abstract interface to tasks that would normally depend heavily on the hardware and operating system.net and java. such as maintaining lists of items or performing complex string parsing. in addition to a set of troubleshooting tools. the Java platform provides a comprehensive set of standard class libraries. Text using these characters can appear in properties files and resource bundles only if the non-ASCII and non-Latin-^1 characters are converted into Unicode escape sequences (\uXXXX notation). In addition. Monitoring and Management Tools / Troubleshooting Tools With Java 1. Like other standard code libraries. for example. So they might disappear in future JDKs. then provide a standard interface for the Java applications to perform those tasks.or resource bundles -. monitoring memory usage during the execution of a Java program. They allow. but international programmers need a full range of character sets. Such files can contain only ASCII and Latin-1 characters. applications cannot rely on any of the existing libraries.) All the monitoring and management and troubleshooting tools are currently marked as "experimental" (which does not affect jdb).Java Programming/Print version 17 The native2ascii tool native2ascii is an important. You can write the international text in an editor using the appropriate character encoding. The task of writing such escape sequences is handled by native2ascii. so it is useful for converting an existing properties file or resource bundle back to some other encoding. Tasks such as network access and file access are often heavily dependent on the native capabilities of the platform. the Java debugger is not categorized as a troubleshooting tool.files containing language translations of text. native2ascii makes most sense when integrated into a build system to automate the conversion.
the . Most notable of these is the Microsoft's . In fact.Java Programming/Print version 18 Similar concepts The success of the Java platform and the concepts of the write once. Due to the lack of interest from developers. Figure 5: Third-party JVM-targeted compilation for non-Java source compilation to Java byte-code. Third-party compilers targeting the JVM The word Java. the . Oracle does not encourage the use of any other languages with the platform. To compare Java with C#. J# is nothing more than a non-standard extension of the Java programming language. For instance. and lists the Java programming language as a core part of the Java 2 platform. The language and runtime are therefore commonly considered a single unit. There are cases where you might want to program using a different language (say. Python) and yet be able to generate Java byte-code (instead of the Python compiled code) to be run with the JVM. J# is normally not supported with the JVM because instead of compiling it in Java byte-code. by itself. . and focused on a similar programming language – C#. usually refers to the Java programming language which was designed for use with the Java platform. Python developers can use Jython compilers to compile Python code to the Java byte-code format (as illustrated below). because J# implements the . Illustrated example shows Python source being compiled to both Python compiled code and Java byte-code. run anywhere principle has led to the development of similar frameworks and platforms.NET framework and its open-source equivalent Mono.NET framework The . thus making J# different from the Java programming language. The .NET framework borrows many of the concepts and innovations of Java – their alternative for the JVM is called the Common Language Runtime (CLR).NET platform compiles the code into CIL.NET platform had an implementation of a Java-like language called Visual J# (formerly known as J++). while their alternative for the byte-code is the Common Intermediate Language (CIL). Microsoft had to withdraw their support for J#. Many third-party programming language vendors provide compilers that can compile code written in their language to Java byte-code. Furthermore. read Comparison of C# and Java. However.NET Base Class Libraries (BCL) instead of the Java Class Libraries. Programming languages are typically outside of the scope of the phrase "platform".
The result is machine code which is then fed to the memory and is executed. which the JVM then executes at runtime. Early Java compilers compiled the byte-code to machine code each time it was used. Some of these languages are also used to extend the functionalities of the Java language itself. to speed up execution. JIT compilers compile byte-code once and the compiled machine code are re-used again and again.Java Programming/Print version Of late. JVM-targeted third-party programming and scripting languages have seen tremendous growth. .class file The Java classes/Byte Codes are compiled to machine code and loaded into memory by the JVM when needed the first time. programs are not compiled into executable files. When the program is to be run. Compiling to bytecode In Java. Java source code is compiled into bytecode when we use the javac compiler. The bytecode gets saved on the disk with the file extension . instead of from byte-code each time it was used. where code is compiled from high level language. the bytecode is converted.class file merge that byte code to that blank *. they are compiled into Bytecode (as discussed earlier). but more modern compilers cache this machine code for reuse on the machine. Here's a recap of the concepts we'd seen earlier and some additional details. before the program could start. using the Just-In-Time(JIT) compiler. Even then. This is different than other languages like C/C++ where the whole program had to be compiled to machine code and linked to create an executable file. So Java has four step compilation: • • • • compiler will check the syntactical error create byte-code create a blank *. A few examples include the following: • • • • Groovy Pizza GJ (Generic Java) – later officially incorporated into Java SE 5. NetREXX 19 Compilation Navigate Getting Started topic: • • • • • Installing Java on Your Computer Compiling programs Running Java programs Developing the first Java program Understanding a Java program We have already discussed compilation basics . java's JIT compiling was still faster than an "interpreter-language".class.
These are referred to generally as 'resources' and stored in directories local to the classes in which they're used. Subdirectories. The build tool would check all the needed dependencies and compile only the needed class for the build. Using Ant you would build your application from the command line by typing: ant build. . if that is missing. The Ant tool is the best and the most popular build tool currently available. if that was not compiled already. it must be in a directory with the same name as the package. A file containing a class in the unnamed package must be in the current/root directory.xml The xml file contains all the information needed to build the application.. Example: Top level package.Java Programming/Print version 20 Automatic Compilation of Dependent Classes In Java. MainClass. because javac does not copy them to wherever the . or you are compiling for servlets or a "jar" package.java.wikibooks. For compilation.. and this continues until all classes are compiled that are needed to run the program. examples of which are listed below.) When this occurrs.en.example. Packages. Java programs often contain non-code files such as images and properties files. IDE stands for Integrated Development Environment. you can't rely on this feature if your program is using reflection to create objects. These automatic compilations are nested.java. this file could be stored as /com/example/resources/icon. A class with this package declaration package example. ServletOne. the class belongs to the unnamed package. The best way is to use a build tool to build your application. and Resources Each Java top level class belongs to a package (covered in the chapter about Packages). if the class belongs to a package. These resources present a problem when a program is complied. In these cases you should list these classes for explicit compilation.png.. it is up to the programmer to move the resource files and directories. if you have used any reference to any other java object. The next most popular way to build applications are using an IDE. your code may appear to compile correctly but the program behaves as if you were using an old copy of the source code (or otherwise reports errors during runtime. The convention is that package names and directory names corresponding to the package consist of only lower case letters. It is usually enough to compile only the high level class. . then the class for that object will be automatically compiled. javac .class files are being complied to (see above).. A class with this package declaration package org. javac . has to be in a directory named example Example: Subpackages. if the class com. since all the dependent classes will be automatically compiled.png file.ExampleApp uses the icon. Note: In rare cases. MainClass. For example. See also the section on how to automate this using ant. the file must be in the right directory structure. This may be declared in a package statement at the beginning of the file. has to be in the org/wikibooks/en directory. you may need to clean your compilation folder by either deleting the class files or using the Clean command from the IDE.java However...
. . One of the advantages in integration with together. views.apcomputerscience. the file contains. and later IBM Rational Software Development Platform are built on Eclipse. With the online error checking only. IBM was behind this free software development and it replaced IBM Visual Age tool.) Code refactoring (global reference update... JBuilder JBuilder is a IDE with proprietary source code. The Java class name is case sensitive. a modeling tool..) Built in debugger (full featured GUI debugger) Source code generation (getters and setters... Microsoft tools give a standard way of developing code in the Microsoft world. .Java Programming/Print version below.... coding can be speed up by at least 50%(coding does not include programming).) As you type error checking (immediate error indications. Compiler Options Additional Tools IDEs This section contains a little about the different IDEs available and their strengths and weaknesses. browsers.) Automatic build (changed source code automatically compiled. . explorers. Standard Eclipse features: • • • • • • • • Standard window management (perspectives.. The idea was to create a standard look and feel that can be extended..com/ide/jcreator/index.. ..) As you type help window (type . The naming convention for the class name is for it to start with a capital letter. sold by Borland. The extendibility has distinguished Eclipse from other IDE tools.. as is the source file name. . .. plus a developer tool framework that can be extended for a particular development need. . with big success so far. The goal for Eclipse are twofold: • Give a standard IDE for developing code • Give a starting point...) • Plug-in-based architecture (be able to build tools that integrate seamlessly with the environment and other tools) • . and the same look and feel for all other more sophisticated tools built on Eclipse IBM's WSAD. or <ctrl> space. for references. Eclipse also meant to compete with Microsoft Visual Studio tools. JCreator There's info at: http://www. There can be only one public class defined per file.htm Eclipse Eclipse is a free IDE. 21 Filename Case The Java source file name must be the same as the public class name.) Searches (for implementation. Eclipse gives a similar standard way of developing code in the Java world..
tek-tools. Some of these tasks may depend upon others having been done previously (not creating a jar unless the program has been complied. com/ javaworld/ jw-06-2000/jw-0602-iw-kawa. Ant is a build management tool designed to replace make as the tool for automated builds of large Java applications. It is basically a Java editor which does not include wizards. web and mobile applications.0. It is easy to install and use straight out of the box. and the MacOS. com/ kawa/ ). Ant makes all of these things easy.bluej. See a javaworld article (http:/ / www. More info can be found at http://www.Java Programming/Print version For more information see.xml file.e.org/products/ide/ BlueJ BlueJ is an IDE that includes templates and will compile and run the applications for you. Ant is designed to be platform independent. Below we give an example build. BlueJ offers an interesting GUI for the creation of packages and programs. you can download it.apache. olabs. That sets the classpath for all compilations that come out of it to include those you have added and the BlueJ libraries. Ant parses this file and executes the tasks therein. please see the Ant Wikibook.xml file.g. running tests and so on. packaging the program into a jar file.eclipseplugincentral.netbeans. Ant For comprehensive information about all aspects of Ant. It might also be a good idea to not execute all tasks every time the program is complied -. The latest version is 4. and GUI tools. please see the project website at http://ant. • Eclipse (http://www. javaworld. Those tasks may include not only compiling the code. For more information about writing your own Ant tasks. BlueJ Homesite (http://www.eclipse. for instance).org/). and the Ant tool will load it at runtime. open-source Integrated Development Environment for software developers. The tasks and their dependencies are defined in a build.org) Kawa Kawa was developed by Tek-Tools. so it can be extended if there is a task you'd like to be done during the build that is not in the pre-defined tasks list. generally kept in the root directory of the java project. . Like Java. Classes are represented as boxes with arrows running between them to represent inheritance/implementation or if one is constructed in another. It is very easy to hook your ant task code to the other tasks: your code only needs to be in the classpath. • Plugincentral (http://www. The NetBeans IDE provides developers with all the tools they need to create professional cross-platform desktop. Solaris. BlueJ adds all those classes (the project) into the classpath at compile time. to only compile changed source files. (http:/ / www. Linux. see Kawa from tek-tools (http:/ / www.html) It looks that there is no new development for Kawa. and unlike make. enterprise. Building a Java application requires certain tasks to be performed. It is best suited to experienced Java programmers in small and midsized development teams. com/ kawa/ ) For more info. The IDE runs on many platforms including Windows. but also copying files.org/. BlueJ is often used by classes because it is not necessary to set classpaths. Ant tool is written in Java and is open source. BlueJ has it's own sets of Library's and you can add your own under preferences.com/) 22 NetBeans The NetBeans IDE is a free.
0"?> 23 <project name="ExampleApp" basedir=".manifest.xml file.dir" value="libraries" /> <property name="build.dir}"/> </target> <target name="compile"> <mkdir dir="${classes.java" /> </copy> </target> <target name="build" depends="compile"> <mkdir dir="${dist.dir}"/> <javac srcdir="${source.dir}" excludes="**/*.Java Programming/Print version Example build.name}.classpath"> <fileset dir="${dist.dir}/lib" includes="*.dir}" destdir="${classes. <?xml version="1.dir}/classes" /> value="${build.name}.manifest.jar"/> </path> <target name="clean"> <delete dir="${build.dir}" classpathref="classpath" /> <copy todir="${classes.classpath" jarfile="${dist.classpath}"/> <attribute name="Main-Class" value="${main-class}" /> </manifest> </jar> </target> ." default="main"> <property name="source.dir}/${ant.jar"> <classpath refid="dist.dir}" includes="**/*.dir}/${ant.dir}/lib" flatten="true"> <path refid="classpath" /> </copy> <path id="dist.project.dir" <property name="main-class" value="build" /> value="${build.jar" > <zipfileset dir="${classes.example.ExampleApp"/> <path id="classpath"> <fileset dir="${libraries.dir}"> <fileset dir="${src.dir" <property name="classes.dir}"/> <copy todir="${dist.project.dir" value="source" /> <property name="libraries.jar" /> </path> <manifestclasspath property="dist.dir}/dist" /> value="com.dir}" /> <manifest> <attribute name="Class-Path" value="${dist.dir" <property name="dist.classpath" /> </manifestclasspath> <jar destfile="${dist.
As Java applications run.name}. When a method is called repeatedly.name}.Java Programming/Print version 24 <target name="run-build" depends="build"> <java jar="${dist. which runs very fast.project. they tend to become faster and faster. the JIT compiler analyzes the bytecode and produces highly efficient machine code. Methods or code blocks which do not run often receive less optimization.dir}/${ant.run"/> </project> The JIT compiler The standard JIT compiler runs on demand.project.dir}" /> </classpath> </java> </target> <target name="clean-build" depends="clean. those which run often (so called hotspots) receive more profiling and optimization.jar" fork="true"> <classpath> <path refid="classpath"/> <path location="${dist. so as the application runs. because the JIT can perform runtime profiling and optimization to the code to meet the execution environment. compilation happens only as needed.dir}/${ant. The JIT compiler is smart enough to recognize when the code has already been compiled. Java Programming Overview Getting Started Fundamentals Index Classes and Objects Collections Exceptions Applets JavaBeans Concurrent Programming Reflection Libraries Execution Navigate Getting Started topic: • • • • • Installing Java on Your Computer Compiling programs Running Java programs Developing the first Java program Understanding a Java program .jar"/> </classpath> </java> </target> <target name="run" depends="compile"> <java classname="${main-class}" > <classpath> <path refid="classpath"/> <pathelement location="${classes.build"/> <target name="main" depends="clean.
JSE (Java Standard Edition) had/has the following in its belt: • GUI capabilities (AWT. either from the file system or from a *.) java Main Java 'jar' class libraries Utility classes. A complex Java application usually uses third party APIs or services. The application stops when the main() method exits. The user interface could be either AWT or Swing. JSE code execution Java language first edition came out in the client-server era. Java first edition. The application may or may not use a database to persist data. by the JVM. he needs to launch the application by the following command line (where Main is the name of the class containing the main() method. and/or third party classes are usually packaged and distributed in Java ' *. These 'jar' files need to be put in the CLASSPATH of the java program from which these classes are going to be used. In this section we list the most popular ways a piece of Java code may be packed together and/or executed. Classes are loaded to memory and compiled as needed. framework classes. it can be run from the command line: java -jar Application.jar . Invocation of Java programs distributed in this manner requires usage of the command line. If a jar file is executable.Java Programming/Print version 25 There are various ways in which Java code can be executed. The application would start with a main() method of a Class.jar file. or if an exception is thrown from the application to the JVM. Once the user has all the Figure 1: Stand alone execution class files. Thick clients were developed with rich GUI interfaces.jar' files. Swing) • Network computing capabilities (RMI) • Multi-tasking capabilities (Threads) With JSE the following Java code executions are possible: Stand alone Java application (Figure 1) Stand alone application refers to a Java program where both the user interface and business modules are running on the same computer.
incompatibilities between browser JVM implementations. and it can be accessed and used by the Browser client. This is the container of rendering HTML text. and the back-end business logic code. Having a standard client. Web Applications For applications needed by lots of client installations. Maintaining and upgrading the hundreds or thousands of clients caused a problem. Java has built-in support to render applets in the browser window. and a back-end part. In this case Java objects are transmitted across the network. it makes sense to create a unified. by the <APPLET> tag. There are three main area of interest in Web Applications. Wikipedia has an article about Java Applets. separate graphic-like animation applets. in this book. with sample code. Client Server applications The client server applications consist of a front-end. More information can be found about applets at the Applet Chapter. the client-server model did not work. • One way is to define a data communication protocol between the two tiers. Text data are sent back and forth between Browser and the Server • The Web server to serve static content. standard client. The communication between the front-end and the back-end can be achieved by two ways. those are: • The Web Browser. standard back-end service as well. Web Application is an application that is running in the Application Server.com (http://java. . • The other way is to use Java Remote Invocation (RMI). Sophisticated GUI clients were found hard to develop. for all applications. Based on the protocol it interprets the request and sends back the result in data form. The idea is that the business logic would be on the back-end part of the program. and that is the Application Server. Here the challenge is to achieve a separation between front-end user interface code. mostly because of download time. and communication Figure 2: Applet Execution requirements back to the server. however. The back-end part would listen for an incoming request. Application server to serve dynamic content and host EJBs. It was not practical.Java Programming/Print version 26 Java Applet code (Figure 2) Java Applets are Java code referenced from HTML pages. at the Client Server Chapter in this book. which would be reused by all the clients. the plugin is still available as a free download from java. The Java code is downloaded from a server and runs in the client browser JVM. and are most commonly used as small. and that is the Browser. More information can be found about client-server programming. Also. With the use of RMI. a remote object can be created and used by the client. and running client scripts • The HTTP protocol. The popularity of Java declined when Microsoft withdrew its Java support from Internet Explorer default configuration. Applets are rarely used today.com/). Wikipedia also has an article about Web application. The solution to this problem was to create a unified. both running on a separate computer.
There are two main solutions to this: • Use a template tool (Store the presentation part in an HTML file. • Connecting to a database if needed. more specifically with J2EE servlets. Depending on the complexity of the data. JSP should be used to render the presentation of the static and dynamic data. with Java version 2. 100% separation is not easy to achieve. Java Server Pages (JSP) code (Figure 4) JSP is an HTML file with embedded Java code inside. J2EE added : • Components Base Architecture. Figure 3: Servlet Execution Figure 4: Jsp Execution Using the JSP technology correctly. This servlet outputs HTML which has inside the result of the business logic computation. JSP. They are meant to replace CGI programming for web pages rendering dynamic content. The servlet is running in a so called servlet-container/web container. marking the areas that need to be replaced after business logic computations). • Create HTML to present to the user through the browser The HTML output represents both the presention logic and the results of the business computations. There are special JSP tags that helps to add data dynamically to the HTML. Using custom tags.Java Programming/Print version 27 J2EE code execution As the focus was shifting from reaching GUI clients to thin client applications. This is advocated also in MVC architecture (see below). This represents a huge problem. business logic computations should not be in the embedded Java part of the JSP. Sun introduced J2EE (Java 2 Extended Edition). Also JSP technology allows to create custom tags. . EJB Containers) With J2EE the following Java component executions are possible: Java Servlet code (Figure 3) Java got its popularity with server side programming. and there is no real application relying only on servlets to handle the presention part of the responsibility. The first time the JSP is accessed. The servlet's responsibility is to: • Handle the request by doing the business logic computation. the JSP is converted to a Java Servlet. (Servlet. Servlets are running in a simple J2EE framework to handle client HTTP requests. • Use JSP (See next section) Wikipedia also has an article about Servlets. however may help to get closer to 100%.
In modern web applications. The Enterprise Java Beans (EJBs) were supposed to address this. a trend started. An EJB should be designed as a separate application in the enterprise. That resulted in many small separate applications in a Company/Enterprise. A new philosophy. with the client server computing. Figure 5: EJB Execution An EJB is an application component that runs in an EJB container. Many times the same data was used in different applications. The client accesses the EJB modules through the container. transaction management.. Figure 6: MVC Execution • The view (User Interface Logic) contains the logic that is necessary to construct the presentation. the separation between data. • The servlet acts as the controller and contains the logic that is necessary to process user events and to select an appropriate response. please see MVC. "Enterprise Computing". EJBs have the same problems as any reusable code: they need to be generic enough to be able to be reused and the changes or maintenance of EJBs can affect existing clients. The idea was to create components that can be reused throughout the Enterprise. The container manages the life cycle of the EJB modules. never directly. This could be handled by EJB technology. • The business logic (model) actually accomplishes the goal of the interaction. Combine J2EE components to create an MVC architecture This leads us to the three layers/tiers as shown in (Figure 6). was created to address these issues. how the data is presented to the user became very important and usually needs the help of a graphic artist. . To help programmers and graphic artists to work together. For more information about MVC. and how it is presented became crucial. that is to move away from Mainfraim computing. .. Many times EJBs are used unnecessarily when they are not really needed. and handles all the issues that arise from network/enterpise computing. code. Some of those are security/access control. fulfilling one function. .Java Programming/Print version 28 EJB code (Figure 5) In the 1990s. object pooling. This might be a query or an update to a database. with lots of static data and nice graphics. This could be handled by JSP technology.
) • Consumers of JINI services do not need prior knowledge of the service's implementation. First Java Program Navigate Getting Started topic: • • • • • Installing Java on Your Computer Compiling programs Running Java programs Developing the first Java program Understanding a Java program Hello World Generally when you first start programming in any language.) A minimal Jini network environment consists of: • One or more services • A lookup-service keeping a list of registered services • One or more consumers Jini is not widely used at the current writing (2006). The basic features of Jini are: • No user intervention is needed when services are brought on or offline.Java Programming/Print version 29 Jini After J2EE Sun had a vision about the next step of network computing. make sure you know how to compile and run Java programs. That is Jini. Jini would allow these services/consumers to interact dynamically with each other in a robust way. There are two possible reasons for it. The other reason is that Microsoft pulled out from Java. without configuration and user intervention. (In contrast to EJBs where the client program has to know the server and port number where the EJB is deployed. as Java is case sensitive. Pay close attention to capitalization. One is Jini a bit complicated to understand and to set it up.java. The main idea is that in a network environment. The implementation is downloaded dynamically and run on the consumer JVM. world!"). Again. Open your IDE and write the following text. You guessed it. (For example. Compile and run it: . you'll start with the traditional Hello World example. it's Hello World! Before starting this exercise. let's start building your first Java program. The implementation of the user interface code would be provided by the service being used. (Services periodically need to renew a lease to indicate that they are still available. which caused the industry to turn to the use of proprietary solutions. the end user may be presented with a slightly different user interface depending upon which service is being used at the time. make sure that the filename is the same case as the class name. That said. to discover.out.println("Hello. in Jini the client is supposed to find. } } Save it as HelloWorld.) • Self healing by adapting when services (consumers of services) come and go. the service in the network. public class HelloWorld { public static void main(String[] args) { System. there would be many independent services and consumers.
length > 0) { who = args[0]. args is a method parameter which will contain the command line arguments used to run the program. The main method is where Java will start executing the program. if (args.java java HelloWorld Your computer should display Hello. you don't need to know a whole lot about them. For more information on modifiers such as public and static. Modifying the Program Now. The System.Java Programming/Print version javac HelloWorld. public static void main(String[] args) { begins a Java method named main. world! 30 Line-by-line Analysis The first line of the class. It should display .println("Hello. " + who + "!"). public class HelloWorld { declares a Java class named HelloWorld.it is available to any other class. } System.println("Hello. The method must be both public and static for the program to run correctly. if you invoke the program as java HelloWorld wikibooks it will print Hello. and modify it to read public class HelloWorld { public static void main(String[] args) { String who. } } Run it again.out. wikibooks! Go back to the program. though at this level. see Access Modifiers. world! to the console (with a line terminator). world!"). The next line. if there is one. along with the greeting. statement sends the text Hello. This class is declared public . we will modify this program to print the first command line argument. For example. The final two braces mark the end of the main method and the end of the class. } else { who = "World".out.
for more information see w:Java (programming language)#Hello world 31 Common Problems If the program does not work as you expect. Visit Understanding a Java Program which presents key Java language features along with a more complete explanation of the syntax and structure of a basic Java program. wikibooks! or. The Next Step Now that you have seen the classic Hello. if you do not pass a command line parameter. let's move on to a more realistic example which highlights the object oriented nature of Java. World program in Java.Java Programming/Print version Hello. World! Wikipedia has a hello world sample for each type of java code. modern IDEs would try coloring the entire source as a quote in this case.java extension. check the following common errors. Are you launching the javac or java correctly? Javac requires the full filename with the . Overview • About this book • Installing the Java platform • History of Java • The Java platform • Java Runtime Environment (JRE) • Java Development Kit (JDK) • Similar concepts • Overview of the Java programming language Getting started • The process of abstraction • Working with objects • Exercise 1: Writing your first Java program Compiling programs Running programs Your first program Anatomy of a Java program • • • • . while java requires the class name itself. • • • • Are you sure all words are spelled correctly and with the exact case as shown? Are there semicolons and brackets in the appropriate spot? Are you missing a quote? Usually. it will simply print Hello.
literals and expressions Types String Classes. objects and data types Syntax Keywords Packages Nested classes Access modifiers Data and variables Generics • Java security Classes and objects • • • • • • • Defining classes Creating objects Interfaces Using static members Destroying objects Overloading methods and constructors Class loading Basic arithmetic • Math functions • Large numbers • Random numbers Collections • Introduction • Arrays • Collection Classes • Collections and maps • Sets. lists and queues • Thread Safe Collections • Comparing Objects .Java Programming/Print version 32 Language fundamentals • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers.
Java Programming/Print version 33 Exceptions • Throwing and catching exceptions • Application exceptions • Runtime exceptions • NullPointerException • Main Exception classes Stack trace Checked exceptions Unchecked exceptions Preventing NullPointerException Chained exceptions(Nesting Exceptions) • • • • • User Interface topics • • • • Basic I/O Event handling Graphics Applets Swing programming • • • • • First Example: Frames and Labels Events and Buttons A Responsive Application Layouts Building a calculator Advanced Topics • • • • • • • • • • • • • Byte code Concurrent programming Networking Database programming Reflection JavaBeans Design patterns Libraries. extensions and frameworks 3D programming Threads Remote Method Invocation (RMI) and Internet Inter-Orb Protocol (IIOP) Profiling Monitoring .
But let's not get too far ahead of ourselves! The Distance Class: Intent. we will get to that soon. int y0. .awt. because the program is intended as a simple introduction.Point point0.Java Programming/Print version 34 Extra Reading • Java IDEs Appendices • Links • Glossary • Index Understanding a Java Program Navigate Getting Started topic: • • • • • Installing Java on Your Computer Compiling programs Running Java programs Developing the first Java program Understanding a Java program This article presents a small Java program which can be run from the console.java. } public static void main(String[] args) { Distance dist = new Distance( intValue(args[0]). then copy the source below and paste it into the file and save the file.Point(x1.Point(x0. } public void printDistance() { System. It computes the distance between two points on a plane. int x1. and later in the module we will show some of these improvements. Source. Also. y0). point1.println("Distance between " + point0 + " and " + point1 + " is " + point0. first create a file named Distance. point1 = new java. y1). public class Distance { private java.out. int y1) { point0 = new java.awt.distance(point1)). public Distance(int x0. intValue(args[1]). intValue(args[2]).awt. so using your favorite editor or Java IDE. it has some room for improvement. and Use This class is named Distance. You need not understand the structure and meaning of the program just yet. intValue(args[3])).
parseInt(data). y1) For example. We will provide here only enough description of the syntax to grasp the above program. you supply it with the x and y coordinates of two points on a plane.y=3] and java.java (If the javac command fails.y=5] and java. someone with understanding of other procedural languages such as C.printDistance(). the command java Distance 0 3 4 0 will compute the distance between the points (0.) To run the program. y0) and (x1.awt. There are different kinds of tokens.awt. you may wish to review the source to see how much you might be able to understand. } } At this point. Detailed Program Structure and Overview As promised.0 The command java Distance -4 5 11 19 will compute the distance between the points (-4. or other OO languages such as C++ or C#. there are word tokens such as class and public which represent keywords .Point[x=-4.3) and (4.y=0] is 5.y=19] is 20. will be able to understand most if not all of the sample program. compile the program: javac Distance.5) and (11. we will now provide a detailed description of this Java program. review the Java installation instructions. We will discuss the syntax and structure of the program and the meaning of that structure. } private static int intValue(String data) { return Integer.19): Distance between java.) The command sequence is java Distance x0 y0 x1 y1 to compute the distance between the points (x0. Once you save the file.518284528683193 35 We'll explain this strange looking output. Java programs consist of a sequence of tokens. (For this version of Distance.0) and print the following: Distance between java. . Introduction to Java Syntax The syntax of a Java class is the characters and symbols and their structure used to code the class using Unicode characters.Point[x=0. and also show how to improve it.Point[x=11.Java Programming/Print version dist.special words with reserved meaning in Java. only integer points are supported.awt. For example. later. A fuller treatment of the syntax elements of Java may be found at Syntax.Point[x=4.awt. While perhaps not being the most literate of programming languages.
such as 1 and 3. such as "Distance between ". These two fields could also have been declared with two separate but more verbose declarations. The public keyword is an access modifier which declares that this class and its members may be accessed from other classes. such as periods .Java Programming/Print version Other words (non keywords such as Distance. the type is java. identifies this declaration as a class. and commas . This means that we are defining a class named Distance. For our example. point1. which we will explain in turn. Three method declarations Example: Instance Fields The declaration private java.awt package. or in our case. The value of point0 in the first Distance object can vary independently of the value of point0 in the second Distance object. whitespace is required between keywords and identifiers: publicstatic is a single identifier with twelve characters. Some tokens are punctuation. Instance fields represent named values that are allocated whenever an instance of the class is constructed. declares two instance fields. 3. When a Java program creates a Distance instance. A class declaration provides the name and visibility of a class. The class declaration is then followed by a block (surrounded by curly braces) which provides the class's definition. The names of the instance fields in a comma separated list.Point. and semicolons . point0.Point point0. the command line. it will contain space for its own point0 and point1 values. The body of a class is one such block. not two Java keywords. For example. public and class followed by the identifier Distance. public class Distance is the class declaration. which means these instance fields are not visible to other classes.the declaration and definitions of the class's members. There are also left and right braces ({ and }) which enclose blocks. 2. and operators such as + and = are used to express basic computation such as addition or String concatenation or assignment. This declaration consists of: 1. Identifiers have many different uses in Java but primarily they are used as names.awt. Two field declarations.. and newlines. to separate tokens. . A constructor declaration 3. String literals. The definition is the implementation of the class . named point0 and point1 2. tabs. You use whitespace such as spaces. these are known as literals. In this case. This class contains exactly six members. and printDistance) are identifiers. obviously. 1. This is the class Point in the java. 36 Declarations and Definitions Sequences of tokens are used to construct the next building blocks of Java classes: declarations and definitions. can refer to the class by this name. The type of the instance fields. The private access modifier. Java also has tokens to represent numbers. The class keyword.awt. x1. consist of zero or more characters embedded in double quotes. Other classes. that instance will contain space for point0 and point1. When another Distance object is created. It consists (in this case) of two keywords. Java also allows declarations of interfaces and (as of Java 5) annotations.
as in private java.Point(x1. y1).Java Programming/Print version private java. The constructor body.awt. The constructor name. public Distance(int x0. The special Java literal.awt. y0). 4. (x0. the constructor is declared public 2.awt. This constructor's body contains two statements. Java integer values are signed. Distance implicitly extends from java. } This constructor accepts four parameters. 5. y1). While you can explicitly assign null values in a declaration.awt. x1 and y1. 37 . Java will implicitly initialize the values of point0 and point1 to null when a Distance instance is created. The constructor parameters.awt. int y0. Java will automatically initialize the fields point0 and point1 to null. y0). The first allocates an object representing the first point. In this case. which is a Java block (enclosed in {}). The second statement allocates a second java. which must match the class name exactly: Distance in this case. The null value means that a reference value does not refer to an object. int x1.Point objects.Point point0. Since the types of these fields is a reference type (i. The optional access modifier(s). The constructor can perform initialization for the object. This constructor does not declare any exceptions. Even if a constructor does not have any parameters. The two assignments in this constructor use Java's new operator to allocate two java.Point point0 = null. point1 = new java.awt.Point point1 = null.awt. private java. The parameter list is required. For example. it is not necessary and most programmers omit such default assignments. y1) and assigns it to the point1 instance variable. null is used to represent the null value in a program.Object. The parameters in the parameter list are separated by commas. y0. The above constructor body is equivalent to the following body with the explicit super constructor call: { super(). The parameter list declares the type and name of each of the method's parameters.awt. It consists of five parts: 1. Java inserts a call to the super constructor as the first executable statement of the constructor if there is not one explicitly coded.lang.e. y0). beyond that which the Java VM does automatically. named x0.Point(x1. point0 = new java.Point(x0. An optional throws clause which declares the exceptions that the constructor may throw. private java.Point point1.Point(x0. which in this example is int for all four parameters. 3. and assigns it to the point0 instance variable (replacing the null value that the instance variable was initialized to). int y1) { point0 = new java.awt. Below is the constructor for this class.Point instance with (x1. Each parameter requires a parameter type declaration. Example: Constructor A constructor is a special method in a class which is used to construct an instance of the class.awt. This is the constructor for the Distance class. a field that refers to or can hold a reference to an object value). 32 bit twos complement integers. point1 = new java. you must specify the empty list ().
(Actually. This method uses the syntax "Distance between " + this. We take advantage of existing behavior already implemented in the Java platform rather than implementing it again.point1 + " is " + this.distance(point1). The body of the printDistance method could also be coded as System.out.point1)). However.point1 + " is " + this. We can change this implementation without altering how clients use and invoke the class. to make the instance field references more explicit. The method operates on point0 (binding this to the object that point0 refers to during the execution of the method) and accepting another Point as a parameter.distance(point1)).println("Distance between " + this.point0.awt.out. such as simply storing the coordinates of the two points and computing the distance as . the key point is that this class uses information hiding. and intValue() The printDistance() method The printDistance() method prints the distance between the two points to the standard output (normally the console). } 38 } This instance method executes within the context of an implicit Distance object. The instance field references.) The result of the distance() method is a double precision floating point number. point0 and point1. We will see later how to make the program more flexible without adding much complexity. how the class stores its state or how it computes the distance is hidden.point0 + " and " + this. main().distance(this.awt. You can also use the special variable this to explicitly reference the current object. Within an instance method. Java binds the name this to the object on which the method is executing.point0.Point class (of which point0 and point1 are instances.Java Programming/Print version } While it is true that this class could be implemented in other ways.Point class. This class contains three methods in which the behavior of the Distance class is defined: printDistance().distance(this.point0 + " and " + this.println("Distance between " + point0 + " and " + point1 + " is " + point0. This choice matches the abstract definition of this class: to print the distance between two points on the plane. This method both computes the distance and prints it in one statement. but we'll explain later. and the type of this is that of the current class. That is. refer to instance fields of that implicit object. it is slightly more complicated than that. because we choose to use object abstractions here.point1) } . distance() is an instance method of the java. this class instead uses the existing java. public void printDistance() { System. The distance is computed with point0. Example: Methods Methods are the third and most important type of class member.
intValue().y=0] is 5. The command java Distance 0 3 4 0 instructs Java to locate the Distance class.parseInt(data).printDistance(). In order to print. intValue(args[3])). } This method is private since. which is the type of the static field out in the class java.Point[x=0.io.) Any Java class that you want to invoke from the command line or desktop shortcut must have a main method with this signature. The intValue() method The intValue() method delegates its job to the Integer.parseInt() directly. The main method could have called Integer. four times. the intValue() method simply makes the main() method slightly more readable. The main() method The main() method is the main entry point which Java invokes when you start a Java program from the command line. the result of this expression for the points (0. The intValue() takes a single string parameter and returns the integer value represented in the string. (We will introduce arrays shortly. intValue(args[1]). This is an instance method from java.0" which the method then prints to System. public static void main(String[] args) { Distance dist = new Distance( intValue(args[0]).out. For example.Point[x=4. This expression is a series of String concatenation methods which concatenates Strings or the String representation of primitive types (such as doubles) or objects.lang. dist.parseInt() method. 39 .awt. and returns a long string.0) is the String "Distance between java. we invoke the println(). private static int intValue(String data) { return Integer. put the four command line arguments into an array of String values.Java Programming/Print version to construct a String to pass to the System. } The main() method invokes the final method. intValue("3") will return the integer 3.out. like the fields point0 and point1.System. then pass those arguments the public static main(String[]) method of the class. intValue(args[2]). The Java VM binds System. it is part of the internal implementation of the class and is not part of the external programming interface of the Distance class.out to the standard output stream when it starts a program.PrintStream. For example.y=3] and java.awt.println().3) and (4.
args[1]. This program uses three separate reference types: 1. Distance 3. The Distance() constructor's four parameters also have the type int.Java Programming/Print version Static vs. the intValue() method has a return type of int. static methods cannot access instance methods or instance fields (such as printDistance()) or point0) directly. and 3 to access elements of the command line arguments. This means a call to that method. args[2]. The main() method can only invoke the instance method printDistance() method via an instance reference such as dist. Also. args[3]) // this is an error Since the type of the args array element is String. and the special variable this is not available. which represents 32 bit signed integer values. Java supports primitive types. void. String literals. The idea is similar to pointers in other languages like C. The static keyword tells the compiler to create a single memory space associated with the class. is an expression of type int. such as intValue(args[0]). The program also implicitly uses double. such a call would result in an error because Java cannot automatically convert values of type String into int values. character. with the reference type java. Each individual object instantiated has its own private state variables and methods but use the same static methods and members common to the single class object created by the compiler when the first class object is instantiated or created. double values are 64 bit IEEE floating point values. 1. primitive types. int. which is the return type of the distance() method of java. int. byte. float and double.String which is most commonly referred to as String. short. This means that the method executes in a static or non-object context . java. Java has several categories of data types: reference types.String (or simply String) 2. This program uses only one primitive type explicitly. Reference Types A reference type is a Java data type which is defined by a Java class or interface. char. such as "Distance between " are constants whose type is String. The main() method uses integer values 0. Instance Methods Both the main() and intValue() methods are static methods. 2. Reference types derive this name because such values refer to an object or contain a reference to an object. . This helps explain why the main method cannot call new Distance(args[0]. Objects and Types. Primitive Types In addition to object or reference types.Point. array types.Point For more information see chapter : Java Programming/Classes.awt.lang. Java represents sequences of character data.lang. and numeric values.there is no implicit separate instance available when the static methods run from various objects. and a special type. and our constructor's parameters must be int. The primitive types are used to represent Boolean. each of which are also Java language keywords. 40 Data Types Most declarations have a data type.awt. Java's primitive types are boolean. or String. As such. java. long.
Java Programming/Print version Array Types Java supports arrays, which are aggregate types which have a fixed element type (which can be any Java type) and an integral size. This program uses only one array, String[] args. This indicates that args has an array type and that the element type is String. The Java VM constructs and initializes the array that is passed to the main method. See arrays for more details on how to create arrays and access their size. The elements of arrays are accessed with integer indices. The first element of an array is always element 0. This program accesses the first four elements of the args array explicitly with the indices 0, 1, 2, and 3. (This program does not perform any input validation, such as verifying that the user passed at least four arguments to the program. We will fix that later.) void void is not a type in Java; it represents the absence of a type. Methods which do not return values are declared as void methods. This class defines two void methods: public static void main(String[] args) { ... } public void printDistance() { ... }
Comments in Java programs
See here for more information on that important topic.
• About this book • Installing the Java platform • History of Java • The Java platform • Java Runtime Environment (JRE) • Java Development Kit (JDK) • Similar concepts • Overview of the Java programming language
Getting started
• The process of abstraction • Working with objects • Exercise 1: Writing your first Java program Compiling programs Running programs Your first program Anatomy of a Java program
• • • •
Java Programming/Print version
Language fundamentals
• • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers, literals and expressions Types String Classes, objects and data types Syntax Keywords Packages Nested classes Access modifiers Data and variables Generics
• Java security
Classes and objects
• • • • • • • Defining classes Creating objects Interfaces Using static members Destroying objects Overloading methods and constructors Class loading
Basic arithmetic
• Math functions • Large numbers • Random numbers
• Introduction • Arrays • Collection Classes • Collections and maps • Sets, lists and queues • Thread Safe Collections • Comparing Objects
Java Programming/Print version
• Throwing and catching exceptions • Application exceptions • Runtime exceptions • NullPointerException • Main Exception classes Stack trace Checked exceptions Unchecked exceptions Preventing NullPointerException Chained exceptions(Nesting Exceptions)
• • • • •
User Interface topics
• • • • Basic I/O Event handling Graphics Applets
Swing programming
• • • • • First Example: Frames and Labels Events and Buttons A Responsive Application Layouts Building a calculator
Advanced Topics
• • • • • • • • • • • • • Byte code Concurrent programming Networking Database programming Reflection JavaBeans Design patterns Libraries, extensions and frameworks 3D programming Threads Remote Method Invocation (RMI) and Internet Inter-Orb Protocol (IIOP) Profiling Monitoring
there are word tokens such as class and public which are keywords. Identifiers have many different uses in Java but primarily they are used as names. literals and expressions Types String Classes.Java Programming/Print version 44 Extra Reading • Java IDEs Appendices • Links • Glossary • Index Syntax Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. String literals. Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security Java derives much of its syntax from the C programming language: basic assignment statement syntax.. will be very familiar to C programmers.. For example. and variable names. class names. literals Java also has tokens to represent numbers. expressions. such as "http://en. such as 1 and 3. Those words can not be used by the programers to name identifiers. method names. Tokens Java programs consist of a sequence of different kinds of tokens. .org/Java_Programming". Keywords Those are special words with reserved meaning in Java. Identifiers Other words (non keywords) are identifiers. . Unicode Java source code are built by Unicode characters. these are known as literals. control flow statements and blocks. consist of zero or more characters embedded in double quotes.wikibooks. etc.
String doubleQuote2 = "\\u0022". // "\u0022" doesn't work since """ doesn't work. The following demonstrate the use of Unicode escape sequences in other Java syntax: // Declare Strings pi and quote which contain \u03C0 and \u0027 respectively: String pi = "\u03C0". . whitespace You use whitespace such as spaces. π (which is the Greek Lowercase Letter pi) is a valid Java identifier. E. For example. in comments. but any Unicode character can be used as part of identifier names. not two Java keywords.PI. but not very readable. See Unicode escape sequences for full details. and newlines. declaration and assignment: double \u03C0 = Math. the following is a valid. double π = Math. A statement is the smallest building block of Java. Separators Some tokens are punctuation.PI. and in a string literal String pi = "π". Note that a Unicode escape sequence functions just like any other character in the source code. Thus. Blocks There are also left and right braces ({ and }) which enclose blocks. and commas . For example. π may also be represented in Java as the Unicode escape sequence \u03C0.Java Programming/Print version Operators And operators such as + and = are used to express basic computation such as addition or String concatenation or assignment. Unicode escape sequences may also be used to express a Unicode character. tabs. ") needs to be quoted in a string just like ". Comments are used to document the code. Comments Comments are not part of the executing code. and semicolons . 45 Unicode Most Java program text consists of ASCII characters. Statements A Block contains one or more Java statement(s). separated by semicolons. and in character and string literals. such as periods . // Declare Strings doubleQuote1 and doubleQuote2 which both contain " (double quote): String doubleQuote1 = "\"". whitespace is required between keywords and identifiers: publicstatic is a single identifier with twelve characters.. The body of a class is one such block. to separate tokens.g.. String quote = "\u0027". \u0022 (double quote.
{ // -. Wherever whitespace is optional in Java. the value zero.. The following method call passes a String literal "int count = 0.This is a nested block --} } Whitespaces Whitespace in Java is used to separate the tokens in a Java source file. or string data. Literals provide a means of expressing specific values in your program. in the following statement. true. • the form feed character (hex 0x0c). • Boolean Literals • Numeric Literals • Character Literals • Integer Literals • Floating Point Literals • String Literals • null Blocks Java has a concept called block that is enclosed between the { and } characters. For example. . After a block is executed all local variables defined inside the block is discarded. The literal 0 represents. • the tab character (hex 0x09). such as between access modifiers. character. zero or more whitespace characters may be used. natually enough. an integer variable named count is declared and assigned an integer value. { ." the boolean literal true and the special null value null to the method parse(): List items = parse("int count = 0.Java Programming/Print version 46 Literals Java Literals are syntactic representations of boolean.". Whitespace is required in some places.This is a block --} Blocks can be nested: { . and can be used where a single statetement is accepted. int count = 0. A block executed as a single statetement. one or more whitespace characters may be used. type names and Identifiers. called curly braces... and is used to improve readability elsewhere. numeric. Wherever whitespace is required in Java. null). go out of scope. // -. Java whitespace consists of the • space character ' ' (0x20)..
As this. Other Unicode space characters. Line separators are special whitespace characters in that they also terminate line comments. even at a higher reading speed. the following is not legal: publicabstractDistance distanceTo(Destination dest).println("Hello World!"). Indentation Java ignores all whitespace in front of a statement. between Distance and distanceTo.) . as the readability is higher. (The method body is easier to distinguish from the head.Java Programming/Print version • the line separators characters newline (hex 0x0a) or carriage return (hex 0x0d) characters. are not allowed as whitespace in Java. including vertical tab. } void printMessage() { System. } However. whereas normal whitespace does not. Whitespace is required between public and abstract. so the above would result in a semantic error. The following is lexically valid publicabstractDistance distanceTo(Destination dest). However. because whitespace is required between keywords and identifiers. and between Destination and dest. } public static void main(String[] args) { printMessage().println("Hello World!").out. but means something completely different: it declares a method which has the return type publicabstractDistance It is unlikely that this type exists. these two code snippets are identical for the compiler: public static void main(String[] args) { printMessage().out. } void printMessage() { System. 47 Required Whitespace Below is the declaration of an abstract method taken from a Java class public abstract Distance distanceTo(Destination dest). the first one's style (with whitespace) is preferred. between abstract and Distance.
if you're lucky) box with your name on it. A tiny (or big. As we have already discussed. Statements can be anything from a single line of code to a complex mathematical equation. Let's put another type of data within it. . The code would look something like this: Listing 1.2: Putting a number with decimal point inside an integer variable. birthdays too. } } The class declaration and function declaration will be described in the upcoming chapters. Think of a present you've ever gotten. short. Just like int. Variables Christmases are usually exciting. float.1. The word int tells us what is inside the age variable. we talked about initialization of a variable and setting its value. int age = 24. Here the variable we have just created is age. This all might sound a bit too technical.1: A simple assignment statement. We explored the statement in Listing 1.Java Programming/Print version 48 Statements Now. we are geared towards understanding how programs are actually made. int actually stands for integer – a number. the number 10. Within that simple definition. it would look similar to this: public class MyProgram { public static void main (String[] args) { int age = 24. If the above statement was the only one in the program. They are tiny little boxes in the computer's memory that save something within themselves. On the right to this variable is the value of the variable which is the number 24. Let's say.5. Now think of variables as something similar. This something within them is called a value. Consider the following line: Listing 1. Data types Take a look at the code in Listing 1. Where do you find statements Java. These instructions are called statements in Java. where the variable held an integer within in. but it will make sense as you read ahead. This line is a simple instruction that tells the system to initialize a variable and set its value as 24. These are hence called data types. boolean. The function in turn is placed within a class declaration. char or String. A program is a set of instructions – very simple tasks provided to a computer. long. What exactly are statements? Statements are a single instruction in a program – a single unit of code. This section helps you understand what statements are and how they work. in the same style as C and C++. double. All these tell us what type of data is within a variable. places statements within functions (or methods). we can use byte. that we have the Java platform on our systems and have run the first program successfully.1. And the one thing that makes them exciting are: you get presents.
4567f.647.036. all the way up to infinity.807. It's at the end of each statement. Now that we know that. Any number between -32. int c = 30.483.775. long l = 1234567890L.036. Assignment statements Up until now.5.56789. Why not float. What would make it right is when you assign a certain type to the variable that would accept numbers with decimal points – numbers with decimal points are called floating points.147. so 10. A clear indicator that a line of code is a statement is its termination with an ending semicolon. there are several reasons why not. did you notice why we used a double in listing 1. double d = 1234. you say? Well. We will look at the others as we go on. 10. int i = 1234567. This is actually wrong.854.372.223.2 as: Listing 1. The only difference between double and float is the addition of an f as a suffix after the float value.5: Multiple assignment statements. Data type byte short int long Any number between -128 and 127. float float f = 123. The above table only list the number data types. hence the statement by virtue of it is wrong. 1. Anything with a decimal point is not an integer. int and long but when it comes to floating point numbers.767. When you assign a value to a variable in a statement.3: The correct way to assign a type to floating point variables double age = 10. short s = 12345.483. 2.5 should be 10.768 and 32.5f as in: Listing 1. Any number between -2. Consider the example below: Listing 1. we can modify the code in Listing 1. If one was to write multiple statements.775. Values accepted Declaration byte b = 123. it is usually done on each separate line ending with a semicolon. we assign a value to those variables. If we'd used a float.3.223. we use float or double. This can be explained further by looking at the table below. Any number between -9.).372. In essence.5 can be anything – a float or a double but by a certain rule.5. and that's just what it is called.854. we would have to append the number with a f as a suffix. we've assumed the creation of variables as a single statement. int b = 20. float age = 10.648 and 2. Did you notice one more thing? The semicolon (. By definition (if you have been awake throughout your mathematics lectures) you'd know that integers are whole numbers: 0. . int a = 10. that statement is called an assignment statement.4: The correct way to define floating point numbers of type float.808 and 9. 49 Whole numbers and floating point numbers The data types that one can use for whole numbers are byte. short.147.Java Programming/Print version int age = 10.5f. and not a float? The answer is pretty simple. double Extremely large numbers beyond the scope of discussion here. Extremely large numbers beyond the scope of discussion here. it is given a certain type. So.
you can actually write a decent Java program that can sum up values. • A statement is a unit of code in programming. Think of your computer's memory as a shelf where you put all those boxes. nothing fancy.6. int b = 20. We've already talked of variables as being similar to gift boxes. before we more on to the next topic.6: Multiple assignment statement on the same line. Whenever you need a box (or variable).. int a = 10. • If we are assigning a variable a value. then it's called a literal. However. it's very difficult to read it back. you need to try and understand what the code below does. That's what the value of the last variable result would be.e. It would look more complex and incomprehensible written as it is in Listing 1. We will look more into the nature of identifiers and values in the section titled Identifiers. It doesn't look that intimidating at first. you can begin writing the next statement where you ended the first one as depicted below: Listing 1. The third however is a bit interesting. literals and expressions later.5 but with different variable names. but once you've got a significant amount of code. Now that we have looked into the anatomy of a simple assignment statement. you call its identifier (that's the name of the variable). 50 . • An assignment statement include three parts: a data type. The part of the third statement where you add the numbers. firstNumber + secondNumber is called an expression and the expression is what decides what the value is to be. calling secondNumber would give you 20 hence when you add the two up. So calling the variable identifier firstNumber gives you the number 10. int result = firstNumber + secondNumber. int c = 30. we can look back at what we've achieved.. Just like English. If it's just a plain value. the only problem with writing such code is. int secondNumber = 20. We know that. With the information you have just attained. Listing 1.Java Programming/Print version You do not necessarily have to use a new line to write each statement. Now. it's usually better to organize it in a way that makes sense. continue reading. i. The first two statements are pretty much similar to those in Listing 1.. variable name (also called an identifier) and the value of a variable. the statement is called an assignment statement. To learn more. the answer should be 30.7: Multiple assignment statements with expressions int firstNumber = 10.
but these methods can't take any action with just the blueprint . telling the car how to park and drive.Java Programming/Print version 51 Classes. characteristics. and speed. The members. Think of an object as a noun. Objects and Types Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. etc. For example. The methods really don't mean much unless associated with the sports car. The blueprint that lets us build our sports car object is called a class. and that they will be say. . as you will need to add methods and member variables first. its members as adjectives describing that noun. and its methods as the verbs that can be performed by or on that noun. describe the object. In Java. "race". "park". Some of the methods of the sports car could be "drive". The methods generally describe the actions associated with a particular object. Some of its members might be its height. but it does tell us that our sports car will have a member representing speed and color. literals and expressions Types String Classes. If you want to have a class called SportsCar. A class doesn't tell us how fast our sports car goes. Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security Objects and Classes An object is composed of members and methods. weight.java. its source file needs to be SportsCar.they need an object to have an effect. The class is created by placing the following in the source file: public class SportsCar { /* Insert your code here */ } The class doesn't do anything yet. a sports car is an object. An object's members just hold data about that object. or properties. a class is located in a file similar to its own name. The class also lays out the methods for us. and the same goes for the members. a number and a word (or hex color code). respectively. or what color it is. acceleration. also called data members. attributes.
Type When an object is created. In general there are four constructor types: default. goes from 0-60 in 12 seconds. Behind the scenes. which allows the method Object. As the name implies.operations(). A non-default constructor is designed to create an object instance with prescribed values for most. Type is basically a list of features/operations. Nothing would stop you.. this means assigning all the members values like null. The object can not be accessed directly in Java. We need this type when passing the object reference to a method as a parameter. and the end results are similar to copy constructor. It's important to understand what it is. See below: (new ClassName()). a reference to the object is also created. from your default sports car color from being red. tops out at 190mph. You can assign the created object reference to the class super class. a constructor builds the object based on the blueprint. zero. it comes with a list of features/operations listed in the user manual that guarantees that those will be there when the car is used. non-default.operations(). and cloning. so just keep the terminology under your belt. or to an interface the class implements: SuperClass objectRef = new ClassName(). This instantiation is brought about by one of the class's methods. a copy constructor creates a new instance to be a duplicate of an already existing one.clone() to perform a field-by-field copy. Generally. however. but this is generally bad programming style. objRefVariable. copy. if not all. // features/operations list are defined by the interface // features/operations list are defined by the SuperClass class . however one can easily create a constructor that do the same as a copy constructor.Java Programming/Print version 52 Instantiation and Constructors In order to get from class to object. which was created from a class. This is possible by implementing the Cloneable interface. that can be performed through that object reference. In Java. because all the object memory is copied at once to destination cloned object. colorless. When you create an object from a class by default its type is the same as its class. and then using the assignment operator to equivocate them. As its name implies. You can assign this to a variable having the same type as the class: ClassName objRefVariable = new ClassName(). Java has the concepts of cloning object. called a constructor. A copy constructor is not included in the Java language. this means that computer memory is being allocated for the instance. of the object's members. This is not possible in all languages though. A default constructor will build the most basic instance. Cloning an object is faster than creation with the new keyword. and can be used. When a car is created. we "build" our object by instantiation. this can be also accomplished by creating the instance with the default constructor. . Instantiation simply means to create an instance of a class. This object reference has a type assigned to it. only through this object reference. but remember that an instance refers to a specific object. or an empty string. Java does strong type checking. Interface inter = new ClassName(). It means that all the features/operations the class defined are there and available. Another programmer would be confused if your basic car came out red instead of say. The object reference type basically is a contract that guarantees that those operations will be there at run time. and values are being assigned to the data members. etc. The car is red. Instance and object are very similar terms and are sometimes interchangeable.
In Java. public class OuterClass { .. When the car key is passed from an adult to a child we replacing the user manuals. Child user manual.. Power user manual. the 'AdditionalClass' can be accessed only in the same package. until we reach the Object • up casting: to an interface the class implements • down casting: until we reach the class the object was created from Type and Type Casting will be covered in more details later at 'Java Programming/Types' module. Each type of user manual describes only those features/operations appropriate for the type of driver. } Because they have the "package (default)" access specifier.Java Programming/Print version In the car analogy. The Power driver may have additional gears to switch to higher speeds.. However.. Average user manual. the created car may have different Type of drivers. casts can occur in three ways: • up casting: going up in the inheritance tree. However. including multiple classes in one file may increase the difficulty in examining the structure of a given application.. that are not available to other type of users. 53 Multiple classes in a Java file Normally. . These "additional" classes compile to separate ". that is called Type Casting. } class AdditionalClass { .. a Java file can have one and only one public Java class. or Handicapped user manual. We create separate user manuals for them. just as if they were in separate source files. a given file can contain additional non-public classes.class" bytecode files when compiled.
yourcompany. When we want to reference a Java class that is defined outside of the current package name space. literals and expressions Types String Classes. the package keyword is used at the top of each class file.YourClass.yourmodule. That is guaranteed to be unique. For example a company called 'Your Company Inc.com/cgi-bin/fun/java-tutorials-main. For Example. Class name collision is likely to happen. In those cases.yourapplicationname. would use a package name something like this: com.yourcompany. This "name space" in Java is called the package. In rare cases it can happen that you need to reference two classes having the same name in different packages. import com.yourapplication. package com.yourmodule. interactive Java Lesson (http://javalessons.YourClass .cgi?ses=ao789& code=ctr&sub=fun) Packages Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. To avoid having to type in the package name each time when we want to reference an outside class. Then we can refer to that class by just using the class name YourClass .yourapplication. The end product can easily contain hundreds of classes. To put a class in a package. To avoid this a Java class can be put in a "name space". . One of them needs to be referenced by typing in the whole package name.yourmodule. Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security Java Package / Name Space Usually a Java application is built by many developers and it is common that third party modules/classes are integrated. you can not use the import keyword for both classes. For Example. we have to specify which package name space that class is in. For Example.yourmodule.yourcompany.YourClass. So we could reference that class with something like com.Java Programming/Print version 54 External links • Constructors.yourcompany.yourapplication. we can declare which package the class belongs to by using the import Java keyword at the top of the file. For that reason Vendors usually use their domain name in reverse order. The Java package needs to be unique across Vendors to avoid name collisions.'.
expanded. If you change this to import javax. Furthermore. . you must ensure that the file is in the current classpath (both at compile.swing.SameClassName herObjectRef = new com.and execution-time). Apart from this requirement. instead of the point where you imported JFrame along with everything else in javax.*..swing. Importing packages from . Even though it seems as if it was imported. similarly: java -classpath libraries/lib..jar files If you are importing library packages or classes that reside in a .yourapplication.JFraim. Because of this.util. The Java package has one more interesting characteristic. the compiler is giving the error report at the first mention of JFraim.hercompany.. the class com. and javax. the package name corresponds where the actual file is stored on the file system. but then type JFraim frame=new JFraim(). is stored on the file system in the corresponding directory : com/yourcompany/yourapplication/yourmodule/YourClass.herapplication. which is half-way through your code. And that is actually how the compiler and the class loader find the Java files on the file system. package names should be lowercase.hermodule. import com.java And then to run it. and demands that MainClass be in the default package.jar source/MainClass (The above is simplified. This particular example is fairly unlikely.SameClassName(). the Java compiler will report an error similar to "Cannot find symbol: JFraim". if you import javax. if you use the above import to use JFrame. your code that uses Queue (java.swing. directory structure. the error will be at the import instead of within your code. but if you are working with non-Sun libraries. importing these packages and classes is the same as if they were in their full.hercompany. 55 Wildcard imports It is possible to import an entire package. it may cause problems if you make a typographical error..*.Java Programming/Print version package com. since in some operating systems the directory names are not case sensitive. and import java.mymodule. For example.yourapplication.. or a package called 'source'.myapplication. SameClassName yourObjectRef = new SameClassName(). it may be more likely to happen.youmodule. which isn't very desirable.yourmodule.YourClass.) .herapplication. .. using an asterisk: import javax.SameClassName.hermodule.Queue is later added in a future version of Java.*.util) will fail to compile.yourcompany. com.yourcompany.mycompany.jar file. While it may seem convenient.swing. For example.swing.jar source/MainClass. Example: To compile and run a class from a project's top directory (that contains the two directories /source and /libraries) you could use the following command: javac -classpath libraries/lib.
util name space. For the other we need to use the fully qualified name.Customer When we need to use both class in the same program file. • com. the HashMap class is in the java. literals and expressions Types String Classes.HashMap. Sometime is can happen that two class has the same name. In the above example. we can use the import keyword only for one of the class.Java Programming/Print version 56 Class Loading / Name Space A fully qualified class name consist of the package name plus the class name. but it can not have on the same package. is seen by the Virtual Machine as two completely different types. otherwise it would be the same class. A class can be nested: • inside another class. The runtime identity of a class in Java 2 is defined by the fully qualified class name and its defining class loader. This means that the same class. It can be said that the two class with the same name is in different name space.Customer • com. • or inside a method . Nested Classes Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. loaded by two different class loaders. Let be two Customer class with different name space (in different package). Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security In Java you can define a class inside an other class. the fully qualified class name of HashMap is java.bluecompany.redcompany.util. For example.
class".. private. This makes it very different from the nested class in C++. . usually with the name of the enclosing class. since the class instance is implied to be this... OuterClass.println( "Print Outer Class Instance Var. OuterClass outer = new OuterClass().this. see below.. even private ones. followed by the name of the inner class. as seen above.out.class" bytecode files. public void testInner() { . An inner object has a reference to the outer object. So for example. . the above inner class would typically be compiled to a file named "OuterClass$InnerClass. you can directly use new InnerClass(). public class OuterClass { private String outerInstanceVar. public class InnerClass { public void printVars() { System.) You can directly access the reference to the outer object from within an inner class with the syntax OuterClass. the nested class' access modifier can be public. which are equivalent to the "static" inner classes.Java Programming/Print version 57 Nest a class inside a class When a class is declared inside another class. Inner classes compile to separate ".InnerClass inner = outer. followed by a "$". The nested object can only be created with a reference to the 'outer' object.protected or package(default). although this is usually unnecessary because you already have access to its fields and methods.new InnerClass(). } } } The inner class has access to the enclosing class instance's variables and methods. See below.:" + outerInstanceVar). } (When in a non-static method of the outer class.
This is because the local class instance might outlive the invocation of the method. above. . By doing that the class does not require a name. and it is a nested class to an existing class. local classes can also access local variables of the enclosing method.Java Programming/Print version 58 Static inner class An inner class can be declared static. and so needs its own copy of the variable. also called local classes. An anonymous class can be defined and instantiated in contexts where a reference can be used. ActionListener clk = new ActionListener() { public void click() { // --. cannot have access modifiers.. Anonymous class is a special case of the local class to a method. } . public interface ActionListener { public void click(). so it cannot be changed. A static inner class has no enclosing instance. return. You do not specify an instance when creating a static inner class. The inner class can be only abstract or final. } }. and therefore cannot access instance variables and methods of the outer class. it is required to be final.. since the class is 'private' to the method. Anonymous Classes In Java a class definition and its instantiation can be combined into a single step. like local variables. To avoid problems with having two different copies of a mutable variable with the same name in the same scope. and hence they also can use final local variables of the enclosing method.. This is equivalent to the inner classes in C++. Anonymous classes are most useful to subclass and upcast to an 'Adapter Class' or to an interface.. Nest a class inside a method These inner classes. public class OuterClass { public void method() { class InnerClass { } } } In addition to instance variables of the enclosing class. but only ones that are declared final.implementation of the click event --. Those classes are called anonymous classes.
// --. class MyApp implements Serializable { BigObjectThatShouldNotBeSerializedWithAButton bigOne.*.event.Serializable. class MyApp implements Serializable { BigObjectThatShouldNotBeSerializedWithAButton bigOne. import java.Serializable. The above code is harder to read than if the class explicitly defined. } } ). Button aButton = new Button(). } } MyApp() { aButton.io.*.println("Hello There").awt.io. } } The following example does the same thing. MyApp() { aButton.out.println("Hello There").awt. import java. but it names the class that implements the action listener. import java. import java. import java.out. The following example uses anonymous class to implement an action listener. import java.*.*. using anonymous class makes sense.awt.Nested class to implement the action listener --class MyActionListener implements ActionListener { public void actionPerformed(ActionEvent e) { System.awt.Java Programming/Print version In the above example the class that implements the ActionListener is anonymous. The class is defined where it is instantiated.addActionListener( new MyActionListener() ).event. } } 59 .addActionListener( new ActionListener() { public void actionPerformed(ActionEvent e) { System. Button aButton = new Button(). so why use it? If many implementations are needed for an interface and those classes are used only in one particular place.
or Member variable Interface public visible from anywhere N/A same as its class same as its class visible from anywhere N/A visible from anywhere protected its class and its subclass only from its package only from its class its class and its subclass. " Interface method signature The following table shows what Access Modifiers are appropriate for classes. " . 60 Access Modifiers Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. These keywords are called the access modifiers in the Java language syntax. Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security Access modifiers You surely would have noticed by now. nested classes.. and methods: Class Nested class Method. and methods. and can be defined as. protected and private at the beginning of class's method declarations used in this book. its member variables.Java Programming/Print version Using anonymous classes is especially preferable when you intend to use many different classes that each implement the same Interface. keywords that help set the visibility and accessibility of a class. literals and expressions Types String Classes. member variables.. default is public only from its class N/A .. and from its package only from its package N/A package (default) private only from its package N/A only from its package N/A N/A. the words public.
If a variable has package visibility. If a class has package visibility. Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security . The same way all member variables defined in the Interface by default will become static final once inherited in a class. Methods Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. For clarity it is considered a good practice to put the public keyword. the class can be referenced by anywhere in the program. You do not need to specify the access modifier it will default to public. If a method has private visibility. If a variable has private visibility. the method can be called anywhere in the application through the class it is defined in. the class can be referenced only in the package where the class is defined. the variable can be accessed only in the class it is defined in. If a class has public visibility.Java Programming/Print version 61 Points to ponder Note that Interface method visibility is public by default. the method can be called only in the class it is defined in. If a class has private visibility. (it can happen only if the class is defined nested in an other class) the class can be accessed only in the outer class. the variable can be reference anywhere in the application through the class it is defined in. literals and expressions Types String Classes. the variable can be referenced only in the same package through the class it is defined in. If a method is defined in a public class and it has public visibility. If a method has package visibility. the method can be called only in the same package through the class it is defined in. If a variable is defined in a public class and it has public visibility.
Method Overloading For the same class we can define two methods with the same name. return retType. For example we may have the operation : runAroundThe. We also need to set its visibility(private.. We can define two methods with the same name. that needs to be declared as well.. but different input parameter type: public void runAroundThe( Building block ) { . this is called method overloading. The syntax of method definition is: class MyClass { . package.. return.. String param2 ) { . If the method throws an Exception..Java Programming/Print version 62 Method Definition A method is an operation on a particular object.. ParamTwoType param2 ) throws ExceptionName { ReturnType retType. When we define a class we define its member variables and its methods. when the executation flow reach the return keyword. In java the method signature contains method name and the input parameter types. For example: private void methodName( String param1. } public void runAroundThe( Park park ) { .. For each method we need to give a name. In the java terminology. the method execution is stopped and the execution flow returns to the caller method.. } When the method returns nothing.. } Related terminology is the method signature. However the parameter types and/or the number of parameters must be different for those two methods. For example the following two method definitions are valid: . . or public). An object is an instance of a class.. the return keyword at the end of the method is optional. we need to define its input parameters and we need to define its return type.. It is useful to use method overloading when we need to do something different based on a parameter type. The java compiler takes the signature for each method and makes sure that each method signature is unique for a class. The return keyword can be used anywhere in the method.. } . } We can declare that the method does not return anything using the void java keyword. public ReturnType methodName( ParamOneType param1.
This feature came from C and C++. Why not? The reason is that a method can be called without assigning its return value to a variable.. String err ) { .. } Because the type order is different. } the compiler would not know which method to call.. Method overriding happens with same name.. } The compiler would give an error for the following method definitions as well: public void logIt( String param ) { . } public String logIt( String param ) { String retType. that would be a problem since the compiler would not be able to distinguish between the two: public void logIt( String param. Method overloading happens with methods with the same name different signature....Java Programming/Print version public void logIt( String param. This is different from method overloading.. If we define a method that exist in the super class then we override the super class method.. same signature between inherited classes. return retValue. String param ) { . String param ) { . . the return type is not part of the unique signature. } Note.. Error err ) { . So for the call: { logIt( msg ).. The same method signature can be defined in different classes.. The terminology for this is method overriding. } public void logIt( String err. If both input parameters were type String. } public void logIt( Error err. . 63 Method Overriding Obviously a method signature has to be unique inside a class.
{ 1 2 3 4 5 } obj. On line 1 it references SubOfMyClass object. or the method that defined in its subclasses. However if you modify the object itself. on line 3 it references MyClass object. In order for the Method Overriding to work. NOTE: A common mistake to think that if we can override methods. but if you do keep in mind that Java does not support variable overriding. and will be looked up in a table. the visibility of the new method that overrides the super class method can not be reduced. Method overriding is related dynamic linking. So the common rule is that the override method must throw the same exception or it is any of its subclasses. It will be decided at runtime. For example: . // -. only its references. Also another rule is that when you do an override. MyClass obj = new SubOfMyClass(). In fact if that is not the same. { 1 2 3 4 5 } String var = obj. Think about the object reference as a pointer to an object. However the 'obj' reference points two different objects. it is not known what reference the 'obj' has. The visibility can be increased. it does not count what object the 'obj' reference points to. This is not the case.The myMemberVar is defined in the MyClass object MyClass obj = new MyClass(). however. MyClass or SubOfMyClass MyClass obj = new MyClass(). that will be permanent. When you pass in an object reference to a method and then you change that inside the method. we could also override member variables. An object cannot be passed to a method. So if the super class method visibility is public. Because the 'obj' reference can point to object and all its sub object. All parameters (those are primitive types and object references) are passed by value. When we override a super class method the return type also must be the same.myMemberVar. that will have no effect on the original object reference. that will stay after the method returns.Java Programming/Print version The return type can cause the same problem we saw above.myMethod(). In other words if you change the passed in parameter values inside the method. In the case of the exception the override method may throw can be the same as the super class or it can be one of that exception inherited class. MyClass obj = new SubOfMyClass(). the method call that is going to be called can not be determined at compilation time. In real examples we rarely use public variables. // -. and that will be known only at runtime. a table needs to be kept with all the possible method address to be called. If you change the object the reference points at. the compiler will give you an error.During compilation. method define in MyClass. In the above example. Parameter Passing We can pass in all the primitive data types or any object references to a method. or private. 64 In the above example 'obj' reference has the type MyClass on both line 1 and line 3. So on line 5 which method will be called. as member variables are not overriden. or runtime binding. because it was declared MyClass type on both line 1 and line 3. the variable in the MyClass object will be referenced. that will have no effect on the original variable that was passed in. the override method can not be package.
. obj. { MyObjOne obj = new MyObjOne().The name attribute was changed to 'Susan' inside the method On line 2. When the variables were passed in to the methods their values were copied. . For example: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 } } void myMethod( MyObjOne obj ) { obj. myMethod( var1. System.setName("Sonya"). ...println( "var1="+var1 +"var2="+var2 ). On line 5 we called the myMethod( obj ).out. // --. int var2 ) 11 { 12 13 14 15 16 } . . Note however that after we reassigned the obj reference to a new object... ... we changed the name to 'Susan' through the passed in object reference. int var1 = 10. that is no effect whatsoever on the passed in object. In java we do not represent an object directly.getName().Java Programming/Print version 1 { 2 3 4 5 6 7 8 } 9 . obj.. myMethod( obj ). ... var2 ). Inside the method... String name = obj. // -. You can think of an object reference as a variable having the address of the object. we represent an object throught an object reference. ..The variable values did not change 65 On line 7 the value of var1 is 10 and the value of var2 is 20. var1 = 0. . This is called passing the parameter by value.setName("Christin"). on line 3 we set its name property to 'Christin'. int var2 = 20. but the object itself is not. So that change will stay. 10 void myMethod( int var1. var2 = 0. we created an object.. obj = new MyObjOne().setName("Susan").. So the object reference passed in by value..
a method may or may not return a value. The best way is to use a nested class. int a_function(double d) { return (int)d. public void setAge( int val ) { this. So a method can return only one value. private String name. if you want to mix primitive data types and object references as the output values from the method.age = val. If the method does not return a value we use the void java keyword. the method can return a primitive type or an object reference.Java Programming/Print version 66 Functions In java. There is a better approach. see example below: public class MyObject { . } Return Parameter So as we can see. so do not use a public class. assign the values and return the reference to this object.. Create that object inside the method. The modified values can be considered as an output value from the method. You could have a problem however. You can always pass in an object reference to the method. } public int getAge() { return age. and let the method modify the object properties. Same as the parameter passing. } public void setName( String val ) { name = val. and cleaner if you create an Object array inside the method. functions (methods really) are just like in C++ except that they must be declared inside a class and objects are passed by reference automatically. You cannot create pointers to a function but Java has events which really are function pointers under the hood for when you need that type of functionality. } public String getName() { . What if you want to return more than one value from a method. assign the return values and return the array to the caller.. This special object is "bound" to this method and used only for returning values. Defines special return object with the needed return values. /** Nested object is for return values from 'getPersonInfoById' method */ public static class ReturnObj { private int age. However better option.
setName( name ). That is the Constructor. you can define more than one constructor with different parameters. . /** * MyClass Constructor... // --. the Constructor There is a special method for each class that will be executed each time an object is created from that class.Get the name and age based on the ID from the database --.. The Constructor can be overloaded.. } 67 Special method. For example: public class MyClass { private String memberField.out.getAge() ). ret.. MyObject.. MyObject obj = new MyObject(). ret.. there is no input parameter */ public MyClass() { . .Java Programming/Print version return name. Constructor does not have a return value and its name is the same as the class name.setAge( age ).getName() ). ReturnObj ret = new ReturnObj(). System. See below how you may use that object: { .println( "Person Name=" + person.. } } In the above example the 'getPersonInfoById' method returns an object reference that contains both values the name and age.End of nested class defination --/** Method using the nested class to return values */ public ReturnObj getPersonInfoById( int ID ) { int age. return ret. } } // --. System..ReturnObj person = obj.getPersonInfoById( 102 ). . String name.out.println( "Person Age =" + person..
will be called 68 In the above example we created two objects from the same class. /** * MyClass Constructor. If we do not define one. The difference between the two is that in the first one the memberVar field is not initialized. Each class must have a constructor.. MyClass obj2 = new MyClass("Init Value"). there is one input parameter */ public MyClass( String param1 ) { memberField = param1.Java Programming/Print version } /** * MyClass Constructor. public class MyClass { /** * MyClass Empty Constructor */ public MyClass() { } } The Constructor is called automatically when an object is created with the new keyword. A constructor may also be called from an other constructor.. and obj2 contains the reference to the object.. You may ask which constructor will be called. see below: public class MyClass { private String memberField.. . or we can also say that obj1 and obj2 both have the same type. } // The constructor with no input parameter will be called // The constructor with one input param. Its depends how the object is created with the new keyword. See below: { . } } In the above example we defined two constructors. MyClass obj1 = new MyClass(). . obj1. one with no input parameter.. in the second one that is initialized to 'Init Value'. there is no input parameter */ public MyClass() { . and one with one input parameter.. the compiler will create a default so called empty constructor automatically.
Because java evolved from the C programming language. // --. No object needs to be created to execute a Static Method. You write Static Method the same way as normal method. but they are not an operation on an object. the constructor with no input parameter calls the other constructor with the default initial value. the only difference is that you can not reference any member variables and any object methods.. } } Static Methods can be referenced anywhere prefixed by the class name. } --System. there is one input parameter */ public MyClass( String param1 ) { memberField = param1. } /** * MyClass Constructor. This gives an option to the user. Static Methods are defined inside a class. Static Method is a left over from a non object oriented language. they are simply global functions.out. } } In the above example.Java Programming/Print version MyClass("Default Value").. . 69 Static Method We defined method above as an operation on an object.Call myStaticMethod . return("I am a Static Method"). However you can create an object an use it inside a Static Method.println( "Output from the myStaticMethod:" + MyObject. See below: { .myStaticMethod() )... with input parameters and a return value... . to create the object with the default value or create the object with a specified value. public class MyObject { public String memberVar. public class MyObject { static public String myStaticMethod() { . Static Methods can reference only Static variables and call only other Static Methods... You can write a non object oriented program by using only Static Methods in java.
memberStaticVar = "Value". static vars. // --. literals and expressions Types String Classes.Java Programming/Print version 70 static private String memberStaticVar. cgi?ses=ao789&code=abs&sub=fun) Primitive Types Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. . obj. can be used // --. just to define different types of numbers in Java.memberVar = "Value". // --. Any data. For instance. there are about six simple types available to programmers – some define whole numbers (integers numbers) and others define numbers with a decimal values (floating point numbers).This is okay. --> ERROR Cannot reference member var..Create an object --MyObject obj = new MyObject(). interactive Java Lesson (http://javalessons. Java also gives freedom to programmers to create complex and customizable data types. We will deal with complex data types in later chapters. be it numeric or a sentence of words. return("I am a Static Method").This is okay since an object is created -. the structure of such data and operations associated with it. It is seen as a way of declaring allowed values for certain data. can have a different data type. static public String myStaticMethod() { memberVar = "Value". Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security Data Types (or simply Types) in Java are a way of telling what certain data is.com/cgi-bin/fun/java-tutorials-main. } } External links • Abstract methods..
This is either declared or inferred and the Java language only allows programs to run if they adhere to type constraints. Operations associated with such data types are those of simple arithmetic (addition. While supporting object oriented (OO) programming. hence making a program more secure and safe once compiled completely and successfully. etc. Primitive types are used for boolean. Java offers both object types and primitive types. there are three further categories of primitives: • Numeric Primitives These primitive data types hold only numeric data. As we have discussed above. However. . Such types serve only one purpose – containing pure. then such declarations would violate Java's type system. we will discuss these three categories in detail. These types serve as the building blocks of data manipulation in Java. joining characters to make words. You can not define a new operation for such primitive types. they come with a number of operations predefined. It then automatically flags this occurrence as an error at compile time. Java performs run time type checking when doing such casts. Java is not a pure OO language like Smalltalk or Ruby. subtration. but unlike numbers. or types that define textual content within your program. Java checks if an expression or data is encountered with an incorrect type or none at all. operations associated with such types are those of textual manipulation (comparing two words. say textual content. is equal to. If you present a numeric type with data that is not numeric. at the expense of flexibility. This gives Java the unique ability of type safety. they do not have operations that serve arithmetic purposes.) • Boolean and Null Primitives About Java Types some type errors can still occur at runtime because Java supports a cast operation which is a way of changing the type of one expression to another. For example. Because these data types are defined into the Java type system by default. Most type-related errors are caught by the Java compiler.Java Programming/Print version 71 Data Types in Java Java is considered a strongly typed programming language in that it is obligatory for all data. Rather. you cannot subclass the primitive types and add new operations to them. expressions and declarations within the code to have a certain type associated with it. Primitives Primitives are the most basic data types available within the Java language. so an incorrect type cast will cause a runtime exception rather than succeeding silently and allowing data corruption. etc. Java is also known as a hybrid language. This allows relatively good performance when manipulating numeric data. In the Java type system. you can have types that define a number. there are three broad categories of data types: • Primitives • Object Reference Types • Arrays In the following section. simple values of a kind. and numeric values and operations. In the Java language. etc.) • Textual Primitives These primitive data types hold characters (which can be alphabets or even numbers). character. Instead.) or of comparions (is greater than.
>.. >=). compareTo(String anotherString) and more. For example. ~). +=. <=. logical xor.lang. one would use the declaration private int length. Example: int The primitive type int represents a signed 32 bit integer value. to declare a private integer instance field named length. In the above example the name object reference's attributes are: • Type is : String • The referenced object is also : String Both the java.String class methods and java. --. >>>). != and assignment =. |. >>.lang. The set of operations that may be performed on int values includes integer arithmetic such as +. bit-wise operations such as logical and. /. !=. ++.lang. Additional details on each are available in other modules.lang. comparison operations (==. private Object name = "Marry Brown". *. toString(). Example: String You use class and interface definition in Java to define new types. which includes length(). The allowed operations on String object.Object. logical or. bit shift operations (<<.lang. In the above example the name object reference's attributes are: • Type is : Object • The referenced object is : String Only the java.. toUpperCase(). hashCode().Object class methods are available for the object reference name. that is created by instantiating a class The String class is one such example. <. however are the set of methods in the java. The operations on a String reference variable are those available for all reference types. assignments (=. %. -=). -. such as comparison operations ==. ^.String class. which is java. negation (&. The null reference is another valid value for a String expression. .Java Programming/Print version 72 Examples of Types Below are two examples of Java types and a brief description of the allowed values and operations for these types. wait(). String objects also inherit the set of operations from the base class that String extends from.Object class methods are available for the object reference name. These operations include methods such as equals(). private String name = "Marry Brown". . An object reference has two main attributes: • Its type associated with a class or an interface • A java object it references. String values are a sequence of 0 or more Unicode characters. conversions to other numeric types and promotion to other integer types. toLowerCase(). In addition. The allowed data values for int are the integers between -2147483648 to 2147483647 inclusive. and getClass(). Class and interface types are associated with object references also sometime refered to as Reference types. notifyAll().
-2 } Arrays are described in more detail in the Arrays section. they behave as a reference but have a few differences allowing them to allow easy access to sub elements. */ private int[] data2 = new int[] { 1. int.) To create an array. primitive values can not be put in a collection. For this reason object wrappers are defined in JDK 'java. It is basically data types similar to what other non-object-oriented languages have. you will need to use the new operator to have it create the Array with the specified number of elements: /* Declares an array named data1. (While you can instead place these square brackets next to the variable name. When one declares an array. As with object types. The type byte is commonly used in binary file input output operations. char c = 65. int i = 65. byte and char can also be used. 99. no operations. There are arrays of primitive types in Java. short s = 65.lang. Size (bits) Example Minimum Value Integer types byte char 8 16 byte b = 65. but less commonly. /* Declares an array named data2 that contains these three elements. long.Java Programming/Print version 73 Array Types Arrays in Java are represented as a built-in Array object. long l = 65L. Primitive Data Types The Java primitive data types contain pure values. . double d = 65. float. the data type is changed to include square brackets.*' package for all the primitive types. but because they are not objects. The character type char is used for text processing. this is not recommended. -- --- --- Boolean Void The types short. -128 0 127 216-1 215-1 231-1 263-1 Byte Character Maximum Value Wrapper Class short int long 16 32 64 -215 -231 -263 Floating-point types Short Integer Long float double 32 64 float f = 65f.55. and double are usually used in arithmetic operations. and has it assigned to an array with 25 elements. char c = 'A'. Other Float Double boolean 1 void -- boolean b = true. */ private int[] data1 = new int[25].
one was expected to 'wrap' one's primitive data type with the corresponding primitive wrapper object. but instead is a special class built into the Java language.long is converted to int. make the programmer's life much easier when it comes to working with the primitive wrapper types.String is not a primitive type. see String. It has been 'automatically boxed up' on your behalf. language features since Java 1. There are two kinds: • Implicit : casting operation is not required.lang.Java Programming/Print version String objects representing literal character strings in Java. int i = (int) l.int is converted to long. For further info. . However. Integer ageObject = new Integer(age). Primitive wrapper objects were Java's way of allowing one to treat primitive data types as though they were objects.lang. The extra step of wrapping the primitive is no longer required.* package. long l = i. Autoboxing Since Java 1. // --. // --. Consider this code fragment: int age = 23. one may write as below and the compiler will automatically create the wrap object. the magnitude of the numeric value is always preserved. java. it shows the casting operation for explicit conversions: from byte to byte to char to short to int to long to float to double to boolean N/A N/A N/A N/A N/A N/A from char (byte) (short) from short (byte) (char) from int (byte) (char) (short) from long (byte) (char) (short) (int) from float (byte) (char) (short) (int) (long) from double (byte) (char) (short) (int) (long) (float) N/A from boolean N/A N/A N/A N/A N/A N/A N/A - Autoboxing/unboxing Autoboxing/unboxing Autoboxing and unboxing. 74 Data Conversion (Casting) Data conversion (casting) can happen between two primitive types. casting is not needed Example for explicit casting: long l = 656666L. casting is needed The following table shows the conversions between primitive types. Consequently. the magnitude of the numeric value may not be preserved Example for implicit casting: int i = 65.5.5. as shown above. in the java. precision may be lost when converting from integer to floating point types • Explicit : casting operation required.
The if statement requires a boolean primitive value. Any data. literals and expressions Types String Classes. Study the following code for a moment. Integer ageObject = age.5 will automatically 'unbox' this. Unboxing Uses the same process in reverse.out. not a performance booster. Consider this feature a programmer convenience. Java also gives freedom to programmers to create complex and customizable data types. Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security Data Types (or simply Types) in Java are a way of telling what certain data is.Java Programming/Print version 75 Points to ponder Keep in mind that the compiler still creates the missing wrapper code. so one doesn't really gain anything performance-wise. For instance. just to define different types of numbers in Java. int age = 23. can have a different data type. No problem! Java 1. be it numeric or a sentence of words. } Types Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. there are about six simple types available to programmers – some define whole numbers (integers numbers) and others define numbers with a decimal values (floating point numbers).5" ). We will deal with complex data types in later chapters. It is seen as a way of declaring allowed values for certain data. Boolean canMove = new Boolean(true). . yet it was given a Boolean wrapper object. if ( canMove ) { System.println( "This code is legal in Java 1. the structure of such data and operations associated with it.
they come with a number of operations predefined.Java Programming/Print version 76 Data Types in Java Java is considered a strongly typed programming language in that it is obligatory for all data. or types that define textual content within your program. hence making a program more secure and safe once compiled completely and successfully. you cannot subclass the primitive types and add new operations to them. Java performs run time type checking when doing such casts. etc. Java offers both object types and primitive types. expressions and declarations within the code to have a certain type associated with it. Java is also known as a hybrid language. Java is not a pure OO language like Smalltalk or Ruby. there are three further categories of primitives: • Numeric Primitives These primitive data types hold only numeric data. operations associated with such types are those of textual manipulation (comparing two words. at the expense of flexibility. so an incorrect type cast will cause a runtime exception rather than succeeding silently and allowing data corruption. Because these data types are defined into the Java type system by default. Operations associated with such data types are those of simple arithmetic (addition. is equal to.) • Boolean and Null Primitives About Java Types some type errors can still occur at runtime because Java supports a cast operation which is a way of changing the type of one expression to another. This is either declared or inferred and the Java language only allows programs to run if they adhere to type constraints. This gives Java the unique ability of type safety. we will discuss these three categories in detail. character. and numeric values and operations. Primitives Primitives are the most basic data types available within the Java language. say textual content.) or of comparions (is greater than. etc. there are three broad categories of data types: • Primitives • Object Reference Types • Arrays In the following section. Such types serve only one purpose – containing pure. In the Java type system. subtration. but unlike numbers. If you present a numeric type with data that is not numeric. In the Java language.) • Textual Primitives These primitive data types hold characters (which can be alphabets or even numbers). While supporting object oriented (OO) programming. you can have types that define a number. then such declarations would violate Java's type system. For example. As we have discussed above. Java checks if an expression or data is encountered with an incorrect type or none at all. These types serve as the building blocks of data manipulation in Java. You can not define a new operation for such primitive types. Instead. This allows relatively good performance when manipulating numeric data. It then automatically flags this occurrence as an error at compile time. etc. Primitive types are used for boolean. However. simple values of a kind. Most type-related errors are caught by the Java compiler. joining characters to make words. . they do not have operations that serve arithmetic purposes. Rather.
!= and assignment =. toString(). which is java. For example. >>>). and getClass(). >=). In the above example the name object reference's attributes are: • Type is : Object • The referenced object is : String Only the java. assignments (=. The allowed data values for int are the integers between -2147483648 to 2147483647 inclusive. compareTo(String anotherString) and more. that is created by instantiating a class The String class is one such example. bit shift operations (<<. In addition. *. --. /. In the above example the name object reference's attributes are: • Type is : String • The referenced object is also : String Both the java. !=. private String name = "Marry Brown". ++. String values are a sequence of 0 or more Unicode characters. -=). bit-wise operations such as logical and.Object class methods are available for the object reference name. logical xor. |. >>.lang.Object. negation (&.Object class methods are available for the object reference name.. Example: int The primitive type int represents a signed 32 bit integer value. +=. conversions to other numeric types and promotion to other integer types. . Class and interface types are associated with object references also sometime refered to as Reference types. toLowerCase(). however are the set of methods in the java. private Object name = "Marry Brown". The null reference is another valid value for a String expression.Java Programming/Print version 77 Examples of Types Below are two examples of Java types and a brief description of the allowed values and operations for these types. The set of operations that may be performed on int values includes integer arithmetic such as +. String objects also inherit the set of operations from the base class that String extends from.lang. ~). Example: String You use class and interface definition in Java to define new types. The operations on a String reference variable are those available for all reference types.lang. . %.lang. to declare a private integer instance field named length. such as comparison operations ==.lang. <. comparison operations (==. hashCode(). <=. logical or. one would use the declaration private int length. wait(). The allowed operations on String object. toUpperCase(). notifyAll().. >.String class methods and java. An object reference has two main attributes: • Its type associated with a class or an interface • A java object it references.String class. -. Additional details on each are available in other modules. which includes length(). These operations include methods such as equals(). ^.
primitive values can not be put in a collection.) To create an array. -2 } Arrays are described in more detail in the Arrays section. Size (bits) Example Minimum Value Integer types byte char 8 16 byte b = 65. short s = 65. float. byte and char can also be used. -128 0 127 216-1 215-1 231-1 263-1 Byte Character Maximum Value Wrapper Class short int long 16 32 64 -215 -231 -263 Floating-point types Short Integer Long float double 32 64 float f = 65f. but less commonly.*' package for all the primitive types.lang. double d = 65. There are arrays of primitive types in Java. and has it assigned to an array with 25 elements. */ private int[] data1 = new int[25]. /* Declares an array named data2 that contains these three elements. no operations. this is not recommended. and double are usually used in arithmetic operations. long.55. . (While you can instead place these square brackets next to the variable name.Java Programming/Print version 78 Array Types Arrays in Java are represented as a built-in Array object. long l = 65L. but because they are not objects. int i = 65. When one declares an array. The type byte is commonly used in binary file input output operations. As with object types. they behave as a reference but have a few differences allowing them to allow easy access to sub elements. Primitive Data Types The Java primitive data types contain pure values. For this reason object wrappers are defined in JDK 'java. Other Float Double boolean 1 void -- boolean b = true. 99. you will need to use the new operator to have it create the Array with the specified number of elements: /* Declares an array named data1. int. char c = 'A'. The character type char is used for text processing. -- --- --- Boolean Void The types short. the data type is changed to include square brackets. char c = 65. */ private int[] data2 = new int[] { 1. It is basically data types similar to what other non-object-oriented languages have.
// --. it shows the casting operation for explicit conversions: from byte to byte to char to short to int to long to float to double to boolean N/A N/A N/A N/A N/A N/A from char (byte) (short) from short (byte) (char) from int (byte) (char) (short) from long (byte) (char) (short) (int) from float (byte) (char) (short) (int) (long) from double (byte) (char) (short) (int) (long) (float) N/A from boolean N/A N/A N/A N/A N/A N/A N/A - Autoboxing/unboxing Autoboxing/unboxing Autoboxing and unboxing.long is converted to int. make the programmer's life much easier when it comes to working with the primitive wrapper types. The extra step of wrapping the primitive is no longer required. language features since Java 1. java.lang. . Integer ageObject = new Integer(age). It has been 'automatically boxed up' on your behalf. long l = i. Primitive wrapper objects were Java's way of allowing one to treat primitive data types as though they were objects. casting is needed The following table shows the conversions between primitive types. For further info. precision may be lost when converting from integer to floating point types • Explicit : casting operation required. the magnitude of the numeric value is always preserved.int is converted to long. Consider this code fragment: int age = 23.Java Programming/Print version String objects representing literal character strings in Java.* package. // --. int i = (int) l. casting is not needed Example for explicit casting: long l = 656666L. There are two kinds: • Implicit : casting operation is not required.String is not a primitive type. in the java.lang. 79 Data Conversion (Casting) Data conversion (casting) can happen between two primitive types. the magnitude of the numeric value may not be preserved Example for implicit casting: int i = 65. one may write as below and the compiler will automatically create the wrap object. see String. one was expected to 'wrap' one's primitive data type with the corresponding primitive wrapper object. Autoboxing Since Java 1. However.5. as shown above. Consequently. but instead is a special class built into the Java language.5.
int age = 23. Consider this feature a programmer convenience. Study the following code for a moment. Unboxing Uses the same process in reverse. } java.Java Programming/Print version 80 Points to ponder Keep in mind that the compiler still creates the missing wrapper code.lang. The if statement requires a boolean primitive value.5 will automatically 'unbox' this.out. if ( canMove ) { System. No problem! Java 1.String Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. yet it was given a Boolean wrapper object. literals and expressions Types String Classes. Boolean canMove = new Boolean(true). Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security . not a performance booster.println( "This code is legal in Java 1. so one doesn't really gain anything performance-wise. Integer ageObject = age.5" ).
are implemented as instances of this class.Java Programming/Print version 81 java. all methods return a new String object.trim().trim().lang. The String class provides a nice set of methods for string manipulation. For example: String str = "First part" + " second part". // --. Most classes override the default behavior to provide more specific (and more useful) data in the returned String. The String class represents character strings. Integers will also be converted to String after the ( + ) operator: String str = "Age=" + 25. The Java language provides special support for the string concatenation operator ( + ). Strings are immutable. This method provides a way to convert objects into Strings. Note that the following would do nothing useful: name. that is.lang package. For example: String str = "This is string literal".Is the same as: String str = "First part second part". they cannot be modified once created. Since String objects are immutable. For example: name = name. Whenever it looks as if a String object was modified.String String is a special class built into the Java language defined in the java. a new String was actually created and the old one was thrown away. String literals in Java programs. Strings are ubiquitous in Java. Its object reference is assigned to the str variable. Study the String class and its methods carefully. . such as "abc". // wrong! This would create a new trimmed string and then throw it away. The trim() method returns a copy of the string with leading and trailing whitespace removed. Each Java object has the String toString() inherited from the Object class. On the right hand side a String object is created represented by the string literal. and for conversion of other objects to strings. it will serve you well to know how to manipulate them skillfully.
println("Match Found"). Once a String is created. use the StringBuffer or StringBuilder class. Comparing Strings Comparing strings is not as easy as it may first seem. "Bar".Java Programming/Print version 82 Using StringBuffer/StringBuilder to concatenate strings Remember that String objects are immutable objects. so you can't use it in more than one thread (see the chapter on Concurrency). "Foo Bar Bam ". use the equals(Object) method inherited by every class and defined by String to return true if and only if the object passed in is a String containing the exact same data. The method creates five Strings ("". } return str. Suppose coll contains the elements ["Foo". } On the ( + ) operation a new String object is created at each iteration. } return buf. Unlike StringBuffer.append(oneElem). buf. "Bam". { System. it can not be modified. where only one StringBuilder object is created: To avoid unnecessary memory use like this. StringBuilders are faster. for(String oneElem : coll) // loops through every element in coll { buf. Be careful of writing a method like this : public String convertToString(Collection<String> coll) { String str = "". Instead use StringBuffer. but store their data in a mutable way. public String convertToString(Collection<String> coll) { StringBuilder buf = new StringBuilder(). "Foo Bar ". takes up memory until garbage collected. using StringBuffer or StringBuilder produces much faster code. We cannot just use a simple equality statement such as: if(myString == "Hello World!") //Can't Use this. .toString(). because it doesn't have to worry about synchronization. "Baz"]. } To test for equality. They provide similar functionality to Strings.append(" "). and "Foo Bar Bam Baz") even though only last one is actually useful.out. } StringBuilder was introduced in Java 5. However. "Foo ". Also because object creation is time consuming. as shown below. StringBuilder isn't thread safe. for(String oneElem : coll) // loops through every element in coll { str = str + oneElem + " ".
. .. Toronto:(416)777-9999". See the following example: String person = "Brown. If myString was to be different. .String)) for more details. The compareTo method returns a negative. equal to. zero. John:100 Yonge Street.4. see Regex.. 4. String[] personData = person. since Java 1. html#compareTo(java. } To order String objects.compareTo("Hello World!") == 0 ) { System.. so you could process the text line by line. which can be accessed wherever we use a String datatype. The result is a positive integer if this String object lexicographically follows the argument string. (For more information on regular expressions. com/ j2se/ 1.. Take a look at the Java API (http:/ / java.. String phone = personData[2].split( ":" ). 2/ docs/ api/ java/ lang/ String. even in the slightest manner we will get a value above or below 0 depending on the exact difference. or greater than the object on which it is called.out.. } This snippet of code is comparing the String variable myString to "Hello World".) The String class has a split() method. use the compareTo() method. //. String address = personData[1].. String name = personData[0].. if(myString.equals("Hello World!")) { //true // . The result is negative if this String object lexicographically precedes the argument string..Java Programming/Print version 83 String greeting = "Hello World!". Splitting a String Sometimes it is useful to split a string into separate strings. if(greeting. based on a regular expression.. } if(greeting.equals("hello world!")) { //false // . sun. that will return a String array. lang. Let's take a look at an example: String myString = "Hello World!". An other useful application could be to 'split' the String text based on the 'new line' character.println("Match Found"). or positive number if the parameter is less than.
This is known as the beginIndex. The two methods that make this possible are toLowerCase() and toUpperCase().Java Programming/Print version 84 Creating substrings It may also be sometimes useful to create substrings. In this example.substring(3). in the typical programming classroom assignment of evaluating whether or not a string is a palindrome. String substr = str. All characters from the beginIndex until the end of the string will be copied into the new substring. It is easy to mistake the method substr() for subStr() (which does not exist and would return with a syntax error on compilation). Note: "Substring" is considered to be one word. String substr = str. . Please note that the endIndex is not inclusive. it is apparent that the character in index 3 is the second "f" in "coffee". Just remember that this style only applies to methods or other elements that are made up of more than one word. In this example. and b. The string returned by substr would be "port". These methods are useful.toLowerCase() would return a result of "wikibooks". in this example. String b = "wikipedia". was copied into the substring. This can be done in two methods. The first method involves creating a substring out of the characters of a string from a given index to the end. every character from index 3 to index 6.toUpperCase() would return "WIKIPEDIA". For example: String str = "coffee". Therefore. or strings using the order of letters from an existing string. String a = "WIKIBOOKS". The second method involves a user-defined beginIndex and endIndex. for example. By counting from there. As previously discussed. inclusive. This means that the last character will be of the index endIndex-1. substr would return "fee". the index of the first character in a string is 0. a call to a. Modifying String cases The String Class also allows for the modification of cases.7). This is why the method name does not seem to follow the common syntax of Java. For example: String str = "supporting".substring(3.
for (int i = 0.print(prompt + " "). i < names.util. // Get rid of this line // so that getString won't read it return integer. public class ArrayExample { static Scanner input = new Scanner(System.nextInt().*.println(names[i]).length.out.out. } .java import java. Look at the following program: ArrayExample.in). } for (int i = 0.nextLine(). keyed by number. String[] names = new String[numNames]. int integer = input. public static void main(String[] args) { int numNames = getInt("Number of names?"). In Java an array is an object like all other objects. } } public static int getInt(String prompt) { System. literals and expressions Types String Classes. Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security Intro to Arrays An array is similar to a table of data.Java Programming/Print version 85 Arrays Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. i < names. ++i) { System.length. i++) { names[i] = getString("Enter name #" + (i+1)). input.
} } public static int getInt(String prompt) { System.*. i < names. and how to access it.out. return input. } public static String getString(String prompt) { System. String[] names = new String[numNames]. // Get rid of this line // so that getString won't read it return integer. . } } Copy friendly import java. Note that all Java arrays start at 0 and go to (array size .util.in). int integer = input. tells Java to create an array of size numNames that will store Strings.println(names[i]).length.print(prompt + " ").Java Programming/Print version 86 public static String getString(String prompt) { System. the highest index is 9.nextLine().out. ++i) { System.nextInt(). public class ArrayExample { static Scanner input = new Scanner(System. for (int i = 0.print(prompt + " "). i < names. input. how to set data. public static void main(String[] args) { int numNames = getInt("Number of names?"). } } Copy the code and compile it. Thus. use names[x] = data where x is the index to access.print(prompt + " "). The code String[] names = new String[numNames]. } for (int i = 0. The program will ask you to enter some names then reprints the names in order. It demonstrates three major aspects of arrays: how to define an array.1). To set data.out. return input.nextLine(). if you dimension an array to size 10.nextLine().out.length. i++) { names[i] = getString("Enter name #" + (i+1)).
In this way an array with any number of dimensions can be created. "22". "01". 24} }. use the syntax variable[i] = data • To find the length of an array. 13. if you know the data beforehand. This is false for boolean's. each element contains an array having 5 elements. Here are examples of two dimensional arrays with initializer blocks: String [][] twoDimArray = { {"00". Since they are arrays of array references. 23. "13". subarrays can have different lenghts). for (int j = 0. 11. and null for all reference types. So for example. "01". "21".item n} • All elements of the array will be automatically initialized with the default value for that datatype. i < twoDimArray.length Two-Dimensional Arrays A two dimensional array is represented by an array of an array. {"20". {10. {"20". all of which are initialized to null. String [] oneDimArray = {"00".. 14}. "11". . Because an array is also an object. "12". item 2. We could create the array having the 5 elements first and use that one in the initialize block. null. . it can be used to create an array where the element of the array is also an array. "12". "03".e. "22". or the subarray reference can even be null. j < twoDimArray[i]. "23".. "21". } In the above example we defined an array which has three elements. "02". "14"}. Consider String [][] weirdTwoDimArray = { {"10".. "11". String [][] twoDimArray = { oneDimArray .length. use the syntax variable[i] where i is the index • To set an item. Alternatively. "03". 12. 0 for all numeric primitive types. {20. "23". 22. j++) twoDimArray[i][j] = "" + i + j. "04"}. {"10". the previous note created an array of DataType references. 02. 04}. "13". 21.. "21". you can write DataType[] variable = {item 1.length. for (int i = 0. "23"} }. "11".. like any other object having the Object as the super class. "23". "24"} }. 03. use the syntax DataType[] variable = new DataType[ArraySize]. int [][] twoDimIntArray = { {00. "22". Note that the above "twoDimArray" is equivalent to the following more verbose code: String [][] twoDimArray = new String[3][].Java Programming/Print version 87 Array Fundamentals • To create an array. these multi-dimensional arrays can be "jagged" (i. • To access an item. "14"}. use the syntax variable. {"10". "04"}. i++) { twoDimArray[i] = new String[5]. "02". 01. "24"} }. {"20". "12"}.
literals and expressions Types String Classes. representing one character like 'A' or '. a program statement like . only int need be of interest to a beginner) floating-point numbers (decimal numbers. declared as byte... the Java language has special features for its String class. short. declared as float or double.') boolean (holding only true or false as values) In addition.Java Programming/Print version 88 Multidimensional Array Going further any number of dimensional array can be defined. a variable is declared to be a particular type of data.. <elementType>[][]. only float need be of interest at first) characters (declared as char. int.[] Java Programming Overview Getting Started Fundamentals Index Classes and Objects Collections Exceptions Applets JavaBeans Concurrent Programming Reflection Libraries Data and Variables Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. and strings can be treated very much like primitives for many purposes. To store a value in a variable.. or long. the syntax for a declaration is: variabletype variablename. Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security A variable in Java can store two kinds of variables: • Java primitive type values • a reference to a Java object Java's primitive types are • • • • integers (whole numbers.[] <arrayName> or <elementType><arrayName>[][]. As in most languages.
Produces 2007 89 Strong Typing Variables in Java are strongly typed. others (such as PL/I) define a conversion for almost all such statements and perform the conversion to complete the assignment.out.Java Programming/Print version variablename = data. System. Be careful! By convention. CaseConventions(String id) { } void method() { } void longMethodName() { } } . (Constructors have to start with a capital. Classes begin with a capital letter. Case Conventions Java is case-sensitive. you cannot assign an integer value to that variable. Thus: package org. To access the data in year. because they must have the same name as the class.) Package names use lowercase. A method called mymethod is completely separate from a method called myMethod. int multipleWordVariable. which means that the compiler checks the type of a variable matches the type of data stored in that variable. named year that stores 2007. Strong typing is intended to prevent mistakes made by unwittingly assigning the wrong kind of value to a variable. and do not use camel case. Some languages (such as C) define an interpretation of such a statement and use that interpretation without any warning. use the variable in place of the number.println(year). If you declare a variable to hold a String. And can reference the variable (and use the data stored in it) by its name. most identifiers that includes more than one word uses camel case. to create an int primitive type value. methods and variables do not. year = 2007.wikibooks. class CaseConventions { int variable. For example. for instance.samplecode. and catching those mistakes when the program is compiled rather than waiting to find it when the program is running.
. A block is any section of code within curly braces. This is the basic data structure rule of Object-Oriented programming. // valid only in this method if(local > 0) { boolean positive = true. which means that a variable is "un-defined" (and becomes useless) at the end of the block in which it is defined. It is valid for and occupies storage for as long as the corresponding object is in memory. and for. • instance variable. Every time the method is called. a new copy of the variable is used. outside any method. and each one gets its own copy of all instance variables. Common blocks include class definitions. if/else blocks. declared within a class but outside any method. • static variable. declared within a method in a class. // valid only in this constructor } void someMethod() { int local = 42. // valid in all of class BlockScope BlockScope(int param) { // param is valid only in this constructor int localVariable = 0. // valid only within the if block } else { // positive is not defined here! } } } There are three basic kinds of scope for variables in Java: • local variable. declared within a class as static. a program can instantiate multiple objects of the class. There is only one copy of such a variable no matter how many objects are instantiated from that class. class BlockScope { int classScope. while. and each instance holds its own data. methods and constructors.Java Programming/Print version 90 Scope Java uses block scope. classes are defined to hold data specific to a "class of objects" in a given system. and do-while loops. valid for (and occupying storage only for) the time that method is executing.
literals and expressions Types String Classes. This means that code using Generics will not compile with Java 1. . public class Person<T> { private Person<T> person. to forbid at compile time that an Integer object is added to a Collection that should only contain Strings). Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security Generics were added to the Java language syntax in version 1. Similar constructs to Java Generics exist in other languages.5. This is now possible since Java 1. . through the constructor.. to pass a Type as a parameter to a class at creation time. Java was long criticized for the need to explicitly type-cast an element when it was taken out of a "container/collection" class.Java Programming/Print version 91 Generics Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers..g.5. When an object is created. before the object is created. we can also pass in Types.. parameters can be passed to the created object. Now with Generics. The type-place-holders will be replaced with the specified type. This has changed by the appearance of Microsoft C#. Java did not have a real competitor.. What are Generics? Generics are so called because this language feature allows methods to be written generically. A better name might have been type parameter argument.4 and less. Type parameter arguments can be set: for a class When an object is created from that class the type-parameter-argument will be replaced with the actual Type. } . With Generics Java is better suited to compete against C#. Because. see w:Generic programming for more information. There was no way to enforce that a "collection" class contains only one type of object (e. In the first couple of years of Java evolution.. it is basically that. with no foreknowledge of the type on which they will eventually be called upon to carry out their behaviors.
. .. When an object was added to a container. . // --. 2 collString.Create a Customer person --Person<Customer> custPerson = new Person<Customer>(). // --. . If that contains Integers then the code is fine. // --. parameterized by one or more type parameters. Many potential problems are caught by the compiler.Java Programming/Print version // --. When it was taken out an explicit cast was needed. Integer integer = (Integer) st. Before Generics were introduced there was no way to ensure that a container object contains only one type of objects.Compilation Error -- But in the case of container classes. That's one of the reasons why it is so easy to use.. we do not know what type of objects collString contains. method declarations can be generic--that is. Integer integer = (Integer) collString. it is okay to use generic classes without the generics type specification thing (<T>). the compiler was not able to catch an invalid type casting. 92 Introduction Java is a strongly typed language. Container objects are objects that contain other objects.Create an Employee person --Person<Employee> emplPerson = new Person<Employee>(). it was automatically cast to Java Object.Compilation Error collString is a container object. // --. RunTime CastException Just looking at line 3.. 1 Collection collString = new ArrayList().. . public static <T> void assign( Person<T> person.. String st = "This is a String". } use of generics is optional For backwards compatibility with pre-Generics code.get(0). The below code using Generic: Collection<String> collString = new ArrayList<String>(). The compiler should also warn about unsafe operations. collString. In such a case. that can contain only String objects.. so when we get out an element it can be casted only to class that normally a String can be casted.get(0). you will have to manually typecast it from type Object to the correct type. One area where Java was criticized was regarding the container objects.No Compilation Error.add( "This is a String" ). T obj ) { person. when you retrieve an object reference from a generic object. nothing else..setPerson( obj ).add( "This is a String" ). . for a method Just like class declarations. 3 Integer integer = (Integer) collString. Normally an explicit cast is checked by the compiler.
For example. } The T type here represents the type that is handed to the Class object. Using Generics with container classes. no matter how many Generic objects are created with different type parameters. that is collCustomer can contain only Customer objects. with each different type parameter. Java strict type checking can be extended to container objects. at runtime. The syntax of Java Generic and C++ Template are also similar. and that collection happened to be empty.Compilation Error Both collObject and collCustomer have the same type.. Collection<Object> collObject = collCustomer. Type. No additional code is generated. Before Generics: Collection collCustomer = new ArrayList(). the type of the parameter type has to be matched too. AnnotatedElement { . The amount of code generated depends on how many different types of classes are created from the Template. The generated code depends on how the Template class is referenced. C++ Templates do not have any run-time mechanisms. // --. // --. There are some differences however. both collObject and collCustomer have the same type 93 With generics: Collection<Customer> collCustomer = new ArrayList<Customer>(). that generates code before compilation. The generics type information is erased during compilation (type erasure). This means... The same Class object handles all the Generic type variations. public final class Class<T> extends Object implements Serializable..add( new Customer() ). and collObject can contain only Object object. . Collection collObject = collCustomer. collCustomer. Collection<String> collString = new ArrayList<String>(). gives an impression that a new container type is created. . BUT it is against the Generic rule. both these objects appear as the same type (both ArrayList's). The Class class itself is generic since Java 1. The C++ template can be seen as a kind of macro.. In fact. Java Generics are built into the language. The compiler creates normal code to substitute the template.add( new Customer() ). similar to any 'hand-written' code.No problem. for example. . In contrast. So there is an additional check to the normal type checking.Java Programming/Print version With Generics. your function would have no way of instantiating another T object. Note for C++ programmers Java Generics are similar to C++ Templates in that both were added for the same reason. The T type will be substituted with the class being loaded. GenericDeclaration.. Collection<Integer> collInteger = new ArrayList<Integer>(). because it doesn't know what T was. There is only one Class object created. collCustomer. that if you had function that takes Collection<T> as an argument.5.
createAnyObject(Customer.class). It is an interesting example of using genericness for something other than a container class..5.. // --. and the type of Serializable.Exception Handling } return ret. you can get more precise types when creating objects reflectively.createAnyObject(Customer.class is Class<String>.. Customer cust = Utility. since the newInstance() method in Class now returns a T. public static <T> T createAnyObject(Class<T> cls) { T ret = null. try { ret = cls. based on how it will be called. without casting.Using reflection. For example. Now we can use the newInstance() method to return a new object with exact type. This can be used to improve the type safety of your reflection code. . the class java.lang.Exception Handling } return ret.class).No casting Get exact type when getting JavaBean property. } catch (Exception e) { // --.Class is generic.class is Class<Serializable>.Casting is needed // . String propertyName) .newInstance().. } // .Java Programming/Print version 94 Class<T> Since Java 1. the type of String. get a Java Bean property by its name --public static <T> T getProperty(Object bean. try { ret = cls. } catch (Exception e) { // --. In particular. public static Object createAnyObject(Class cls) { Object ret = null. .newInstance(). } And the above code without Generics: Customer cust = (Customer) Utility. using reflection See the following code where the method will return the exact type of the Java Bean property.
} return argList. java. this was mostly done by passing an array.reflect. for (int i = 0. } catch (Exception e) { // --. T property = null. null). null).Based on the property name build the getter method name --String methodName = "get" + propertyName.length() == 0) { return null. property = (T) m.lang.substring(0. for example: .length.invoke(bean.lang. this was not possible in Java—if a likewise feature was needed.Handle exception -} return property.Class c = bean.1). args) { List<T> argList = new ArrayList<T>(). Before generics. it is very easy to define a method with a variable number of arguments. i < args.Java Programming/Print version { if (bean == null || propertyName == null || propertyName. } // --. The following code illustrates a method that can be called with a variable number arguments: /** * Method using variable-length argument list * @param <T> * @param args */ public static <T> List<T> makeAList(T.add(args[i]).. i++) { argList..substring(1).toUpperCase() + propertyName.Method m = c.getClass(). try { java.getMethod(methodName. } 95 Variable Argument Using Generics. The only requirement for using a variable number of arguments using Generics is that the arguments in the list must have the same type. } The above method can be called with a variable number of arguments.
or Any-Type.. not only Classes. This Unknown type matches anything. specify: Collection<? extends Serializable> serColl = new ArrayList<String>(). The following collection can only contain objects that extend the class Animal. new LinkedList()). We can only take out elements of type Object from it. We have also seen that in addition to the normal type checking. If we write <? extends Object> instead of T. For example. to create a collection that may only contain "Serializable" objects. the type parameter has to match as well when we assign generics variables.util. while ( iter. the String class is serializable.iterator(). See also: java. } } Wildcard for a specific type of classes "<? extends ClassName>" specifies a restriction on the types of classes that may used.hasNext() ) { System. "Two". "Two". new StringBuffer(). So now we can define a collection whose element type matches anything.. and pronounced Unknown. "Three".Arrays. regardless of the parameter type it holds? Wildcard The wildcard type is represented by the character <?>. See below: Collection<?> collUnknown. if it is used only by itself. Any-Type can be express also by <? extends Object>. The above code is valid because. In the above example calls. So what is the use of this variable if we can not add anything to the collection it represents? The use of this new construct will be clear when you want to create a generic method that takes any collection. Use of a class that is not serializable would cause a compilation error. "Three"). public static void printElements( Collection<?> anycoll ) { Iterator<?> iter = coll. a) Wildcard Types As we have seen above.print( iter. 10. List<String> list2 = makeAList("One". Any-Type includes Interfaces. generics give the impression that a new container type is created with each different type parameter.Java Programming/Print version List<String> list1 = makeAList("One". then we can pass any kind of objects. 96 Note: the number 10 in the above code will be converted (autoboxed) to Integer.out. . In some cases this is too restrictive.asList(T. "Four").next() ). Note that we can not add anything to this collection. What if we would like to relax this additional checking? What if we would like to define a collection variable that can hold any generic collection. the arguments must be of type String. regardles of their type: List<? extends Object> list3 = makeAList("One".
to declare a Comparator that can compare Dogs. For example. the Animal class is a supertype of the Dog class.compare(dog1.. Animal b) { //. static int compareTwoDogs(Comparator<? super Dog> comp.Create "Animal Collection" variable --Collection<? extends Animal> animalColl = new ArrayList<Dog>(). but comparators for any strict subclass cannot. } ..Java Programming/Print version class Dog extends Animal { .. dog2). 97 Defining Classes Navigate Classes and Objects topic: • • • • • • • Defining classes Creating objects Interfaces Using static members Destroying objects Overloading methods and constructors Class Loading .. Use of a class that is not a supertype would cause a compilation error. Comparators for any superclass of Dog can also compare Dog.. } The above code is valid because. "<? super ClassName>" specifies a restriction on the types of classes that may be used. you use Comparator<? super Dog> myComparator. // --. Dog dog2) { return comp. Comparator<Animal> myAnimalComparator = new AnimalComparator(). Dog dog1. Now suppose you define a comparator that can compare Animals: class AnimalComparator implements Comparator<Animal> { int compare(Animal a.. you can use this comparator to compare Dogs also. } } Since Dogs are Animals.
A special method that generates a new object. an employee might have an ID number.Static fields are common to any object of the same class. where a preprocessor evaluates the macros. 'parameterized types' can be assigned to a class during definition. an employee might have a method to issue his paycheck or to access his name.Sometimes it is useful to contain a class within another one if it is useless outside of the class or should not be accessed outside the class.Static methods are methods that do not affect a specific object. • static methods . .Since 1.5. It is similar to the C language macro '#define' statement. a static field within the Employee class could keep track of the last ID number issued. It is done by the compiler.Fields are variables that hold data specific to each object. For example. • member methods . For example. • Parameterized types .Java Programming/Print version 98 Fundamentals Every class in Java can be composed of the following elements • fields . • Constructors . Only one static field exists for one class. The 'parameterized types' will be substituted with the types specified at the class's instantiation. For example.) There is one field for each object of a class. (They are also called member variables.Member methods perform operations on an object. • static fields . • other classes .
public Employee(String name) pass a name to the constructor { created Employee object. } public static int getNextID() returns the next ID { another Employee is created. { that class private static int nextID.Java Programming/Print version 99 // This defines the Employee // The public modifier indicates // it can be accessed by any other public class Employee class. modifier indicates that class can access it. The private // only code inside the Employee // This is a constructor. } public String getName() returns the { return myName. private field myName. You can // and it will give you a newly // Automatically assign an ID to // Increment the ID counter // This is a member method that // Employee object's name. copy of this will exist. myID = nextID. // Note how it can access the // This is another member method. // This is a static method that // that will be assigned if . return nextID. myName = name. Only one // no matter how many Employees // Define fields that will be // for each Employee. private int myID. } } The following Java code // Define a static field. stored private String myName. } public int getID() { return myID. the object nextID++. are created.
An object is created based on its class. After a while the heap memory will be full of unused objects. template.getName()). Employee b = new Employee("Jane Smith"). memory is allocated to hold the object properties.getName()). There is one Class created for each type of Java class.getID() + " : " + b. keeps track of the usage of object references. To use the object in the future.getNextID()).println(b.Java Programming/Print version 100 public class EmployeeList { public static void main(String[] args) { System. } } would produce this output: 0 3 0 : John Doe 1 : Jane Smith 2 : Sally Brown Creating Objects Navigate Classes and Objects topic: • • • • • • • Defining classes Creating objects Interfaces Using static members Destroying objects Overloading methods and constructors Class Loading Introduction Before a Java object can be created the class byte code must be loaded from the file system (with . The JVM collects those garbage objects and frees the memory they allocated. You can consider a class as a blueprint. the object can not be used any more and becomes garbage. that object reference has to be stored as a local variable or as an object member variable. If there are no more reference to the object. System. This process of locating the byte code for a given class name and converting that code into a Java Class class instance is known as class loading.println(Employee. An object reference pointing to that memory location is also created. See below a simple example: { // --.getNextID()).out. When an object is created.println(a. Employee c = new Employee("Sally Brown").out.println(Employee. Employee a = new Employee("John Doe"). The Java Virtual Machine (JVM).println(c. System.class extension) to memory.out. so the memory can be reused again when a new object is created. All objects in java programs are created on heap memory.getID() + " : " + c.Create an object --- . System. or a description how to create an object.getName()).out.getID() + " : " + a. System.out.
. } The obj contains the object reference pointing to an object created from the MyObject class.class' file is found. // --.. There is no limit on how many objects from the same class can be created.class' file.class'. and executes any static initialize block. Memory is not allocated for the object member variables at this point. the package name will be translated to a directory path.printMyValues(). and do the copying variable by variable? Lets see: public class MyObject { private int memberVar. Thus. The file has the extention of '*. . The obj object reference is in scope inside the { }.Create an 'MyObject' for the first time the application started -MyObject obj = new MyObject(). as all Java objects inherit the protected Object clone() method.. 101 Creating object with the new keyword 99% of new objects are created using the new keyword. The JVM stores the code in memory.. When the 'MyObject.. } . memory will be allocated for them when an instance of the class. .memberVar = obj. You may ask why we need this clone method. that is the Java byte code. This base method would allocate the memory and do the bit by bit copying of the object's states. } When an object from the MyObject class is created for the first time.. { // --. Depending on which package the class belongs to.Java Programming/Print version MyObject obj = new MyObject(). Creating object by cloning an object Cloning is not automatically available to classes. is created. The CLASSPATH environment variable contains locations where Java classes are stored.memberVar. Code and static variables are stored only once. no matter how many objects are created. Memory is allocated for the object member variables when the object is created. The JVM is looking for the 'MyObject. MyObject( MyObject obj ) { this. allocates memory for the static variables. Object references can be passed in to methods. and creates a Class object. the size of an object is determined not by its code's size but by the memory it needs for its member variables to be stored. . After the } the object becomes garbage. the JVM's class loader loads the class in memory. The JVM searches the file system for the definition of the class. an object. Couldn't I create a constructor and just passing in the same object. There is some help though. object references can be returned from methods.Use the object --obj.
Lets say we want to store Customer objects in a collection. The solution: • Make the Activity class also cloneable .clone()method at the beginning of your clone method. /** Clone Customer object for performance reason */ public Customer createCustomerObject() { // --. So before adding the object. return obj. } Now. the added object won't.Java Programming/Print version } You might think that accessing the private memberVar variable of obj would fail but as this is in the same class this code is legal.put( "Customer".clone(). } return template. lets see how to make the Customer object cloneable.. Override the clone() method in all the subclasses of Customer. • • • • First the Customer class needs to implement the Cloneable Interface. 102 public class Customer implements Cloneable { .. Now we are facing a problem.. as that is protected in the Object class.Create template --template = new Customer(). we clone them. An other use of cloning could be to take a snapshot of an object that can change in time. See below a factory method that will return a new object using cloning.. Object creation with the new keyword is expensive. . if ( template == null ) { // --. public Object clone() throws CloneNotSupportedException { Object obj = super.get( "Customer" ). Override and make the clone() method public. but we want to disassociate them from the 'live' objects . This is much faster than using the new keyword. template ). it is not enough to clone the Customer object. we also need to clone the referenced objects. performance will be better if you create one object and clone new ones from it.See if a template object exists in our cache --Customer template = _cacheTemplate. so if you need to create lots of objects with the same type. HashTable _cacheTemplate = new HashTable.clone(). Call the super. _cacheTemplate. so if the original object changes from that point forward. Also lets say that the Customer object has a reference to an Activity object that contains the customer activities. The clone() method copies the whole object's memory in one operation. } } In the above example we used cloning for speed up object creation.
By default. private void readObject(java. you need to provide an implementation for two methods: private void writeObject(java. At the later time the object can be re-created from that stream of bytes.clone() ).io. The only requirement is that the same class has to be available at both times. otherwise the object won't be able to be re-created. // -. however. the object needs to be recreated at the receiving host. then the same class must be available on both servers. as seen below • Change the Customer class clone() method as follows: public class Customer implements Cloneable { Activity _activity.Clone the object referenced objects --if ( _activity != null ) { clonedCustomer. This is a maintenance problem to those applications where java serialization is used to persist object or sent the object through the network. When a class is modified.clone(). . a class must first implement the Serializable interface. } return clonedCustomer.) If additional handling is required beyond the default of writing all fields. The byte stream can be stored on the file system. there could be a problem re-creating those objects that were serialized using an earlier version of the class.Java Programming/Print version • Make sure that if the Activity class has other 'changeable' object references.ObjectInputStream in) throws IOException.setActivity( (Activity) _activity. those has to be cloned as well. Java has built in support for serialization. using the Serializable interface. a class will have all of its fields serialized when converted into a data stream (with transient fields being skipped. ClassNotFoundException. .. 103 Creating object receiving from a remote source When an object is sent through a network.. Same class means that exactly the same version of the class must be available. public Customer clone() throws CloneNotSupportedException { Customer clonedCustomer = (Customer) super. when the object is serialized and also when the object is re-created.io.ObjectOutputStream out) throws IOException. If that happens in different servers. References to unchangeable objects such as String be used in the cloned object without worry. Object Serialization The term Object Serialization refers to the act of converting the object to a byte stream. or can be sent through a network. } } Note that only mutable objects needs to be cloned.
If the object needs to write or provide a replacement object during serialization. without the abstract key word. The major benefit of that is that all java objects can have a common ancestor. it actually means the following: class MyObject extends Object // -.The compiler adds 'extends Object'.. which are similar to abstract classes. // This method must be implemented by the class that implements this interface . All java classes can be up-casted to Object.. a minor change to the class can cause the serialization to fail. Object readResolve() throws ObjectStreamException. 104 Interfaces Navigate Classes and Objects topic: • • • • • • • Defining classes Creating objects Interfaces Using static members Destroying objects Overloading methods and constructors Class Loading Interfaces Java does not allow you to create a subclass from two classes. Normally. if not specified { . public String methodOne(). This makes the language simpler. That class is called Object. In interfaces all methods are abstract by default. with any access specifier: Object writeReplace() throws ObjectStreamException. } When you type the above code.Java Programming/Print version private void readObjectNoData() throws ObjectStreamException. You can still allow the class to be loaded by defining the serialization version id: private static final long serialVersionUID = 42L. it needs to impement the following two methods.. public interface MyInterface { public static final String CONSTANT = "This value can not be changed". To mimic multiple inheritance. Example: class MyObject { . Interfaces have no implementation and no variables.. } So it can be guaranteed that certain methods are available in all java classes. java offers interfaces. a single class can implement as many interfaces as required.however. but constant values can be defined in interfaces . There is no multiple inheritance.
com/cgi-bin/fun/java-tutorials-main.. no instance needs to be created..cgi?ses=ao789& code=if2&sub=fun) Using Static Members Navigate Classes and Objects topic: • • • • • • • Defining classes Creating objects Interfaces Using static members Destroying objects Overloading methods and constructors Class Loading What does static mean? When you declare a • method. public class MyObject implements MyInterface { // Implement MyInterface interface public String methodOne() { . } 105 .. it is independent of any particular.cgi?ses=ao789& code=if1&sub=fun) • Interfaces 2. interactive Java Lesson (http://javalessons. but rather it is shared among all instances of a class. . The static keyword is used to declare a method. To access a static method or member variable.. interactive Java Lesson (http://javalessons.com/cgi-bin/fun/java-tutorials-main. } } External links • Interfaces.Java Programming/Print version . or • member variable static... or member variable static.
int par2 ) { return par1 + par2. . . } } Danger of static variables Use static variables only for: • data sharing (be careful) • defining global constants Use static methods for: • utility functions Using static variables and/or method for other purposes goes against object orientation. public static int addTwoNumbers( int par1.. See below: static public final String CONSTANT_VAR = "Const".. as these become GLOBAL variables that can be accessed from everywhere in the program. Public static variable should not be used.... } } The counter variable is incremented each time an object is created. public AClass() { .. For example: public Match { . counter += 1. public int getNumberOfObjectsCreated() { return counter. For example to implement a counter that stores the number of objects created at a given time can be defined as so: public AClass { static private int counter. • Static methods can be used for utility functions or for functions that do not belong to any particular object.. } . Global constants can be used..Java Programming/Print version 106 What can it be used for? • Static variables can be used as data sharing amongst objects of the same class. however. and will make your code either harder to read or maintain.
the finalize() method may never be called. .Java Programming/Print version 107 External links • Static. In particular. To accomplish this. Java performs automatic garbage collection . interactive Java Lesson (http://javalessons. protected void finalize() throws Throwable { try { doCleanup(). The garbage-collector thread runs in a lower priority than the other threads. the finalize() method is used. such as closing any open input/output streams. finalize() When an object is garbage-collected. The finalize method is required only if there are resources beyond the direct control of the Java Virtual Machine that need to be cleaned up. the program can run out of memory.any unreferenced objects are automatically erased from memory . //Call finalize on the parent object } } // Perform some cleanup. we can not rely on when the finalize() method will be called. } finally { super. Instead. If the application creates objects faster than the garbage-collector can claim back memory. Also. Note that finalize() should never be manually called.com/cgi-bin/fun/java-tutorials-main. there is no need to explicitly close an OutputStream.finalize(). If it fails for some reason. since the OutputStream will close itself when it gets finalized. the programmer may want to manually perform cleanup.and prohibits the user from manually destroying objects. it is ignored. If the java application exits before the object is garbage-collected.cgi?ses=ao789& code=stc&sub=fun) Destroying Objects Navigate Classes and Objects topic: • • • • • • • Defining classes Creating objects Interfaces Using static members Destroying objects Overloading methods and constructors Class Loading Unlike in many other object-oriented programming languages. the finalize method is used to release either native or remote resources controlled by the class. except to call a super class' finalize method from a derived class' finalize method.
There is no required relationship between the return types or between the throws clauses of two methods with the same name but different signatures. If. Arrays Navigate Language Fundamentals topic: • • • • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. the subclass still inherits the other method. the Java programming language differs from C++. using dynamic method lookup. the number of actual arguments and the compile-time types of the arguments are used. and a subclass overrides one of them. Methods are overridden on a signature-by-signature basis. This fact causes no difficulty and never of itself results in a compile-time error. at compile time. then the method name is said to be overloaded. When a method is invoked. If the method that is to be invoked is an instance method. a class declares two public methods with the same name. or one declared and one inherited) have the same name but different signatures. Objects and Types Syntax Assertions Keywords Packages Nested classes Access modifiers Data and variables Generics Java Security . the actual method to be invoked will be determined at run time. for example. to determine the signature of the method that will be invoked . or both inherited by a class.Java Programming/Print version 108 Overloading Methods and Constructors Navigate Classes and Objects topic: • • • • • • • Defining classes Creating objects Interfaces Using static members Destroying objects Overloading methods and constructors Class Loading If two methods of a class (whether both declared in the same class. In this respect. literals and expressions Types String Classes.
i < names. for (int i = 0. } . Look at the following program: ArrayExample. i < names.out.nextInt(). i < names. String[] names = new String[numNames]. i++) { names[i] = getString("Enter name #" + (i+1)). public static void main(String[] args) { int numNames = getInt("Number of names?").*. } } public static int getInt(String prompt) { System. input. public static void main(String[] args) { int numNames = getInt("Number of names?"). String[] names = new String[numNames].in).out. public class ArrayExample { static Scanner input = new Scanner(System.nextLine().out.java import java. int integer = input. // Get rid of this line // so that getString won't read it return integer. i++) { names[i] = getString("Enter name #" + (i+1)).*.length.util.print(prompt + " "). public class ArrayExample { static Scanner input = new Scanner(System. } } Copy friendly import java. ++i) { System.nextLine(). } public static String getString(String prompt) { System.length. In Java an array is an object like all other objects.util. keyed by number. for (int i = 0. } for (int i = 0.println(names[i]).in). return input.Java Programming/Print version 109 Intro to Arrays An array is similar to a table of data.print(prompt + " ").length.
Java Programming/Print version for (int i = 0; i < names.length; ++i) { System.out.println(names[i]); } } public static int getInt(String prompt) { System.out.print(prompt + " "); int integer = input.nextInt(); input.nextLine(); // Get rid of this line // so that getString won't read it return integer; } public static String getString(String prompt) { System.out.print(prompt + " "); return input.nextLine(); } } Copy the code and compile it. The program will ask you to enter some names then reprints the names in order. It demonstrates three major aspects of arrays: how to define an array, how to set data, and how to access it. The code String[] names = new String[numNames]; tells Java to create an array of size numNames that will store Strings. To set data, use names[x] = data where x is the index to access. Note that all Java arrays start at 0 and go to (array size - 1). Thus, if you dimension an array to size 10, the highest index is 9.
Array Fundamentals
• To create an array, use the syntax DataType[] variable = new DataType[ArraySize]. Alternatively, if you know the data beforehand, you can write DataType[] variable = {item 1, item 2,...item n} • All elements of the array will be automatically initialized with the default value for that datatype. This is false for boolean's, 0 for all numeric primitive types, and null for all reference types. So for example, the previous note created an array of DataType references, all of which are initialized to null. • To access an item, use the syntax variable[i] where i is the index • To set an item, use the syntax variable[i] = data • To find the length of an array, use the syntax variable.length
Two-Dimensional Arrays
A two dimensional array is represented by an array of an array. Because an array is also an object, like any other object having the Object as the super class, it can be used to create an array where the element of the array is also an array. In this way an array with any number of dimensions can be created. Here are examples of two dimensional arrays with initializer blocks: String [][] twoDimArray = { {"00", "01", "02", "03", "04"}, {"10", "11", "12", "13", "14"}, {"20", "21", "22", "23", "23"} }; ... int [][] twoDimIntArray = { {00, 01, 02, 03, 04}, {10, 11, 12, 13, 14},
Java Programming/Print version {20, 21, 22, 23, 24} }; Note that the above "twoDimArray" is equivalent to the following more verbose code: String [][] twoDimArray = new String[3][]; for (int i = 0; i < twoDimArray.length; i++) { twoDimArray[i] = new String[5]; for (int j = 0; j < twoDimArray[i].length; j++) twoDimArray[i][j] = "" + i + j; } In the above example we defined an array which has three elements, each element contains an array having 5 elements. We could create the array having the 5 elements first and use that one in the initialize block. String [] oneDimArray = {"00", "01", "02", "03", "04"}; String [][] twoDimArray = { oneDimArray , {"10", "11", "12", "13", "14"}, {"20", "21", "22", "23", "24"} }; Since they are arrays of array references, these multi-dimensional arrays can be "jagged" (i.e. subarrays can have different lenghts), or the subarray reference can even be null. Consider String [][] weirdTwoDimArray = { {"10", "11", "12"}, null, {"20", "21", "22", "23", "24"} };
Multidimensional Array
Going further any number of dimensional array can be defined. <elementType>[][]...[] <arrayName> or <elementType><arrayName>[][]...[]
Java Programming Overview Getting Started Fundamentals
Index Classes and Objects
Collections Exceptions Applets JavaBeans
Concurrent Programming Reflection Libraries
Java Programming/Print version
Collection Classes
Navigate Collection topic: • • • Introduction Arrays Collection Classes Collection or Map Collection Map Set or List or Queue Set List Queue Map Classes Thread Safe Collections Classes Diagram (UML) • Comparing Objects
Collections are a group of objects bound together by a common characteristic. Java has various built-in classes to support the collection of objects, either of the same type or general. The most basic construct to work with is the array of which you will come to know of in a section later in this chapter.
Introduction to Collections
The most basic collection interface is called Collection. This interface gives the user a generic usage of a collection. import java.util.Collection; // Interface import java.util.ArrayList; // Implementation import java.util.HashSet; // Implementation ... Collection coll1 = new ArrayList(); Collection coll2 = new HashSet(); ... < Use coll1 & coll2 > In the above there are two collections. The usage of the collections are the same, the implementations are different. If the existing collection implementations do not meet your needs, you can write your version of the implementation. Your version of the implementation just needs to implement the same java.util.Collection interface, then you can switch to using your implementation and the code that is using the collection does not need to be changed. import java.util.Collection; import com.yourcomp.util.YourCollectionImpl; ... Collection coll1 = new YourCollectionImpl(); Collection coll2 = new YourCollectionImpl(); ... < Use coll1 & coll2 >
util. import java. parameterized type <E> All collection implementations since 1.Hashtable. If a collection contains different types of objects. import java. Because of that.util.util.5 an enhancement to the type system of the Java language has been added. the object that is 'in' the collection also 'changes'.Java Programming/Print version The Java JDK collection implementations are quite powerful and good. . which one needs to be faster.. Map map = new Hashtable(). . In that case when you are adding an object that could change any time you need to make a copy or clone of the object. so it is unlikely that you will need to write your own. All collections contain object references. This is an issue only if you put an actively used object in the collection. A new object will be created and its reference will be put in the collection.5 now have one 'parameterized type <E>' added. are duplicates allowed? Can my collection contain the null element? Should the collection maintain the order of the elements? Is the order important in any way? How do you want to access an element? By index. The 'E' refers to an Element type. Objects put into a collection are upcasted to Object class. key or just with an iterator? Does the collection need to be synchronized? From a performance perspective. only the object reference is. Generics Since JDK version 1. if the object is changed after it was put in the collection. or a 'Map'. (Iterating through the collection) 113 Before selecting a particular collection implementation. In that case there will be no object references outside of the collection. When a collection is created the actual 'Element type' will replace the E. updates or reads? From a usage perspective. we will have difficulty finding out the type of the objects obtained from a collection at run . so the objects 'inside' the collection can only be changed if we take out an object reference from the collection.put( key. For this reason we usually add objects of the same type to a collection.util. value ). the Java JDK has the java. The object is not really in the collection.. map. All collections need to have the same basic operations. It is called Generics.e. It also means that you need to know the type of the object when you taking it out. It is not guaranteed that the objects 'inside' the collections won't change... Those are: • • • • Adding element(s) to the collection Removing element(s) from the collection Obtaining the number of elements in the collection Listing the contents of the collection. Most often Generics will be used with the collection classes. Aside from the java. ask the following question: • • • • • • • Can my collection contain the same elements.Map interface as well. i. This defines key value mappings. which operation will be more frequent. Instead they contain collections of key->value mappings.Collection interface.Map. It means that you need to cast the object reference back when you get an element out from the collection. But first decide if you need a 'Collection'. Implementations of the Map interface do not contain collections of objects. updates or reads? Once you know your needs. you can select an exsisting implementation.
as we talked about above. ageList. Lets have a closer look at this magic. Maps are used for mapping key/value pairs. No casting is required when we take out an element. removeAll( Collection c ). remove( Object o ). a YourImpOfCollection.Collection • java. a HashSet. it is easy to switch implementation • It makes it easy to convert one type of collection to an other. Collection Use the Collection interface if you need to keep related (usually the same type of) objects together in a collection where you can: • Search for a particular element • List the elements • Maintain and/or change the order of the elements by using the collection basic operations (Add. Remove. addAll( Collection c ). You can pass in a Vector. retainAll( Collection c ). Update. an ArrayList. // --. The following code is a possible implementation of addAll() method of the .. Collection<Integer> ageList = new ArrayList<Integer>().Map Implementations for those interfaces are not interchangeable. Collections are used for collecting Java objects. Collection or Map The Java JDK contains two high level Interfaces: • java. This is the beauty of using the Collection interface. For more information about Generics.) • Access the elements by an index number The advantages of using the Collection interface are: • Gives a generic usage. The conversion is easy because the Collection interface defines a standard way of looping through the elements. Integer age = ageList.add( new Integer(46) ). passing in a list. The methods above return true if the collection has changed due to the operation.Integer can be added // --.get(0)..Java Programming/Print version time. the 'Element-type' that can be put into the collection can be specified when the collection object is created. You can have a LinkedList and just call the addAll(list) method.. The Collection interface defines the following basic operations: • • • • • boolean boolean boolean boolean boolean add( E o ). please go to Java Programming/Generics. Note that in addAll() we can add any type of collection. All those different type of collections will be magically converted to a LinkList..util.Compilation error. ageList can have only Integers inside 114 ageList is a collection that can contain only Integer objects as elements. ageList.util.add( "50" ). With the use of the 'parameterized type <E>'. a TreeSet. .
What you might need to code for is to loop through a 'Customer' collection: import java..util.iterator(). buf.. public String printCustomerNames( Collection customerColl ) { StringBuffer buf = new StringBuffer().next() ). } Notice two things: • The above code will work for all type of collections.size().hasNext() ) { _linkList. } else { return false.. public String addAll( Collection coll ) { int sizeBefore = _linkList.. Iterator iter = coll. } } The above code just iterates through the passed in collection and adds the elements to the link list.yourcompany. while( iter.getName() ). because we call a method on it.Customer . • We have to know the type of objects inside the collection.Collection import java.Iterator import java.util. import java.util.append( "\n" ). while( iter. You do not have to do that.Java Programming/Print version LinkList.add( iter.size() ) { return true. } if ( sizeBefore > _linkList.next().toString(). } return buf. 115 . buf.Iterator . Iterator iter = customerColl. since that is already defined.util.append( cust.iterator().Collection import java.hasNext() ) { Customer cust = (Customer) iter.
util. so duplicates not allowed. plus the elements in the SortedSet are sorted java. first out) queues. The key is usually a non-mutable object.util.util.Map<K. extraction. The value object however can be a mutable object. Best used for producer-consumer queues. java. first out) and LIFO (last in. each key can map to at most one value. and can be accessed by an index.List<E> elements are put in the list in a certain order. collection of values. The Collection interface has five sub interfaces. and inspection operations.V> maps keys to values.util.util.V> same as the Map interface.Queue<E> queues provide additional insertion. A map cannot contain duplicate keys. the same element can be added to a list. java. or set of key-value mappings. It is similar to a mathematical Set. Figure 1:Map Interfaces Set or List or Queue There is no direct implementation for the java.util. java.SortedMap<K.util. Figure 2: . Duplicates are allowed. java. Those are: java.Collection interface.Set<E> contains unique elements.SortedSet<E> same as the Set interface. plus the keys in the Map are sorted. and waits for space to become available in the queue when storing an element. There are FIFO (first in. The Map interface provides three collection views. can be thought of as an array where the index need not be an integer.BlockingQueue<E> waits for the queue to become non-empty when retrieving an element. Use the Map interface if you need to keep related objects together in a Map where you can: • Access an element by a key object • Map one object to other java.util.Java Programming/Print version 116 Map A map. sometimes also called an Associated Array or a Dictionary. which allow a map's contents to be viewed as a set of keys.
util.util. java. Add. each java object has the boolean equal(Object obj). java. The equal() method is used to compare objects only with the same value of hashCode. There is a problem.Java Programming/Print version 117 Set The basic implementation of the Set interface is the HashSet. They can not be compared. delete are expensive. You may wonder how duplicates are detected when we are adding an object to the Set. Put Fruits in the Set.HashSet<E> Not synchronized.. There are two solutions to this: • Solution one : Override the int hashCode() method and return the same values for the same type of objects and return different values for different type of objects. though. Detecting duplicate objects in Sets Set cannot have duplicates in it. fortunately. Enum sets are represented internally as bit vectors. java. a fresh copy is created during modification operation. let's call it Fruit class. call the equal() methods passing in the candidate object • if any of them returns true. plus defines the iteration ordering. That method will be called by the Set implementation to compare the two objects to see if they are equal or not. So before an object is added. method available inherited from Object. update. Allows the null elements java. Calling the equal() method would cause a ClassCastException. You need to do the following: • Do not override the equals() and hashCode() methods in the Apple and Orange classes • Create appleEquals() method in the Apple class. To do that. the object is not added to the Set.TreeSet<E> Elements are synchronized. and create orangeEquals() method in the Orange class • Override the hashCode() method in the Fruit class and return the same value. the Set implementation needs to: • find all the objects in the Set that has the same hashCode as the candidate object hashCode • and for those.CopyOnWriteArraySet<E> Thread safe.EnumSet<E extends Enum<E>> All of the elements in an enum set must come from a single enum type that is specified. which is the order in which elements were inserted into the set. sorted. null not not .util.LinkedHashSet<E> Same as HashSet. so the equals() is called by the Set implementation • Override the equals() method in the Fruit class for something like this. • Solution two : Create a super class for the Apple and Orange. We have to see if that object exists in the Set or not. allowed java. It is not enough to check the object references.util. the objects' values have to be checked as well. I put an Apple and an Orange. What if I put two different type of objects to the Set. You need to override it. when the set is created. explicitly or implicitly.util.
This is so because the Set interface is defined in terms of the equals operation. if ( this instanceof Apple && obj instanceof Apple ) { ret = this.orangeEquals(obj). E toElement). to the exclusive toElement SortedSet tailSet(E fromElement). } Note: • only the objects that have the same hashCode will be compared.) . inclusive. -. exclusive. -.returns from the inclusive fromElement to the end SortedSet subSet(E fromElement. Overriding the default hashCode() method may affect performance there. Those are: • • • • • E first().returns elements range from fromElement. The default implementations in Object won't work. the returned sorted set is empty. -.Java Programming/Print version 118 public boolean equals( Object obj ) { boolean ret = false. so if you decide not to override the hashCode() method. there is no point overriding the equal() method. equal. as it won't be called. Note that the ordering maintained by a sorted set must be consistent with equals if the sorted set is to correctly implement the Set interface. futher more all elements must be mutually comparable. } return ret. SortedSet The SortedSet interface extends the Set Interface. } else if ( this instanceof Orange && obj instanceof Orange ) { ret = this. so two elements that are deemed equal by this method are. All elements in the SortedSet must implement the Comparable Interface. • Do not override the hashCode() method if you know that the values of your objects are different.returns the last element SortedSet headSet(E toElement). } else { // --. like in a Hashtable to find an object fast.returns the first element E last(). (If fromElement and toElement are equal. to toElement. The SortedSet interface has additional methods due to the sorted nature of the 'Set'. -.returns from the first. or if you only want to prevent adding the exactly same object. but a sorted set performs all element comparisons using its compareTo (or compare) method. • Only override the hashCode() method if you want to eliminate value duplicates. • you are responsible to override the equal() and hashCode() methods. from the standpoint of the sorted set. • the default hashCodes are unique for each object created. • Beware that the hashCode() may be used in other collection implementaions. -.appleEquals(obj).Can not compare Orange to Apple --ret = false.
The name LinkList implies a special data structure where the elements/nodes are connected by pointers.LinkedList<E> Non-syncronized.util. To remove an element from the link list the pointers need to be rearranged.ArrayList<E> Non-syncronized. Head ______ | Size | |______| | elem | | _________________ | point | | _______________ | point | | _____________ | | Node 1 Node 2 Node n | First|-------->| Data | to next |------>| Data | to next|-. javax. Vector is syncronized. When only one thread is accessing the list. update operation is faster than other lists. to provide roles successfully retrieved. because of the extra overhead to make it thread safe.util.RoleUnresolvedList<E> A RoleUnresolvedList represents a list of RoleUnresolved objects. double-ended queues. there are extra overhead to reindex the list.. consider using the LinkedList. Whenever you insert or remove an element from the list. use in multiple thread access.util. and you have lots of insert and remove. It represents a last-in-first-out (LIFO) stack of objects. and when trying to set several roles in a relation (via 'setRoles()' method). It is used as parameter when creating a relation.management. use in single thread environment. The methods used for the insertion of Attribute objects in the AttributeList overrides the corresponding methods in the superclass ArrayList. The ArrayList is not syncronized. When you have a large list. otherwise use ArrayList java. easy to use for stacks.Java Programming/Print version 119 List The List has the following implemenations: java.relation. -->| Data | null | |______|_________| |______|________| |______|______| .RoleList<E> A RoleList represents a list of roles (Role objects). The basic implemenation of the List interface is the ArrayList. queues. javax.. otherwise use Vector java..AtributeList<E> Represents a list of values for attributes of an MBean.management.Stack<E> It extends class Vector with five operations that allow a vector to be treated as a stack. and thread safe. This is needed in order to insure that the objects contained in the AttributeList are only Attribute objects. java. It is returned as part of a RoleResult. Vector is slower. javax. not thread safe.util.Vector<E> Syncronized. use the ArrayList. representing roles not retrieved from a relation due to a problem encountered when trying to access (read or write to roles).relation.management.
This method differs from the peek method only in that it throws an exception if this queue is empty Inserts the specified element into this queue. returning null if this queue is empty Retrieves and removes the head of this queue.________ _______________ | point | | Data | to next| |______|________| _|____________ | | | | Data | null | |______|______| | Node n | First|-------->| Data | to next |---|______|_________| Queue The Queue interface adds the following operations to the Collection interface: E element() Retrieves.SyncronousQueue<E> each put must wait for a take.. the head of this queue.concorrent.util.1__| | elem | |______| | Last |----------------------------------------------------------------|_elem_| | _________________ | point | | | | Node 1 Node 2 ___________________. java.concorrent. This method differs from the poll method in that it throws an exception if this queue is empty.ArrayBlockingQueue<E> orders elements FIFO. or null if this queue is empty Retrieves and removes the head of this queue. an element is only present when you try to take it. java. if possible. thread safe. boolean offer(E o) E peek() E poll() E remove() Retrieves. and vice versa. but does not remove. null element is not allowed.util. the head of this queue. java.PriorityQueue<E> orders elements according to an order/priority specified at construction time. syncronized.Java Programming/Print version |______| | Last |----------------------------------------------------------------|_elem_| | 120 After removing Node 2: Head ______ | Size | |_. does not have any internal capacity.util. you cannot add an element (using any method) unless another thread is trying to remove it . but does not remove. not even a capacity of one..
Most of the popular collection classes have implementations for both single thread and multiple thread environments.concurrent. This package supplies a few Collection implementations designed for use in multithreaded environments. explicitly or implicitly. In other words. java. when you make sure that only one thread updating the collection at any given time.util.util.util. java. Nonsyncronized. when the map is created.WeakHashMap<E> entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use.5. sorted according to the natural order for the key's class. not-syncronized. java.) Not-syncronized. The following table list all the syncronized collection classes: .util.Java Programming/Print version 121 Map Classes The Map interface has the following implementations: java.TreeMap<E> guarantees that the map will be in ascending key order. Not-syncronized. The non-synchronized implementations are always faster. Note that insertion order is not affected if a key is re-inserted into the map.util. You can use the non-synchronized implementations in multiple thread environments.LinkedHashMap<E> This linked list defines the iteration ordering.concurrent.util.util.ConcurrentHashMap same Hashtable.HashMap<E> is roughly equivalent to Hashtable. two keys k1 and k2 are considered equal if and only if (k1==k2). which is normally the order in which keys were inserted into the map (insertion-order). Enum maps are represented internally as arrays.util. that is java.IdentityHashMap This class implements the Map interface with a hash table.util.Hashtable<E> Syncronized. using reference-equality in place of object-equality when comparing keys (and values). null can not be used as key java.EnumMap All of the keys in an enum map must come from a single enum type that is specified. plus retrieval operations (including get) generally do not block. java. Thread Safe Collections It is also called Concurrent Collections. in an IdentityHashMap.equals(k2)). A new Java JDK package was introduced at Java 1. This representation is extremely compact and efficient. except that it is unsynchronized and permits nulls java. (In normal Map implementations (like HashMap) two keys k1 and k2 are considered equal if and only if (k1==null ? k2==null : k1. so may overlap with update operations (including put and remove). java.
util.Hashtable java.HashSet java.CopyOnWriteArrayList Set java.util.concurrent.CopyOnWriteArraySet Map java.Stack non-syncronized java.util.util.util.Vector java.concurrent.util.util.util.util. .util.util.util.IdentityHashMap java.Java Programming/Print version 122 syncronized List java.TreeMap java.TreeSet java.EnumMap Classes Diagram (UML) The following UML class diagram shows the Collection interfaces and their implementations.util.LinkList java.util.util.concurrent.HashMap java.util.ArrayList java.ConcurrentHashMap java.LinkedHashMap java.LinkHashSet java.
however there are some errors that only become apparent when the program is executed.Java Programming/Print version 123 The following UML class diagram shows the Map interfaces and their implementations.length > 1) { . However.1.com/docs/books/tutorial/collections/interfaces/collection. 2) ). int b) { return a / b.java public class ExceptionTutorial01 { public static int divide(int a. External links • Java Tutorial on Collections (http://java. where the program relies upon the provided command line arguments to generate a division operation. here. } public static void main(String[] args) { System. consider the next statement.println( divide(4. Consider the code in Listing 1. What if the user provides the number zero (0) as the second argument. It can safely be assumed that when the divide(4. ///: exceptions/ExceptionTutorial01. if(args.out.1: A simple division operation. it would return the number 2. Listing 1. but the compiler couldn't possibly have anticipated the user providing zero as an argument. the program defines a method divide that does a simple division operation taking two integers as parameter arguments and returning the result of their division.sun. 2) statement is called. We all know that division by zero is impossible.html) Throwing and Catching Exceptions Language compilers are adept at pointing out most of the erroneous code in a program.
2: Instance of an exception object with the default constructor. However. an exception object is returned. This information is known as a stack trace. throw new Exception(). the type of exception .where the exception occurred. 2.out. arg1) ). although you get into trouble if you take that analogy too far.parseInt(args[0]).println( divide(arg0.parseInt(args[1]). which allocates storage and calls a constructor./ by zero. you always create exceptions on the heap using new. Anything after the throw statement would not be executed. in effect. a comprehensible display message . As with any object in Java. as you'll see. System. In either case.4: Throwing an exception results in an unexpected return from the method. Listing 1. because where you return is some place completely different from where you return for a normal method call. and the exact methods and the line numbers where the exception may have occurred. Listing 1. new Exception("Something unexpected happened"). When the Java interpreter encounters an exceptional code.main(ExceptionTutorial01.lang. The keyword throw produces a number of interesting results.e. i. } } } /* Output for: java ExceptionTutorial01 1 0 2 Exception in thread "main" java. the exception object for java. .lang. new Exception(). you give the resulting reference to throw.3: Instance of an Exception object by passing string in constructor. This string can later be extracted using various methods.ArithmeticException: / by zero at ExceptionTutorial01. After creating an exception object with new.. The object is. Any similarity to an ordinary return from a method ends here. and. you end up in an appropriate exception handler that might be far away .java:15) *///:~ Such exceptional code that results in erroneous interpretations at program runtime usually results in errors that are called exceptions in Java.Java Programming/Print version int arg0 = Integer. There are two constructors in all standard exceptions: 1. it halts execution and displays information about the error that occurs. A simplistic way to think about exception handling is as a different kind of return mechanism.java. there are times when you would need to explicitly create your own exceptions. You can also exit from ordinary scopes by throwing an exception.from where the exception was thrown.java:6) at ExceptionTutorial01. The stack trace in the above example tells us more about the error. The default constructor.lang. "returned" from the method. such as the thread . and the method or scope exits.many levels on the call stack .ArithmeticException. 124 Exception arguments In the code above.ArithmeticException was generated by the Java interpreter itself. Listing 1. even though that object type isn't normally what the method is designed to return. A constructor taking a string argument so that you can place pertinent information in the exception. unless the thrown exception is handled properly.divide(ExceptionTutorial01. int arg1 = Integer."main" .
that method will exit in the process of throwing. even if you call the same method several times. you'd have to surround every method call with setup and error-testing code. This is called the try block because you "try" your various method calls there. Exception handlers immediately follow the try block and are denoted by the keyword catch: Listing 1. you can throw any type of Throwable.6: Exception handling with catch blocks. } catch(NullPointerException ex) { // Exception handling code for the NullPointerException } catch(NumberFormatException ex) { // Exception handling code for the NumberFormatException . If you don't want a throw to exit the method. you put everything in a try block and capture all the exceptions in one place. With exception handling. which is the exception root class. Listing 1. Exception handlers Of course. you can set up a special block within that method to capture the exception. and there's one for every exception type you want to catch. This is a section of code that might produce exceptions and is followed by the code to handle those exceptions. The information about therror is represented both inside the exception object and implicitly in the name of the exception class. Often.Java Programming/Print version In addition. 125 Catching an exception To see how an exception is caught. and nothing meaningful is stored within the exception object. Typically.5: A basic try block. you'll throw a different class of exception for each different type of error. The try block If you are inside a method and you throw an exception (or another method that you call within this method throws an exception). This means your code is much easier to write and read because the goal of the code is not confused with the error checking. the only information is the type of exception. the thrown exception must end up some place. The try block is an ordinary scope preceded by the keyword try. try { // Suppose the code here throws two exceptions: // NullPointerException and NumberFormatException. you must first understand the concept of a guarded region. // then each is handled in a separate catch block. This "place" is the exception handler. try { // Code that might generate exceptions } If you were checking for errors carefully in a programming language that didn't support exception handling. so someone in the bigger context can figure out what to do with your exception.
Listing 1.. } return result. the program would now not abruptly terminate but continue its execution whilst returning zero for a division by zero operation. Using the code in Listing 1.1 below talks about the various exception classes within the java.7. } catch(ArithmeticException ex) { result = 0. it saved your program from terminating abnormally. try { result = a / b.6.1: The Java exception classes Figure 1. Box 1.lang package of the JCL. but hey. 126 Exception classes in the JCL The box 1. int result = 0.1 as under. I know it's not the correct answer.1: The exception classes and their inheritance model in the JCL. .Java Programming/Print version } // etc. we can write the potentially problematic division code in Listing 1..7: Catching 'division by zero' errors. Using the syntax in Listing 1.
Java Programming/Print version
Throwable The Throwable class is the superclass of all errors and exceptions in the Java language. Only objects that are instances of this class (or one of its subclasses) are thrown by the Java Virtual Machine or can be thrown by the Java throw statement. A throwable contains a snapshot of the execution stack of its thread at the time it was created. It can also contain a message string that gives more information about the error. Finally, it can contain a cause: another throwable that caused this throwable to get thrown. The cause facility is new in release 1.4. It is also known as the chained exception facility, as the cause can, itself, have a cause, and so on, leading to a "chain" of exceptions, each caused by another Error An Error indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions. Exception The class Exception and its subclasses are a form of Throwable that indicates conditions that a reasonable application might want to catch. Also this is the class that a programmer may want to extend when adding business logic exceptions. RuntimeException RuntimeException is the superclass of those exceptions that can be thrown during the normal operation of the Java Virtual Machine. A method is not required to declare in its throws clause any subclasses of RuntimeException that might be thrown during the execution of the method but not caught.
Catch clauses
There are three distinct types of catch clauses used by Java programmers. We have already explored the first rule in the code listings above. Given below are all three clauses; we will explain all three in greater detail in the next sections. 1. The catch-list clause 2. The catch-any clause 3. The multi-catch clause
Java Programming/Print version
The catch-list clause
Using the catch-list clause, you provide one handle for every one type of exception. So, for instance if a guarded block generates a NullPointerException and an ArithmeticException, you have two provide two exception handling catch blocks - one for each exception. Refer to code listing 1.6 for an example.
The catch-any clause
There are times when too many blocks are just verbose, especially when all exceptions are handled in a similar manner. What you need here is a catch-any clause, where all exceptions are handled in a single catch block. Because all exceptions in Java are the sub-class of java.lang.Exception class, you can have a single catch block that catches an exception of type Exception only. Hence the compiler is fooled into thinking that this block can handle any exception. Using this, the code in Listing 1.6 can be rewritten as follows: Listing 1.8: The catch-any clause. try { // ... } catch(Exception ex) { // Exception handling code for ANY exception }
The multi-catch clause
As of JDK 7, Java added another convenient feature in their exception handling routines - the multi-catch clause. This is a combination of the previous two catch clauses and let's you handle exceptions in a single handler while also maintaining their types. So, instead of being boxed into a parent Exception super-class, they retain their individual types. Listing 1.9: The multi-catch clause. try { // ... } catch(NullPointerException | NumberFormatException ex) { // Exception handling code specifically for the NullPointerException // and the NumberFormatException }
Example of handling exceptions
Let's examine the following code: public void methodA() throws SomeException, AnotherException { //methodbody } public void methodB() throws CustomException
Java Programming/Print version { //Methodbody } public void methodC() { methodB(); methodA(); } In the code sample, methodC is invalid. Because methodA and methodB pass (or throw) exceptions, methodC must be prepared to handle them. This can be handled in two ways: a try - catch block, which will handle the exception within the method and a throws clause which would in turn throw the exception to the caller to handle. The above example will cause a compilation error, as Java is very strict about exception handling. So the programmer forced to handle any possible error condition at some point. A method can do two things with an exception. Ask the calling method to handle it by the throws declaration. Or handle the exception inside the method by the try-catch block. To construct a throws declaration, add throws ExceptionName (additional exceptions can be added with commas). To construct a try - catch block, use the following syntax try { // Guarded region that will catch any exception that // occurs within this code. } catch(Exception ex) { // Caught exceptions are handled here } finally { // This class is optional. Code here is executed // regardless of exceptions being thrown } The original code can be modified to work correctly in multiple ways. For example, the following: public void methodC() throws CustomException, SomeException { try { methodB(); } catch(AnotherException e) { // Handle caught exceptions. } methodA(); }
lang. it must be caught by the application some point. } catch(Exception e) { // NullPointerException is subclass of the Exception class.Java Programming/Print version The AnotherException from methodB will be handled locally. If any of those Exception object is thrown. } catch(Exception e) { // The NullPointerException thrown in the code above is caught here.lang. The catching blocks evaluated sequentially one by one.Exception and it is not the java. can be caught by the application. } catch(Exception ex) { // The "thrown" object is the same what is specified at the catch-block } try { throw new NullPointerException("This will be caught below"). by a catch-block.. java. while CustomException and SomeException will be thrown to the caller to handle it. 130 Application Exceptions Application Exception classes should extend the java. Lets see what is the catching criteria for a catch block to catch the "thrown" exception.lang. } There can be more than one catch-block for a try-block.Exception class.RuntimeException or its inherited objects. If a catch-block catch the exception. However.. Some of the JDK classes also throw exception objects inherited from java.RuntimeException or its inherited objects. Example: try { throw new NullPointerException("This will be caught below"). The compiler will enforce that there is a catch-block associated with an exception thrown. A catch-block will "catch" a thrown exception if and only if: • the thrown exception object is the same as the exception object specified by the catch-block • the thrown exception object is the subtype of the exception object specified by the catch-block try { throw new Exception("This will be caught below"). if the thrown exception object is inherited from java. but that is not enforced by the compiler. the others will not be evaluated.lang.Exception.lang. } catch(NullPointerException e) .
NullPointerException is one of the most common exceptions thrown in Java.RuntimeException exception class is inherited from java. because catching this exception class or its subclasses are not enforced by the Java compiler. Runtime exceptions are not business related exceptions. If the reference type is a subtype of Throwable. These should not be able to be triggered by the software's user(s). a special null can be assigned to an object reference.toString().. Object obj = null.. NullPointerException NullPointerException is a RuntimeException. It is a special exception class. having the null value.lang. Applications should throw instances of this class to indicate other illegal uses of the null object. and the most common source of bugs. like arithmetic overflow. . . } catch(NullPointerException e) { // The above NullPointerException will be caught here.. Instead the above code should be rewritten as follows: try { throw new NullPointerException("This will be caught below").Exception. . NullPointerException is thrown when an application attempts to use an object reference. runtime exceptions should not occur. These include: • • • • • Calling an instance method on the object referred by a null reference. If the reference type is an array type.. throwing a null reference.. runtime exception Runtime exceptions are usually caused by data errors..lang. If the reference type is an array type.Java Programming/Print version { // . } 131 Runtime Exceptions The java. All NullPointerExceptions will be caught by the first catch-block.while this code is never executed and the compiler returns an error } Because NullPointerException is subclass of the Exception class. In a well debugged code. In Java. Runtime exceptions should only be used in the case that the exception could be thrown by and only by something hard-coded into the program. divide by zero. } catch(Exception e) { // . // This statement will throw a NullPointerException The above code shows one of the pitfall of Java. accessing or modifying the slots of a null reference. Accessing or modifying an instance field of the object referred by a null reference. taking the length of a null reference. obj.while other exception are caught here. No object is created and the compiler does not detect it.
Object references cannot exist without a value. to make you more aware of null values. so you can't even treat a final field as being completely trusted during the object's . but we also want to return the object reference to the caller. before the object itself is created. } In the above code we want to create the Customer inside the try-block. Note that this list is not complete . so we need to create the object reference outside of the try-block. } catch(Exception ex) { . } return customer.there are no rules for preventing NullPointerException entirely in Java. because of the scoping rule in Java. It is of some use.. try { . so we assign the null value to it. because the standard libraries have to be used. It does not describe general techniques for how you should program Java.. and to be more careful about generating them yourself.. it is possible to observe an uninitialised final field in Java. This is one of the pitfall of Java. Keyword references • • • • try catch throws throw Navigate Exceptions topic: • Throwing and catching exceptions Application exceptions Runtime exceptions NullPointerException Main Exception classes • • • • • Stack trace Checked exceptions Unchecked exceptions Preventing NullPointerException Chained exceptions(Nesting Exceptions) This page describes some techniques for preventing NullPointerException.. and they can cause NullPointerExceptions. Also. customer = createCustomer(). public Customer getCustomer() { Customer customer = null. .Java Programming/Print version 132 Why do we need null? The reason we need it is because many times we need to create an object reference...
Check all references obtained from 'untrusted' methods Many methods that can return a reference can return a null reference. Make sure you check these. File[] files=file. but default values bring about bugs caused by default values being left in place.5. so use collections instead. For example: File file=new File("/etc"). if (files!=null) { stuff . and become competent with that. You can replace this with: boolean done=false. 133 Minimize the use of the keyword 'null' in assignment statements This means not doing things like: String s=null. as it allows the runtime to tell you about the bug. or: Object[] objects. rather than just continue with a default value. if (s!=null) something3(s).Java Programming/Print version creation. while (!done && something) if (something2) { done=true. but they don't replace the need to know about NullPointerExceptions. or explicitly fill the array at initialisation with: Object[] objects={"blah". objects=new Object[]{"blah".listFiles(). A good approach is to learn how to deal with NullPointerExceptions first.new File("/usr/bin")}.5. That's 10 more than we want. something3("yep"). A NullPointerException is actually better. while (something) if (something2) s="yep". These suggestions will help you to cause less NullPointerExceptions. } You might also consider replacing null with "" in the first example.new File("/usr/bin")}. Minimize the use of the new Type[int] syntax for creating arrays of objects An array created using new Object[10] has 10 null pointers.
com/) • Java Certification Mock Exams (http://www.htm) • Two Semesters of College-Level Java Lectures--Free (http://curmudgeon99.com/) • Java Lessons .equals( "OK" ) ) { .html) • The Java Tutorial (http://java.com) .4 version of the exam) • Java Language Specification.epractizelabs.com/forum/viewtopic.zeusedit.certification4career.sun. instead of throwing an exception.5. Some methods that don't specify that they might return null.com/docs/books/tutorial/index. html).sun.net/Programmer/393/ITCS-393.sun.com) 500+ questions with exam simulator (this is the older 1.mindview..com/developer/onlineTraining/new2java/index.sun.com/docs/books/jls/third_edition/html/j3TOC.alnaja7. but that's an assumption you're making.5.googlepages.0/docs/api/index. actually do.php?t=10) • Java 5 API Documentation (http://java. but not in the first one.html) • Java 5 SDK Documentation in CHM Format (http://www..Interactive Java programming tutorials based on examples (http://javalessons.Java Programming/Print version } File.com/j2se/1.0/docs/index. Links External References • Java Certification Preparation Guides (http://www. • Thinking in Java (http://www.kidwaresoftware.net/Books/TIJ/) • Java 5 SDK Documentation (http://java. you get a NullPointerException in the second example.sun. You can decide to trust some methods not to return null. 3rd Edition (http://java.com) • Java Tutorials for Kids and Adults (http://www. if you like..com/j2se/1.html) • Sun Developer Network New to Java Center (http://java. 134 Comparing string variable with a string literal When you compare a variable with a string literal. For example do: if ( "OK".listFiles() can return null if "/etc" is not a directory. } and do not do: <strike> if ( state. } </strike> If the 'state' variable is null. always put the string literal first.equals( state ) ) { . html) • A simple Java Tutorial (http://www..
SCBCD.sun. driven by Sang Shin from Sun • beanshell (http://www.org/) .com/vendors/Sun.com/java.Learn Java Programming by Examples (http://www. Java Course.org/ShowBook.apl.com/docs/books/vmspec/ 2nd-edition/jvms-clarify.html) • More resources (http://findshell.com/javaintro/) . tricks and best practices for Java Swing development • JavaTips (http://www.php?subcategoryid=17) • Full Java Tutorial (http://www.net/e-books/ share_ebook-310-220-sun-certified-developer-for-java-web-services) .akkidi.com/jgloss/jgloss. Second Edition (http://java.com) .netbeans.sun.roseindia.doc.org) .info/ download-free-java-ebooks) .javabeat.swingwiki.edu/~hall/java/FAQs-and-Tutorials.sun.com/docs/books/jls/third_edition/html/ j3TOC.net) • Java Programming FAQs and Tutorials (http://www.eclipse.SCEA (http://www.net/bin/view/Javapedia/) • Bruce Eckel Thinking in Java Third edition -.mindprod.com/docs/white/langenv/) • Complete Java Programming Tutorials (http://www.org/) • Games Programming Wiki (http://gpwiki.java.html) • Free Java and J2EE eBooks (http://www.html) • Kode Java .freeonlinebookstore.zeusedit.kodejava.Blog project containing best JAVA tips and tricks • Free Java/ Advanced Java Books (http://www.beanshell.sun.net .net/java/) • Javapassion.Examples and tutorials in Java • Download Free java ebooks from 83 ebooks collection (http://www.freebookcentre.sun.org/wiki/Java/Introduction_to_Java) .javapassion.0.techbooksforfree. Third Edition (http://java.info • Download Free Sun Certified Developer for Java Web Services (http://ebooks.Free Java Ebooks to download from ebooks.Java tutorials and lessons based on game programming • WikiJava (http://www.html) • Official Java Home Site (http://java.com/docs/books/vmspec/ 2nd-edition/html/VMSpecTOC.com) • Java lessons (http://www.mindview.landofcode.The Javapassion Site.jhu.org) • Zeus for Windows IDE (http://www.com) • Original Java Whitepaper (http://java.wikijava.net/Books/TIJ/) (Bruce has an C/C++ free book available on-line too) • JavaGameDevelopment (http://javagamedevelopment.org/) .Open documentation project containing tips.com/cgi/wiki?JavaLanguage) • NetBeans IDE (http://www.com/java/) • Online Java Tutorial (http://computer.asp?cat=55) • Java books available for free downloads (http://www. Java course (http://www.html) and amendments (http://java." • The Java Virtual Machine Specification.net) Daily news and articles on Java Game Development • Java Certifications Site(SCJP.Java Programming/Print version 135 External links • Java Certification Mock Exams (http://www.meshplex.Free Java Ebooks to download from ebookslab.html) "This book attempts a complete specification of the syntax and semantics of the language.mzwriter.org) Interpreted version • The Java Language Specification.A collection of free premium programming tutorials • Java Certification Practice Tests and Articles (http://www.html) A comprehensive reference that's also an excellent starting point for beginners • C2: Java Language (http://c2.com/) • Javapedia project (http://wiki.mzwriter.com/default.SCWCD.shtml) • Roedy Green's Java & Internet Glossary (http://www.html) • A pure java desktop (http://www.org) • Eclipse IDE (http://www.net/JavaTech/javaCategory.bestebooksworld.com/java.com) 500+ questions with exam simulator • SwingWiki (http://www.certification4career.ebookslab.(http://www.ucertify.Java 5.jdistro.
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literals and expressions Types String Classes. and with the Back-Cover Texts being LIST. with the Front-Cover Texts being LIST. merge those two alternatives to suit the situation. If you have Invariant Sections without Cover Texts. such as the GNU General Public License. 142 Overview • About this book • Installing the Java platform • History of Java • The Java platform • Java Runtime Environment (JRE) • Java Development Kit (JDK) • Similar concepts • Overview of the Java programming language Getting started • The process of abstraction • Working with objects • Exercise 1: Writing your first Java program Compiling programs Running programs Your first program Anatomy of a Java program • • • • Language fundamentals • • • • • • • • • • • • • • • Variables Flow control Arrays Methods Statements Identifiers. or some other combination of the three. objects and data types Syntax Keywords Packages Nested classes Access modifiers Data and variables • Generics • Java security . If your document contains nontrivial examples of program code. to permit their use in free software. we recommend releasing these examples in parallel under your choice of free software license.Java Programming/Print version with the Invariant Sections being LIST THEIR TITLES.
Java Programming/Print version 143 Classes and objects • • • • • • • Defining classes Creating objects Interfaces Using static members Destroying objects Overloading methods and constructors Class loading Basic arithmetic • Math functions • Large numbers • Random numbers Collections • Introduction • Arrays • Collection Classes • Collections and maps • Sets. lists and queues • Thread Safe Collections • Comparing Objects Exceptions • Throwing and catching exceptions • Application exceptions • Runtime exceptions • NullPointerException • Main Exception classes Stack trace Checked exceptions Unchecked exceptions Preventing NullPointerException Chained exceptions(Nesting Exceptions) • • • • • .
extensions and frameworks 3D programming Threads Remote Method Invocation (RMI) and Internet Inter-Orb Protocol (IIOP) Profiling Monitoring Extra Reading • Java IDEs Appendices • Links • Glossary • Index .Java Programming/Print version 144 User Interface topics • • • • Basic I/O Event handling Graphics Applets Swing programming • • • • • First Example: Frames and Labels Events and Buttons A Responsive Application Layouts Building a calculator Advanced Topics • Byte code • Concurrent programming • • • • • • • • • • • Networking Database programming Reflection JavaBeans Design patterns Libraries.
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