text:
This action might not be possible to undo. Are you sure you want to continue?
Introduction to Programming I
Student's Manual
Version 1.3 June 2006
Introduction to Programming I
Author Florence Tiu Balagtas Team Joyce Avestro Florence Balagtas Rommel Feria Reginald Hutcherson Rebecca Ong John Paul Petines Sang Shin Raghavan Srinivas Matthew Thompson
Requirements For the Laboratory Exercises
Supported Operating Systems The NetBeans IDE 5.5 runs on operating systems that support the Java VM. • Microsoft Windows XP Professional SP2 or newer
Solaris™ 10 Operating System Update 1 (SPARC® and x86/x64 Platform Edition) NetBeans Enterprise Pack is also known to run on the following platforms: • Microsoft Windows 2000 Professional SP4
• • •
Mac OS X 10.4.5 or newer Red Hat Fedora Core 3
• •
Solaris™ 8 OS (SPARC and x86/x64 Platform Edition) and Solaris 9 OS (SPARC and x86/x64 Platform Edition) Various other Linux distributions
Minimum Hardware Configuration Note: The NetBeans IDE's minimum screen resolution is 1024x768 pixels. Operating System Microsoft Windows Linux Solaris OS (SPARC) Processor 500 MHz Intel Pentium III 500 MHz Intel Pentium III workstation or equivalent UltraSPARC II 450 MHz Memory 512 MB 512 MB 512 MB 512 MB 512 MB Disk Space 850 MB of free disk space 450 MB of free disk space 450 MB of free disk space 450 MB of free disk space 450 MB of free disk space
Solaris OS (x86/x64 AMD Opteron 100 Series Platform Edition) 1.8 GHz Macintosh OS X operating system PowerPC G4
Recommended Hardware Configuration Operating System Microsoft Windows Linux Solaris OS (SPARC) Processor Memory Disk Speed 1 GB of free disk space 850 MB of free disk space 850 MB of free disk space 850 MB of free disk space 850 MB of free disk space 1.4 GHz Intel Pentium III 1 GB workstation or equivalent 1.4 GHz Intel Pentium III 1 GB workstation or equivalent UltraSPARC IIIi 1 GHz 1 GB
Solaris OS (x86/x64 AMD Opteron 100 Series 1 GB Platform Edition) 1.8 GHz Macintosh OS X operating system PowerPC G5 1 GB
Required Software NetBeans Enterprise Pack 5.5 Early Access runs on the Java 2 Platform Standard Edition Development Kit 5.0 Update 1 or higher (JDK 5.0, version 1.5.0_01 or higher), which consists of the Java Runtime Environment plus developer tools for compiling, debugging, and running applications written in the Java language. Sun Java System Application Server Platform Edition 9 has been tested with JDK 5.0 update 6. • For Solaris, Windows, and Linux, you can download the JDK for your platform from http://java.sun.com/j2se/1.5.0/download.html • For Mac OS X, Java 2 Platform Standard Edition (J2SE) 5.0 Release 4, is required. You can download the JDK from Apple's Developer Connection site. Start here: http://developer.apple.com/java (you must register to download the JDK). For more information, please visit: http://www.netbeans.org/community/releases/40/relnotes.html
Introduction to Programming I
Table of Contents
1 Introduction to Computer Programming.............................................................. 10 1.1 Objectives............................................................................................... 10 1.2 Introduction............................................................................................. 10 1.3 Basic Components of a Computer................................................................ 11 1.3.1 Hardware.......................................................................................... 11 126.96.36.199 The Central Processing Unit.......................................................... 11 22.214.171.124 Memory .................................................................................... 11 22.214.171.124 Input and Output Devices............................................................. 12 1.3.2 Software........................................................................................... 12 1.4 Overview of Computer Programming Languages........................................... 13 1.4.1 What is a Programming Language?....................................................... 13 1.4.2 Categories of Programming Languages.................................................. 13 1.5 The Program Development Life Cycle........................................................... 14 1.5.1 Problem Definition............................................................................. 15 1.5.2 Problem Analysis............................................................................... 15 1.5.3 Algorithm design and representation.................................................... 16 22.214.171.124 Flowcharting Symbols and their meanings...................................... 17 1.5.4 Coding and Debugging....................................................................... 18 1.6 Number Systems and Conversions.............................................................. 19 1.6.1 Decimal............................................................................................ 19 1.6.2 Binary.............................................................................................. 19 1.6.3 Octal................................................................................................ 19 1.6.4 Hexadecimal..................................................................................... 19 1.6.5 Conversions...................................................................................... 20 126.96.36.199 Decimal to Binary / Binary to Decimal............................................ 20 220.127.116.11 Decimal to Octal (or Hexadecimal)/Octal (or Hexadecimal) to Decimal.... 21 184.108.40.206 Binary to Octal / Octal to Binary.................................................... 22 18.104.22.168 Binary to Hexadecimal / Hexadecimal to Binary............................... 23 1.7 Exercises................................................................................................. 24 1.7.1 Writing Algorithms............................................................................. 24 1.7.2 Number Conversions.......................................................................... 24 2 Introduction to Java........................................................................................ 25 2.1 Objectives............................................................................................... 25 2.2 Java Background...................................................................................... 25 2.2.1 A little Bit of History .......................................................................... 25 2.2.2 What is Java Technology?................................................................... 25 126.96.36.199 A programming language............................................................. 25 18.104.22.168 A development environment......................................................... 25 22.214.171.124 An application environment.......................................................... 25 188.8.131.52 A deployment environment........................................................... 26 2.2.3 Some Features of Java........................................................................ 26 220.127.116.11 The Java Virtual Machine.............................................................. 26 188.8.131.52 Garbage Collection...................................................................... 26 188.8.131.52 Code Security............................................................................. 27 2.2.4 Phases of a Java Program.................................................................... 28 3 Getting to know your Programming Environment................................................. 29 3.1 Objectives............................................................................................... 29 3.2 Introduction............................................................................................. 29 3.3 My First Java Program............................................................................... 29 3.4 Using a Text Editor and Console................................................................. 30
Introduction to Programming I
3.4.1 Errors .............................................................................................. 40 184.108.40.206 Syntax Errors............................................................................. 40 18.104.22.168 Run-time Errors.......................................................................... 41 3.5 Using NetBeans........................................................................................ 42 3.6 Exercises................................................................................................. 55 3.6.1 Hello World!...................................................................................... 55 3.6.2 The Tree........................................................................................... 55 4 Programming Fundamentals............................................................................. 56 4.1 Objectives............................................................................................... 56 4.2 Dissecting my first Java program................................................................ 56 4.3 Java Comments........................................................................................ 58 4.3.1 C++-Style Comments......................................................................... 58 4.3.2 C-Style Comments............................................................................. 58 4.3.3 Special Javadoc Comments................................................................. 58 4.4 Java Statements and blocks....................................................................... 59 4.5 Java Identifiers........................................................................................ 60 4.6 Java Keywords......................................................................................... 61 4.7 Java Literals............................................................................................ 62 4.7.1 Integer Literals ................................................................................. 62 4.7.2 Floating-Point Literals ........................................................................ 62 4.7.3 Boolean Literals ................................................................................ 62 4.7.4 Character Literals .............................................................................. 63 4.7.5 String Literals ................................................................................... 63 4.8 Primitive data types.................................................................................. 64 4.8.1 Logical - boolean............................................................................... 64 4.8.2 Textual – char................................................................................... 64 4.8.3 Integral – byte, short, int & long.......................................................... 65 4.8.4 Floating Point – float and double.......................................................... 66 4.9 Variables................................................................................................. 67 4.9.1 Declaring and Initializing Variables....................................................... 67 4.9.2 Outputting Variable Data.................................................................... 68 4.9.3 System.out.println() vs. System.out.print() ......................................... 68 4.9.4 Reference Variables vs. Primitive Variables............................................ 69 4.10 Operators.............................................................................................. 70 4.10.1 Arithmetic operators......................................................................... 70 4.10.2 Increment and Decrement operators................................................... 73 4.10.3 Relational operators......................................................................... 75 4.10.4 Logical operators.............................................................................. 78 188.8.131.52 && (logical AND) and & (boolean logical AND)............................... 79 184.108.40.206 || (logical OR) and | (boolean logical inclusive OR)......................... 81 18.104.22.168 ^ (boolean logical exclusive OR).................................................. 83 22.214.171.124 ! (logical NOT).......................................................................... 84 4.10.5 Conditional Operator (?:).................................................................. 85 4.10.6 Operator Precedence........................................................................ 87 4.11 Exercises............................................................................................... 88 4.11.1 Declaring and printing variables......................................................... 88 4.11.2 Getting the average of three numbers................................................. 88 4.11.3 Output greatest value....................................................................... 88 4.11.4 Operator precedence........................................................................ 88 5 Getting Input from the Keyboard....................................................................... 89 5.1 Objectives............................................................................................... 89 5.2 Using BufferedReader to get input.............................................................. 89 5.3 Using JOptionPane to get input................................................................... 93
Introduction to Programming I
7 Exercises......6 switch statement................ 96 6.4 Powers.................................................. 109 6............4.........................................................................5 Exercises....E................................................................................................3 Hundred Times................. 115 7........... 114 6.........1 Last 3 words (BufferedReader version).................................................................................7 Example for switch..........................................................3 Repetition Control Structures............................................4......................................2 Decision Control Structures..............1 Objectives........... 128 8..................................4 Accessing an array element............................................2 if-else statement......... 116 7........................................5 Example for if-else-else if...............................................................................2 Introduction to arrays.....................4..................2 Arithmetic Operations.......5........................................................ 100 6............................ 114 6...........................4 Exercises....................... 120 7...................................4.................... 114 6............. 110 6................... 122 8....................................................................................................................1 Print arguments...................5.... 110 6................... 112 6.......2..........................................1 Objectives.................................................1.4 Branching Statements...... 105 6.................. 110 6.................................1 Days of the Week....................................................... 115 7........... 106 6.....3.............................................................4................................................2...................................3 Addressbook Entries................. 111 6........................... 102 6.......................................2............................4....................................................................................3................................................................ 121 7...................7....2 Labeled break statement.............2 Last 3 words (JOptionPane version).. 122 8..........1 Grades............................1 Objectives............1 Unlabeled break statement....................................................................................................2............. 108 6....... 101 6....... 130 Introduction to Programming I 5 .2 continue statement..................2.................6 Multidimensional Arrays..............7..................................................................... 128 8....................................................1 if statement......................4 Common Errors when using the if-else statements:...... 122 8..........................................................................2.... 98 6. 121 7................2....................4................................ 118 7...............................7............................................................................3 Declaring Arrays...........................4..................D.... 119 7................................. 112 6............................................................................... 124 8...3......... 113 6.. 130 9.........4.............. 115 7.3 Command-line arguments in NetBeans.......4..I 6 7 8 9 5..... 96 6.......................................5 Array length....... 103 6..............................................................3..........................................................................................................................1 while loop....... 112 6................................................................................................. 106 6................................................... 114 Java Arrays.....2........................................................ 129 9............3 for loop...............1 Difference Between Classes and Objects...................................................................................J............................3 return statement. 95 5............ 121 Command-line Arguments.......................1................. 129 9.........................................................................................4 Exercises.................................................................................... 95 Control Structures..................2 Greatest number...............................................................................3 Classes and Objects......................................... 96 6...1 break statement......................................................5.......................................................1 Objectives... 128 Working with the Java Class Library.............. 129 9............................2...........2 Introduction to Object-Oriented Programming.................2 Number in words.......................5........................... 114 6..2 Labeled continue statement...1 Unlabeled continue statement............ 121 7.....................4.................... 95 5.2 Command-line arguments.......................................2 do-while loop................................................................................... 96 6.....................................3 if-else-if statement...............................
.................. 150 10........................................D..........................................................................................10 Exercises........ 165 10................ 162 10....................................... 141 9........ 163 10.....3......................................... 138 9...........3....................................................................................... 162 10.................... Converting and Comparing Objects.........8 Packages.....3 protected access.................................................10............................ 164 10..................4 Calling Static Methods....1 Casting Primitive Types. 168 10.. 156 10.....................................3 Class Variables and Methods..................8................................................................9.........2 Calling Instance Methods and Passing Variables..............1 Instance Variables........5.............................................................. 149 10 Creating your own Classes.. Polymorphism and Interfaces....................................................................... 151 10....5 Determining the Class of an Object................... 171 Introduction to Programming I 6 ......................................... 156 10.................4........3.... 153 10.......4 Declaring Methods... 148 9....................4.............................................................................................................................6 Overloading Methods..........3 Converting Primitive Types to Objects and Vice Versa.................................2 public access............... 149 9.......................4 Comparing Objects....6.. 133 9...........3 Declaring Attributes..................................2 Creating your own packages...................................... 146 9.................................. 165 10.................................................. 168 10........3................................... 170 10..........7............................................................................................5........4................4 Static methods................ 171 11................4...............2 Encapsulation..................................................................5.....................................................................................................4........................... 165 10..7....................2 Casting Objects.....................................................................2 Java Scavenger Hunt.....................6................... 162 10....7....4 The this() Constructor Call.................8................. 141 9........ 170 11 Inheritance.........................................2 Defining your own classes......3 Using Constructors.......................................2 Class Variables or Static Variables........................ 157 10...........4......................134 9.......................4...1 Objectives.....4..........3 Setting the CLASSPATH................E............................................................................2 Pass-by-reference....................7 Declaring Constructors.................................1 default access (also called package accessibility)............................... 131 9.................................7....................5.....5 The this reference........ 131 9..................J.......10............................................... 169 10................................................................ 150 10...... 169 10.........................4.........1 Address Book Entry.................... 132 9...............................1 Importing Packages.....5 Casting..... 166 10...................... 136 9............2 AddressBook..3...3 Multiple Return statements................................4 Class Instantiation......... 160 10............I 9.........................3................. 170 10......1 Pass-by-value.........................................4 Methods.......................................2 Overloading Constructors............ 153 10........1 Defining terms. 137 9.................................................................................1 Accessor methods..................................................................5..........9........................6 Exercises............................................................................2 Mutator Methods........................ 135 9................................................9 Access Modifiers..............5 Sample Source Code for StudentRecord class.... 145 9.............................. 154 10.....9..................... 155 10......................4.....1 What are Methods and Why Use Methods?................... 133 9................1 Objectives.........3................... 159 10.................................. 149 9.... 152 10....................8............................................................. 152 10........... 143 9..........................................................................................4 private access.... 135 9...........................5 Scope of a variable................................4.....................................3 Passing Variables in Methods....................................9.....1 Default Constructor........... 168 10..........................................................................................................................................................................4............
.................................................. 175 11.................6 Exercises................................ 176 11........6 Inheritance among Interfaces................................. 182 11............................................................D............................................. 242 Introduction to Programming I 7 .......................................6.............................. 240 Machine Problem 2: Minesweeper.... 229 Using NetBeans......................................2...........5 Relationship of an Interface to a Class.... 179 11..... 181 11.................E..........2 What are Exceptions?.......................1 Why do we use Interfaces?....5.................................... 196 Installing NetBeans in Ubuntu Dapper........4........................................................................................................5........................1 Extending StudentRecord............................................ 181 11...................................................................................... Class........................................................................................................................................ 186 12...........................................................................3 Polymorphism..................................................................1 Defining Superclasses and Subclasses............................................................................................... 189 12............................................... 215 Using a Text Editor and Console.......................5 Interfaces....................................................................... 185 12 Basic Exception Handling......... 172 11......6.................. 182 11..................J..............................2 Inheritance.........4 Exercises........ 191 Installing Java in Windows............................................. 190 Installing Java in Ubuntu Dapper.......................3 Overriding Methods...................... 200 Installing NetBeans in Windows...........4 Abstract Classes......................5..... 215 My First Java Program................................................3 Handling Exceptions. 184 11.................2 The super keyword......2 Interface vs....................................... 189 12..............................3 Interface vs.............................. 185 11........................................ 240 Machine Problem 1: Phone Book.................................. 189 Appendix A : Java and NetBeans Installation.....................................................................................................................................................5..................... 186 12.2............I 11............................................ 208 Appendix B: Getting to know your Programming Environment (Windows XP version)....................................................................... 216 Setting the Path...................4 Final Methods and Final Classes.............................. 181 11........4 Creating Interfaces............... 230 Appendix D : Machine Problems..................2 Catching Exceptions 2........................................................ 185 11.............. 171 11.. 186 12....... 174 11................ 184 11. 186 12..................................................................................5.............................................5...............1 Objectives.............. 241 Machine Problem 3: Number Conversion...................................................... Abstract Class................................................................................. 177 11...2.........................................................................................4.......2..................................................1 Catching Exceptions1................2 The Shape abstract class...................................................................................
2 Change Netbeans/netbeans to NetBeans List of Java keywords Chapter 10: Creating your own classes Coding guidelines: filenames should have the same name as the public class name Master Documents For Version 1.J. Chapter 3:Getting to know your programming environment Appendix F: Additional Exercises Details Switch to Netbeans 5. Interfaces and Polymorphism Polymorphism section • Added example that uses another class whose method can receive a reference variable Interface • Added sections • Why do we use Interfaces? Introduction to Programming I 8 .D.1 to 1.I Revision History For Version 1.E.0 to 1.1 August 2005 Section Version Number Revision History Appendix E: Hands-on Lab Exercises Details Change from 1.3 June 2006 Section Appendix A and B. Chapter 3:Getting to know your programming environment Appendix A.1 Added Added (c/o Sang) Added to list of references Chapter 10: Creating Your own classes Added subsection on How to set classpath at packages section Chapter 11: Inheritance.5 Beta Version Switch from Redhat Linux to Ubuntu Dapper Added (Teacher's manual)-c/o JEDI member school teachers For Version 1.2 January 2006 Section Version Number Chapter 3: Gettting to know your programming environment Appendix A Chapter 4: Programming Fundamentals Details Change from 1.
E.I Section Details • • • • Interface vs. Abstract Class Interface vs.J.D. Class Relationship of an Interface to a Class Inheritance among Interfaces Introduction to Programming I 9 .
Introduction to Programming I 10 . Finally. different number systems and conversions from one type to another will be discussed. It consists of data and the computer programs.E.D.J. The computer has two major components.I 1 Introduction to Computer Programming 1.2 Introduction A computer is a machine that performs a variety of tasks according to specific instructions. It is composed of electronic and mechanical parts. The first one is the Hardware which is the tangible part of the computer. the student should be able to: • • • • Identify the different components of a computer Know about programming languages and their categories Understand the program development life cycle and apply it in problem solving Learn the different number systems and their conversions 1. we will be discussing the basic components of a computer. At the end of the lesson.1 Objectives In this section. It is a data processing machine which accepts data via an input device and its processor manipulates the data according to a program. We will also be giving a brief overview of programming languages and the program development life cycle. The second major component is the software which is the intangible part of a computer. both hardware and software.
This means that once the computer is turned off. that the processor is actively working with.J. Secondary memory is considered as non-volatile storage.1 Hardware 1. It is used to hold programs and data for long term use. It is divided into several storage locations which have corresponding addresses. It does the fundamental computing within the system. It is sometimes called the RAM (Random Access Memory).3. Examples of secondary memory are hard disks and cd-rom. Main Memory The main memory is very closely connected to the processor.3 Basic Components of a Computer 1. The CPU accesses the memory with the use of these addresses.2 Memory The memory is where data and instructions needed by the CPU to do its appointed tasks can be found. Athlon and SPARC. The Secondary Memory The secondary memory is connected to main memory. 1.3. Examples of processors are Pentium.1. all information residing in the main memory is erased.1. This means that information residing in secondary memory is not erased after the computer is turned off. Main Memory Fast Expensive Low Yes Secondary Memory Slow Cheap High No Property Speed Price Capacity Volatile Table 1: Comparison between main memory and secondary memory Introduction to Programming I 11 . It contains millions of extremely tiny electrical parts.D. The computer's main memory is considered as volatile storage. 2.1 The Central Processing Unit The processor is the "brain" of the computer.E. It is used to hold programs and data. 1.3.I 1. It is not used for long-term storage.
The data that the computer uses can be anything that a program needs. for this purpose. mice and microphones. Solaris. there exists compilers.D. Compilers • The computer understands only one language: machine language.J. printers and speakers.3. Since it is highly impractical for people to create programs out of zeros and ones. Programs acts like instructions for the processor. there must be a way of translating or converting a language which we understand into machine language. 1. It is kept on some hardware device like a hard disk.I 1. Windows. MacOS 2. Systems Programs • • Programs that are needed to keep all the hardware and software systems running together smoothly Examples: • Operating Systems like Linux. Application Programs • • Programs that people use to get their work done Examples: • Word Processor • Game programs • Spreadsheets 3.3. but it itself is intangible. Machine language is in the form of ones and zeros. Examples of output devices are monitors.1.2 Software A software is the program that a computer uses in order to function.3 Input and Output Devices Input and output devices allows a computer system to interact with the outside world by moving data into and out of the system. Unix. Examples of input devices are keyboards.E. Introduction to Programming I 12 . Some Types of Computer Programs: 1.
C. but they are much easier to program in because they allow a programmer to substitute names for numbers. A programming statement may be translated into one or several machine instructions by a compiler.I 1.4. High-level Programming Languages • A high-level programming language is a programming language that is more userfriendly. Low-level Assembly Language • Assembly languages are similar to machine languages.4 Overview of Computer Programming Languages 1. Like human languages. how these data will be stored/transmitted. assembly language was considered low-level and COBOL. and each assembly instruction is translated into one machine instruction by an assembler program. and abstract from low-level computer processor operations such as memory accesses. Basic.2 Categories of Programming Languages 1. Assembly languages are available for each CPU family. Fortran • 2. Many programmers today might refer to these latter languages as low-level. Examples are Java.J. C++. these instructions are translated into machine language that can be understood by computers. to some extent platform-independent.4. Programming languages enable a programmer to precisely specify what data a computer will act upon. but regardless of what language you use. each language has its own syntax and grammar. There are different types of programming languages that can be used to create programs.D.1 What is a Programming Language? A programming language is a standardized communication technique for expressing instructions to a computer. Note: The terms "high-level" and "low-level" are inherently relative. were considered high-level. Introduction to Programming I 13 . C. 1.E. and precisely what actions to take under various circumstances. etc. Originally.
they follow an organized plan or methodology.5 The Program Development Life Cycle Programmers do not sit down and start writing code right away when trying to make a computer program.J.D. 2.I 1. Introduction to Programming I 14 . let us define a single problem that we will solve step-by-step as we discuss the problem solving methodologies in detail. 4. The problem we will solve will be defined in the next section. Instead. 3. that breaks the process into a series of tasks. Problem Definition Problem Analysis Algorithm design and representation (Pseudocode or flowchart) Coding and debugging In order to understand the basic steps in solving a problem on a computer. Here are the basic steps in trying to solve a problem on the computer: 1.E.
Before a program can be designed to solve a particular problem.2 Problem Analysis After the problem has been adequately defined.J. this step involves breaking up the problem into smaller and simpler subproblems.I 1. Example Problem: Determine the number of times a name occurs in a list Input to the program: list of names. Let us now define our example problem: "Create a program that will determine the number of times a name occurs in a list.D.E.5.5. name to look for Output of the program: the number of times the name occurs in a list Introduction to Programming I 15 . the problem must be well and clearly defined first in terms of its input and output requirements. Computer programming requires us to define the problem first before we even try to create a solution." 1. the simplest and yet the most efficient and effective approach to solve the problem must be formulated. Usually. A clearly defined problem is already half the solution.1 Problem Definition A programmer is usually given a task in the form of a problem.
how do we express our general solution in such a way that it is simple yet understandable? Expressing our solution through Human language: 1. through a graphical representation like a flowchart or through a pseudocode. Get the name to look for. If the keyname is the same with a name in the list. Compare the keyname to each of the names in the list 4.1: Example of a flow chart Introduction to Programming I 16 . let's call this the keyname 3.E. If all the names have been compared. Tagalog). It may be expressed in either Human language (English.5. Now given the problem defined in the previous sections. which is a cross between human language and a programming language.3 Algorithm design and representation Once our problem is clearly defined. add 1 to the count 5.I 1. it is normally required to express our solution in a step-by-step manner. In computer programming. Get the list of names 2. An Algorithm is a clear and unambiguous specification of the steps needed to solve a problem. output the result Expressing our solution through a flowchart: YES Figure 1.J. we can now set to finding a solution.D.
Also functions as the default symbol when no other symbol is available. or on the right. or location of information. Decision Symbol Terminal Symbol Represents the beginning. and the arrowheads are mandatory only for right-to-left and bottom-totop flow. Here are some guidelines for commonly used symbols in creating flowcharts.2: Example of a pseudocode 1.E. form.D.The lines connect Flowline Symbol other symbols. which makes data available for processing (input) or displaying (output)of processed information. as shown.J.I Expressing our solution through pseudocode: Let nameList = List of Names Let keyName = the name to be sought Let Count = 0 For each name in NameList do the following if name == keyName Count = Count + 1 Display Count Figure 1. You can use any symbols in creating your flowcharts. as long as you are consistent in using them. Rather. Symbol Name Meaning Represents the process of executing a defined operation or groups of operations that results in a Process Symbol change in value. the end. Input/Output (I/O) Symbol Represents the sequence of available information and executable operations. or explanatory notes as clarification. comments. Introduction to Programming I 17 . The vertical line and the broken line may be placed on the left.3.1 Flowcharting Symbols and their meanings A flowchart is a design tool used to graphically represent the logic in a solution. Annotation Symbol Represents a decision that determines which of a number of alternative paths is to be followed. Represents the addition of descriptive information.5. Flowcharts typically do not display programming language commands. they state the concept in English or mathematical notation. or a point of interruption or delay in a program. Represents an I/O function.
Predefined Process Symbol Represents a named process consisting of one or more operations or program steps that are specified elsewhere. Most of the time. and unfortunately. the wrong thing happens. Introduction to Programming I 18 . the source code can now be written using the chosen programming language. The first one is compile-time error. It's something the compiler can detect as an error. But when you follow the code's logic. when the end-user runs the program.E.5. etc. This type of error is called runtime error. and therefore. and the other is runtime error. the program (or even their whole computer) freezes up due to an infinite loop. At this point. The programmer has to add some fixes to the program in case of errors (also called bugs) that occurs in the program. This is especially true for logic errors such as infinite loops. For example. There are two types of errors that a programmer will encounter along the way. Forgetting a semi-colon at the end of a statement or misspelling a certain command. another part of the flowchart. compilers aren't really smart enough to catch all of these types of errors at compile-time. This process of is called debugging.J. Other types of run-time errors are when an incorrect value is computed. it is now possible to create the source code.D. after the programmer has written the program. is a compile-time error.I Symbol Name Connector Symbol Meaning Represents any entry from. The compiler will detect the error and the program won't even compile. the actual syntax of the code looks okay. the program isn't 100% working right away. Compile-Time Errors occur if there is a syntax error in the code. Compilers aren't perfect and so can't catch all errors at compile time. the program compiles fine into an executable file. the programmer is unable to form an executable that a user can run until the error is fixed. Table 2: Flowchart Symbols 1. In such a case. the same piece of code keeps executing over and over again infinitely so that it loops. Also serves as an off-page connector. or exit to. Using the algorithm as basis. However. for example.4 Coding and Debugging After constructing the algorithm.
I 1.J. We need to write the subscript 2 to indicate that the number is a binary number. Here are examples of numbers written in binary form: 11111102 10112 1. This means that the only digits that appear are 0-9. Here are examples of numbers written in hexadecimal form: 7E16 B16 Hexadecimal Decimal Equivalent 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 A B C D E F 1 9 10 11 12 13 14 5 Table 3: Hexadecimal Numbers and their Equivalence to decimal numbers Decimal 12610 1110 Binary 11111102 10112 Octal 1768 138 Hexadecimal 7E16 B16 Table 4: Summary of Examples Introduction to Programming I 19 . This means that the only legal digits are 09 and the letters A-F (or a-f.2 Binary Numbers in binary form are in base 2.1 Decimal We normally represent numbers in their decimal form. This means that the only legal digits are 0-7.6. 1.3 Octal Numbers in octal form are in base 8. Here are examples of numbers written in decimal form: 12610 (normally written as just 126) 1110 (normally written as just 11) 1. This means that the only legal digits are 0 and 1. The representation depends on what is called the BASE.6. lowercase or uppercase does not matter). Numbers in decimal form are in base 10.4 Hexadecimal Numbers in hexadecimal form are in base 16. The following are the four most common representations.6 Number Systems and Conversions Numbers can be represented in a variety of ways.6. We need to write the subscript 8 to indicate that the number is an octal number.D. Here are examples of numbers written in octal form: 1768 138 1. We need to write the subscript 16 to indicate that the number is a hexadecimal number.E.6.
We then add all the products to get the resulting decimal number.6. we multiply the binary digit to "2 raised to the position of the binary number". For Example: 12610 = ? 2 126 63 31 15 7 3 1 / / / / / / / 2 2 2 2 2 2 2 = = = = = = = Quotient 63 31 15 7 3 1 Remainder 0 1 1 1 1 1 1 So. Get the quotient and divide that number again by 2 and repeat the whole process until the quotient reaches 0 or 1.D. and get that number as a digit of the binary form of the number.1 Decimal to Binary / Binary to Decimal To convert a decimal number to binary.5 Conversions 1.6. continuously divide the number by 2 and get the remainder (which is either 0 or 1). For Example: 11111102 = ? Position Write it this way 10 Binary Digits 6 5 4 3 2 1 0 1 1 1 1 1 1 0 0 x 20 = 0 1 x 21 = 2 1 x 22 = 4 1 x 2 3= 8 1 x 24= 16 1 x 25 = 32 1 x 26 = 64 TOTAL: 126 Introduction to Programming I 20 .E.5. writing the remainders from the bottom up. We then get all the remainders starting from the last remainder. we get the binary number 11111102 To convert a binary number to decimal. and the result is the binary form of the number.I 1.J. NOTE: For the last digit which is already less than the divisor (which is 2) just copy the value to the remainder portion.
E. writing the remainders from the bottom up. For Example (Octal): 12610 = ? 8 Quotient 126 / 8 = 15 / 8 = 1/8= 15 1 Remainder 6 7 1 Write it this way So. instead of having 2 as the divisor. writing the remainders from the bottom up.6. we get the octal number 1768 For Example (Hexadecimal): 12610 = ? 16 Quotient 126 / 16 = 7 / 16 = 7 Remainder 14 (equal to hex digit E) 7 Write it this way So.I 1. For Example (Octal): 1768 = ? 10 Position Octal Digits 2 1 1 7 0 6 6 x 80 = 6 7 x 81 = 56 1 x 82 = 64 TOTAL: 126 Introduction to Programming I 21 . we will just replace the base number 2 with 8 for Octal and 16 for hexadecimal.2 Decimal to Octal (or Hexadecimal)/Octal (or Hexadecimal) to Decimal Converting decimal numbers to Octal or hexadecimal is basically the same as converting decimal to binary. To do that.J. you replace it with 8(for octal) or 16 (for hexadecimal). However. we get the hexadecimal number 7E16 *** Converting octal or hexadecimal numbers is also the same as converting binary numbers to decimal.5.D.
3 Binary to Octal / Octal to Binary To convert from binary numbers to octal. we partition the binary number into groups of 3 digits (from right to left). Simply convert each octal digit into its binary representation (given the table) and concatenate them. Octal Digit 0 1 2 3 4 5 6 7 Binary Representation 000 001 010 011 100 101 110 111 Table 5: Octal Digits and their corresponding binary represenation For Example: 11111102 = ? 8 0 0 1 1 1 1 7 1 1 1 6 0 Equivalent octal number Converting octal numbers to binary is just the opposite of what is given above. Introduction to Programming I 22 .5. The result is the binary representation.E.6. We then convert each partition into its corresponding octal digit. and pad it with zeros if the number of digits is not divisible by 3.J.I For Example (Hexadecimal): 7E16 = ? 10 Position Hex Digits 1 7 0 E 14 x 160 = 14 7 x 161 = 112 TOTAL: 126 1. The following is a table showing the binary representation of each octal digit.D.
The result is the binary representation.4 Binary to Hexadecimal / Hexadecimal to Binary To convert from binary numbers to hexadecimal. we partition the binary number into groups of 4 digits (from right to left). The following is a table showing the binary representation of each hexadecimal digit.6.I 1. Introduction to Programming I 23 . Hexadecimal Digit 0 1 2 3 4 5 6 7 8 9 A B C D E F Binary Representation 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 Table 6: Hexadecimal Digits and their corresponding binary represenation For Example: 11111102 = ? 16 0 1 7 1 1 1 1 E 1 0 Equivalent Hexadecimal number Converting hexadecimal numbers to binary is just the opposite of what is given above. and pad it with zeros if the number of digits is not divisible by 4. Simply convert each hexadecimal digit into its binary representation (given the table) and concatenate them.E.D.J.5. We then convert each partition into its corresponding hexadecimal digit.
Getting the average of three numbers 1. create an algorithm to accomplish the following tasks. You may write your algorithms using pseudocodes or you can use flowcharts.J. 1. 43F16 to binary.E.D.I 1. hexadecimal and octal 3. hexadecimal and octal 2.1 Writing Algorithms Given the following set of tasks. 198010 to binary. Baking Bread 2. 768 to binary. hexadecimal and decimal 4.2 Number Conversions Convert the following numbers: 1.7.7 Exercises 1.7. decimal and octal Introduction to Programming I 24 . 10010011012 to decimal. Logging into your laboratory's computer 3.
E. At the end of the lesson.2 Java Background 2.I 2 Introduction to Java 2. a class file packaging tool. Java technology provides you with a large suite of tools: a compiler. Java can create all kinds of applications that you could create using any conventional programming language.2 What is Java Technology? 2.2. 2. Gosling et. its name was changed to Java because there was already a language called Oak. Initially called Oak.3 An application environment Java technology applications are typically general-purpose programs that run on any machine where the Java runtime environment (JRE) is installed. Introduction to Programming I 25 . 2.2 A development environment As a development environment.2. a documentation generator. of Sun Microsystems. We will also discuss the phases that a Java program undergoes.2. One of the first projects developed using Java was a personal hand-held remote control named Star 7. an interpreter. and so on. The original motivation for Java was the need for platform independent language that could be embedded in various consumer electronic products like toasters and refrigerators.2. 2.2. At about the same time. realized that Java could be used for Internet programming. the World Wide Web and the Internet were gaining popularity.1 A little Bit of History Java was created in 1991 by James Gosling et al. garbage collection and code security Describe the different phases of a Java program 2. we will be discussing a little bit of Java history and what is Java Technology. in honor of the tree outside Gosling's window.1 Objectives In this section.2.2.J.2. the student should be able to: • • Describe the features of Java technology such as the Java virtual machine. al.1 A programming language As a programming language.D.
2. The garbage collection thread is responsible for freeing any memory that can be freed. This can be difficult at times since there can be instances wherein the programmers forget to deallocate memory and therefor result to what we call memory leaks.3 Some Features of Java 2. Introduction to Programming I 26 . Most commercial browsers supply a Java technology interpreter and runtime environment.E.2. there should be a way to deallocate that memory block in order for other programs to use it again.2.2.J.4 A deployment environment There are two main deployment environments: First. and so on. However. no matter what type of computer the program was compiled on. GUI component classes. The bytecode is independent of any particular computer hardware. the programmer is freed from the burden of having to deallocate that memory themselves by having what we call the garbage collection thread. after using that allocated memory.2.2 Garbage Collection Many programming languages allows a programmer to allocate memory during runtime. This specification enables the Java software to be platform-independent because the compilation is done for a generic machine known as the JVM.I 2. 2. so any computer with a Java interpreter can execute the compiled Java program. which includes basic language classes.2.1 The Java Virtual Machine The Java Virtual Machine is an imaginary machine that is implemented by emulating software on a real machine. C++ and other languages the programmer is responsible for this.3. The JVM provides the hardware platform specifications to which you compile all Java technology code. The other main deployment environment is on your web browser. In C. In Java.3. A bytecode is a special machine language that can be understood by the Java Virtual Machine (JVM). This happens automatically during the lifetime of the Java program.D. the JRE supplied by the Java 2 Software Development Kit (SDK) contains the complete set of class files for all the Java technology packages.
3 Code Security Code security is attained in Java through the implementation of its Java Runtime Environment (JRE). code verification (through the bytecode verifier) and finally code execution. The Class Loader is responsible for loading all classes needed for the Java program. After all of these have been done. It adds security by separating the namespaces for the classes of the local file system from those that are imported from network sources.3. Introduction to Programming I 27 . the bytecode verifier then tests the format of the code fragments and checks the code fragments for illegal code that can violate access rights to objects.E. the code is then finally executed.J. After loading the class and layouting of memory. This limits any Trojan horse applications since local classes are always loaded first.I 2. the memory layout of the executable is then determined.2. This adds protection against unauthorized access to restricted areas of the code since the memory layout is determined during runtime. The JRE runs code compiled for a JVM and performs class loading (through the class loader).D. After loading all the classes.
D. The .E.J. compile the program by using the Java Compiler. vi. After creating and saving your Java program.1: Phases of a Java Program The first step in creating a Java program is by writing your programs in a text editor. Examples of text editors you can use are notepad. emacs.class extension (Java bytecodes) Program Output Table 7: Summary of Phases of a Java Program Introduction to Programming I 28 .java.class file is then interpreted by the Java interpreter that converts the bytecodes into the machine language of the particular computer you are using. Figure 2.class. etc.4 Phases of a Java Program The following figure describes the process of compiling and executing a Java program. This file is stored in a disk file with the extension .java extension File with .I 2. Task Write the program Compile the program Run the program Tool to use Any text editor Java Compiler Java Interpreter Output File with .2. The output of this process is a file of Java bytecodes with the file extension .
a compiler and/or interpreter and a debugger. Make sure that before you do this tutorial. There are two ways of doing this.D. The second one is by using NetBeans which is an Integrated Development Environment or IDE. } } Before we try to explain what the program means. compile and run Java programs. 3. For the Windows XP version of this section.I 3 Getting to know your Programming Environment 3. the student should be able to: • • • Create a Java program using text editor and console in the Linux (Ubuntu Dapper) environment Differentiate between syntax-errors and runtime errors Create a Java program using NetBeans 3.println("Hello world!").J.E. This tutorial uses Ubuntu Dapper as the operating system.2 Introduction An IDE is a programming environment integrated into a software application that provides a GUI builder. let us first take a look at the first Java program you will be writing. please refer to Appendix B. let's first try to write this program in a file and try to run it. please refer to Appendix A. we will be discussing on how to write.out. Introduction to Programming I 29 . At the end of the lesson. Before going into details. the first one is by using a console and a text editor.3 My First Java Program public class Hello { /** * My first java program */ public static void main(String[] args) { //prints the string "Hello world" on screen System. you have installed Java and NetBeans in your system. For instructions on how to install Java and NetBeans. a text or code editor.1 Objectives In this section.
3.4 Using a Text Editor and Console
For this example, we will be using a text editor to edit the Java source code. You will also need to open the Terminal window to compile and execute your Java programs. Step 1: Start the Text Editor To start the Text Editor in Linux, click on Applications->Accessories->Text Editor.
Figure 3.1: Text Editor Application in Linux
Step 2: Open Terminal To open Terminal in Linux, click on Applications-> Accessories-> Terminal.
Figure 3.2: Terminal in Linux
Introduction to Programming I
Step 3: Write your the source code of your Java program in the Text Editor
Figure 3.3: Writing the Source Code with the Text Editor
Introduction to Programming I
Step 4: Save your Java Program We will save our program on a file named "Hello.java", and we will be saving it inside a folder named MYJAVAPROGRAMS. To open the Save dialog box, click on the File menu found on the menubar and then click on Save. After doing the procedure described above, a dialog box will appear as shown in Figure below.
Figure 3.4: Save As Dialog
Introduction to Programming I
Click on the browse button, and then click on the Create Folder button.
Introduction to Programming I
You will see a similar figure as shown below after you clicked on MYJAVAPROGRAMS.D. Introduction to Programming I 34 .J. which is "Hello. Now.I Name the new folder MYJAVAPROGRAMS. type in the filename of your program. in the Selection textbox. and then click on the SAVE button. click on the MYJAPROGRAMS folder in order to get inside that folder.java". The folder should be empty for now since it's a newly created folder and we haven't saved anything in it yet. Now.E.
Figure 3. you can just edit it. notice how the title of the frame changes from "Untitled Document 1 (modified) – gedit" to "Hello.I Now that you've saved your file.5: New Window After Saving Introduction to Programming I 35 .java (~/MYJAVAPROGRAMS) . Take note that if you want to make changes in your file. and then save it again by clicking on File -> Save.E.gedit".D.J.
E.java program.J. Go to the Terminal window we just opened a while ago. when you open the terminal window. since the name of our directory is MYJAVAPROGRAMS. In this case. it opens up and takes you directly to what is called your home folder. What you will see is a list of files and folders inside your home folder. you type in: cd MYJAVAPROGRAMS Figure 3. Typically.D. type ls and then press ENTER. To go inside a directory. you can see here that there is a folder named "MYJAVAPROGRAMS" which we have created a while ago. Now. the next step is to compile your program. and where we saved our Hello. To see what is inside that home folder. The "cd" command stands for. Now let's go inside that directory. you type in the command: cd [directory name]. change directory.I Step 5: Compiling your program Now.6: Changing the Directory Introduction to Programming I 36 .
7: List of Files Inside the New Directory Introduction to Programming I 37 .J. Figure 3. Take note that.E. In order to do that. you should make sure that the file is inside the folder where you are in.I Once inside the folder where your Java programs are.D. let us now start compiling your Java program. execute the "ls" command again to see if your file is inside that folder.
Hello.I To compile a Java program. we type in the command: javac [filename].class.D.E. So in this case.J. javac adds a file to the disk called [filename]. Introduction to Programming I 38 . or in this case. type in: javac Hello. Figure 3.8: Compiling Java File During compilation.java.class. which is the actual bytecode.
E. we are now ready to run your program. "Hello world!".D. type in the command: java [filename without the extension]. so in the case of our example. assuming that there are no problems during compilation (we'll explore more of the problems encountered during compilation in the next section).I Step 6: Running the Program Now. To run your Java program.J. Figure 3. type in: java Hello You can see on the screen that you have just run your first Java program that prints the message.9: Running Class File Introduction to Programming I 39 .
4. The second one is the runtime error.J. Figure 3. The first one is a compile-time error or also called as syntax error.1 Errors What we've shown so far is a Java program wherein we didn't encounter any problems in compiling and running. As what we have discussed in the first part of this course. our Hello. the use of incorrect special characters. However. You may have misspelled a command in Java or forgot to write a semi-colon at the end of a statement.1. Let's take for example.I 3.D. this is not always the case. we usually encounter errors along the way.4. spelling. 3. Java attempts to isolate the error by displaying the line of code and pointing to the first incorrect character in that line. Other common mistakes are in capitalization.E. and omission of correct punctuation.1 Syntax Errors Syntax errors are usually typing errors. However.java program wherein we intentionally omit the semicolon at one statement and we try to type the incorrect spelling of a command. As discussed before. the problem may not be at the exact point. there are two types of errors.10: Source Code With Errors Introduction to Programming I 40 .
The first error message suggests that there is an error in line 6 of your program.1. It pointed to the next word after the statict. Even programs that compile successfully may display wrong answers if the programmer has not thought through the logical processes and structures of the program. 3. if you encounter a lot of error messages.E. and try to compile the program again.2 Run-time Errors Run-time errors are errors that will not display until you run or execute your program.I See the error messages generated after compiling the program. Doing so may reduce the total number of errors dramatically. try to correct the first mistake in a long list.4. which should be spelled as static. Introduction to Programming I 41 .D. The second error message suggests that there is a missing semicolon after your statement. Figure 3.11: Compiling the Source Code with Errors As a rule of thumb.J.
and type: netbeans Figure 3. a compiler and/or interpreter and a debugger.I 3. which is an Integrated Development Environment or IDE. One is through command-line using terminal. In this part of the lesson.5 Using NetBeans Now that we've tried doing our programs the complicated way. Step 1: Run NetBeans There are two ways to run NetBeans. we will be using NetBeans. Open terminal (see steps on how to run terminal in the previous discussion).J. An IDE is a programming environment integrated into a software application that provides a GUI builder. or by jst clicking on the shortcut button found on the desktop. To run NetBeans using command-line.E. let's now see how to do all the processes we've described in the previous sections by using just one application.D. a text or code editor.12: Running NetBeans with the Command-Line Introduction to Programming I 42 .
J. is by clicking on the shortcut icon found on your Desktop.13: Running NetBeans using shortcut icon on desktop Introduction to Programming I 43 .I The second way to run NetBeans.E. Figure 3.D.
E.D.I After you've open NetBeans IDE.14: Window After Openning NetBeans Introduction to Programming I 44 . Figure 3. you will see a graphical user interface (GUI) similar to what is shown below.J.
E. a New Project dialog will appear. Figure 3.J.15: Choosing Project Type Introduction to Programming I 45 . After doing this. Now click on Java Application and click on the NEXT button.D. Click on File-> New Project.I Step 2: Make a project Now. let's first make a project.
J.16: Setting the Project Information Introduction to Programming I 46 .I Now.D. Figure 3. a New Application dialog will appear.E.
D. Now double-click on the Introduction to Programming I 47 . by clicking on the BROWSE button.17: Setting the Project Location The contents of the root folder is then displayed. Figure 3. A Project Location dialog will then appear. Double-click on your home folder. MYJAVAPROGRAMS folder and click on the OPEN button.E.I Now try to change the Application Location.J.
and then click on the FINISH button.J. on the Create Main Class textfield.E.I See now that the Project Location /home/florence/MYJAVAPROGRAMS. type in Hello as the main class' name.D. and Project Folder is changed to Finally.18: Window after Setting the Project Location to MYJAVAPROGRAMS/Setting the Main Class of the Project to Hello Introduction to Programming I 48 . Figure 3.
J. As shown below. This can all be found in your MYJAVAPROGRAMS folder. let us first describe the main window after creating the project. you can see a list of folders and files that NetBeans generated after creating the project.19: View of the Created Project Introduction to Programming I 49 . where you set the Project location. On the left side of the window. NetBeans automatically creates the basic code for your Java program. Figure 3.E. You can just add your own statements to the generated code.D.I Step 3: Type in your program Before typing in your program.
J.I Now.D.out. //TODO code application logic here.println("Hello world!").E. after the statement. Ignore the other parts of the program for now. as we will explain the details of the code later. Insert the code: System. try to modify the code generated by NetBeans.20: Inserting the Code Introduction to Programming I 50 . Figure 3.
D.J.I Step 4: Compile your program Now. you could also use the shortcut button to compile your code. just click on Build -> Build Main Project. Introduction to Programming I 51 . to compile your program. Or.E.
I If there are no errors in your program. you will see a build successful message on the output window.21: View after a Successful Compilation Introduction to Programming I 52 . Figure 3.D.E.J.
J.D.22: Running with NetBeans Introduction to Programming I 53 . click on Run-> Run Main Project. Figure 3.I Step 5: Run your program To run your program.E. Or you could also use the shortcut button to run your program.
J. Figure 3.I The output of your program is displayed in the output window.E.23: View after a Successful Run Introduction to Programming I 54 .D.
create a class named: [YourName].1 Hello World! Using NetBeans.6. Introduction to Programming I 55 . A tree whose hungry mouth is pressed Against the Earth's sweet flowing breast.I 3.E. The program should output the following lines on the screen: I think that I shall never see.J. create a class named: TheTree.D. The program should output on the screen: Welcome to Java Programming [YourName]!!! 3. a poem as lovely as a tree.2 The Tree Using NetBeans.6.6 Exercises 3.
public class Hello indicates the name of the class which is Hello.1 Objectives In this section. In Java. the class uses an access specifier public. we placed the curly brace at the next line after the class declaration. } } The first line of the code. We will be covering packages and access specifiers later. In addition. We will start by trying to explain the basic parts of the Hello.println("Hello world!").D.J. primitive data types. which indicates that our class in accessible to other classes from other packages (packages are a collection of classes). The next line which contains a curly brace { indicates the start of a block. In this code.I 4 Programming Fundamentals 4. the student should be able to: • • • Identify the basic parts of a Java program Differentiate among Java literals. we will be discussing the basic parts of a Java program. At the end of the lesson. all code should be placed inside a class declaration. So. however. We do this by using the class keyword. we can also place this next to the first line of our code. we'll try to the dissect your first Java program: public class Hello { /** * My first java program */ public static void main(String[] args) { //prints the string "Hello world" on screen System. variable types .out.java program introduced in the previous section. We will also be discussing some coding guidelines or code conventions along the way to help in effectively writing readable programs.identifiers and operators Develop a simple valid Java program using the concepts learned in this chapter 4. we could actually write our code as: public class Hello { public class Hello { or Introduction to Programming I 56 .2 Dissecting my first Java program Now.E.
The next line is also a Java comment. you should save it in a file called Hello. and the other one is by writing // at the start of the comment. or what a certain method do. and are treated as comments.java extension. but used for documentation purposes. public static void main(String[] args) { or can also be written as. //prints the string "Hello world" on screen Now. It is good programming practice to add comments to your code.D. The last two lines which contains the two curly braces is used to close the main method and class respectively.E.java. Coding Guidelines: 1.out. public static void main(String[] args) { indicates the name of one method in Hello which is the main method. So for example. 2.println("Hello world!"). The main method is the starting point of a Java program.I The next three lines indicates a Java comment. we learned two ways of creating comments. The next line. 3. The next line.println(). A comment is something used to document a part of a code. prints the text "Hello World!" on screen. Anything within these delimiters are ignored by the Java compiler. It is not part of the program itself. /** * My first java program */ A comment is indicated by the delimiters "/*" and "*/".J.out. Your Java programs should always end with the . Make sure to follow the exact signature. System. Introduction to Programming I 57 . if the name of your public class is Hello. prints the text enclosed by quotation on the screen. Filenames should match the name of your public class. You should write comments in your code explaining what a certain class does. The command System. All programs except Applets written in Java start with the main method. The first one is by placing the comment inside /* and */.
All the text after // are treated as comments. C-style multiline comments and special javadoc comments.3. All text in between the two delimeters are treated as comments. It uses tags like: @author Florence Balagtas @version 1. Those text are not part of the program and does not affect the flow of the program. For example. It can also contain certain tags to add more information to your comments. Like C-style comments. Unlike C++ style comments. // This is a C++ style or single line comments 4. 4. For example.D.3 Java Comments Comments are notes written to a code for documentation purposes. /* this is an exmaple of a C style or multiline comments */ 4. For example. Java supports three types of comments: C++-style single line comments.3.2 C-Style Comments C-style comments or also called multiline comments starts with a /* and ends with a */.2 */ Introduction to Programming I 58 . /** This is an example of special java doc comments used for \n generating an html documentation. it can span multiple lines.3. You can create javadoc comments by starting the line with /** and ending it with */.1 C++-Style Comments C++ Style comments starts with //.3 Special Javadoc Comments Special Javadoc comments are used for generating an HTML documentation for your Java programs.J.I 4. it can also span lines.E.
out. like. System. A block is one or more statements bounded by an opening and closing curly braces that groups the statements as one unit. Block statements can be nested indefinitely.4 Java Statements and blocks A statement is one or more lines of code terminated by a semicolon.println("world").out. public static void main( String[] args ){ or you can place the curly brace on the next line. System. In creating blocks.out. public static void main( String[] args ){ System.J. An example of a single statement is. } Introduction to Programming I 59 .I 4.D.out.for example.println("Hello"). you can place the opening curly brace in line with the statement.E. You should indent the next statements after the start of a block.out. public static void main( String[] args ) { 2. like for example.println("world"). An example of a block is. } Coding Guidelines: 1. Any amount of white space is allowed.println("Hello world"). System. public static void main( String[] args ){ System.println("Hello").
an underscore "_". Subsequent characters may use numbers 0 to 9. charArray. Letters may be lower or upper case.E. In case of multi-word identifiers. System. capitalize the first letter of the class name. etc. For example. classes. Identifiers must begin with either a letter.I 4. Java identifiers are case-sensitive. For names of methods and variables. main. Identifiers cannot use Java keywords like class. public.For example: ThisIsAnExampleOfClassName thisIsAnExampleOfMethodName 2. For names of classes.J. This means that the identifier: Hello is not the same as hello. ClassName. use capital letters to indicate the start of the word except the first word. Introduction to Programming I 60 . Examples of identifiers are: Hello. Coding Guidelines: 1. etc.D. We will discuss more about Java keywords later. methods. void. fileNumber. out. 3.5 Java Identifiers Identifiers are tokens that represent names of variables. the first letter of the word should start with a small letter. or a dollar sign "$". Avoid using underscores at the start of the identifier such as _read or _write.
so you cannot use them as names in your programs either Introduction to Programming I 61 . methods …etc. Here is a list of the Java Keywords. and null are not keywords but they are reserved words. classes.J.E. Note: true. You cannot use keywords as names for your variables.6 Java Keywords Keywords are predefined identifiers reserved by Java for a specific purpose. Figure 4.D.1: Java Key Words We will try to discuss all the meanings of these keywords and how they are used in our Java programs as we go along the way. false.I 4.
we have no special notations. For hexadecimal numbers. while in hexadecimal. To use a smaller precision (32-bit) float. and in octal. In using integer literals in our program. In some cases. The different types of literals in Java are: Integer Literals. For example. Character Literals and String Literals. it should be preceeded by "0x" or "0X". 4. It's decimal representation is 12. For decimal numbers. we have to follow some special notations. Integer literals default to the data type int. A long is a signed 64-bit value. For example. hexadecimal (base 16). Introduction to Programming I 62 . Floating point literals can be expressed in standard or scientific notations. Floating-Point Literals. Boolean Literals. while 5.7 Java Literals Literals are tokens that do not change or are constant. An example is 3. Floating point literals default to the data type double which is a 64-bit value.1 Integer Literals Integer literals come in different formats: decimal (base 10).J. they are preceeded by "0". 583.1415. For octals. 4. you may wish to force integer literal to the data type long by appending the "l" or "L" character. We will cover more on data types later.45 is in standard notation. and octal (base 8).7. true or false.7. We just write a decimal number as it is. An int is a signed 32-bit value. consider the number 12.8345e2 is in scientific notation.7. just append the "f" or "F" character.2 Floating-Point Literals Floating point literals represent decimals with fractional parts.I 4.E.3 Boolean Literals Boolean literals have only two values. 4.D. it is equivalent to 014. it is 0xC.
'\r' for the carriage return. is represented as 'a'.4 Character Literals Character Literals represent single Unicode characters.D. '\b' for backspace. '\n' for the newline character. An example of a string literal is. "Hello World". enclose the character in single quote delimiters. a backslash is used followed by the character code. Introduction to Programming I 63 . Unicode allows the inclusion of symbols and special characters from other languages. To use special characters such as a newline character. For example. A Unicode character is a 16-bit character set that replaces the 8-bit ASCII character set. To use a character literal. 4.7.5 String Literals String literals represent multiple characters and are enclosed by double quotes.J.E. the letter a.7. For example.I 4.
'a' '\t' //The letter a //A tab To represent special characters like ' (single quotes) or " (double quotes). char (for textual). It must have its literal enclosed in single quotes(' '). declares a variable named result as boolean type and assigns it a value of true. An example is. use the escape character \.I 4.8.D. String message="Hello world!" Introduction to Programming I 64 .8.2 Textual – char A character data type (char).1 Logical . For example. it is a class. For example.J. boolean (for logical). The following are.boolean A boolean data type represents two states: true and false. String is not a primitive data type (it is a Class). For example. It is not a primitive data type. represents a single Unicode character. '\'' '\"' //for single quotes //for double quotes Although. short. A String represents a data type that contains multiple characters. 4. long (integral). boolean result = true.E. The example shown above.8 Primitive data types The Java programming language defines eight primitive data types. It has it's literal enclosed in double quotes(""). 4. int. double and float (floating point). we will just introduce String in this section. byte.
Introduction to Programming I 65 .D.8. a lowercase L is not recommended because it is hard to distinguish from the digit 1.E. short. int & long Integral data types in Java are. You can define its long value by appending the letter l or L. octal or hexadecimal. Examples decimal value 2 leading 0 indicates an octal value //The leading 0x indicates a hexadecimal value Integral types has int as default data type.3 Integral – byte. 2 //The 077 //The 0xBACC uses three forms – decimal.I 4.J. Integral data type have the following ranges: Integer Length 8 bits 16 bits 32 bits 64 bits Name or Type byte short int long -27 -215 -231 -263 Range to to to to 27-1 215-1 231-1 263-1 Table 8: Integral types and their ranges Coding Guidelines: In defining a long value.
02E+23.J.14 //A simple floating-point value (a double) 6. 3.8. the 23 after the E in the second example is implicitly positive.4E+306D //A large double value with redundant D In the example shown above.02E23 //A large floating-point value 2. Floating-point data types have the following ranges: Float Length 32 bits 64 bits Name or Type float double -231 -263 Range to to 231-1 263-1 Table 9: Floating point types and their ranges Introduction to Programming I 66 . That example is equivalent to 6.4 Floating Point – float and double Floating point types has double as default data type. E or e //(add exponential value) F or f //(float) D or d //(double) Examples are.E.I 4.D. Floating-point literal includes either a decimal point or one of the following.718F //A simple float size value 123.
//assign 'C' to option //declare a data type with variable name //grade. double data type and initialized //to 0. It always good to initialize your variables as you declare them. the variable declarations. } } Coding Guidelines: 1. Here is a sample program that declares and initializes some variables. The data type indicates the type of value that the variable can hold.1 Declaring and Initializing Variables To declare a variable is as follows. 4.9. The variable name must follow rules for identifiers. Use descriptive names for your variables.J. Like for example. double quiz=10. 3. if you want to have a variable that contains a grade for a student.9 Variables A variable is an item of data used to store state of objects. grade and not just some random letters you choose. double exam=0. grade=0. Note: Values enclosed in <> are required values. For example.D. Declare one variable per line of code. quiz=10.E.0 double grade = 0. while those values enclosed in [] are optional. double grade = 0. public class VariableSamples { public static void main( String[] args ){ //declare a data type with variable name // result and boolean data type boolean result. A variable has a data type and a name.I 4.0. option = 'C'. Introduction to Programming I 67 . name it as. <data type> <name> [=initial value]. double exam=0. is preferred over the declaration. 2. //declare a data type with variable name // option and char data type char option.
System. These statements will output the following on the screen.print() What is the difference between the commands System.I 4.out.out. System.J.2 Outputting Variable Data In order to output the value of a certain variable. } } The program will output the following text on screen. These statements will output the following on the screen. System.out.println() and System.println() System. Hello world! Introduction to Programming I 68 .out. Consider the statements. System. System.out.println("Hello "). while the latter doesn't. Hello world! Now consider the following statements.println("world!").out. 10 The value of x=A 4. x = 'A'.D. System.print("world!").out.println( value ).9. System.out.out. we can use the following commands.println( "The value of x=" + x ). public class OutputVariable { public static void main( String[] args ){ int value = 10.out.print() Here's a sample program.out. System.3 System.print("Hello ").out.E. char x.9.print()? The first one appends a newline at the end of the data to output.println() vs.
suppose we have two variables with data types int and String. Introduction to Programming I 69 . int num = 10. for the primitive variable num. These are reference variables and primitive variables.I 4. wherein you have the address of the memory cells. They store data in the actual memory location of where the variable is. Memory Address 1001 : 1563 : : 2000 name Variable Name num Data 10 : Address(2000) : : "Hello" As you can see. you are actually declaring a reference variable to the object with that certain class. String name = "Hello" Suppose. the variable name and the data they hold. the illustration shown below is the actual memory of your computer.9. Primitive Variables We will now differentiate the two types of variables that Java programs have. the data is on the actual location of where the variable is. For the reference variable name.D. It points to another memory location of where the actual data is. the variable just holds the address of where the actual data is. Primitive variables are variables with primitive data types. For example. When you declare a variable of a certain class.J.E. Reference variables are variables that stores the address in the memory location.4 Reference Variables vs.
10.J.D. relational operators.op2 Description Adds op1 and op2 Multiplies op1 by op2 Divides op1 by op2 Computes the remainder of dividing op1 by op2 Subtracts op2 from op1 Table 10: Arithmetic operations and their functions Introduction to Programming I 70 .E. there are different types of operators. 4.I 4. logical operators and conditional operators. These operators follow a certain kind of precedence so that the compiler will know which operator to evaluate first in case multiple operators are used in one statement.1 Arithmetic operators Here are the basic arithmetic operators that can be used in creating your Java programs.10 Operators In Java. There are arithmetic operators. Operator + * / % Use op1 + op2 op1 * op2 op1 / op2 op1 % op2 op1 .
..out.out.println(" x * y = " + (x //dividing numbers System.println(" i = " + i).println(" i * x = " + (i * x)).."). System.").out.out..out..println("Mixing types. / y)).println("Computing the remainder.println(" i * j = " + (i System.out.D. System.out..")..println("Multiplying. System.out..out.").println(" i + j = " + (i + j)). } } Introduction to Programming I 71 . //multiplying numbers System. //adding numbers System.475.println(" i ...out. double x = 27.y = " + (x .y)). * y)). System. System.println(" i % j = " + (i % j)).println("Variable values."). double y = 7. / j)).E.println(" y = " + y).out.println("Dividing..22.out.j = " + (i ..out.out.println(" i / j = " + (i System.println(" x % y = " + (x % y)). System.out.out.out.J. System. System.").println("Subtracting.out.. System.println(" x . //computing the remainder resulting from dividing System.println(" j + y = " + (j + y)). //subtracting numbers System.out. System. System.println(" x + y = " + (x + y)).out. //mixing types System.I Here's a sample program in the usage of these operators: public class ArithmeticDemo { public static void main(String[] args) { //a few numbers int i = 37. System.out. int j = 42.println("Adding.println(" x / y = " + (x numbers * j)). System.println(" x = " + x).out. System. System..out.").println(" j = " + j).j)).
.y = 20. j + y = 49.815 Mixing types...22 i * x = 1016. i = 37 j = 42 x = 27.255 Multiplying..37 Dividing.J.695 Subtracting. i ... i + j = 79 x + y = 34. Variable values.j = -5 x .D.22 Adding... i * j = 1554 x * y = 198.. Introduction to Programming I 72 .. i / j = 0 x / y = 3. the result is a floating point.8054 Computing the remainder.475 y = 7.. The integer is implicitly converted to a floating-point number before the operation takes place...I Here is the output of the program.58 Note: When an integer and a floating-point number are used as operands to a single arithmetic operation. i % j = 37 x % y = 5.E..
When used before an operand.E. //will result to k = 4+10 = 14 Introduction to Programming I 73 . int k = 0.J. it causes the variable to be incremented or decremented by 1. int j = 3. count = count + 1. For example. int i = 10. and then the new value is used in the expression in which it appears. k = ++j + i. For example.10. Java also includes a unary increment operator (++) and unary decrement operator (--). evaluates to the value of op before it was incremented Increments op by 1. Increment and decrement operators increase and decrease a value stored in a number variable by 1. evaluates to the value of op before it was decremented Decrements op by 1. is equivalent to. evaluates to the value of op after it was incremented Decrements op by 1. count++. the expression. evaluates to the value of op after it was decremented //increment the value of count by 1 -- op-- -- --op Table 11: Increment and Decrement operators The increment and decrement operators can be placed before or after an operand. Operator ++ ++ Use op++ ++op Description Increments op by 1.2 Increment and Decrement operators Aside from the basic arithmetic operators.D.I 4.
int j = 3. //will result to k = 3+10 = 13 Coding Guideline: Always keep expressions containing increment and decrement operators simple and easy to understand. the old value of the variable will be used in the expression where it appears.D.E. k = j++ + i.J.I When the increment and decrement operators are placed after the operand. int k = 0. Introduction to Programming I 74 . int i = 10. For example.
Operator > >= < <= == != Use op1 > op2 op1 >= op2 op1 < op2 op1 <= op2 op1 == op2 op1 != op2 Description op1 is greater than op2 op1 is greater than or equal to op2 op1 is less than op2 op1 is less than or equal to op2 op1 and op2 are equal op1 and op2 are not equal Table 12: Relational Operators Introduction to Programming I 75 .E. The output of evaluation are the boolean values true or false.J.I 4.10.3 Relational operators Relational operators compare two values and determines the relationship between those values.D.
//false //less than or equal to System.println("Greater than System.println(" k <= j = " + (k <= j)). System.out.out..out.println(" i > System. System.out. //false System.")..out.println(" i >= j = System.J.println("Not equal to. //false i = " + (j > i)).out.println(" k < j = " + (k < j)). //false " + (j >= i))."). //false } } Introduction to Programming I 76 . System.out. //false System.println(" i = " + i).println(" j <= i = " + (j <= i)).out.").out.println(" j < i = " + (j < i)).println(" i == j = " + (i == j))."). System.. //true //not equal to System.println(" k != j = " + (k != j)).")..").println(" i != j = " + (i != j)). //greater than System.E.println(" k >= j = //less than System.println(" j = " + j). int j = 42.. //true //equal to System.println("Variable values.println("Less than or equal to.out.println(" k > than.out.out.println("Less than.out. //false System.println("Equal to.println(" i < j = " + (i < j)).. //true System.out.println("Greater System. System..I Here's a sample program that uses relational operators.println(" k = " + k).out.out.. //true j = " + (k > j)).. //true " + (k >= j)).out.println(" i <= j = " + (i <= j)).println(" j > System.out. j = " + (i > j)). //true //greater than or equal to System.out.out..")..println(" k == j = " + (k == j)).println(" j >= i = System..D.. //true System. //true System. " + (i >= j)).out. System. System.out.out. public class RelationalDemo { public static void main(String[] args) { //a few numbers int i = 37.out.out. int k = 42.out. //false or equal to. System..
E....J.I Here's the output from this program: Variable values.. i != j = true k != j = false Introduction to Programming I 77 . i >= j = false j >= i = true k >= j = true Less than.... i == j = false k == j = true Not equal to.. i <= j = true j <= i = false k <= j = true Equal to. i < j = true j < i = false k < j = false Less than or equal to. i > j = false j > i = true k > j = false Greater than or equal to... i = 37 j = 42 k = 42 Greater than.D.....
and ! (logical NOT). Introduction to Programming I 78 . x2 can be boolean expressions. There are six logical operators: && (logical AND). variables or constants.E.D. summarize the result of each operation for all possible combinations of x1 and x2. and op is either &&. & (boolean logical AND).4 Logical operators Logical operators have one or two boolean operands that yield a boolean result. || (logical OR). ||. x1 op x2 where x1. | (boolean logical inclusive OR).I 4. | or ^ operator. ^ (boolean logical exclusive OR). The truth tables that will be shown next.J. The basic expression for a logical operation is.10. &.
while & doesn't.out. What does this mean? Given an expression.E.println(j).println(test).out.4.J. boolean test= false. System. System. public class TestAND { public static void main( String[] args ){ int i = 0. If exp1 is false.println(test). In contrast.out. System.D. System. //demonstrate && test = (i > 10) && (j++ > 9). and immediately return a false value is exp1 is false.out. x1 TRUE TRUE FALSE FALSE x2 TRUE FALSE TRUE FALSE Result TRUE FALSE FALSE FALSE Table 13: Truth table for & and && The basic difference between && and & operators is that && supports short-circuit evaluations (or partial evaluations).println(i). exp1 && exp2 && will evaluate the expression exp1. System.out.out.10.I 4.println(i). int j = 10.println(j). //demonstrate & test = (i > 10) & (j++ > 9). Here's a sample source code that uses logical and boolean AND. } } Introduction to Programming I 79 . the & operator always evaluates both exp1 and exp2 before returning an answer.1 && (logical AND) and & (boolean logical AND) Here is the truth table for && and &. the operator never evaluates exp2 because the result of the operator will be false regardless of the value of exp2. System.
E.D. that the j++ on the line containing the && operator is not evaluated since the first expression (i>10) is already equal to false.J.I The output of the program is. 0 10 false 0 11 false Note. Introduction to Programming I 80 .
x1 TRUE TRUE FALSE FALSE x2 TRUE FALSE TRUE FALSE Result TRUE TRUE TRUE FALSE Table 14: Truth table for | and || The basic difference between || and | operators is that || supports short-circuit evaluations (or partial evaluations).out.4. In contrast. } } Introduction to Programming I 81 . System.println(i). the | operator always evaluates both exp1 and exp2 before returning an answer. System. System.10.out. System. public class TestOR { public static void main( String[] args ){ int i = 0.out.D.println(test). What does this mean? Given an expression.out. //demonstrate || test = (i < 10) || (j++ > 9).out. exp1 || exp2 || will evaluate the expression exp1.J. System. the operator never evaluates exp2 because the result of the operator will be true regardless of the value of exp2. int j = 10.println(j). System.out.println(i). //demonstrate | test = (i < 10) | (j++ > 9). and immediately return a true value is exp1 is true. boolean test= false. Here's a sample source code that uses logical and boolean OR. If exp1 is true.println(j).E.println(test). while | doesn't.2 || (logical OR) and | (boolean logical inclusive OR) Here is the truth table for || and |.I 4.
that the j++ on the line containing the || operator is not evaluated since the first expression (i<10) is already equal to true. Introduction to Programming I 82 .I The output of the program is. 0 10 true 0 11 true Note.E.D.J.
x1 TRUE TRUE FALSE FALSE x2 TRUE FALSE TRUE FALSE Table 15: Truth table for ^ Result FALSE TRUE TRUE FALSE The result of an exclusive OR operation is TRUE.out. } } The output of the program is. Here's a sample source code that uses the logical exclusive OR operator.E.println(val1 ^ val2). System.I 4.10.D. val1 = true. val1 = false. Note that both operands must always be evaluated in order to calculate the result of an exclusive OR. System. System.J. System. false true false true Introduction to Programming I 83 . public class TestXOR { public static void main( String[] args ){ boolean val1 = true. val2 = true. boolean val2 = true.4. val1 = false.println(val1 ^ val2). val2 = false.3 ^ (boolean logical exclusive OR) Here is the truth table for ^. if and only if one operand is true and the other is false.out.println(val1 ^ val2).out.out.println(val1 ^ val2). val2 = false.
variable or constant.I 4.10. x1 TRUE FALSE Result FALSE TRUE Table 16: Truth table for ! Here's a sample source code that uses the logical NOT operator.J. false true Introduction to Programming I 84 .4. } } The output of the program is.E. boolean val2 = false. public class TestNOT { public static void main( String[] args ){ boolean val1 = true. wherein that argument can be an expression.println(!val1). System.out.4 ! (logical NOT) The logical NOT takes in one argument.println(!val2).D. Here is the truth table for !.out. System.
I 4. The structure of an expression using a conditional operator is.out. //print status System.E. If exp1 is true.println( status ).D. If it is false. exp2 is the value returned. Passed Introduction to Programming I 85 . This means that it takes in three arguments that together form a conditional expression. //get status of the student status = (grade >= 60)?"Passed":"Fail". then exp3 is returned. public class ConditionalOperator { public static void main( String[] args ){ String status = "". exp1?exp2:exp3 wherein exp1 is a boolean expression whose result must either be true or false.5 Conditional Operator (?:) The conditional operator ?: is a ternary operator. int grade = 80.J.10. } } The output of this program will be. given the code. For example.
} } score = (answer == 'a') ? 10 : 0.out.println("Score = " + score ). Score = 10 Introduction to Programming I 86 . System.D. The output of the program is. class ConditionalOperator { public static void main( String[] args ){ int char score = 0. answer = 'a'. Figure 4.I Here is the flowchart of how ?: works.2: Flowchart using the ?: operator Here is another program that uses the ?: operator.E.J.
I 4. Coding Guidelines To avoid confusion in evaluating mathematical operations.D.J. 6%2*5+4/2+88-10 we can re-write the expression and place some parenthesis base on operator precedence.10. ((6%2)*5)+(4/2)+88-10.3: Operator Precedence Given a complicated expression. Introduction to Programming I 87 . keep your expressions simple and use parenthesis.E.6 Operator Precedence Operator precedence defines the compiler's order of evaluation of operators so as to come up with an unambiguous result. Figure 4.
D. 1. your program should output. 20 and 45.11. declare the following variables with the corresponding data types and initialization values. Let the values of the three numbers be.J. Variable name number letter result str Data Type integer character boolean String Initial value 10 a true hello The following should be the expected screen output. r ^ s * t / u – v + w ^ x – y++ Introduction to Programming I 88 .11.1 Declaring and printing variables Given the table below. 3 * 10 *2 / 15 – 2 + 4 ^ 2 ^ 2 3. a / b ^ c ^ d – e + f – g * h + i 2. given the numbers 10. number 1 = 10 number 2 = 23 number 3 = 5 The highest number is = 23 4. 23 and 5. write a program that outputs the number with the greatest value among the three. re-write them by writing some parenthesis based on the sequence on how they will be evaluated.4 Operator precedence Given the following expressions.11 Exercises 4.11. The expected screen output is. Output to the screen the variable names together with the values.3 Output greatest value Given three numbers.2 Getting the average of three numbers Create a program that outputs the average of three numbers.I 4.11. 10. Use the conditional ?: operator that we have studied so far (HINT: You will need to use two sets of ?: to solve this). For example.E. Number = 10 letter = a result = true str = hello 4. number 1 = number 2 = number 3 = Average is 10 20 45 = 25 4.
} catch( IOException e ){ System.readLine(). At the end of the lesson.E. the first one is through the use of the BufferedReader class and the other one involves a graphical user interface by using JOptionPane.I 5 Getting Input from the Keyboard 5.J. Declare a temporary String variable to get the input.*.io. } Introduction to Programming I 89 . Here are the steps to get input from the keyboard: 1. try{ String temp = dataIn. we will use the BufferedReader class found in the java. In this section.2 Using BufferedReader to get input In this section. let's make our programs more interactive by getting some input from the user.println("Error in getting input"). the student should be able to: • • • Create an interactive Java program that gets input from the keyboard Use the BufferedReader class to get input from the keyboard using a console Use the JOptionPane class to get input from the keyboard using a graphical user interface 5.in) ). 3.io package in order to get input from the keyboard.D. You have to type it inside a try-catch block. and invoke the readLine() method to get input from the keyboard. 2.out. Add this statement: BufferedReader dataIn = new BufferedReader( new InputStreamReader( System. we'll be discussing two methods of getting input. Add this at the top of your code: import java.1 Objectives Now that we've studied some basic concepts in Java and we've written some simple programs.
and then we can use those classes inside our program.BufferedReader. } Now let's try to explain each line of code: The statements. String name = "".io package contains classes that allow programs to input and output data. The statements can also be rewritten as.println("Hello " + name +"!"). public class GetInputFromKeyboard { public static void main( String[] args ){ BufferedReader dataIn = new BufferedReader(new InputStreamReader( System. which will load all the classes found in the package.in) ).D.IOException.InputStreamReader.BufferedReader.*. Packages contain classes that have related purpose. the java. import java.io.out. }catch( IOException e ){ System. Just like in our example. import java.I Here is the complete source code: import java. import java.io. System. import java. Introduction to Programming I 90 .println("Error!"). These classes are organized into what we call packages.io package. The Java Application Programming Interface (API) contains hundreds of predefined classes that you can use in your programs. indicate that we want to use the classes BufferedReader. import java.io.readLine().J.out. try{ name = dataIn.io.io. InputStreamReader and IOException which is inside the java.io.InputStreamReader.out.IOException.print("Please Enter Your Name:"). } } System.E. import java.io.
J.readLine(). We will cover more about exception handling in the latter part of this course.in) ).println("Error!"). will be catched. It is always good to initialize your variables as you declare them. String name = "".print("Please Enter Your Name:"). }catch( IOException e ){ System. System. Now. Now. The variable name is initialized to an empty String "".D. BufferedReader dataIn = new BufferedReader(new InputStreamReader( System.out. the following block defines a try-catch block. The next line just outputs a String on the screen asking for the user's name. just take note that you need to add this code in order to use the readLine() method of BufferedReader to get input from the user. In the statement. try{ name = dataIn.I The next two statements.out. but for now.E. we are declaring a variable named dataIn with the class type BufferedReader. This means we declare a class named GetInputFromKeyboard and we declare the main method.readLine(). } This assures that the possible exceptions that could occur in the statement name = dataIn. This is where we will store the input of the user. Don't worry about what the syntax means for now. Introduction to Programming I 91 . public class GetInputFromKeyboard { public static void main( String[] args ){ were already discussed in the previous lesson. we are declaring a String variable with the identifier name. We will cover more about this later in the course.
println("Hello " + name + "!"). gets input from the user and will return a String value.readLine(). which we will use in our final statement to greet the user.D.E.readLine().I Now going back to the statement. This value will then be saved to our name variable. System. dataIn.J.out. the method call. name = dataIn. Introduction to Programming I 92 .
msg).showMessageDialog(null.swing.2: Input florence on the JOptionPane Figure 5. Given the following code. } This will output.swing package. } Figure 5. name = JoptionPane.3 Using JOptionPane to get input Another way to get input from the user is by using the JOptionPane class which is found in the javax.JOptionPane.J.1: Getting Input Using JOptionPane Figure 5. JOptionPane. public class GetInputFromKeyboard { public static void main( String[] args ){ String name = "".I 5.E. import javax. String msg = "Hello " + name + "!". JOptionPane makes it easy to pop up a standard dialog box that prompts users for a value or informs them of something.D.3: Showing Message Using JOptionPane Introduction to Programming I 93 .showInputDialog("Please enter your name").
The next line displays a dialog which contains a message and an OK button.I The first statement. which we will store in the msg variable.JOptionPane. The statement. Now we create the welcome message.E. import javax. name = JOptionPane. String msg = "Hello " + name + "!". which will display a dialog with a message. creates a JOptionPane input dialog.swing.showInputDialog("Please enter your name").D. import javax.swing. a textfield and an OK button as shown in the figure. Introduction to Programming I 94 . msg).*.swing package. indicates that we want to import the class JOptionPane from the javax.showMessageDialog(null.J. This returns a String which we will save in the name variable. We can also write this as. JOptionPane.
1 Last 3 words (BufferedReader version) Using BufferedReader. ask for three words from the user and output those three words on the screen.2 Last 3 words (JOptionPane version) Using JOptionPane. For example.5: Second Input Figure 5.4. For example. Enter word1:Goodbye Enter word2:and Enter word3:Hello Goodbye and Hello 5.7: Show Message Introduction to Programming I 95 . ask for three words from the user and output those three words on the screen.D.J.6: Third Input Figure 5.I 5.4 Exercises 5.4.E.4: First Input Figure 5. Figure 5.
. continue. switch) which allows selection of specific sections of code to be executed Use repetition control structures (while. } where. The if-statement has the form.D. else. statement2. Introduction to Programming I 96 . we have given examples of sequential programs. for) which allow executing specific sections of code a number of times Use branching statements (break. .I 6 Control Structures 6. do-while.1 Objectives In the previous sections. In this section.1 if statement The if-statement specifies that a statement (or block of code) will be executed if and only if a certain boolean statement is true. 6.2 Decision Control Structures Decision control structures are Java statements that allows us to select and execute specific blocks of code while skipping other sections. boolean_expression is either a boolean expression or boolean variable. which allows us to change the ordering of how the statements in our programs are executed. At the end of the lesson. if( boolean_expression ) statement. wherein statements are executed one after another in a fixed order.2. the student should be able to: • • • Use decision control structures (if.E. . we will be discussing control structures. return) which allows redirection of program flow 6.J. or if( boolean_expression ){ statement1.
Indent the statements inside the if-block.I Figure 6. 2.out.out.E.println("You passed!"). or int grade = 68. } Introduction to Programming I 97 . } Coding Guidelines: 1. if( boolean_expression ){ //statement1. The boolean_expression part of a statement should evaluate to a boolean value. That means that the execution of the condition should either result to a value of true or a false. System.out.J.1: Flowchart of If-Statement if( grade > 60 ) System.println("Congratulations!").println("Congratulations!"). if( grade > 60 ){ System. //statement2.For example.D.
if( grade > 60 ) System.out. or int grade = 68.D. else System. statement2. . statement2.println("Congratulations!"). . .println("Congratulations!"). System.2. or can also be written as.println("You passed!").println("Sorry you failed"). given the code snippet. } else{ System. and a different statement if the condition is false. if( boolean_expression ) statement. if( grade > 60 ){ System.println("Sorry you failed").out.E. } For example. .out. int grade = 68. if( boolean_expression ){ statement1. .2 if-else statement The if-else statement is used when we want to execute a certain statement if a condition is true. } else{ statement1. } Introduction to Programming I 98 . else statement. The if-else statement has the form.J.out. .I 6.out.
if( boolean_expression ){ if( boolean_expression ){ . } Introduction to Programming I 99 . .2: Flowchart of If-Else Statement Coding Guidelines: 1.D. To avoid confusion. } } else{ .E.J. You can have nested if-else blocks. This means that you can have other if-else blocks inside another if-else block. 2. always place the statement or statements of an if or if-else block inside brackets {}..I Figure 6. ..For example.
D.E. If boolean_expression2 is true. Take note that you can have many else-if blocks after an if-statement. The if-else if statement has the form.2.3 if-else-if statement The statement in the else-clause of an if-else block can be another if-else structures. This cascading of structures allows us to make more complex selections.I 6. The else-block is optional and can be omitted. else if( boolean_expression2 ) statement2. if( boolean_expression1 ) statement1.3: Flowchart of If-Else-If Statement Introduction to Programming I 100 . In the example shown above. Figure 6. else statement3. then the program executes statement 2 and skips to the statements following statement3. if boolean_expression1 is true.J. then the program executes statement1 and skips the other statements.
} else if( grade > 60 ){ System. if( grade > 90 ){ System.out.println("Very good!"). //WRONG int number = 0.2.4 Common Errors when using the if-else statements: 1. For example. if( number == 0 ){ //some statements here } 3. } 6.out. if( number ){ //some statements here } The variable number does not hold a Boolean value. } else{ System.I For example.println("Very good!"). given the code snippet.E.out. Introduction to Programming I 101 . 2. For example. The condition inside the if-statement does not evaluate to a boolean value. int grade = 68. Using = instead of == for comparison. //CORRECT int number = 0. Writing elseif instead of else if.D. //WRONG int number = 0. if( number = 0 ){ //some statements here } This should be written as.J.println("Sorry you failed").
} else if( (grade < 80) && (grade >= 60)){ System.2. you failed.out."). } else{ System.out. if( grade >= 90 ){ System.out. } } } Introduction to Programming I 102 .println("Good job!" ).0.J.println("Study harder!" ).println( "Excellent!" ).println("Sorry.I 6.E.out. } else if( (grade < 90) && (grade >= 80)){ System.5 Example for if-else-else if public class Grade { public static void main( String[] args ) { double grade = 92.D.
. //block 1 . Introduction to Programming I 103 . switch( switch_expression ){ case case_selector1: statement1. NOTES: • Unlike with the if statement.2. Not only that. A switch statement can have no default block. .E. case_selector2 and so on. The switch statement has the form. // break. . and jumps to the case whose selector matches the value of the expression.6 switch statement Another way to indicate a branch is through the switch keyword.J. . case_selector1. the multiple statements are executed in the switch statement without needing the curly braces. //block 2 . .I 6. . • When a case in a switch statement has been matched. // statement2. } where. // statement2. the default block is executed. the statements associated with the succeeding cases are also executed. When a switch is encountered. that the default part is optional. // break. all the statements associated with that case are executed. The program executes the statements in order from that point on until a break statement is encountered. we use a break statement as our last statement. The switch construct allows branching on multiple outcomes. . • To prevent the program from executing statements in the subsequent cases. // break. .D. If none of the cases are satisfied. switch_expression is an integer or character expression and. case case_selector2: statement1. skipping then to the first statement after the end of the switch structure. //block n . Take note however. . Java first evaluates the switch_expression. default: statement1. are unique integer or character constants. // statement2.
Figure 6.E. the value provided to each case statement must be unique. An if statement can be used to make decisions based on ranges of values or conditions.D. 2.J. whereas a switch statement can make decisions based only on a single integer or character value. Also. You can decide which to use. based on readability and other factors.I Coding Guidelines: 1.4: Flowchart of Switch Statements Introduction to Programming I 104 . Deciding whether to use an if statement or a switch statement is a judgment call.
println("Good job!" ). break. default: System. you failed. } } } Introduction to Programming I 105 .E."). case 90: System.out.out.D. case 80: System.I 6. break.println( "Excellent!" ).println("Study harder!" ).2. switch(grade){ case 100: System. break.out.J.out.7 Example for switch public class Grade { public static void main( String[] args ) { int grade = 92.println("Sorry.
} The sample code shown will print 4321 containing the statement i--. 6. statement2. the while.D. that does not terminate. or a loop using while loops or any kind of repetition some statements that will allow your loop to Introduction to Programming I 106 . For example. Take note that if the line this will result to an infinite loop. on the screen.print(i). while ( i > 0 ){ System.E.J.3. int i = 4. do-while and for loops. . while( boolean_expression ){ statement1. There are three types of repetition control structures. given the code snippet. } The statements inside the while loop are executed as long as the boolean_expression evaluates to true. i--.3 Repetition Control Structures Repetition control structures are Java statements that allows us to execute specific blocks of code a number of times.I 6. is removed. Therefore. make sure that you add terminate at some point.1 while loop The while loop is a statement or block of statements that is repeated as long as some condition is satisfied. The while statement has the form.out. . when control structures. .
} Example 2: //infinite loop while(true) System.I The following are other examples of while loops.println("hello"). Example 1: int x = 0.J. x++.out.out.E. Introduction to Programming I 107 . Example 3: //no loops // statement is not even executed while (false) System.println(x).out.D. while (x<10) { System.println("hello").
6.3.2 do-while loop
The do-while loop is similar to the while-loop. The statements inside a do-while loop are executed several times as long as the condition is satisfied. The main difference between a while and do-while loop is that, the statements inside a do-while loop are executed at least once. The do-while statement has the form, do{ statement1; statement2; . . . }while( boolean_expression ); The statements inside the do-while loop are first executed, and then the condition in the boolean_expression part is evaluated. If this evaluates to true, the statements inside the do-while loop are executed again. Here are a few examples that uses the do-while loop: Example 1: int x = 0; do { System.out.println(x); x++; }while (x<10); This example will output 0123456789 on the screen. Example 2: //infinite loop do{ System.out.println("hello"); } while (true); This example will result to an infinite loop, that prints hello on screen. Example 3: //one loop // statement is executed once do System.out.println("hello"); while (false); This example will output hello on the screen.
Introduction to Programming I
Coding Guidelines: 1. Common programming mistakes when using the do-while loop is forgetting to write the semi-colon after the while expression. do{ ... }while(boolean_expression) //WRONG->forgot semicolon ; 2. Just like in while loops, make sure that your do-while loops will terminate at some point.
6.3.3 for loop
The for loop, like the previous loops, allows execution of the same code a number of times. The for loop has the form, for (InitializationExpression; LoopCondition; StepExpression){ statement1; statement2; . . . } where, InitializationExpression -initializes the loop variable. LoopCondition - compares the loop variable to some limit value. StepExpression - updates the loop variable.
A simple example of the for loop is, int i; for( i = 0; i < 10; i++ ){ System.out.print(i); } In this example, the statement i=0, first initializes our variable. After that, the condition expression i<10 is evaluated. If this evaluates to true, then the statement inside the for loop is executed. Next, the expression i++ is executed, and then the condition expression is again evaluated. This goes on and on, until the condition expression evaluates to false. This example, is equivalent to the while loop shown below, int i = 0; while( i < 10 ){ System.out.print(i); i++; }
Introduction to Programming I
6.4 Branching Statements
Branching statements allows us to redirect the flow of program execution. Java offers three branching statements: break, continue and return.
6.4.1 break statement
The break statement has two forms: unlabeled (we saw its unlabeled form in the switch statement) and labeled. 126.96.36.199 Unlabeled break statement The unlabeled break terminates the enclosing switch statement, and flow of control transfers to the statement immediately following the switch. You can also use the unlabeled form of the break statement to terminate a for, while, or do-while loop. For example, String names[] = {"Beah", "Bianca", "Lance", "Belle", "Nico", "Yza", "Gem", "Ethan"}; String boolean searchName = "Yza"; foundName = false;
for( int i=0; i< names.length; i++ ){ if( names[i].equals( searchName )){ foundName = true; break; } } if( foundName ){ System.out.println( searchName + " found!" ); } else{ System.out.println( searchName + " not found." ); } In this example, if the search string "Yza" is found, the for loop will stop and flow of control transfers to the statement following the for loop.
Introduction to Programming I
22.214.171.124 Labeled break statement The labeled form of a break statement terminates an outer statement, which is identified by the label specified in the break statement. The following program searches for a value in a two-dimensional array. Two nested for loops traverse the array. When the value is found, a labeled break terminates the statement labeled search, which is the outer for loop. int[][] numbers = {{1, 2, 3}, {7, 8, 9}}; int searchNum = 5; boolean foundNum = false; searchLabel: for( int i=0; i<numbers.length; i++ ){ for( int j=0; j<numbers[i].length; j++ ){ if( searchNum == numbers[i][j] ){ foundNum = true; break searchLabel; } } } if( foundNum ){ System.out.println( searchNum + " found!" ); } else{ System.out.println( searchNum + " not found!" ); } The break statement terminates the labeled statement; it does not transfer the flow of control to the label. The flow of control transfers to the statement immediately following the labeled (terminated) statement. {4, 5, 6},
Introduction to Programming I
4. //skip next statement } } count++. int count = 0.length. i++ ){ if( !names[i]. //message2 In this example.I 6.2 Labeled continue statement The labeled form of the continue statement skips the current iteration of an outer loop marked with the given label.2. for( int i=0.equals("Beah") ){ continue. System. The following example counts the number of "Beah"s in the array. while or do-while loop. "Lance".D. "Bianca". basically skipping the remainder of this iteration of the loop. outerLoop: for( int i=0. j<5. "Beah"}.E. } } System.J.2 continue statement The continue statement has two forms: unlabeled and labeled. i<5. //message1 if( j == 2 ) continue outerLoop.out.out. j++ ){ System.println("Inside for(j) loop"). 6. message 2 never gets printed since we have the statement continue outerloop which skips the iteration. i++ ){ for( int j=0. String names[] = {"Beah". You can use the continue statement to skip the current iteration of a for.println("Inside for(i) loop"). Introduction to Programming I 112 . 6.4. i<names.2.1 Unlabeled continue statement The unlabeled form skips to the end of the innermost loop's body and evaluates the boolean expression that controls the loop.4.out.println("There are " + count + " Beahs in the list").
To return a value. or return ++count. For example.3 return statement The return statement is used to exit from the current method. use the form of return that doesn't return a value. We will cover more about return statements later when we discuss about methods.4. The flow of control returns to the statement that follows the original method call. When a method is declared void. return.D. The return statement has two forms: one that returns a value and one that doesn't. The data type of the value returned by return must match the type of the method's declared return value. Introduction to Programming I 113 . simply put the value (or an expression that calculates the value) after the return keyword.E. return "Hello".I 6.J. For example.
1.5.1 Grades Get three exam grades from the user and compute the average of the grades. output. 1. "Invalid number". 6.J. Output the average of the three exams.2 Number in words Get a number as input from the user. a do-while loop and a for-loop.5 Exercises 6.3 Hundred Times Create a program that prints your name a hundred times. Introduction to Programming I 114 .D. If the user inputs a number that is not in the range.out to output the result. The number inputted should range from 1-10. Together with the average.I 6. Do three versions of this program using a while loop.5. Do three versions of this program using a while loop. Use a switch statement to solve this problem 6. Use BufferedReader to get input from the user. Use an if-else statement to solve this problem 2. 2.5.4 Powers Compute the power of a number given the base and exponent. also include a smiley face in the output if the average is greater than or equal to 60.E. a do-while loop and a for-loop. Use JOptionPane to get input from the user and to output the result. 6. otherwise output :-(. and System. and output the equivalent of the number in words.5.
we have here three variables of type int with different identifiers for each variable.2 Introduction to arrays In the previous sections. the student should be able to: • • • • Declare and create arrays Access array elements Determine the number of elements in an array Declare and create multidimensional arrays 7. For example. int number1. This type of variable is called an array.D. in a contiguous block of memory. Think of an array as a stretched variable – a location that still has one identifier name. there is one capability wherein we can use one variable to store a list of data and manipulate them more efficiently. First. At the end of the lesson. number2 = 2. we call it by its identifier name. Introduction to Programming I 115 . we will be discussing about Java Arrays. In Java and other programming languages. number3 = 3. it seems like a tedious task in order to just initialize and use the variables especially if they are used for the same purpose.I 7 Java Arrays 7. In order to use the variable.1 Objectives In this section. we have discussed on how to declare different variables using the primitive data types. In declaring variables. divided into a number of slots. we often use a unique identifier or name and a datatype. number1 = 1. but can hold more than one value.J. Figure 7.1: Example of an Integer Array An array stores multiple data items of the same datatype. As you can see.E. int number2. and then we are going to discuss on how to declare and use them. we are going to define what arrays are. int number3.
J. For example. //declaration int ages[]. We will cover more about instantiating objects and constructors later. int []ages. Instead of using the new keyword to instantiate an array. you list the data type. Figure 7. For example. followed by the identifier name.2: Instantiating Arrays Introduction to Programming I 116 . For example. This process in Java is called instantiation (the Java word for creates). In the example. we need to use a constructor for this. you can also automatically declare. When declaring an array. In order to instantiate an object. construct and assign values at once. that the size of an array cannot be changed once you've initialized it. //instantiate object ages = new int[100]. the declaration tells the Java Compiler that the identifier ages will be used as the name of an array containing integers. //declare and instantiate object int ages[] = new int[100]. After declaring. followed by a set of square brackets[]. and to create or instantiate a new array containing 100 elements. or.D. can also be written as.I 7. we must create the array and specify its length with a constructor statement. Take note. int ages[].E. or you can place the brackets after the identifier.3 Declaring Arrays Arrays must be declared like all variables.
true. This array contains 4 elements that are //initialized to values {true. //creates an array of 4 double variables initialized //to the values {100. double []grades = {100.D. 75}. "Wed". true. 90.E. Introduction to Programming I 117 . 75}. false} boolean results[] ={ true.I Examples are. false. 80. "Sat". "Tue". //creates an array of boolean variables with ientifier //results. This array contains 7 elements String days[] = { "Mon". false. false }. "Thu". 90.J. //creates an array of Strings with identifier days and //initialized. "Fri". "Sun"}. 80.
i<100. we have //assigns 10 to the first element in the array ages[0] = 10. This uses a for loop. you must populate the String arrays explicitly. so our code is shorter. given the array we declared a while ago. int []arr. //prints the last element in the array System.I 7. for( int i=0. allowing the program and the programmer to access individual values when necessary. However. You cannot resize an array. They begin with zero and progress sequentially by whole numbers to the end of the array. The following is a sample code on how to print all the elements in the array. the declaration. Therefore. For example.E. 3. int []arr = new int[100]. is preferred over. you use a number called an index or a subscript. reference data types such as Strings are not initialized to blanks or an empty string "". It is usually better to initialize or instantiate the array right away after you declare it. For example. } } Coding Guidelines: } 1. An index number or subscript is assigned to each member of the array. Note that there is no array element arr[n]! This will result in an array-index-out-of-bounds exception. Take note that once an array is declared and constructed. 2. Index numbers are always integers.J. Take note that the elements inside your array is from 0 to (sizeOfArray-1). public class ArraySample{ public static void main( String[] args ){ int[] ages = new int[100].out. The elements of an n-element array have indexes from 0 to n-1. the stored value of each member of the array will be initialized to zero for number data. arr = new int[100].D. i++ ){ System.print(ages[99]).out.print( ages[i] ). or a part of the array.4 Accessing an array element To access an array element. Introduction to Programming I 118 .
7.5 Array length
In order to get the number of elements in an array, you can use the length field of an array. The length field of an array returns the size of the array. It can be used by writing, arrayName.length For example, given the previous example, we can re-write it as, public class ArraySample { public static void main( String[] args ){ int[] ages = new int[100]; for( int i=0; i<ages.length; i++ ){ System.out.print( ages[i] ); }
} Coding Guidelines:
1. When creating for loops to process the elements of an array, use the array object's length field in the condition statement of the for loop. This will allow the loop to adjust automatically for different-sized arrays. 2. Declare the sizes of arrays in a Java program using named constants to make them easy to change. For example, final int ARRAY_SIZE = 1000; //declare a constant ... int[] ages = new int[ARRAY_SIZE];
Introduction to Programming I
7.6 Multidimensional Arrays
Multidimensional arrays are implemented as arrays of arrays. Multidimensional arrays are declared by appending the appropriate number of bracket pairs after the array name. For example, // integer array 512 x 128 elements int[][] twoD = new int[512][128]; // character array 8 x 16 x 24 char[][][] threeD = new char[8][16][24]; // String array 4 rows x 2 columns String[][] dogs = {{ "terry", "brown" }, { "Kristin", "white" }, { "toby", "gray"}, { "fido", "black"} }; To access an element in a multidimensional array is just the same as accessing the elements in a one dimensional array. For example, to access the first element in the first row of the array dogs, we write, System.out.print( dogs[0][0] ); This will print the String "terry" on the screen.
Introduction to Programming I
7.7 Exercises
7.7.1 Days of the Week
Create an array of Strings which are initialized to the 7 days of the week. For Example, String days[] = {"Monday", "Tuesday"….}; Using a while-loop, print all the contents of the array. (do the same for do-while and forloop)
7.7.2 Greatest number
Using BufferedReader or JOptionPane, ask for 10 numbers from the user. Use an array to store the values of these 10 numbers. Output on the screen the number with the greatest value.
7.7.3 Addressbook Entries
Given the following multidimensional array that contains addressbook entries: String entry = {{"Florence", "735-1234", "Manila"}, {"Joyce", "983-3333", "Quezon City"}, {"Becca", "456-3322", "Manila"}}; Print the following entries on screen in the following format: Name : Florence Tel. # : 735-1234 Address : Manila Name : Joyce Tel. # : 983-3333 Address : Quezon City Name : Becca Tel. # : 456-3322 Address : Manila
Introduction to Programming I
8 Command-line Arguments
8.1 Objectives
In this section, we will study on how to process input from the command-line by using arguments pass onto a Java program. At the end of the lesson, the student should be able to: • • • Know and explain what a command-line argument is Get input from the user using command-line arguments Learn how to pass arguments to your programs in NetBeans
8.2 Command-line arguments
A Java application can accept any number of arguments from the command-line. Command-line arguments allow the user to affect the operation of an application for one invocation. The user enters command-line arguments when invoking the application and specifies them after the name of the class to run. For example, suppose you have a Java application, called Sort, that sorts five numbers, you run it like this:
Figure 8.1: Running with Command-Line Arguments
Take note that the arguments are separated by spaces.
Introduction to Programming I 122
if (args. it must convert a String argument that represents a number. } parseInt throws a NumberFormatException (ERROR) if the format of args[0] isn't valid (not a number).length > 0){ firstArg = Integer.parseInt(args[0]). int firstArg = 0. int numberOfArgs = args. Here's a code snippet that converts a command-line argument to an integer. For example. If your program needs to support a numeric command-line argument.E. public static void main( String[] args ) The arguments that are passed to your program are saved into an array of String with the args identifier. such as "34". to a number. You can derive the number of command-line arguments with the array's length attribute.J.D. Coding Guidelines: Before using command-line arguments. when you invoke an application. Remember the declaration for the main method. Introduction to Programming I 123 . "4". always check if the number of arguments before accessing the array elements so that there will be no exception generated. "3". the runtime system passes the command-line arguments to the application's main method via an array of Strings. In the previous example. "2" and "1". Each String in the array contains one of the command-line arguments. the command-line arguments passed to the Sort application is an array that contains five strings which are: "5".length.I In the Java language.
8.3 Command-line arguments in NetBeans
To illustrate on how to pass some arguments to your programs in NetBeans, let us create a Java program that will print the number of arguments and the first argument passed to it. public class CommandLineExample { public static void main( String[] args ){ System.out.println("Number of arguments=" + args.length); System.out.println("First Argument="+ args[0]);
Now, run netbeans and create a new project and name this CommandLineExample. Copy the code shown above and compile the code. Now, follow these steps to pass arguments to your program using NetBeans. Click on Projects (encircled below).
Figure 8.2: Opening Project File
Introduction to Programming I
Right-click on the CommandLineExample icon, and a popup menu will appear. Click on Properties.
Figure 8.3: Opening Properties
The Project Properties dialog will then appear.
Figure 8.4: Properties Dialog Introduction to Programming I 125
Now, click on Run-> Running Project.
Figure 8.5: Click On Running Project
On the Arguments textbox, type the arguments you want to pass to your program. In this case we typed in the arguments 5 4 3 2 1. Then, click on the OK button.
Figure 8.6: Set the Command-Line Arguments
Introduction to Programming I
Now try to RUN your program.
Figure 8.7: Running the Program in with the Shortcut Button
As you can see here, the output to your program is the number of arguments which is 5, and the first argument which is 5.
Figure 8.8: Program Output
Introduction to Programming I
if the user entered. product and quotient of the two numbers.1 Print arguments Get input from the user using command-line arguments and print all the arguments to the screen. java ArithmeticOperation 20 4 your program should print sum = 24 difference = 16 product = 80 quotient = 5 Introduction to Programming I 128 .4.4 Exercises 8. java Hello world that is all your program should print Hello world that is all 8.2 Arithmetic Operations Get two numbers from the user using command-line arguments and print sum. For example. For example.4. if the user entered.D.I 8.J.E. difference.
For now.J. At the end of the lesson. we will discuss later on how to create your own classes. braking and accelerating. and how to use these classes and their members. When we compare this to the physical world. lion.1 Objectives In this section. a car object has the properties.E. These objects are characterized by their properties (or attributes) and behaviors.D. For example. Later on. we can find many objects around us. Comparison. Similarly. such as cars. Please refer to the table below for the examples. we will discuss the concept of classes and objects.2 Introduction to Object-Oriented Programming Object-Oriented programming or OOP revolves around the concept of objects as the basic elements of your programs. we can define different properties and behavior of a lion. the student should be able to: • • • • • • • Explain object-oriented programming and some of its concepts Differentiate between classes and objects Differentiate between instance variables/methods and class(static) variables/methods Explain what methods are and how to call and pass parameters to methods Identify the scope of a variable Cast primitive data types and objects Compare objects and determine the class of an objects 9. people and so on. we will focus on using classes that are already defined in the Java class library. Its behaviors are turning. Object Car Properties type of transmission manufacturer color Weight Color hungry or not hungry tamed or wild Table 17: Example of Real-life Objects Behavior turning braking accelerating roaring sleeping hunting Lion Introduction to Programming I 129 .I 9 Working with the Java Class Library 9. conversion and casting of objects will also be covered. we will introduce some basic concepts of object-oriented programming. type of transmission. manufacturer and color.
and it also consists of a set of methods (behavior) that describes how an object behaves. These data and methods could even be used in programming games or interactive software to simulate the real-world objects! An example would be a car software object in a racing game or a lion software object in an educational interactive software zoo for kids. What we have here is a Car Class which can be used to define several Car Objects.E. a prototype or a blueprint of an object. Classes provide the benefit of reusability. It can be thought of as a template. Car A and Car B are objects of the Car class. The class is the fundamental structure in object-oriented programming.J. Each object is composed of a set of data (properties/attributes) which are variables describing the essential characteristics of the object. To differentiate between classes and objects. Fields specifiy the data types defined by the class. An object is an instance of the class. The class has fields plate number. It consists of two types of members which are called fields (properties or attributes) and methods. Car Class Instance Variables Plate Number Color Manufacturer Current Speed Blue Object Car A ABC 111 Mitsubishi 50 km/h Accelerate Method Turn Method Brake Method Table 18: Example of Car class and its objects Object Car B XYZ 123 Red Toyota 100 km/h When instantiated.3 Classes and Objects 9. the objects in the physical world can easily be modeled as software objects using the properties as data and the behaviors as methods. an object is a software bundle of variables and related methods. Introduction to Programming I Instance Methods 130 . The Car has also some methods Accelerate. which will be discussed later.I With these descriptions. The variables and methods in a Java object are formally known as instance variables and instance methods to distinguish them from class variables and class methods. while methods specify the operations. and current speed which are filled-up with corresponding values in objects Car A and Car B.3. Software programmers can use a class over and over again to create many objects. Thus. color. 9. an object is a software component whose structure is similar to objects in the real world. In the table shown below. let us discuss an example. manufacturer. However.D. Turn and Brake.1 Difference Between Classes and Objects In the software world. the method implementations are shared among objects of the same class. each object gets a fresh set of state variables.
all of the objects of the Car class will have the value 2 for their Count variable. Suppose that our Car class has one class variable called Count. By placing a boundary around the properties and methods of our objects.E. which are variables that belong to the whole class.D. To clearly describe class variables. This means that it has the same value for all the objects in the same class. 9. They are also called static member variables.3. let's go back to our Car class example. If we change the value of Count to 2.3. it is also possible to define class variables. we can prevent our programs from having side effects wherein programs have their variables changed in unexpected ways.3 Class Variables and Methods In addition to the instance variables. Car Class Instance Variables Plate Number Color Manufacturer Current Speed Blue Mitsubishi 50 km/h Count = 2 Accelerate Method Turn Method Brake Method Table 19: Car class' methods and variables Object Car A ABC 111 Red Toyota Object Car B XYZ 123 100 km/h Introduction to Programming I Instance Methods Variable Class 131 .I 9.2 Encapsulation Encapsulation is the method of hiding certain elements of the implementation of a certain class. We can prevent access to our object's data by declaring them declaring them in a certain way such that we can control access to them. We will learn more about how Java implements encapsulation as we discuss more about classes.J.
you actually invoke the class' constructor.1: Classs Instantiation The new operator allocates a memory for that object and returns a reference of that memory location to you.D.I 9. For example.E. Introduction to Programming I 132 . we use the new operator. if you want to create an instance of the class String. The constructor is a method where you place all the initializations. String str2 = "Hello". we write the following code.4 Class Instantiation To create an object or an instance of a class.J.3. it has the same name as the class. str2 = new String("Hello world!"). String or also equivalent to. Figure 9. When you create an object.
• After the method has finished execution. Introduction to Programming I 133 . and that is the main() method. we only have one method. we can define many methods which we can call from different methods. why do we need to create methods? Why don't we just place all the code inside one big method? The heart of effective problem solving is in problem decomposition. manageable pieces is critical to writing large programs.J. We can do this in Java by creating methods to solve a specific part of the problem. A method is a separate piece of code that can be called by a main program or any other method to perform some specific function.4 Methods 9.E.D.I 9. Parameters are also called function arguments. In Java.1 What are Methods and Why Use Methods? In the examples we discussed before. The following are characteristics of methods: • It can return one or no values • It may accept as many parameters it needs or no parameter at all. Now. it goes back to the method that called it. Taking a problem and breaking it into small.4.
D. Two strings are considered equal ignoring case if they are of the same length. and so on. Method declaration public char charAt(int index) Definition Returns the character at the specified index. Let's take two sample methods found in the class String.4. but for now. to illustrate how to call methods. nameOfObject. we will create our own methods.charAt(0). Later on. x = str2.1. let us use what is available.E. let's use the String class as an example.nameOfMethod( parameters ). String char String str1 = "Hello".I 9. An index ranges from 0 to length() . Introduction to Programming I 134 . and corresponding characters in the two strings are equal ignoring case.equalsIgnoreCase( str1 ). ignoring (String anotherString) case considerations. as for array indexing. You can use the Java API documentation to see all the available methods in the String class. //this will return a boolean value true boolean result = str1. The first character of the sequence is at index 0.J. //will return the character H //and store it to variable x str2 = "hello". we write the following. the next at index 1. To call an instance method. public boolean equalsIgnoreCase Compares this String to another String. Table 20: Sample Methods of class String Using the methods.2 Calling Instance Methods and Passing Variables Now.
4.out. } public static void test( int j ){ //change value of parameter j j = 33. } In the given example. For example. we already tried passing variables to methods. all primitive data types when passed to a method are pass-by-value.4. The method cannot accidentally modify the original argument even if it modifies the parameters during calculations. it will not affect the variable value if i in main since it is a different copy of the variable. The value of i is copied to the variable of the method j. the method makes a copy of the value of the variable passed to the method.1 Pass-by-value When a pass-by-value occurs. Since j is the variable changed in the test method. Introduction to Programming I 135 . we haven't differentiated between the different types of variable passing in Java.E.out. i not changed System. //call method test //and pass i to method test test( i ). By default.3.println( i ).I 9. pass-byreference.J.println( i ). public class TestPassByValue { public static void main( String[] args ){ int i = 10. we called the method test and passed the value of i as parameter. //print the value of i System. 9. Pass i as parameter which is copied to j } //print the value of i. There are two types of passing data to methods.D.3 Passing Variables in Methods In our examples. the first one is pass-by-value and then. However.
} } public static void test( int[] arr ){ //change values of array for( int i=0. //print array values for( int i=0. i++ ){ System. However.D.E.length. the reference to an object is passed to the calling method. since.out. This means that.4. although different references are used in the methods. the method makes a copy of the reference of the variable passed to the method.println( ages[i] ).out. the location of the data they are pointing to is the same. the method can modify the actual object that the reference is pointing to. i++ ){ System.3.2 Pass-by-reference When a pass-by-reference occurs. For example. i<ages. } //call test and pass reference to array test( ages ). unlike in pass-by-value.length. } } Introduction to Programming I 136 . class TestPassByReference { public static void main( String[] args ){ //create an array of integers int []ages = {10. 11. i<arr.println( ages[i] ).J.I 9. i<ages.length. i++ ){ arr[i] = i + 50. 12}. Pass ages as parameter which is copied to variable arr } //print array values again for( int i=0.
9.J.' but it passes object references to methods 'by value.toHexString( 10 ). just type. Examples of static methods.4. Introduction to Programming I 137 . //converts the String 10.'" As a result.out. //Returns a String representation of the integer argument as an //unsigned integer base 16 String hexEquivalent = Integer.staticMethodName(params).E. to an integer int i = Integer. Static methods belongs to the class as a whole and not to a certain instance (or object) of a class. //prints data to screen System.D.4 Calling Static Methods Static methods are methods that can be invoked without instantiating a class (means without invoking the new keyword). you cannot write a standard swap method to swap objects.println("Hello world"). Static methods are distinguished from instance methods in a class definition by the keyword static.parseInt("10"). To call a static method. Take note that Java manipulates objects 'by reference. we've used so far in our examples are. Classname.2: Pass-by-reference example Coding Guidelines: A common misconception about pass-by-reference in Java is when creating a swap method using Java references.I Figure 9.
Given the variables i. The outer curly braces is called the outer blocks. The scope also determines the lifetime of a variable or how long the variable can exist in memory.m and n. D C Introduction to Programming I 138 . starting from the point where it is declared.j.D and E. n is E. you cannot expect the outer block to see it.e..D. int k = 0. However.}.5 Scope of a variable In addition to a variable's data type and name. k is C. The scope is determined by where the variable declaration is placed in the program.k.. m = 0.B. //. A variable's scope is inside the block where it is declared. n = 0. j is B. and the inner curly braces is called inner blocks. and in the inner blocks. a variable has scope. m is D. given the following code snippet. and the five scopes A. public class ScopeExample { public static void main( String[] args ){ int i = 0.E. If you declare variables in the outer block.C. For example. The scope determines where in the program the variable is accessible.I 9. if you declare variables in the inner block.. they are visible (i. some code here B A { int int E } } The code we have here represents five scopes indicated by the lines and the letters representing the scope.4. To simplify things. usable) by the program lines inside the inner blocks. we have the following scopes for each variable: The The The The The scope scope scope scope scope of of of of of variable variable variable variable variable i is A.J. int j = 0. just think of the scope as anything between the curly braces {..
i<arr.D.scope A i in B .E.length.scope D i in E . i++ ){ arr[i] = i + 50. ages[] . the scope ofthe variables are.I Now. //print array values for( int i=0. 12}. given the two methods main and test in our previous examples. arr[] .out. } } D In the main method.out.scope E Introduction to Programming I 139 . class TestPassByReference { public static void main( String[] args ){ //create an array of integers int []ages = {10. i++ ){ System. 11. } B A C } E } public static void test( int[] arr ){ //change values of array for( int i=0.J. i++ ){ System.println( ages[i] ). i<ages. the scope of the variables are. //print array values again for( int i=0.scope B i in C – scope C In the test method.length.length.println( ages[i] ). } //call test and pass reference to array test( ages ). i<ages.
if they are not declared in the same block. For the second System.D.print is invoke. int test = 20.print( test ).out. { } int test = 10. That means that if you have the following declaration. your compiler will generate an error since you should have unique names for your variables in one block. you can have two variables of the same name.out. the value 20 is printed since it is the closest test variable seen at that scope. System. System.print.. int test = 0.E.out.out. However.J. Introduction to Programming I 140 .I When declaring variables. it prints the value of the first test variable since it is the variable seen at that scope. //. only one variable with a given identifier or name can be declared in a scope. } When the first System. For example.some code here { int test = 20.print( test ). Coding Guidelines: Avoid having variables of the same name declared inside one method to avoid confusion.
I 9. If the variable i has the value 65. Typecasting or casting is the process of converting a data of a certain data type to another data type. double numDouble = numInt. The numeric code associated with a capital A is 65. the cast (char)i produces the character value 'A'. For example. Another example is when we want to typecast an int to a char value or vice versa. And finally. For example.print( valInt ). An example of typecasting is when you want to store an integer data to a variable of data type double. char valChar = 'A'. System. int numInt = 10. and that is the boolean data type. //implicit cast In this example.1 Casting Primitive Types Casting between primitive types enables you to convert the value of one data from one type to another primitive type. This commonly occurs between numeric types. we are going to learn how to compare objects. //explicit cast: output 65 Introduction to Programming I 141 . We will also learn how to convert primitive data types to objects and vice versa. Converting and Comparing Objects In this section.D. There is one primitive data type that we cannot do casting though. the data is implicitly casted to data type double.E.out.5.J. and Java adopted this as part of its character support.5 Casting. int valInt = valChar. we are going to learn how to do typecasting. 9. A character can be used as an int because each character has a corresponding numeric code that represents its position in the character set. since the destination variable (double) holds a larger value than what we will place inside it. according to the ASCII character set.
Explicit casts take the following form: (dataType)value where.D. For example.J.12.I When we convert a data that has a large type to a smaller type. we must use an explicit cast. //convert valDouble to int type double x = 10. int int result = (int)(x/y). int y = 2. is the name of the data type you're converting to value. is an expression that results in the value of the source type. dataType. int valInt = (int)valDouble.E.2. double valDouble = 10. //typecast result of operation to Introduction to Programming I 142 .
To cast an object to another class. you could assign objects of that class or any of its subclasses to that variable without casting. but you gain all the methods and variables that the subclass defines. however: Because subclasses contain more behavior than their superclasses. consider a method that takes two arguments. with one restriction: The source and destination classes must be related by inheritance. using an object of class Number won't include many methods specified in Integer. For example. not just inside method calls. you must cast them explicitly. Analogous to converting a primitive value to a larger type. To use superclass objects where subclass objects are expected. Introduction to Programming I 143 .I 9.D.3: Sample Class Hierarchy This is true in the reverse. you can pass in its subclasses. there's a loss in precision involved. is a reference to the source object. and you can use a superclass when a subclass is expected. Those superclass objects might not have all the behavior needed to act in place of a subclass object. is the name of the destination class object. because a subclass contains all the same information as its superclass. classname. Errors occur if you try to call methods that the destination object doesn't have. (classname)object where. some objects might not need to be cast explicitly.E. such as Dialog. If you had a variable defined as class Window. Figure 9. You won't lose any information in the cast. one class must be a subclass of the other.2 Casting Objects Instances of classes also can be cast into instances of other classes. There is a catch.J. you use the same operation as for primitive types: To cast. We'll cover more about inheritance later. For the Window argument. you can use an instance of a subclass anywhere a superclass is expected. one of type Object and another of type Window. This is true anywhere in a program. and Frame. FileDialog. You can pass an instance of any class for the Object argument because all Java classes are subclasses of Object. if you have an operation that calls methods in objects of the class Integer. In particular. For example.5.
which here defines that the VicePresident has executive washroom privileges.I • Note: that casting creates a reference to the old object of the type classname. VicePresident is a subclass of Employee with more information.J.4: Class Hierarchy for superclass Employee The following example casts an instance of the class VicePresident to an instance of the class Employee. Employee emp = new Employee(). Figure 9. // must cast explicitlyCasting Introduction to Programming I 144 .D. // no cast needed for upward use veep = (VicePresident)emp.E. the old object continues to exist as it did before. VicePresident veep = new VicePresident(). emp = veep.
Boolean.lang package includes classes that correspond to each primitive data type: Float. and so on.D. int penn = Integer. Primitive types and objects are very different things in Java.parseInt(pennsylvania). • CAUTION: The Void class represents nothing in Java. except that the class names begin with a capital letter (Short instead of short. It's a placeholder for the void keyword.5. //The following statement converts an Integer object to // its primitive data type int. Double instead of double. The result is an int with //value 7801 int newCount = dataCount.J. Byte. Using the classes that correspond to each primitive type. (Called Wrapper Classes) Java treats the data types and their class versions very differently. such as an int // Object->primitive String pennsylvania = "65000". or vice versa. you can create an object that holds the same value. Also. Examples: //The following statement creates an instance of the Integer // class with the integer value 7801 (primitive -> Object) Integer dataCount = new Integer(7801). so there's no reason it would be used when translating between primitive values and objects. two classes have names that differ from the corresponding data type: Character is used for char variables and Integer for int variables.I 9. the java.intValue(). As an alternative. Introduction to Programming I 145 .E.3 Converting Primitive Types to Objects and Vice Versa One thing you can't do under any circumstance is cast from an object to a primitive data type. and the like). Most of these classes have the same names as the data types. and you can't automatically cast between the two or use them interchangeably. // A common translation you need in programs // is converting a String to a numeric type. which is used in method definitions to indicate that the method does not return a value. and a program won't compile successfully if you use one when the other is expected.
out. str2 = str1. Although their contents match.println("String2: " + str2). To see whether two String objects have matching values. System. and so on. When applied to objects.println("String1: " + str1). less than. str1 = "Free the bound periodicals.E.D.println("String2: " + str2). a method of the class called equals() is used. } } This program's output is as follows. Instead of checking whether one object has the same value as the other object. these operators don't do what you might first expect. If you try to use other values as operands.out. System.I 9.4 Comparing Objects In our previous discussions. however.println("Same object? " + (str1 == str2)).J.println("Same value? " + str1. System. str2.". The exceptions to this rule are the operators for equality: == (equal) and != (not equal). The following code illustrates this. System. they determine whether both sides of the operator refer to the same object.println("Same object? " + (str1 == str2)).equals(str2)). To compare instances of a class and have meaningful results. str2 = new String(str1). the Java compiler produces errors. periodicals. not equal. we learned about operators for comparing values—equal.out. class EqualsTest { public static void main(String[] arguments) { String str1. System.out. System. not on objects. Most of these operators work only on primitive types.5. OUTPUT: String1: Free the bound String2: Free the bound Same object? true String1: Free the bound String2: Free the bound Same object? false Same value? True periodicals. System. they are not the same object. you must implement special methods in your class and call those methods. periodicals. The method tests each character in the string and returns true if the two strings have the same values. If you were to employ the == operator to compare these objects. they would be considered unequal. It is possible to have two different String objects that contain the same values. Introduction to Programming I 146 . A good example of this is the String class. periodicals.println("String1: " + str1).out.out.out.
Figure 9.5: Both references point to the same object The first part of this program declares two variables (str1 and str2). and then assigns that value to str2.E. if you create a string using a literal and then use another literal with the same characters. str1 and str2 now point to the same object. str2. rather than using new? String literals are optimized in Java." to str1.". Introduction to Programming I 147 .D. Both strings are the same objects. As you learned earlier. Figure 9.J. both with the same value.6: References now point to different objects • NOTE: Why can't you just use another literal when you change str2. assigns the literal "Free the bound periodicals. str1 = "Free the bound periodicals. and the equality test proves that. you create a new String object with the same value as str1 and assign str2 to that new String object. you have to go out of your way to create two separate objects.I Now let's discuss the code. String str1. Java knows enough to give you the first String object back. In the second part of this program. Now you have two different string objects in str1 and str2. Testing them using the equals() method also returns the expected answer: true—they have the same values. Testing them to see whether they're the same object by using the == operator returns the expected answer: false—they are not the same object in memory. str2 = new String(str1).
boolean ex1 = "Texas" instanceof String.E. getName() returns a string representing the name of the class.5 Determining the Class of an Object Want to find out what an object's class is? Here's the way to do it for an object assigned to the variable key: 1. The getClass() method returns a Class object (where Class is itself a class) that has a method called getName().getName(). 2. 10). boolean ex2 = pt instanceof String. The instanceOf operator The instanceOf has two operands: a reference to an object on the left and a class name on the right.I 9. For Example. // true Object pt = new Point(10. // false Introduction to Programming I 148 .5.J. String name = key. In turn.D.getClass(). For Example. The expression returns true or false based on whether the object is an instance of the named class or any of that class's subclasses.
if the given string is "Hello". 4. In cases where you can find more methods that can accomplish the task. Your task is to help Pipoy look for the APIs (Application Programming Interface). the method declaration and a sample usage of the said method. Pipoy does not have a copy of the Java Documentation.D. For example. 6. define the following terms: Class Object Instantiate Instance Variable Instance Method Class Variables or static member variables Constructor 9. 5. the method should return true the suffix given is "lo".1 Defining terms In 1.I 9. (Hint: All methods are in the java. Look for the method that gets the floor of a double value. For example. 2.6 Exercises 9.13. Look for a method that checks if a certain String ends with a certain suffix. Look for the method that terminates the currently running Java Virtual Machine 4.parseInt( strValue ). 2. Look for the method that determines if a certain character is a digit. Introduction to Programming I 149 . so as not to confuse Pipoy.6. You should state the class where the method belongs. it returns the value true. and he's eager to try them out. and he also doesn't have an internet access.E.6. if Pipoy wants to know the method that converts a String to integer. For example. so there's no way for him to view the Java APIs. the method should return the value 3. if I input a 3. your own words.2 Java Scavenger Hunt Pipoy is a newbie in the Java programming language. Now let's start the search! 1. if I input '3'. and false if the given suffix is "alp". give only one.lang package). For example.J. Make sure that the snippet of code you write in your sample usage compiles and outputs the correct answer. int value = Integer. The problem is. 7. 3. 5. He just heard that there are already ready-to-use APIs in Java that one could use in their programs. For example. 3. since F is the hexadecimal representation for the number 15 (base 10). the method would return the character F. Look for the method that determines the character representation for a specific digit in the specified radix. your answer should be: Class: Integer Method Declaration: public static int parseInt( String value ) Sample Usage: String strValue = "100". and the radix is 16. if the input digit is 15.
E. . the student should be able to: • • • • • • Create their own classes Declare attributes and methods for their classes Use the this reference to access instance data Create and call overloaded methods Import and create packages Use access modifiers to control access to class members Introduction to Programming I 150 . We will create a class that contains information of a Student and operations needed for a certain student record.indicates that you have to substitute an actual value for this part instead of typing it as it is. we will now be studying on how to write our own classes.I 10 Creating your own Classes 10. we will make a sample class wherein we will add more data and functionality as we go along the way.D. Things to take note of for the syntax defined in this section and for the other sections: * <description> [] .indicates that this part is optional At the end of the lesson. For this section.1 Objectives Now that we've studied on how to use existing classes from the Java class library. . in order to easily understand how to create classes.J.means that there may be 0 or more occurrences of the line whereit was applied to.
and list all the information or properties that you want your class to contain.D. <modifier> class <name> { <attributeDeclaration>* <constructorDeclaration>* <methodDeclaration>* } where <modifier> is an access modifier.means that our class is accessible to other classes outside the package . which may be combined with other types of modifier. public class StudentRecord { //we'll add more code here later } where. if no access modifier prefixes the class keyword). we can now name it. An appropriate name for our class would be StudentRecord.E.e. 3. to define our class we write.J. Don't just call your class XYZ or any random names you can think of. Coding Guidelines: Remember that for a top-level class. public class StudentRecord Coding Guidelines: 1. Now. we write. In this section. Think of an appropriate name for your class. the only valid access modifiers are public and package (i. The filename of your class should have the SAME NAME as your public class name. we will be creating a class that will contain a student record. Think of an appropriate name for the class. Class names should start with a CAPITAL letter.a unique identifier that describes our class Introduction to Programming I 151 .this is the keyword used to create a class in Java . Also list down the methods that you will be using for your class.2 Defining your own classes Before writing your class. To define a class.I 10.. . think first on where you will be using your class and how your class will be used. Since we've already identified the purpose of our class. 2.
name address age math grade english grade science grade average grade String String int double double double double You can add more information if you want to. Now.D. For each information. private String address. Coding Guidelines: 1. Instance variables. Use an appropriate data type for each variable you declare. or a String for a student's grade.J. The following are some sample information we want to add to the student record. private double englishGrade. private int age. Since we want these attributes to be unique for each object (or for each student).3. For example.1 Instance Variables Now that we have a list of all the attributes we want to add to our class.E. public class StudentRecord { private String name. We will cover more about accessibility later.3 Declaring Attributes To declare a certain attribute for our class. private double mathGrade. 4. let us now add them to our code. you don't want to have a data type int for a student's name. For example. private here means that the variables are only accessible within the class. <modifier> <type> <name> [= <default_value>]. private double average. 3. also list what data types would be appropriate to use. we should declare them as instance variables. 10.I 10. 2. like any other variables should start with a SMALL letter. Declare one variable for each line. we write. it's all really up to you. Declare instance variables as private so that only class methods can access them directly. 5. //we'll add more code here later } where. Other objects cannot access these variables directly. Introduction to Programming I 152 . Declare all your instance variables on the top of the class declaration. let us write down the list of attributes that a student record can contain. we will be using these information. But for this example. private double scienceGrade.
public class StudentRecord { //instance variables we have declared private static int studentCount. private double englishGrade.I 10.D. } //we'll add more code here later we use the keyword static to indicate that a variable is a static variable.J. To declare a static variable. we can also declare class variables or variables that belong to the class as a whole. The value of these variables are the same for all the objects of the same class. <modifier> <returnType> <name>(<parameter>*) { <statement>* } where. private double mathGrade. <modifier> can carry a number of different modifiers <returnType> can be any data type (including void) <name> can be any valid identifier <parameter> ::= <parameter_type> <parameter_name>[. } //we'll add more code here later 10. private static int studentCount.] Introduction to Programming I 153 . To declare methods we write. public class StudentRecord { private String name. So far.E.4 Declaring Methods Before we discuss what methods we want our class to have. Let us call this as studentCount. we can declare one static variable that will hold this value. private double average. our whole code now looks like this. let us first take a look at the general syntax for declaring methods. Now suppose.2 Class Variables or Static Variables Aside from instance variables. private double scienceGrade. private int age. private String address. we want to know the total number of student records we have for the whole class.3.
However.java:14: incompatible types found : int required: java. we create accessor methods. we declare the fields or attributes of our classes as private. : : public String getName(){ return name. return name.E.J. address. we want an accessor method that can read the name.I 10. in our program signifies that it will return the value of the instance variable name to the calling method.lang. StudentRecord. Accessor methods are used to read values from class variables (instance/static).String return age. You usually encounter the following error if the two does not have the same data type.is the return type of the method. } } . english grade. In order to do this. public class StudentRecord { private String name.means that the method can be called from objects outside the class .this means that our method does not have any parameters where.D.4.1 Accessor methods In order to implement encapsulation. ^ 1 error Introduction to Programming I 154 . Now let's take a look at one implementation of an accessor method. An accessor method usually starts with a get<NameOfInstanceVariable>. It also returns a value. For our example. Take note that the return type of the method should have the same data type as the data in the return statement. there are times wherein we want other objects to access private data. public String getName () The statement. that is. math grade and science grade of the student.the name of the method . This means that the method should return a value of type String . we don't want any objects to just access our data anytime.
means that the method can be called from objects outside the class imeans that the method does not return any value the name of the method parameter that will be used inside our method Introduction to Programming I 155 . what if we want other objects to alter our data? What we do is we provide methods that can write or change values of our class variables (instance/static). mutator methods. assigns the value of temp to name and thus changes the data inside the instance variable name.I Another example of an accessor method is the getAverage method. public class StudentRecord { private String name. Take note that mutator methods don't return values. result = ( mathGrade+englishGrade+scienceGrade )/3.2 Mutator Methods Now. } The getAverage method computes the average of the 3 grades and returns the result. it contains some program argument or arguments that will be used inside the method. public void setName (String temp) The statement. : : public double getAverage(){ double result = 0.E.J. Now let's take a look at one implementation of a mutator method. A mutator method is usuallyu written as set<NameOfInstanceVariable>. public class StudentRecord { private String name. name = temp. } } where. However.4. : : public void setName( String temp ){ name = temp. We call these methods.D. 10. } return result.
You can also use constants to return values instead of variables. we call the method StudentRecord. Method names should be verbs 3.[methodName].is the return type of the method.means that the method can be called from objects outside the class . public String getNumberInWords( int num ){ String defaultNum = "zero". 10. consider the method. Coding Guidelines: 1.getStudentCount() .J. This means that the method should return a value of type int .4. Always provide documentation before the declaration of the method.the name of the method .this means that our method does not have any parameters For now. if( num == 1 return } else if( num return } ){ "one".I 10. getStudentCount will always return the value zero since we haven't done anything yet in our program in order to set its value.[ClassName]. public static int getStudentCount(){ return studentCount. //return a constant == 2){ "two".means that the method is static and should be called by typing. Please see example. public class StudentRecord { private static int studentCount. Introduction to Programming I 156 .3 Multiple Return statements You can have multiple return statements for a method as long as they are not on the same block. For example.4.E. } } where. You can use javadocs style for this. For example. in this case. We will try to change the value of studentCount later on when we discuss constructors. 2. we can create a static method to access its value.D. public static int getStudentCount () . //return a constant } //return a variable return defaultNum. Method names should start with a SMALL letter.4 Static methods For the static variable studentCount.
private static int studentCount. englishGrade.4. } // other code here .. public class { private private private private private private private StudentRecord String String int double double double double name..D. } /** * returns the number of instances of StudentRecords */ public static int getStudentCount(){ return studentCount. } /** * Changes the name of the student */ public void setName( String temp ){ name = temp. /** * Returns the name of the student */ public String getName(){ return name. /** * Computes the average of the english. address.J. scienceGrade. mathGrade. average. age. result = ( mathGrade+englishGrade+scienceGrade )/3. } return result.E.. } Introduction to Programming I 157 .5 Sample Source Code for StudentRecord class Here is the code for our StudentRecord class.I 10. math and science * grades */ public double getAverage(){ double result = 0.
beahRecord. StudentRecord(). //print number of students System.getName() ).E.out. Introduction to Programming I 158 .I Now.getStudentCount()).out. crisRecord. public class StudentRecordExample { public static void main( String[] args ){ //create three objects for Student StudentRecord annaRecord = new StudentRecord beahRecord = new StudentRecord crisRecord = new //set the name of the students annaRecord. Anna Student Count = 0 record StudentRecord().setName("Beah").D.setName("Anna"). StudentRecord().println( annaRecord.println("Count="+StudentRecord. } } The output of this program is.J.setName("Cris"). here's a sample code of a class that uses our StudentRecord class. //print anna's name System.
To use the this reference.5 The this reference The this reference is used to access the instance variables shadowed by the parameters. NOTE: You can only use the this reference for instance variables and NOT static or class variables. public void setAge( int age ){ this.age = age. the value of the parameter age will be used.<nameOfTheInstanceVariable> So for example. Since the parameter age is the closest declaration to the method. Suppose we have the following declaration for setAge. In order to correct this mistake. we are just assigning the value of the parameter age to itself! This is not what we want to happen in our code.J. we type. this. //WRONG!!! } The parameter name in this declaration is age. To understand this better. public void setAge( int age ){ age = age. age = age. let's take for example the setAge method. which has the same name as the instance variable age. we use the this reference.I 10. Introduction to Programming I 159 .D. we can now rewrite our code to. } This method will then assign the value of the parameter age to the instance variable of the object StudentRecord. So in the statement.E.
in our StudentRecord class we want to have a method that prints information about the student.out. System. System. to have different implementations and return values of different types. For example.out. double sGrade) System. we want the print method to print things differently depending on the parameters we pass to it. System. we want the print method to print out the name.println("Name:" + name).println("Address:" + address).E. System. Method overloading allows a method with the same name but different parameters. we want the method to print the student's name and grades.I 10. System. when we pass a String.out. We have the following overloaded methods inside our StudentRecord class. } Introduction to Programming I 160 . However.out. and it is called Method Overloading.out. public void print( String temp ){ System.D. Rather than invent new names all the time.out.out. address and age of the student. we want to sometimes create methods that has the same names but function differently depending on the parameters that are passed to them.println("Science Grade:" + sGrade). method overloading can be used when the same operation has different implementations.println("Age:" + age). This capability is possible in Java.println("Math Grade:" + mGrade).println("Name:" + name). } public void print(double eGrade. For example. When we pass 3 double values.6 Overloading Methods In our classes. double mGrade.J.println("English Grade:" + eGrade).
print( annaRecord. annaRecord.0 English Grade:95. //overloaded methods annaRecord.setEnglishGrade(95.setMathGrade(80).E.0 Always remember that overloaded methods have the following properties.getEnglishGrade(). annaRecord. annaRecord. public static void main( String[] args ) { StudentRecord annaRecord = new StudentRecord(). Name:Anna Math Grade:80.setAge(15).print( annaRecord.setScienceGrade(100). Name:Anna Address:Philippines Age:15 we will have the output for the second call to print. annaRecord.D.5). } we will have the output for the first call to print.setName("Anna"). annaRecord.getName() ).J.I When we try to call this in the following main method. annaRecord.5 Science Grade:100. – the same name – different parameters – return types can be different or the same Introduction to Programming I 161 . annaRecord.getScienceGrade()).getMathGrade(). annaRecord.setAddress("Philippines"). annaRecord.
however only the following information can be placed in the header of the constructor. You cannot call a constructor directly. englishGrade = eGrade. It is a method where all the initializations are placed. public StudentRecord() { //some code here } 10.I 10.1 Default Constructor Every class has a default constructor. constructors can also be overloaded..E. the default constructor would look like this. Constructors are important in instantiating an object. then an implicit default constructor is created. double sGrade){ mathGrade = mGrade.D.. String address){ this. <modifier> <className> (<parameter>*) { <statement>* } 10.7. To declare a constructor. scope or accessibility identifier (like public. for example. } public StudentRecord(double mGrade.). A constructor is just like an ordinary method.2 Overloading Constructors As we have mentioned. double eGrade. constructor's name and parameters if it has any. Constructors have the same name as the class 2.7. this. For example. it can only be called by using the new operator during class instantiation.name = name. The following are the properties of a constructor: 1. in our StudentRecord class. If the class does not specify any constructors.J. we have here four overloaded constructors. 3.7 Declaring Constructors We have discussed before the concept of constructors.address = address. } public StudentRecord(String name. Constructors does not have any return value 4.name = temp. public StudentRecord(){ //some initialization code here } public StudentRecord(String temp){ this. } Introduction to Programming I 162 . we write. The default constructor is the constructor without any parameters. scienceGrade = sGrade.
studentCount++. public static void main( String[] args ) { //create three objects for Student record StudentRecord annaRecord=new StudentRecord("Anna").address = address. double eGrade. let us go back to the static variable studentCount we have declared a while ago. scienceGrade = sGrade. we have the following code. everytime an object of class StudentRecord is instantiated. //add a student } StudentRecord Introduction to Programming I 163 . this. what we want to do here is. "Philippines").name = name. //add a student } public StudentRecord(String temp){ this. because it is always called everytime an object is instantiated.E.3 Using Constructors To use these constructors.J. before we move on. studentCount++. //some code here } Now.100).I 10. So.90. public StudentRecord(){ //some initialization code here studentCount++. A good location to modify and increment the value of studentCount is in the constructors. englishGrade = eGrade. we increment the value of studentCount. The purpose of the studentCount is to count the number of objects that are instantiated with the class StudentRecord. studentCount++. //add a student } public StudentRecord(double mGrade. double sGrade){ mathGrade = mGrade.D. For example.name = temp. String address){ this.7. beahRecord=new StudentRecord("Beah". //add a student } public StudentRecord(String name. StudentRecord crisRecord=new StudentRecord(80.
Introduction to Programming I 164 .7.E.4 The this() Constructor Call Constructor calls can be chained.name = temp. 8: } 9: 10: public static void main( String[] args ) 11: { 12: 13: StudentRecord annaRecord = new StudentRecord(). There are a few things to remember when using the this constructor call: 1. it will call the default constructor line 1. We use the this() call for this. 14: } Given the code above. given the following code. The this call can then be followed by any other relevant statements. For example. meaning. you can call another constructor from inside another constructor. It can ONLY BE USED IN A CONSTRUCTOR DEFINITION. 1: public StudentRecord(){ 2: this("some string"). it will then call the constructor that has a String parameter (in line 6). IT MUST OCCUR AS THE FIRST STATEMENT in a constructor 2. When statement in line 2 is executed. When using the this constructor call. when the statement at line 13 is called.I 10.D. 3: 4: } 5: 6: public StudentRecord(String temp){ 7: this.J.
: Packages can also be nested. After copying. The syntax for importing packages is as follows.1 Importing Packages To be able to use classes outside of the package you are currently working in.Color color. if you want to use the class Color inside package awt. Introduction to Programming I 165 .D. This powerful feature provides for a convenient mechanism for managing a large group of classes and interfaces while avoiding potential naming conflicts. The first thing you have to do is create a folder named schoolClasses. For example.awt package.J. Suppose we want to create a package where we will place our StudentRecord class. private String address. package schoolClasses.I 10. private int age. public class StudentRecord { private String name. By default. Another way to import classes from other packages is through explicit package referencing. we write. In this case.8. import <nameOfPackage>. Copy all the classes that you want to belong to this package inside this folder. together with other related classes. import java.8 Packages Packages are Java's means of grouping related classes and interfaces together in a single unit (interfaces will be discussed later).awt. 10. the Java interpreter expects the directory structure containing the executable classes to match the package hierarchy. import java.2 Creating your own packages To create our own package.awt. that is why you can use classes like String and Integers inside the program eventhough you haven't imported any packages. add the following code at the top of the class file.awt. schoolClasses. you need to import the package of those classes.*. all your Java programs import the java. We will call our package. java. The first statement imports the specific class Color while the other imports all of the classes in the java. This is done by using the package name to declare an object of a class. For example.* package. 10.8. package <packageName>.lang. you have to type the following.E.Color.
run(Unknown Source) at java. let us take a look at an example on what will happen if we don't set the classpath.lang.loadClassInternal(Unknown Source) We encounter a NoClassDefFoundError which means that Java did not know where to look for your class. C:\schoolClasses> java schoolClasses.defineClass(Unknown Source) at java.J.StudentRecord For Unix base systems.ClassLoader. C:\schoolClasses> set classpath=C:\ where C:\ is the directory in which we have placed the packages.defineClass(Unknown Source) at java.D.lang.I 10. The reason for this is that your class StudentRecord now belongs to a package named studentClasses.ClassLoader. suppose we have our classes in the directory /usr/local/myClasses.lang.URLClassLoader$1.security. we type this at the command prompt.defineClass1(Native Method) at java. we write. we must set the classpath.lang. We also have to tell JVM where to look for our packages.findClass(Unknown Source) at java.URLClassLoader.loadClass(Unknown Source) at java.loadClass(Unknown Source) at sun.URLClassLoader.defineClass(Unknown Source) at java.SecureClassLoader.lang.8.security.access$100(Unknown Source) at java. C:\schoolClasses>javac StudentRecord. We need to set the classpath to point to that directory so that when we try to run it.ClassLoader.doPrivileged(Native Method) at java.Launcher$AppClassLoader. After setting the classpath.net. Suppose we compile and then run the StudentRecord class we wrote in the last section.net. export classpath=/usr/local/myClasses Introduction to Programming I 166 .net. we can now run our program anywhere by typing. If we want to run our class. which in this case is in location C:\.misc.URLClassLoader.lang.3 Setting the CLASSPATH Now. To do this.E. To set the classpath in Windows.ClassLoader. the JVM will be able to see where our classes are stored. we jave to tell Java about its full class name which is schoolClasses.loadClass(Unknown Source) at java.NoClassDefFoundError: StudentRecord (wrong name: schoolClasses/StudentRecord) at java. suppose we place the package schoolClasses under the C:\ directory.ClassLoader.net.AccessController.StudentRecord. Before we discuss how to set the classpath.java C:\schoolClasses>java StudentRecord Exception in thread "main" java.
set classpath=C:\myClasses.J.(for windows) and : (for Unix based systems).E:\MyPrograms\Java and for Unix based systems.D. For example.D:\.I Take note that you can set the classpath anywhere. You can also set more than one classpath.E. we just have to separate them by . export classpath=/usr/local/java:/usr/myClasses Introduction to Programming I 167 .
9. For example. the instance variable name and the method getName() can be accessed from other objects. for the fourth one which is default. we want to implement some kind of restriction to access these data. public class StudentRecord { //default access to instance variable public int name. In Java. //default access to method String getName(){ return name. Any object that interacts with the class can have access to the public members of the class.E. public class StudentRecord { //default access to instance variable int name. you may want to hide this from other objects using your class. //default access to method public String getName(){ return name. private. } } In this example. we have what we call access modifiers in order to implement this. both inside and outside the class. 10.1 default access (also called package accessibility) This specifies that only classes in the same package can have access to the class' variables and methods.9 Access Modifiers When creating our classes and defining the properties and methods in our class. There are four different types of member access modifiers in Java: public. The first three access modifiers are explicitly written in the code to indicate the access type.J. as long as the object belongs to the same package where the class StudentRecord belongs to.I 10. protected and default. 10.D. There are no actual keyword for the default modifier. Introduction to Programming I 168 .2 public access This specifies that class members are accessible to anyone. the instance variable name and the method getName() can be accessed from other objects. } } In this example. it is applied in the absence of an access modifier. For example. no keyword is used. For example. if you want a certain attribute to be changed only by the methods inside the class.9.
For example. //default access to method private String getName(){ return name.3 protected access This specifies that the class members are accessible only to methods in that class and the subclasses of the class. } } In this example. Coding Guidelines: The instance variables of a class should normally be declared private. the instance variable name and the method getName() can be accessed only from methods inside the class.I 10. For example. We will discuss about subclasses on the next chapter. the instance variable name and the method getName() can be accessed only from methods inside the class and from subclasses of StudentRecord. and the class will just provide accessor and mutator methods to these variables. } } In this example.D. 10. //default access to method protected String getName(){ return name.4 private access This specifies that the class members are only accessible by the class they are defined in.9.9. public class StudentRecord { //default access to instance variable private int name. public class StudentRecord { //default access to instance variable protected int name.J. Introduction to Programming I 169 .E.
2 AddressBook Create a class address book that can contain 100 entries of AddressBookEntry objects (use the class you created in the first exercise).D.10. You should provide the following methods for the address book. 2. 3.E.10. The following table describes the information that an adressbook entry has.10 Exercises 10. 4. Provide the necessary accessor and mutator methods for all the attributes. 1.I 10.1 Address Book Entry Your task is to create a class that contains an address book entry. Constructors 10. 2. create the following: 1. Add entry Delete entry View all entries Update an entry Introduction to Programming I 170 . Attributes/Properties Name Address Telephone Number Email Address Description Name of the person in the addressbook Address of the person Telephone number of the person Person's Email address Table 21: Attributes and Attributes Descriptions For the methods.J.
We will also be discussing a special property of Java wherein it can automatically apply the proper methods to each object regardless of what subclass the object came from.1 Objectives In this section. At the end of the lesson. Figure 11. you can encode a method only once and they can be used by all subclasses. Any class above a specific class in the class hierarchy is known as a superclass. While any class below a specific class in the class hierarchy is known as a subclass of that class. A subclass only need to implement the differences between itself and the parent. we will be discussing on how a class can inherit the properties of an existing class. including the classes that make up the Java API. Polymorphism and Interfaces 11. A class that does this is called a subclass and its parent class is called the superclass. A sample class hierarchy is shown below. that behavior is automatically inherited by all subclasses. Finally. the student should be able to: • • • Define super classes and subclasses Override methods of superclasses Create final methods and final classes 11. we are going to discusss about interfaces that helps reduce programming effort. are subclassed from the Object superclass. Inheritance is a major advantage in object-oriented programming since once a behavior (method) is defined in a superclass.2 Inheritance In Java.E. Thus. all classes. This property is known as polymorphism.I 11 Inheritance.J.1: Class Hierarchy Introduction to Programming I 171 .D.
Take note that all the properties of a superclass that are declared as public.out. the attributes name and address are declared as protected. Introduction to Programming I 172 .println("Inside Person:Constructor"). the subclasses won't be able to use them. } /** * Constructor with 2 parameters */ public Person( String name. If we declare this as private. } /** * Accessor methods */ public String getName(){ return name. } public String getAddress(){ return address.address = add. /** * Default constructor */ public Person(){ System. } public void setAddress( String add ){ this. The reason we did this is that.1 Defining Superclasses and Subclasses To derive a class.D. In order to illustrate this.E.address = address. public class Person { protected String protected String name. name = "". String address ){ this.J.name = name. protected and default can be accessed by its subclasses.2. let's create a sample parent class. address = "". } } Notice that.I 11.name = name. address. this. } public void setName( String name ){ this. we use the extends keyword. we want these attributes to be accessible by the subclasses of the superclass. Suppose we have a parent class called Person.
To do this. } In the code.I Now. we decide to just extend the class Person. consider the following code. //some code here } } // some code here When a Student object is instantiated. we write.D.2: Program Flow Introduction to Programming I 173 .J. public static void main( String[] args ){ Student anna = new Student(). we want to create another class named Student. the default constructor of its superclass is invoked implicitly to do the necessary initializations. so that we can inherit all the properties and methods of the existing class Person. the statements inside the subclass are executed.println("Inside Student:Constructor"). we create an object of class Student. After that. Figure 11. public class Student extends Person { public Student(){ System. The output of the program is. To illustrate this.E.out. Inside Person:Constructor Inside Student:Constructor The program flow is shown below. Since a student is also a person.
given our previous example classes Person and Student.J. we show an example of a super constructor call. based on the arguments passed.D.println("Inside Student:Constructor"). public Student() { super. The super() call MUST OCCUR THE FIRST STATEMENT IN A CONSTRUCTOR. 2.2 The super keyword A subclass can also explicitly call a constructor of its immediate superclass.out. } This code calls the default constructor of its immediate superclass (which is Person) and executes it. public Student(){ super( "SomeName". A super constructor call in the constructor of a subclass will result in the execution of relevant constructor from the superclass. } Introduction to Programming I 174 .I 11. Given the following code for Student. } This code calls the second constructor of its immediate superclass (which is Person) and executes it.address = "some address".2. The super() call can only be used in a constructor definition. "SomeAddress" ).name = "somename". Another use of super is to refer to members of the superclass (just like the this reference ).println("Inside Student:Constructor"). System. Another sample code shown below. public Student(){ super().out. super. This implies that the this() construct and the super() calls CANNOT BOTH OCCUR IN THE SAME CONSTRUCTOR. For example.E. System. 3. For example. This is done by using the super constructor call. There are a few things to remember when using the super constructor call: 1.
public class Person { : : public String getName(){ System.E. overriding methods could prove to be very useful. return name. public class Student extends Person { : : public String getName(){ System.println("Student: getName").println("Parent: getName").out. the overridden method would be called. Suppose we have the following implementation for the getName method in the Person superclass. when we invoke the getName method of an object of class Student. and the output would be.3 Overriding Methods If for some reason a derived class needs to have a different implementation of a certain method from that of the superclass. } : } So. A subclass can override a method defined in its superclass by providing a new implementation for that method.I 11.out.2. the getName method in the subclass Student. } : } To override.D. Student: getName Introduction to Programming I 175 .J. return name. we write.
To declare a class to be final. Introduction to Programming I 176 .J.E. if we want the getName method in class Person to be declared final. These classes are called final classes. To declare a method to be final. public final String getName(){ return name. it is also possible to declare classes that can no longer be subclassed. we write. For example.D. It is also possible in Java to create methods that cannot be overridden. we write. we just add the final keyword in the class declaration. Examples of these classes are Integer. public final class Person { //some code here } Many of the classes in the Java API are declared final to ensure that their behavior cannot be overridden. if we want the class Person to be declared final.I 11. This means that you cannot override them. we just add the final keyword in the method declaration.2. For example. } Static methods are automatically final. These methods are what we call final methods.4 Final Methods and Final Classes In Java. Double and String.
For example. we add another subclass of Person which is Employee.J. } } public class Student extends Person { public String getName(){ System.E.println("Student Name:" + name). we can create a reference that is of type superclass to an object of its subclass. public static main( String[] args ) { Person ref. Student Employee studentObject = new Student(). //Person ref points to a // Student object } //some code here Now suppose we have a getName method in our superclass Person.println("Employee Name:" + name). return name.3 Polymorphism Now. and we override this method in both the subclasses Student and Employee.out. return name.D.3: Hierarchy for Person class and it's classes In Java. Below is the class hierarchy for that. employeeObject = new Employee(). Person Student Employee Figure 11. } } public class Employee extends Person { public String getName(){ System.println("Person Name:" + name). ref = studentObject. public class Person { public String getName(){ System. given the parent class Person and the subclass Student of our previous example.I 11. } } Introduction to Programming I 177 .out. return name.out.
println( temp ). employeeObject = new Employee(). Another example that exhibits the property of polymorphism is when we try to pass a reference to methods. the getName method of Employee will be called. Employee employeeObject = new Employee(). } This ability of our reference to change behavior according to what object it is holding is called polymorphism. . while automatically selecting the proper methods to apply to a particular object based on the subclass it belongs to. //getName of Employee //class is called System.J. printInformation( studentObject ). we can actually pass a reference of type Employee and type Student to this method as long as it is a subclass of the class Person. if we assign ref to an Employee object. //Person reference points to an // Employee object String temp = ref. . Polymorphism allows multiple objects of different subclasses to be treated as objects of a single superclass. .out. ref = employeeObject. public static main( String[] args ) { Student studentObject = new Student().out. public static printInformation( Person p ){ . Now.E.I Going back to our main method. when we try to call the getName method of the reference Person ref. //Person reference points to a // Student object String temp = ref. //getName of Student //class is called System. the getName method of the Student object will be called.getName().D. Suppose we have a static method printInformation that takes in a Person object as reference. Student Employee studentObject = new Student(). public static main( String[] args ) { Person ref. } Introduction to Programming I 178 . ref = studentObject. } printInformation( employeeObject ).println( temp ).getName().
there are some methods in this superclass wherein we cannot generalize the behavior.J. To create an abstract method. Introduction to Programming I 179 . just write the method declaration without the body and use the abstract keyword. while other living things like dogs walk on four legs.D. the walk method. Take for example. It often appears at the top of an object-oriented programming class hierarchy. For example. we want to create a superclass named LivingThing. However. eat. For example. public abstract void someMethod(). that is why we want to create a general superclass for this. Not all living things walk the same way. Figure 11. Those methods in the abstract classes that do not have implementation are called abstract methods. sleep and walk. and some methods wherein we just want to be overridden by its subclasses. An abstract class is a class that cannot be instantiated.4 Abstract Classes Now suppose we want to create a superclass wherein it has certain methods in it that contains some implementation. This kind of class is called an abstract class. we can create a superclass that has some methods with implementations and others which do not.E. there are many characteristics that living things have in common. we humans walk on two legs. However.4: Abstract class In order to do this. defining the broad types of actions possible with objects of all subclasses of the class. This class has certain methods like breath.I 11. Take the humans for instance.
"). it is required to override the abstract method walk().J..I Now.out.println("Living Thing breathing. that subclass will also become an abstract class.. and use its subclasses to provide implementation details of the abstract class..println("Living Thing eating. Human. public abstract class LivingThing { public void breath(){ System. public class Human extends LivingThing { public void walk(){ System..println("Human walks. let's create an example abstract class.. } /** * abstract method walk * We want this method to be overridden by subclasses of * LivingThing */ public abstract void walk(). and therefore cannot be instantiated.java:1: Human is not abstract and does not override abstract method walk() in LivingThing public class Human extends LivingThing ^ 1 error Coding Guidelines: Use abstract classes to define broad types of behaviors at the top of an object-oriented programming class hierarchy.D. we would encounter the following error message."). or else."). } } If the class Human does not override the walk method. Introduction to Programming I 180 . For example. } public void eat(){ System. } When a class extends the LivingThing abstract class.E.out..out.
we need to use interfaces to model multiple inheritance which allows a class to have more than one superclass. Let's take as an example a class Line which contains methods that computes the length of the line and compares a Line object to objects of the same class. Abstract Class The following are the main differences between an interface and an abstract class: interface methods have no body. suppose we have another class MyInteger which contains methods that compares a MyInteger object to objects of the same class. Object b). 11. both of the classes have some similar methods which compares them from other objects of the same type. we can use an interface for this. but present in other object-oriented languages like C++.I 11. Now. since program may call an interface method and the proper version of that method will be executed depending on the type of object passed to the interface method call. Finally. Interfaces define a standard and public way of specifying the behavior of classes. 11. public interface Relation { public boolean isGreater( Object a. but they are not related whatsoever.J.2 Interface vs. we can actually capture similarities among unrelated classes without artificially forcing a class relationship. As we can see later on the section Interface vs. to implement common behaviors. public boolean isLess( Object a. Classes.5. As we can see here. they are defined independently. an interface can only define constants and an interface have no direct inherited relationship with any particular class. Multiple inheritance is not present in Java. Object b). we can actually use an interface as data type.E. Object b). let's say interface Relation which has some comparison method declarations. Interfaces define the signatures of a set of methods without the body. They allow classes. regardless of their location in the class hierarchy.5. We can create an interface class.5 Interfaces An interface is a special kind of block containing method signatures (and possibly constants) only. public boolean isEqual( Object a. Introduction to Programming I 181 . Thru interfaces. } Another reason for using an object's programming interface is to reveal an object's programming interface without revealing its class. In order to enforce a way to make sure that these two classes implement some methods with similar signatures.1 Why do we use Interfaces? We need to use interfaces if we want unrelated classes to implement similar methods.D. Our interface Relation can be declared as. Note that interfaces exhibit polymorphism as well.
/** * This class defines a line segment */ public class Line implements Relation { private double x1. Object b).5. Object b).x2 = x2. we write. } public double getLength(){ Introduction to Programming I 182 . you cannot create an instance from an interface. this. given a class Person and an interface PersonInterface.4 Creating Interfaces To create an interface.3 Interface vs. This means that an interface can be used in places where a class can be used. let's create an interface that defines relationships between two objects according to the "natural order" of the objects. the following declarations are valid: PersonInterface Person pi = new Person(). this. For example. public boolean isLess( Object a.x1 = x1. this. double y1. private double y2. we use the implements keyword.y1 = y1. public boolean isEqual( Object a. private double y1.5. public interface Relation { public boolean isGreater( Object a. public interface [InterfaceName] { //some methods without the body } As an example. Object b). double x2. For example.y2 = y2. //COMPILE //ERROR!!! Another common characteristic is that both interface and class can define methods.J.I 11. double y2){ this. An example of this is: PersonInterface pi = new PersonInterface(). Class One common characteristic of an interface and class is that they are both types.E. an interface does not have an implementation code while the class have one. } Now. private double x2.D. However. to use the interface. public Line(double x1. However. pc = new Person(). 11.
java.J.sqrt((x2-x1)*(x2-x1) + (y2-y1)* (y2-y1)). return (aLen < bLen).lang. Once a set of standard method definition is created.getLength().I } double length = Math.getLength(). Object b){ double aLen = ((Line)a). public boolean isGreater( Object a.getLength().getLength(). double bLen = ((Line)b). } } When your class tries to implement an interface. double bLen = ((Line)b). Object b){ double aLen = ((Line)a).Object) in Relation public class Line implements Relation ^ 1 error Coding Guidelines: Use interfaces to create the same standard method definitions in may different classes.getLength().D. Line. you can write a single method to manipulate all of the classes that implement the interface. Object b){ double aLen = ((Line)a).java:4: Line is not abstract and does not override abstract method isGreater(java. return length.lang. you would encounter this error. Introduction to Programming I 183 . } public boolean isEqual( Object a. double bLen = ((Line)b). return (aLen == bLen). return (aLen > bLen). always make sure that you implement all the methods of that interface.Object. or else. } public boolean isLess( Object a.E.getLength().
} public interface StudentInterface extends PersonInterface { .I 11.5. 11.J. . If StudentInterface extends PersonInterface.5. For example. public class Person implements PersonInterface. . Another thing to note about the relationship of interfaces to classes is that. it will inherit all of the method declarations in PersonInterface. Unrelated classes can implement the same interface. suppose we have two interfaces StudentInterface and PersonInterface.E. LivingThing. LivingThing { } //some code here Take note that an interface is not part of the class inheritance hierarchy. However. public class ComputerScienceStudent extends Student implements PersonInterface. interfaces can have inheritance relationship among themselves. a class can implement an interface as long as it provides the implementation code for all the methods defined in the interface. } Introduction to Programming I 184 . WhateverInterface { } //some code here Another example of a class that extends one super class and implements an interface is. a class can only EXTEND ONE super class.D. public interface PersonInterface { . .5 Relationship of an Interface to a Class As we have seen in the previous section. An example of a class that implements many interfaces is. but it can IMPLEMENT MANY interfaces. .6 Inheritance among Interfaces Interfaces are not part of the class hierarchy.
Your task is to extend the StudentRecord class that was implemented in the previous lessons. Introduction to Programming I 185 . Add some attributes and methods that you think are needed for a Computer Science student record.2 The Shape abstract class Try to create an abstract class called Shape with abstract methods getArea() and getName().6.E. we want to create a more specialized student record that contains additional information about a Computer Science student. 11.1 Extending StudentRecord In this exercise.J.6 Exercises 11.D. Write two of its subclasses Circle and Square. You can add additional methods to its subclasses if you want to.6.I 11. Try to override some existing methods in the superclass StudentRecord. if you really need to.
I 12 Basic Exception Handling 12. .2 What are Exceptions? An exception is an event that interrupts the normal processing flow of a program. the student should be able to: • • Define exceptions Handle exceptions using a simple try-catch-finally block 12.parseInt method. which occurs if we try to access a non-existent array element. . we are going to study a technique used in Java to handle unusual conditions that interrupt the normal operation of the program. This event is usually some error of some sort. try{ //write the statements that can generate an exception //in this block } catch( <exceptionType1> <varName1> ){ } .D. The general form of a try-catch-finally block is.1 Objectives In this section.J.3 Handling Exceptions To handle exceptions in Java. Some examples of exceptions that you might have encountered in our previous exercises are: ArrayIndexOutOfBounds exceptions. we use a try-catch-finally block.E. 12. This causes our program to terminate abnormally. which occurs when we try to pass as a parameter a non-number in the Integer. or maybe a NumberFormatException. catch( <exceptionTypen> <varNamen> ){ //write the action your program will do if an //exception of a certain type occurs } finally{ //add more cleanup code here } //write the action your program will do if an exception //of a certain type occurs Introduction to Programming I 186 . At the end of the lesson. What we do in our programs is that we place the statements that can possibly generate an exception inside this block. This technique is called exception handling.
I Exceptions thrown during execution of the try block can be caught and handled in a catch block. Figure 12. • Each catch block defines an exception handle. or both.java:5) Introduction to Programming I 187 . The following are the key aspects about the syntax of the try-catch-finally construct: • The block notation is mandatory.lang.J.E.D. • A try block must be followed by at least one catch block OR one finally block. • The catch blocks and finally blocks must always appear in conjunction with the try block. which is the exception its block is willing to handle. The code in the finally block is always executed. and in the above order. • For each try block.ArrayIndexOutOfBoundsException: 1 at ExceptionExample. The exception must be of the Throwable class or one of its subclasses. there is no checking inside your code for the number of arguments and we just access the second argument args[1] right away. Suppose. The header of the catch block takes exactly one argument. we'll get the following exception. but only one finally block. there can be one or more catch blocks.main(ExceptionExample.1: Flow of events in a try-catch-finally block Let's take for example a code that prints the second argument when we try to run the code using command-line arguments. Exception in thread "main" java.
} } try{ } So when we try to run the program again without arguments. public class ExceptionExample { public static void main( String[] args ){ System.E.println("Exception caught!"). }catch( ArrayIndexOutOfBoundsException exp ){ System. we won't use the finally block.println( args[1] ).I To prevent this from happening.D. we can place the code inside a try-catch block. The finally block is just optional.out. the output would be.out.J. For this example. Exception caught! Introduction to Programming I 188 .
i++ ){ System. } } } Compile and run the TestExceptions program. they simply halted the execution of your code.lang.out.ArrayIndexOutOfBoundsException: 3 at TestExceptions.1 Catching Exceptions1 Given the following code: public class TestExceptions{ public static void main( String[] args ){ for( int i=0. The output of the program after catching the exception should look like this: javac TestExceptions one two three args[0]=one args[1]=two args[2]=three Exception caught: java. The output should look like this: javac TestExceptions one two three args[0]=one args[1]=two args[2]=three Exception in thread "main" java.ArrayIndexOutOfBoundsException: 3 Quiting..4 Exercises 12.J.java:4) Modify the TestExceptions program to handle the exception.E.2 Catching Exceptions 2 Chances are very good that some programs you've written before have encountered exceptions.I 12.4.4. Introduction to Programming I 189 . Go back to those programs and implement exception handling. Since you didn't catch the exceptions.println("args["+i+"]="+ args[i]).lang. 12.. true.D.main(1.
J.I Appendix A : Java and NetBeans Installation In this section. copy the installers in your hard disk first. For Windows: Just copy the installers in any temporary directory. If you are not provided with the Java 5. we will discuss on how to install Java and NetBeans in your system (Ubuntu Dapper/Windows XP).com/) for Java and http://www.NetBeans.org/downloads/ for NetBeans). Before starting with the installation.5 installers by your instructor.sun.D. For Ubuntu Dapper: Copy all the installers inside the /usr folder. you can download a copy of the installers from the Sun Microsystems website (http://java.E. Introduction to Programming I 190 .0 and NetBeans 5.
click on the Permissions tab. and then Click on the execute box.E.I Installing Java in Ubuntu Dapper Step 1: Go to the folder where you have your installers Step 2: Before running your installer. right click on the installer. To do this. make sure it is executable. Close the window.J.D. Introduction to Programming I 191 .
After pressing ENTER.bin.I Step 3: Double click on the file jdk-1_5_0_06-linux-i586. A dialog box will display.D.E. you will see the license agreement displayed on the console.J. Introduction to Programming I 192 . click on the button 'Run In Terminal'.
Introduction to Programming I 193 . Just wait for the installer to finish unpacking all its contents and installing Java.I Just press enter. Just type: yes and press ENTER.J.D.E. until you see the question: Do you agree to the above license terms? [yes or no].
D. go to the directory:/usr/local/bin.I Step 4: Creating symbolic links In order to run java commands anywhere. Type: cd /usr/local/bin Introduction to Programming I 194 .E. To do this.J. we need to create symbolic links for all the commands in JDK inside the /usr/local/bin directory.
type: sudo ln -s /usr/java/jdk1.I To make the symbolic links to the commands.J.E.D. Introduction to Programming I 195 .5.0_01/bin/* .
I Installing Java in Windows Step 1: Using Windows Explorer.D.J. go to the folder where your Java installer is located Figure 12.2: Folder containing installers Introduction to Programming I 196 .E.
D.J.E.I Introduction to Programming I 197 .
E.D. Figure 12. A J2SE installer dialog will then appear. just double-click on the installer icon.I Step 2: Run the installer To run the installer.J. Click on the radio button labeled "I accept the terms in the license agreement" and press NEXT.3: License agreement Introduction to Programming I 198 .
4: Custom setup Click on FINISH to complete installation.J. Figure 12.5: Finish installation Introduction to Programming I 199 .E.I Click on NEXT to continue installation. Figure 12.D.
D.I Installing NetBeans in Ubuntu Dapper Step 1: Go to the folder where you have your installers Introduction to Programming I 200 .E.J.
I Step 2: Before running your installer.D. Click on Run in Terminal. Introduction to Programming I 201 . and then Click on the execute box. click on the Permissions tab. make sure it is executable.J.bin. Close the window. right click on the installer. Step 3: Double click on the file netbeans-5_5-beta-linux.E. To do this.
Introduction to Programming I 202 .5 dialog will then appear.I A Netbeans 5.J. Click on NEXT.E.D.
I Click on the radio button that says "I accept the terms in the license agreement".E.D. then click on NEXT. Introduction to Programming I 203 . And then click on NEXT. change it to: /usr/java/netbeans-4.0.J. For the directory name.
The next dialog just shows information about NetBeans thatyou will install.0_01.I For the JDK directory. choose /usr/java/jdk1. and then click on NEXT.D.5. Just click again on NEXT. Introduction to Programming I 204 .E.J.
Introduction to Programming I 205 .J. Click on FINISH to complete the installation.D. just wait for NetBeans to finish its installation.E.I Now.
D.I Introduction to Programming I 206 .E.J.
To do this. you can run NetBeans in any directory by typing: netbeans & Introduction to Programming I 207 . Type: cd /usr/local/bin Make a symbolic link to the NetBeans executable by typing: sudo ln -s /home/florence/netbeans/bin/netbeans . go first to the directory:/usr/local/bin.E.D.I Step 4: Creating symbolic links In order to run NetBeans anywhere. we need to create symbolic link for it. Now.J.
6: NetBeans installer file Introduction to Programming I 208 .D.J. go to the folder where your NetBeans installer is located Figure 12.I Installing NetBeans in Windows Step 1: Using Windows Explorer.E.
just double-click on the installer icon.7: NetBeans installation Introduction to Programming I 209 .D. Figure 12.J.E. After clicking on the netbeans5_5-beta-bin-windows icon. the NetBeans installation wizard will appear.I Step 2: Run the installer To run the installer. Click on NEXT to enter installation process.
I The agreement page will the appear.D.8: License Agreement Introduction to Programming I 210 . Figure 12.J. Choose to ACCEPT and click NEXT to continue.E.
I Then you will be given the choice on which directory to place the NetBeans.J.E. You can move on by clicking NEXT or you can click on BROWSE to choose a different directory.D.9: Choose directory where to install NetBeans Introduction to Programming I 211 . Figure 12.
E.J. Click on NEXT to continue.D.I Next is choosing the Standard Edition JDKs from your machine.0_01 chould appear from your choices. the jdk1.10: Choose JDK to use Introduction to Programming I 212 . Figure 12.5. If you have finished installing Java.
I It will then inform you the location and size of NetBeans which will be installed to your machine.E. Click on NEXT to finish installation.11: Installation Summary Introduction to Programming I 213 . Figure 12.J.D.
Click on FINISH to complete installation.E.D. Figure 12.12: Successful installation Introduction to Programming I 214 .J.I You have installed NetBeans on your computer.
we will be discussing on how to write. a text or code editor. The second one is by using NetBeans which is an Integrated Development Environment or IDE. let's first try to write this program in a file and try to run it. Before going into details.I Appendix B: Getting to know your Programming Environment (Windows XP version) In this section.J.out.println("Hello world!"). a compiler and/or interpreter and a debugger. There are two ways of doing this. let us first take a look at the first Java program you will be writing. My First Java Program public class Hello { /** * My first java program */ public static void main(String[] args) { //prints the string "Hello world" on screen System.D. the first one is by using a console and a text editor. Introduction to Programming I 215 . } } Before we try to explain what the program means. An IDE is a programming environment integrated into a software application that provides a GUI builder.E. compile and run Java programs.
You can use other text editors if you want to. Figure 12. Step 1: Start Notepad To start Notepad in Windows.D. click on start-> All Programs-> Accessories-> Notepad. we will be using the text editor "Notepad"(for Windows) to edit the Java source code. You will also need to open the MS-DOS prompt window to compile and execute your Java programs.J.13: Click on start-> All Programs-> Accessories -> Notepad Introduction to Programming I 216 .I Using a Text Editor and Console For this example.14: Notepad Application Figure 12.E.
J.E.16: MSDOS Command Prompt Introduction to Programming I 217 . Figure 12. click on start-> All programs-> Accessories-> Command Prompt.D.15: start-> All programs-> Accessories -> Command Prompt Figure 12.I Step 2: Open the Command Prompt window To open the MSDOS command prompt in Windows.
and we will be saving it inside a folder named MYJAVAPROGRAMS.J.D. click on the File menu found on the menubar and then click on Save.I Step 3: Write your the source code of your Java program in Notepad Step 4: Save your Java Program We will save our program on a file named "Hello. Introduction to Programming I 218 .E. To open the Save dialog box.java".
E.D.J. Figure 12.I After doing the procedure described above.17: This Dialog appears after clicking on File -> Save Click on the MY DOCUMENTS button to open the My Documents folder where we will be saving all your Java programs. a dialog box will appear as shown in Figure below. Introduction to Programming I 219 .
E. we'll create a new folder inside the My Documents folder where we will save your programs.18: Click on the button encircled. Figure 12.D. This will open your "My Documents" folder Figure 12.19: Clicking on the encircled button will create a New Folder. Click on the button encircled in the figure below to create the folder. Introduction to Programming I 220 .J. We shall name this folder MYJAVAPROGRAMS.I Now.
I After the folder is created. you can type in the desired name for this folder. In this case.J. Introduction to Programming I 221 . and then press ENTER.D. type in MYJAVAPROGRAMS.E.
Introduction to Programming I 222 .E.D. double click on that folder to open it.I Now that we've created the folder where we will save all the files.J. The folder should be empty for now since it's a newly created folder and we haven't saved anything in it yet. You will see a similar figure as shown below.
which is "Hello. type in the filename of your program.J. Now.E.I Now click on the drop down list box "Save as type". Click on the "All Files" option. so that we can choose what kind of file we want to save.D. and then click on the SAVE button. in the Filename textbox. Introduction to Programming I 223 .java".
D. notice how the title of the frame changes from UntitledNotepad to Hello. you can just edit it. and then save it again by clicking on File -> Save.E. Introduction to Programming I 224 .java-Notepad. Take note that if you want to make changes in your file.I Now that you've saved your file.J.
the next step is to compile your program. Figure 12. type DIR or dir and then press ENTER.E. it opens up and takes you directly to what is called your home folder. Now let's go inside that directory. you can see here that there is a folder named "My Documents" where we created your MYJAVAPROGRAMS folder. Go to the MSDOS command prompt window we just opened a while ago.I Step 5: Compiling your program Now.J. when you open the command prompt window. Typically. Introduction to Programming I 225 . To see what is inside that home folder. What you will see is a list of files and folders inside your home folder.20: List of files and folders shown after executing the command DIR. Now.D.
and tell me what you see. since the name of our directory is My Documents. try typing in the "dir" command again.E. In this case. The "cd" command stands for. Figure 12.22: The contents of My Documents Now perform the same steps described before to go inside the MYJAVAPROGRAMS folder. you type in: cd My Documents.J. change directory.I To go inside a directory. Figure 12. you type in the command: cd [directory name].21: Inside the My Documents folder Now that you are inside the "My Documents" folder. Introduction to Programming I 226 .D.
or in this case.I Once inside the folder where your Java programs are. Hello. type in: javac Hello.class. To compile a Java program. let us now start compiling your Java program. Introduction to Programming I 227 . So in this case. you should make sure that the file is inside the folder where you are in.java.24: Compile program by usingthe javac command During compilation.J. In order to do that.D. which is the actual bytecode.23: Inside the MYJAVAPROGRAMS folder Figure 12. execute the dir command again to see if your file is inside that folder. Take note that.E.class. Figure 12. we type in the command: javac [filename]. javac adds a file to the disk called [filename].
Figure 12. we are now ready to run your program.E.I Step 6: Running the Program Now. so in the case of our example. type in the command: java [filename without the extension].J. "Hello world!". type in: java Hello You can see on the screen that you have just run your first Java program that prints the message. assuming that there are no problems during compilation (we'll explore more of the problems encountered during compilation in the next section).D.25: Output of the program Introduction to Programming I 228 . To run your Java program.
2_04\bin folder. try setting the PATH variable to point to where the Java commands are installed.D. To do this. After doing this.27: Setting the path and running java Introduction to Programming I 229 .2_04\bin. operable program or batch file. This will tell your system to look for the commands in the C:\j2sdk1. you can now use the Java commands.4. you encounter the message: 'javac' is not recognized as an internal or external command. type in the command: set PATH=C:\j2sdk1. which is usually the default location wherein your Java files are placed during installation. This means that either you haven't installed Java in your system yet.26: System did not recognize the javac command If you are sure that you've already installed Java in your system.J.E. Figure 12.I Setting the Path Sometimes. when you try to invoke the javac or java command.4. Figure 12. or you have to configure the path on where the Java commands are installed so that your system will know where to find them.
In this part of the lesson.J.I Using NetBeans Now that we've tried doing our programs the complicated way.D. a compiler and/or interpreter and a debugger. click on start-> All Programs-> NetBeans 5. An IDE is a programming environment integrated into a software application that provides a GUI builder.E.5 Beta -> NetBeans IDE Introduction to Programming I 230 . which is an Integrated Development Environment or IDE. let's now see how to do all the processes we've described in the previous sections by using just one application. we will be using NetBeans. Step 1: Run NetBeans To run NetBeans. a text or code editor.
28: NetBeans IDE Introduction to Programming I 231 .J. you will see a graphical user interface (GUI) similar to what is shown below.D.I After you've open NetBeans IDE.E. Figure 12.
a New Project dialog will appear. Click on File-> New Project.D.E.I Step 2: Make a project Now. Introduction to Programming I 232 . let's first make a project. After doing this.J.
J.D.I Now click on Java Application and click on the NEXT button.E. Introduction to Programming I 233 .
E.I Now. a New Application dialog will appear.29: Change Project Name Introduction to Programming I 234 . Edit the Project Name part and type in "HelloApplication".D. Figure 12.J.
J. Finally. on the Create Main Class textfield. by clicking on the BROWSE button.E. type in Hello as the main class' name.I Now try to change the Application Location. and then click on the FINISH button. Follow the steps described in the previous section to go to your MYJAVAPROGRAMS folder.D. Introduction to Programming I 235 .
NetBeans automatically creates the basic code for your Java program. You can just add your own statements to the generated code.println("Hello world!"). try to modify the code generated by NetBeans. As shown below. Ignore the other parts of the program for now. let us first describe the main window after creating the project.E.J. Now. after the statement. Insert the code: System. Introduction to Programming I 236 . where you set the Project location.I Step 3: Type in your program Before typing in your program. This can all be found in your MYJAVAPROGRAMS folder. //TODO code application logic here. as we will explain the details of the code later. you can see a list of folders and files that NetBeans generated after creating the project. On the left side of the window.out.D.
30: Shortcut button to compile code Introduction to Programming I 237 . you could also use the shortcut button to compile your code. Figure 12. to compile your program.J.D.I Step 4: Compile your program Now. just click on Build -> Build Main Project.E. Or.
I If there are no errors in your program.J. Figure 12.31: Output window just below the window where you type your source code Introduction to Programming I 238 . you will see a build successful message on the output window.E.D.
click on Run-> Run Main Project. Figure 12.E.java Introduction to Programming I 239 .D. Figure 12. Or you could also use the shortcut button to run your program.J.33: Output of Hello.I Step 5: Run your program To run your program.32: Shortcut button to run program The output of your program is displayed in the output window.
D. the user should have a choice. the user should also have an option to search entries by name or by telephone numbers. MAIN MENU 1 .increasing order of telephone numbers 4 .J. delete entries. In viewing all entries. wherein you can add entries in the phonebook.by telephone number 5 – Quit The following will appear when one of the choices in the main menu is chosen. Add phonebook entry Enter Name: Enter Telephone number: (* if entry already exists. the user should also have an option if he/she wants to search by first name or last name.View all entries a . In searching by name.E.Delete phonebook entry 3 . view all entries and search for entries. warn user about this) View all entries Displays all entries in alphabetical order Displays all entries in increasing order of telephone #s Search entries Search phonebook entry by name Search phonebook entry by telephone number Quit close phonebook Introduction to Programming I 240 .I Appendix D : Machine Problems Machine Problem 1: Phone Book Write a program that will create an phonebook.by name b .alphabetical order b .Search entries a . In searching for entries.Add phonebook entry 2 . whether to view the entries in alphabetical order or in increasing order of telephone numbers.
0. For example. If the user chooses a cell that does not contain a bomb. given the array: int bombList5by5[][]={{0. 1. a number appears at that location indicating the number of neighbors that contain bombs. {0. 0. 0. The game should end when all the cells that do not contain bombs have been marked (player wins) or when the user steps on a bomb(player loses). and a 0 if none. If the user chooses a cell that contains a bomb. {0. (2. {0. we have 6 bombs on our list. (3.col) cells. First. 1. given the bombList5by5.2).J. 0. Welcome to Minesweeper! Choose size of grid (Press 1 for 5x5. Press 2 for 10x10): 1 [][][][][] [][][][][] [][][][][] [][][][][] [][][][][] Enter row and column of the cell you want to open[row col]: 1 1 [][][][][] [ ] [2] [ ] [ ] [ ] [ ] [ ] [] [ ] [ ] [][][][][] [][][][][] Enter row and column of the cell you want to open[row col]: 3 2 [][][][][] [ ] [2 ] [ ] [] [ ] [][][][][] [ ] [ ] [4 ] [ ] [ ] [][][][][] Enter row and column of the cell you want to open[row col]: 0 2 [] [ ] [ ] [ ] [ ] [ ] [2] [ ] [] [ ] [ ] [X ] [ ] [ ] [ ] [ ] [ ] [4] [ ] [ ] [][][][][] Ooppps! You stepped on a bomb. 0}. 0. 0}. 0. Here's a sample output of the game. the game ends and all the bombs are displayed. 0. (4. You have two 2-dimensional arrays that contains information about your grid. 1. 0}}.1). 1.3). Given the bomb list. {0. An entry in the array should either contain a 0 or 1.D.1) and (4. (0. Sorry. 0. A 1 signifies that there is a bomb in that location. 0.4). 0.I Machine Problem 2: Minesweeper This is a one player game of a simplified version of the popular computer game minesweeper.E.2). The bombs are located in (row. 0}. 1}. (3. 1. the user is asked if he or she wants to play on a 5x5 grid or 10x10 grid. game over! Introduction to Programming I 241 .
Your program should output the following menu on screen.D.I Machine Problem 3: Number Conversion Create your own scientific calculator that will convert the inputted numbers to the four number representations ( Decimal. MAIN MENU: Please type the number of your choice: 1 – Binary to Decimal 2 – Decimal to Octal 3 – Octal to Hexadecimal 4 – Hexadecimal to Binary 5 – Quit The following will appear when one of the choices in the main menu is chosen.J. as long as the program outputs the correct conversion of numbers. Hexadecimal ). Octal.E. Introduction to Programming I 242 . Choice 1: Enter a binary number: 11000 11000 base 2 = 24 base 10 (goes back to main menu) Choice 2: Enter a Decimal number: 24 24 base 10 = 30 base 8 (goes back to main menu) Choice 3: Enter an Octal number: 30 30 base 8 = 18 base 16 (goes back to main menu) Choice 4: Enter a Hexadecimal number: 18 18 base 16 = 11000 base 2 Choice 1: Enter a binary number: 110A Invalid binary number! Enter a binary number: 1 1 base 2 = 1 base 10 (goes back to main menu) (user chooses 5) Goodbye! You can be more creative with your user interface if you want to. Binary.
Programming Language.javaworld.com at http://www. From Webopedia at http://www.geocities.Inheritance and Polymorphism. High-Level Programming Language. From Answers.php/983081 22.pattonpatton.E.com/TERM/I/integrated_development_environment.Does Java pass by reference or pass by value? Why can't you swap in Java? Available at http://www. Joy L. Thomas J. Available at http://www. Java Programming Complete Concepts and Techniques. Programming Language.html 3.ca/~ve3ll/jatutor7.D.org/wiki/High-level_programming_language 5. 2004 24.com/tech/article. Writing Abstract Classes and Methods.wikipedia.com/basic_flow_chart_symbols. Course Technology Thomson Learning.com/javaworld/javaqa/2000-05/03-qa-0526-pass.Sun Java Programming Student Guide SL-275. 28.com/SiliconValley/Park/3230/java/javl1002.html. Chapman. Defining Flowchart Symbols. Java How to Program 5th Edition. 25.html 7. Shelly. February 2001.developer.sun.ca/~ve3ll/jatutor4.Gary B.wikipedia. Cashman.cogeco.I References 1.htm.J. Java for Engineers and Scientists 2nd Edition. Available at http://java.Java Branching Statements. Sun Microsystems. Pearson Prentice Hall. 2001.com/docs/books/tutorial/java/javaOO/abstract.html 20. 23. From Webopedia at http://www.sun. Programming Language. 26.htm 21.Stephen J.htm 6.html 8. Available at http://www. Variables and Expressions. Integrated Development Environment.sun. Available at http://home. Defining an Interface. From Wikipedia at http://en.answers.html 27.com/topic/programming-language 4.cogeco. Available at http://www.com/docs/books/tutorial/java/interpack/interfaceDef.com/docs/books/tutorial/java/nutsandbolts/branch.webopedia. Available at http://java.com/TERM/p/programming_language.html 9. Available at http://home. Runtime Polymorphism through Inheritance.webopedia. Starks.Encapsulation. From Wikipedia at http://en. Available at http://java.The Essence of OOP using Java. Introduction to Programming I 243 .org/wiki/Programming_language 2.Deitel & Deitel.
This action might not be possible to undo. Are you sure you want to continue?
We've moved you to where you read on your other device.
Get the full title to continue reading from where you left off, or restart the preview.