coupling between objects, which can often be desirable for maximum maintainability in a design. The authors employ the term 'toolkit' where others might today use 'class library', as in C# or Java. In their parlance, toolkits are the object-oriented equivalent of subroutine libraries, whereas a 'framework' is a set of cooperating classes that make up a reusable design for a specific class of software. They state that applications are hard to design, toolkits are harder, and frameworks are the hardest to design. Patterns by type Creational Creational patterns are ones that create objects, rather than having to instantiate objects directly. This gives the program more flexibility in deciding which objects need to be created for a given case. Abstract factory groups object factories that have a common theme. Builder constructs complex objects by separating construction and representation. Factory method creates objects without specifying the exact class to create. Prototype creates objects by cloning an existing object. Singleton restricts object creation for a class to only one instance. Structural These concern class and object composition. They use inheritance to compose interfaces and define ways to compose objects to obtain new functionality. Adapter allows classes with incompatible interfaces to work together by wrapping its own interface around that of an already existing class. Bridge decouples an abstraction from its implementation so that the two can vary independently. Composite composes zero-or-more similar objects so that they can be manipulated as one object. Decorator dynamically adds/overrides behaviour in an existing method of an object. Facade provides a simplified interface to a large body of code. Flyweight reduces the cost of creating and manipulating a large number of similar objects. Proxy provides a placeholder for another object to control access, reduce cost, and reduce complexity. Behavioral Most of these design patterns are specifically concerned with communication between objects. Chain of responsibility delegates commands to a chain of processing objects. Command creates objects that encapsulate actions and parameters. Interpreter implements a specialized language. Iterator accesses the elements of an object sequentially without exposing its underlying representation. Mediator allows loose coupling between classes by being the only class that has detailed knowledge of their methods. Memento provides the ability to restore an object to its previous state (undo). Observer is a publish/subscribe pattern, which allows a number of observer objects to see an event. State allows an object to alter its behavior when its internal state changes.