Gang of Four Design Patterns (2)

The Gang of Four are the four authors of the book, "Design Patterns: Elements of Reusable Object-Oriented Software". In this article their twenty-three design patterns are described with links to UML diagrams, source code and real-world examples for each.

Behavioural Patterns

The final type of design pattern is the behavioural pattern. Behavioural patterns define manners of communication between classes and objects.

• Chain of Responsibility. The chain of responsibility pattern is used to process varied requests, each of which may be dealt with by a different handler.
• Command. The command pattern is used to express a request, including the call to be made and all of its required parameters, in a command object. The command may then be executed immediately or held for later use.
• Interpreter. The interpreter pattern is used to define the grammar for instructions that form part of a language or notation, whilst allowing the grammar to be easily extended.
• Iterator. The iterator pattern is used to provide a standard interface for traversing a collection of items in an aggregate object without the need to understand its underlying structure.
• Mediator. The mediator pattern is used to reduce coupling between classes that communicate with each other. Instead of classes communicating directly, and thus requiring knowledge of their implementation, the classes send messages via a mediator object.
• Memento. The memento pattern is used to capture the current state of an object and store it in such a manner that it can be restored at a later time without breaking the rules of encapsulation.
• Observer. The observer pattern is used to allow an object to publish changes to its state. Other objects subscribe to be immediately notified of any changes.
• State. The state pattern is used to alter the behaviour of an object as its internal state changes. The pattern allows the class for an object to apparently change at run-time.
• Strategy. The strategy pattern is used to create an interchangeable family of algorithms from which the required process is chosen at run-time.
• Template Method. The template method pattern is used to define the basic steps of an algorithm and allow the implementation of the individual steps to be changed.
• Visitor. The visitor pattern is used to separate a relatively complex set of structured data classes from the functionality that may be performed upon the data that they hold.