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GOF Design Patterns

posted 22 Nov 2010, 13:00 by Sanjeev Kumar   [ updated 30 Dec 2010, 13:56 by Sanjeev Kumar ]

In software engineering, a design pattern is a general reusable solution to a commonly occurring problem in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that can be used in many different situations. Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved.Design Patterns are broadly classified into three categories.

Creational patterns

These patterns have to do with class instantiation. They can be further divided into class-creation patterns and object-creational patterns. While class-creation patterns use inheritance effectively in the instantiation process, object-creation patterns use delegation to get the job done.
  • 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 patterns

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 patterns

Most of these design patterns are specifically concerned with communication between objects.
  • Command creates objects which encapsulate actions and parameters.
  • 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.
  • Strategy allows one of a family of algorithms to be selected on-the-fly at runtime.
  • Template method defines the skeleton of an algorithm as an abstract class, allowing its subclasses to provide concrete behavior.
  • Visitor separates an algorithm from an object structure by moving the hierarchy of methods into one object.
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