W. Kandinsky, Improvisation VI(African), 1909

Design Patterns : Elements of Reusable Object - Oriented Software

by Erich Gamma, Richard Helm, Ralh Johnson, John Vlissides

QA76.64.D47 1994

What is a design pattern?

Each pattern describes a problem which occurs over and over again in our enviroment, and then describes the core of the solution to that problem, in such a way that yhou can use this soution a million times over, without ever doing it the same way twice.

In general, a pattern has four essential elements :

1. The pattern name is handle we can use to describe a design problem, its solutions and consequences in a word or two.
2. The problem describes when to apply the pattern. It explains the problem and its context.
3. The solution describes the elements that make up the design, their relationships, responsibilities, and collaborations.
4. The consequences are the results and trade-offs of applying the pattern.

The Catalog of Design Patterns

• Abstract Factory Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
• Adapter Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
  Bridge Decouple an abstraction from its implementation so that the two can vary independently.

  Builder Separate the construction of a complex object from its representation so that the same construction process can create different representations.

  Chain of Responsibility Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

  Command Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

  Composite Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

  Decorator Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

  Facade Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

  Factory Method Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclass.

  Flyweight Use sharing to support large numbers of fine-grained objects efficiently.

  Interpreter Given a language, define a represention for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

  Iterator Provide a way to access the elements of an aggregate object sequentially withot exposing its underlying representation.

  Mediator Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

  Memento Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

  Observer Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

  Prototype Specify the kinds of objects to create using a prototypical instance, and create new objects by coping this prototype.

  Proxy Provide a surrogate or placeholder for another object to control access to it.

  Singleton Ensure a class only has one instance, and provide a global point of access to it.

  State Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

  Strategy Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independenly from clients that use it.

  Template Method Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorighms without changing the algorithm's structure.

  Visitor Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.