Gang of Four/Design Patterns

If you're into software engineering, especially reusable object-oriented software engineering, you're a big fan of the "Gang of Four" book, "Design Patterns." Here's a quick outline of what's in it:

Creational Patterns

• Abstract Factory: Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
  
• Builder: Separate the construction of a complex object from its representation so that the same construction process can create different representations.
  
• 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 subclasses.
  
• Prototype: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
  
• Singleton: Ensure a class only has one instance, and provide a global point of access to it.
  

Structural Patterns

• 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.
  
• 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.
  
• Flyweight: Use sharing to support large numbers of fine-grained objects efficiently.
  
• Proxy: Provide a surrogate or placeholder for another object to control access to it.
  

Behavioral Patterns

• 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 parametrize clients with different requests, queue or log requests, and support undoable operations.
  
• Interpreter: Given a language, define a representation 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 without 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.
  
• 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 independently 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 algorithm 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.

There's also thourough Wikipedia coverage of the topic: Design Patterns