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Gang of Four Design Patterns Cheat Sheet

The 23 Gang of Four Design Patterns (Cheat Sheet)

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 incompatibility 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 processes can create different representations.

Chain of Responsibility

  • Avoid coupling the sender of a request to its receiver by giving more then 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.

Façade

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

Factory Method

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

Flyweight

  • Use sharing to support large numbers of fine-grained objects efficiently. A flyweight is a shared object that can be used in multiple contexts simultaneously. The flyweight acts as an independent object in each context; it’s indistinguishable from an instance of the object that’s not shared.

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 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 copying 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

  • Defines a family of algorithms, encapsulates each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients who use it.

Template

  • Define a 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 algorithms 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.
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