Cheat Sheet
The 23 Gang of Four Design Patterns
Here are the original 23 Gang of Four design patterns (from Design Patterns: Elements of Reusable Object-Oriented Software, Addison-Wesley Professional Computing Series, by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides).
1. The Strategy design pattern: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
2. The Decorator design pattern: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
3. The Factory Method design pattern: Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
4. The Observer design pattern: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
5. The Chain of Responsibility design pattern: 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.
6. The Singleton design pattern: Ensure a class only has one instance, and provide a global point of access to it.
7. The Flyweight design pattern: 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.
8. The Adapter design pattern: Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.
9. The Facade design pattern: Provide a unified interface to a set of interfaces in a system. Facade defines a higher-level interface that makes the subsystem easier to use.
10. The Template design patter: 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.
11. The Builder design pattern: Separate the construction of a complex object from its representation so that the same construction processes can create different representations.
12. The Iterator design pattern: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
13. The Composite design pattern: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
14. The Command design pattern: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
15. The Mediator design pattern: 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.
16. The State design pattern: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
17. The Proxy design pattern: Provide a surrogate or placeholder for another object to control access to it.
18. The Abstract Factory design pattern: Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
19. The Prototype design pattern: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
20. The Bridge design pattern: Decouple an abstraction from its implementation so that the two can vary independently.
21. The Interpreter design pattern: Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
22. The Memento design pattern: Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.
23. The Visitor design pattern: 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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