Gang of Four Design Patterns

The 23 design patterns catalogued by Gamma, Helm, Johnson and Vlissides in Design Patterns: Elements of Reusable Object-Oriented Software (1994).

Creational

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1. Abstract Factory — provide an interface for creating families of related objects without specifying their concrete classes.
2. Builder — separate the construction of a complex object from its representation, so the same process can create different representations.
3. Factory Method — define an interface for creating an object, but let subclasses decide which class to instantiate.
4. Prototype — create new objects by cloning an existing instance.
5. Singleton — ensure a class has only one instance and provide a global point of access to it.

Structural

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6. Adapter — convert the interface of a class into another interface clients expect.
7. Bridge — decouple an abstraction from its implementation so the two can vary independently.
8. Composite — compose objects into tree structures to represent part-whole hierarchies, letting clients treat individual objects and compositions uniformly.
9. Decorator — attach additional responsibilities to an object dynamically, as an alternative to subclassing.
10. Facade — provide a simplified interface to a complex subsystem.
11. Flyweight — use sharing to support large numbers of fine-grained objects efficiently.
12. Proxy — provide a surrogate or placeholder for another object to control access to it.

Behavioral

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13. Chain of Responsibility — pass a request along a chain of handlers until one processes it.
14. Command — encapsulate a request as an object, allowing parameterization, queuing, and undo.
15. Interpreter — define a grammar and an interpreter that uses the grammar to interpret sentences.
16. Iterator — access elements of a collection sequentially without exposing its underlying representation.
17. Memento — capture an object’s internal state so it can be restored later without violating encapsulation.
18. Mediator — define an object that encapsulates how a set of objects interact, promoting loose coupling.
19. Observer — define a one-to-many dependency so that when one object changes state, all dependents are notified.
20. Strategy — define a family of algorithms, encapsulate each one, and make them interchangeable at runtime.
21. State — allow an object to alter its behavior when its internal state changes, appearing to change its class.
22. Template Method — define the skeleton of an algorithm in a base class, letting subclasses override specific steps.
23. Visitor — represent an operation to be performed on elements of an object structure without changing the classes.