There are 23 listed design patterns in GoF:
- Strategy: Defines a family of algorithms, encapsulates each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients who use it.
- Decorator: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
- 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.
- Observer: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
- 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.
- Singleton: Ensure a class only has one instance and provide a global point of access to it.
- 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.
- Adaptor: Convert the interface of a class into another interface clients expect. Adaptor lets classes work together that couldn’t otherwise because of incompatibility interfaces.
- Facade: 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.
- Template: Define a skeleton of an algorithm in an operation, deferring some steps to sub-classes. Template Method lets sub-classes redefine certain steps of an algorithm without changing the algorithms’ structure.
- Builder: Separate the construction of a complex object from its representation so that the same construction processes can create different representations.
- Iterator: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
- Composite: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
- Command: Encapsulate a request as an object, thereby letting you parameterise clients with different requests, queue or log requests, and support: un-doable operations.
- 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.
- State: Allow an object to alter its behaviour when its internal state changes. The object will appear to change its class.
- Proxy: Provide a surrogate or placeholder for another object to control access to it.
- Abstract Factory: Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
- Bridge: Decouple an abstraction from its implementation so that the two can vary independently.
- Interpreter: Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
- Memento: Without violating encapsulation, capture and externalise an object’s internal state so that the object can be restored to this state later.
- Prototype: Specify the kinds of objects to create using a prototypical instance and create new objects by copying this prototype.
- 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.