Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
Attach additional responsibilities to an object dynamically. [This pattern] provide a flexible alternative to subclassing for extending functionality.
Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Connect the receiving objects and pass the request along the connection until an object handles it.
Chain of Responsibility
Compose objects into tree structures to represent part-whole hierarchies. [This pattern] lets clients treat individual objects and compositions of objects uniformly.
Convert the interface of a class into another interface clients expect. [This pattern] lets classes work together that couldn’t otherwise because of incompatible interfaces.
Decouple an abstraction from its implementation so that the two can vary independently.
Define a family of algorithms, encapsulate each one, and make them interchangeable.
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Define an interface for creating an object, but let subclasses decide which class to instantiate. [This pattern] lets a class defer instantiation to subclasses.
Define an object that encapsulates how a set of objects interact. [This pattern] promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. [This pattern] lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.
Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
Ensure a class only has one instance, and provide a global point of access to it.
Given a language, define a representation for its grammar along with an [pattern] that uses the representation to [pattern] sentences in the language.
Provide a surrogate or placeholder for another object to control access to it.
Provide a unified interface to a set of interfaces in a subsystem. [This pattern] defines a higher-level interface that makes the subsystem easier to use.
Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
Represent an operation to be performed on the elements of an object structure. [This pattern] lets you define a new operation without changing the classes of the elements on which it operates.
Separate the construction of a complex object from its representation so that the same construction process can create different representations.
Specify the kinds of objects to create using a [pattern] instance, and create new objects by copying this [pattern].
Use sharing to support large numbers of fine-grained objects efficiently.
Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.