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.
Attach additional responsibilities to an object dynamically. [This pattern] provide a flexible alternative to subclassing for extending functionality.
Define an interface for creating an object, but let subclasses decide which class to instantiate. [This pattern] lets a class defer instantiation to subclasses.
Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
Ensure a class only has one instance, and provide a global point of access to it.
Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
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.
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.
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.
Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
Compose objects into tree structures to represent part-whole hierarchies. [This pattern] lets clients treat individual objects and compositions of objects uniformly.
Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
Provide a surrogate or placeholder for another object to control access to it.