**Intent:** Separate the construction of a complex object from its representation so that the same construction process can create different representations.
**Intent:** Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
**Applicability:** Use the Prototype pattern when a system should be independent of how its products are created, composed and represented; and
* when the classes to instantiate are specified at run-time, for example, by dynamic loading; or
* to avoid building a class hierarchy of factories that parallels the class hierarchy of products; or
* when instances of a class can have one of only a few different combinations of state. It may be more convenient to install a corresponding number of prototypes and clone them rather than instantiating the class manually, each time with the appropriate state
**Intent:** Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
* you want to use an existing class, and its interface does not match the one you need
* you want to create a reusable class that cooperates with unrelated or unforeseen classes, that is, classes that don't necessarily have compatible interfaces
* you need to use several existing subclasses, but it's impractical to adapt their interface by subclassing every one. An object adapter can adapt the interface of its parent class.
* you want to avoid a permanent binding between an abstraction and its implementation. This might be the case, for example, when the implementation must be selected or switched at run-time.
* both the abstractions and their implementations should be extensible by subclassing. In this case, the Bridge pattern lets you combine the different abstractions and implementations and extend them independently
* changes in the implementation of an abstraction should have no impact on clients; that is, their code should not have to be recompiled.
* you have a proliferation of classes. Such a class hierarchy indicates the need for splitting an object into two parts. Rumbaugh uses the term "nested generalizations" to refer to such class hierarchies
* you want to share an implementation among multiple objects (perhaps using reference counting), and this fact should be hidden from the client. A simple example is Coplien's String class, in which multiple objects can share the same string representation.
**Intent:** Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
* you want to represent part-whole hierarchies of objects
* you want clients to be able to ignore the difference between compositions of objects and individual objects. Clients will treat all objects in the composite structure uniformly
**Intent:** Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
* to add responsibilities to individual objects dynamically and transparently, that is, without affecting other objects
* for responsibilities that can be withdrawn
* when extension by subclassing is impractical. Sometimes a large number of independent extensions are possible and would produce an explosion of sublasses to support every combination. Or a class definition may be hidden or otherwise unavailable for subclassing
**Intent:** Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
* you want to provide a simple interface to a complex subsystem. Subsystems often get more complex as they evolve. Most patterns, when applied, result in more and smaller classes. This makes the subsystem more reusable and easier to customize, but is also becomes harder to use for clients that don't need to customize it. A facade can provide a simple default view of the subsystem that is good enough for most clients. Only clients needing more customizability will need to look beyond the facade.
* there are many dependencies between clients and the implementation classes of an abstraction. Introduce a facade to decouple the subsystem from clients and other subsystems, thereby promoting subsystem independence and portability.
* you want to layer your subsystems. Use a facade to define an entry point to each subsystem level. If subsystems are dependent, the you can simplify the dependencies between them by making them communicate with each other solely through their facades
**Applicability:** The Flyweight pattern's effectiveness depends heavily on how and where it's used. Apply the Flyweight pattern when all of the following are true
* an application uses a large number of objects
* storage costs are high because of the sheer quantity of objects
* most object state can be made extrinsic
* many groups of objects may be replaced by relatively few shared objects once extrinsic state is removed
* the application doesn't depend on object identity. Since flyweight objects may be shared, identity tests will return true for conceptually distinct objects.
**Applicability:** Proxy is applicable whenever there is a need for a more versatile or sophisticated reference to an object than a simple pointer. here are several common situations in which the Proxy pattern is applicable
* a remote proxy provides a local representative for an object in a different address space.
* a virtual proxy creates expensive objects on demand.
* a protection proxy controls access to the original object. Protection proxies are useful when objects should have different access rights.
**Intent:** 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.
**Intent:** Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
**Intent:** Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
**Intent:** 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.
**Intent:** Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.
**Intent:** Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
**Intent:** Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
##Template method
**Intent:** 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.
**Intent:** 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.
