java-design-patterns

a system should be independent of how its products are created, composed and represented
a system should be configured with one of multiple families of products
a family of related product objects is designed to be used together, and you need to enforce this constraint
you want to provide a class library of products, and you want to reveal just their interfaces, not their implementations

##Builder Intent: Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Applicability: Use the Builder pattern when

the algorithm for creating a complex object should be independent of the parts that make up the object and how they're assembled
the construction process must allow different representations for the object that's constructed

##Factory Method 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 Factory Method pattern when

a class can't anticipate the class of objects it must create
a class wants its subclasses to specify the objects it creates
classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate

##Prototype Intent: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

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

##Singleton Intent: Ensure a class only has one instance, and provide a global point of access to it.

Applicability: Use the Singleton pattern when

the must be exactly one instance of a class, and it must be accessible to clients from a well-known access point
when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code

##Adapter 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.

Applicability: Use the Adapter pattern when

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.

##Bridge Intent: Decouple an abstraction from its implementationso that the two can vary independently.

Applicability: Use the Bridge pattern when

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.

##Composite Intent: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Applicability: Use the Composite pattern when

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

##Decorator Intent: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Applicability: Use Decorator

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

##Facade 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.

##Flyweight Intent: Use sharing to support large numbers of fine-grained objects efficiently.

##Proxy Intent: Provide a surrogate or placeholder for another object to control access to it.

##Chain of responsibility 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.

##Command Intent: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

##Interpreter Intent: Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

##Iterator Intent: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

##Mediator 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.

##Memento Intent: Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

##Observer Intent: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

##State Intent: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

##Strategy 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.

##Visitor 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.