What and When Design Pattern?

 

Pattern	What & When
Decorator	To provide a way of attaching new state and behavior to an object dynamically. This pattern allows to add new functionality an existing object without altering its structure by creating wrapper class. You have: An existing component class that may be unavailable for subclassing You wants: Attach additional state and behavior to an object dynamically. Make changes to some objects in a class without affecting others. Avoid subclassing because too many classes could result.
****************** Proxy	It Supports objects that control the creation of and access to other objects In Proxy pattern, a class represents functionality of another class. You have: Are expensive to create Need access control Access remote sites. Need to perform some actions when they are accessed. You wants: Create objects when their operations are requested. Perform checks on objects whenever accessed. Have a local objects that will refers to a remote object. Implement access rights on objects as their operation are requested.
****************** Bridge	It decouples an abstraction from its implementation, enabling them to vary independently. You have: Identify that there are operations that do not always need to be implemented in the same way. You wants: Completely hide implementation from client. Combine different part of system at runtime.
****************** Composite	The Composite pattern arranges structured hierarchies so that single components and groups of components can be treated in the same way. The beauty of this pattern is its ability to manipulate the different types of the elements equally. You have: An irregular structure of objects and composites of objects. You wants: Clients to ignore all but the essential differences between individual objects and composites of objects. To treat all objects in a composite uniformly.
****************** Flyweight	The Flyweight pattern promotes an efficient way to share common information present in small objects that occurs in a system in large number. It thus helps reduce storage requirement when many values are duplicated. You have(There are): Many objects to deal with in memory. Different kind of state, which can be handled differently to achieve space saving. Group of objects that share state. Ways of computing some of the state at runtime. You wants: Implement a system despite server memory constraints
****************** Adapter	It is versatile pattern that joins together types that were not design to work each other. It is useful pattern when dealing with legacy code. It enables a system to use classes whose interfaces don't quite match its requirements. You have: A Domain specific Interface A class to connect to with a mismatching interface. You wants: Create a reusable class to cooperate with yet-to-be-build classes. Change the name of methods as called and as implemented. Support different sets of methods for different purpose.
****************** Facade	To provide different high level views of subsystems whose details are hidden from users. Facade pattern hides the complexities of the system and provides an interface to the client using which the client can access the system.
****************** Prototype	It creates new objects by cloning of a few stored prototypes. It speeds up the instantiation of very large, dynamically loaded classes. It refers to creating duplicate object while keeping performance in mind. This pattern is used when creation of object directly is costly. For example, an object is to be created after a costly database operation. We can cache the object, returns its clone on next request and update the database as and when needed thus reducing database calls. You wants: Hide concrete classes from the client. Add and remove new classes (via prototypes) at runtime. Keep the number of classes in the system to a minimum. Adapt to changing structure of data at runtime. Note: In C# cloning by deep copying is absolutely straightforward.
****************** Factory method	It is a way of creating objects, but letting subclasses decide exactly which class to instantiate. You wants: Flexibility is important. Objects can be extended in subclasses There is a specific reason why one subclass would be chosen over another-this logic forms part of the Factory method.
****************** Singleton	The purpose of the Singleton Pattern is to ensure that there is only one instance of a class, and there is a global access point to that object. The pattern ensures that the class is instantiated only once and that all requests are directed to that one and only object. Moreover, the object should not be created until it is really needed. You wants: You need to ensure that only one instance of a class. Controlled access to that instance is essential. You might need more than one instance at a larger stage.
Abstract Factory