Overview of Design Patterns

Overview of Design Patterns

As per WIKI, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design.

Why do we need them?
Well, rather than having to re-invent the wheel, we can re-use an existing solution recognized across organizations as something that simply works. Since it is well documented, it is comparatively easier to solve the problem and solution in our mind.

How many are there? 
There is no definitive list of the patterns. People working on various challenges document them and spread the word about a possible solution. Over a period of time, this solution is recognized as a standard pattern for solving a particular problem. Obviously the most common patterns are the ones for Gang of Four or you can also look at Wikipedia for design patterns.

Design pattern is divided based on purpose and scope.

		Purpose
		Creational	Structural	Behavioural
Scope	Class	Factory Method	Adapter	Interpreter Template Method
	Object	Abstract Factory Builder Prototype Singleton	Bridge Composite Decorator Façade Flyweight Proxy	Chain of Responsibility Command Iterator Mediator Memento Observer State Strategy Visitor

Design patterns are divided into three fundamental groups.

• Behavioral patterns are those patterns that are most specifically concerned with communication between objects.
• All of the creational patterns deal with the best way to create instances of objects. This is important because your program should not depend on how objects are created and arranged.
• Structural patterns describe how classes and objects can be combined to form larger structures. The difference between class patterns and object patterns is that class patterns describe how inheritance can be used to provide more useful program interfaces. Object patterns, on the other hand, describe how objects can be composed into larger structures using object composition, or the inclusion of objects within other objects. 

Each fundamental groups are further divided into multiple ways:

Creational Patterns
Abstract Factory	Creates an instance of several families of classes
Builder	Separates object construction from its representation
Factory Method	Creates an instance of several derived classes
Prototype	A fully initialized instance to be copied or cloned
Singleton	A class of which only a single instance can exist

Structural Patterns
Adapter	Match interfaces of different classes
Bridge	Separates an object’s interface from its implementation
Composite	A tree structure of simple and composite objects
Decorator	Add responsibilities to objects dynamically
Facade	A single class that represents an entire subsystem
Flyweight	A fine-grained instance used for efficient sharing
Proxy	An object representing another object

Behavioral Patterns
Chain of Resp.	A way of passing a request between a chain of objects
Command	Encapsulate a command request as an object
Interpreter	A way to include language elements in a program
Iterator	Sequentially access the elements of a collection
Mediator	Defines simplified communication between classes
Memento	Capture and restore an object's internal state
Observer	A way of notifying change to a number of classes
State	Alter an object's behavior when its state changes
Strategy	Encapsulates an algorithm inside a class
Template Method	Defer the exact steps of an algorithm to a subclass
Visitor	Defines a new operation to a class without change

Useful websites:
GoF Patterns
DoFactory