Java Design Patterns

Introduction :

Design patterns represent the best practices used by experienced object-oriented software developers. Design patterns are solutions to general problems that software developers faced during software development. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that can be used in many different situations.
These solutions were obtained by trial and error by numerous software developers over quite a substantial period of time.

In 1994, four authors Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides published a book titled Design Patterns - Elements of Reusable Object-Oriented Software which initiated the concept of Design Pattern in Software development.These authors are collectively known as Gang of Four (GOF).

As per the design pattern reference book Design Patterns - Elements of Reusable Object-Oriented Software, there are 23 design patterns which can be classified in three categories:

1. Creational (the way of creating objects)
2. Structural (simplifies the structure by identifying the relationships)
3. Behavioral (interaction between the objects should be in such a way that they can easily talk to each other and still should be loosely coupled)

Creational Patterns :

These design patterns provide a way to create objects while hiding the creation logic, rather than instantiating objects directly using new operator. This gives more flexibility to the program in deciding which objects need to be created for a given use case.
It provides instantiation mechanisms, making it easier to create objects in a way that suits the situation.

There are following 6 types of Creational design patterns

1. Factory Method : loose-coupling, defines an interface or abstract class for creating an object but let the subclasses decide which class to instantiate. 
2. Abstract Factory : create families of related to objects but without specifying their concrete sub-classes
3. Singleton : only one instance and provides a global point of access to it
4. Prototype : cloning of an existing object instead of creating new one and can also be customized as per the requirement.
5. Builder : construct a complex object from simple objects using step-by-step approach
6. Object Pool : to reuse the object that are expensive to create 

Structural Patterns :

These design patterns concern class and object composition. Concept of inheritance is used to compose interfaces and define ways to compose objects to obtain new functionalities. It generally deal with relationships between entities, making it easier for these entities to work together.

There are following 7 types of structural design patterns.

1. Adapter   : Adapting an interface into another according to client expectation.
2. Bridge    :       Separating abstraction (interface) from implementation.
3. Composite : Allowing clients to operate on hierarchy of objects.
4. Decorator : Adding functionality to an object dynamically.
5. Facade  :       Providing an interface to a set of interfaces.
6. Flyweight :   Reusing an object by sharing it.
7. Proxy :         Representing another object.

Behavioral Patterns :

These design patterns are specifically concerned with communication between objects.These are used in communications between entities and make it easier and more flexible for these entities to communicate.

1. Chain of Responsibility 
2. Command 
3. Interpreter 
4. Iterator 
5. Mediator 
6. Memento 
7. Observer
8. State
9. Strategy
10. Template
11. Visitor