Design Pattern:

·       Design Patterns are reusable solutions to the problems that as a developer we encounter in our day to day programming.

·       Writing the code with design patterns will make your applications more Reliable, Scalable and Maintainable.

·       Used to solve the problems of object generation and integration.

History of Design Pattern:

·       Concept came from the book “Elements of Reusable Object-Oriented Software” by 4 authors known as Gang of four.

·       Book divided into 2 parts

o   First part à pros & cons of OOPs

o   Second partà the evolution of 23 classic software design patterns.

Types of Design Pattern:

1.       Creational Design pattern

2.       Structural Design pattern

3.       Behavioral Design Pattern

1.      Creational Design Pattern

ü  Deals with object creation & initialization

ü  Gives the program more flexible in deciding which object need to be created for a given case

ü  Best suit for a project with a lot of classes with a lot of objects, then objects are scattered on client code which leads complicated logic at client code i.e. not centralized code. So this pattern centralized the object creation logic. 

ü  Exp. Factory , Abstract factory.. etc

2.      Structural Design Pattern

ü  Deals with class & object composition

ü  Focused on decoupling interface & implementation of class & its objects.

ü  change in structure of a class or relationship between the classes but you don’t want the project to be affected.

ü  e.g. customer and product class and the product class is used inside the customer class (one to many relationships). Later, the customer wants to keep away the product class as they want to use the product and customer class independently. This is structural change and you don’t want this structural change to affect your project. This is where the Structural Design Pattern helps us.

ü  Exp: Adaptive, Bridge. etc.

3.      Behavioral Design Pattern

ü  Deals with communication between classes & objects.

ü  change in behavior of a class and again it affects other classes of the project.

ü  E.g. Invoice class currently applying taxes as 18%. Later if to add another extra tax. That means changing the behavior of a class.

ü  Exp: Chain of Responsibility, Command etc.