Design patterns in java

Design Pattern is a reusable and named solution to a recurring problem in a context. There are 3 categories of patterns when classified by the purpose:

• Structural Design Patterns
  • Strategy Pattern: Defines a family of algorithms, encapsulates them and makes them interchangeable thus Strategy lets algorithm vary independently from the clients that use it. Example: Collections Java, List
• Creational Design Patterns: Allows creational of the objects
  • Abstract Design factory pattern
• Behavioural Design Patterns

Reference Books for Design patterns:

• The Gang of Four: The Definitive Design Patterns Book
• Head first Design patterns

Object Oriented Programming

Building blocks of OOP:

• Abstraction:
  • Showing the essential details by hiding the complexity.
  • In Java, achieved by Interfaces, Abstract classes.
• Encapsulation:
  • Hiding the information. Can be field level or behavioural level.
  • In Java, achieved by access specifiers.
• Inheritance:
  • Inhering properties, methods of another class to avoid code duplication.
  • In Java, achieved Extending a class or implementing any interface.
• Polymorphism:
  • Allows the actual object to be decided at runtime, basically a subclass/interface can stand in for super class.
  • In Java, achieved Extending a class or implementing any interface.

SOLID Principles

• Single Responsibility Priniciple
  • One class should own a single responsibilty.
• Open-Closed Priniciple
  • classes should be open for extension, closed for modification.
  • Can be achieved by inheritance.
• Liskov Substitution Principle
  • When you inherit a class, you should be able to substitute super class methods on sub class methods without any hussle.
• Interface segregation Principle
  • Seggregate the interfaces in a way client can use.
• Dependency injection Principle
  • High level module should not depend on low level module.

Other OOP principles:

• Favor composition over inheritence.
• Program to an interface not implementation.
• DRY: Donot repeat yourself