Object Oriented Programming is a method of implementation in which programs are organized as cooperative collection of objects, each of which represents an instance of a class, and whose classes are all members of a hierarchy of classes united via inheritance relationships.

OOP Concepts

Four principles of Object Oriented Programming are

Abstraction
Encapsulation
Inheritance
Polymorphism

Abstraction

         Abstraction denotes the essential characteristics of an object that distinguish it from all other kinds of objects and thus provide crisply defined conceptual boundaries, relative to the perspective of the viewer.

Encapsulation

        Encapsulation is the process of compartmentalizing the elements of an abstraction that constitute its structure and behavior ; encapsulation serves to separate the contractual interface of an abstraction and its                          implementation.

       * Hides the implementation details of a class.
       * Forces the user to use an interface to access data
       * Makes the code more maintainable.

Inheritance

        Inheritance is the process by which one object acquires the properties of another object.

Polymorphism

        Polymorphism is the existence of the classes or methods in different forms or single name denoting different implementations.

Since Java is an object oriented programming language it has following features:

Reusability of Code

Emphasis on data rather than procedure

Data is hidden and cannot be accessed by external functions

Objects can communicate with each other through functions

New data and functions can be easily added                       

Simple
Reusability of Code

Portable (Platform Independent)
Distributed
Robust
Secure
High Performance
Dynamic
Threaded
Interpreted