Learning Java OOP Concepts

1. Introduction to Object-Oriented Programming

Object-Oriented Programming (OOP) is a programming paradigm that uses objects and classes to structure the code. It aims to implement real-world entities like inheritance, polymorphism, abstraction, and encapsulation in programming. Java is a widely-used language that follows the OOP principles.

2. Classes and Objects


A class in Java is a blueprint for creating objects. It defines a datatype by bundling data and methods that work on the data into one single unit.


An object is an instance of a class. When a class is instantiated, memory is allocated for the object and its methods become available for use.

3. Inheritance

Inheritance is a mechanism in which one class acquires the properties and behaviors of another class. It provides a way to reuse code and establish a relationship between classes.

4. Polymorphism

Polymorphism means “many forms”, and it allows one interface to be used for a general class of actions. The most common use of polymorphism in Java is when a parent class reference is used to refer to a child class object.

Compile-Time Polymorphism

Also known as method overloading, it occurs when multiple methods in the same class have the same name but different parameters.

Run-Time Polymorphism

Also known as method overriding, it occurs when a subclass provides a specific implementation of a method that is already defined in its superclass.

5. Abstraction

Abstraction is the concept of hiding the internal details and showing only the functionalities. It helps in reducing programming complexity and effort.

Abstract Class

An abstract class cannot be instantiated and may contain abstract methods, which are methods without a body.


An interface in Java is a blueprint of a class. It has static constants and abstract methods.