OOP concepts in Java

What is the Class? 
The class is a group of similar entities. It is extensible program code for creating objects and to define data types as well as methods.

What is Object in OOP?
Object refers to a particular instance of a class whereas objects can be a combination of variables, functions and data structures.

What are the 4 principles in OOP?
Encapsulation, Data Abstraction, Polymorphism and Inheritance.

What is Inheritance?
Deriving a new class from an existing class and forming them into a hierarchy of classes.

What is Encapsulation? 
 It is used to hiding the implementation details.

What is Constructor overloadin? 
It is a feature that allows a class to have more than one constructor having different argument lists.

What is the difference between method overloading vs method overriding?

Why do we need to use Method Overloading?
Suppose that you have a class that can use calligraphy to draw various types of data (strings, integers, and so on) and that contains a method for drawing each data type. It is difficult to use a new name for each method—for example, drawString, drawInteger, drawFloat, and so on.

In the Java programming language, you can use the same name for all the drawing methods but pass a different argument list to each method. Thus, the data drawing class might declare four methods named draw, each of which has a different parameter list.

public class DataArtist {
    ...
    public void draw(String s) {
        ...
    }
    public void draw(int i) {
        ...
    }
    public void draw(double f) {
        ...
    }
    public void draw(int i, double f) {
        ...
    }
}

Why do we need to use Method Overriding?  

Declaring a method in the subclass which is already present in the parent class is known as method overriding. Overriding is done so that a child class can give its own implementation to a method which is already provided by the parent class.

The implementation in the subclass overrides (replaces) the implementation in the superclass by providing a method that has the same name, same parameters or signature, and same return type as the method in the parent class. The version of a method that is executed will be determined by the object that is used to invoke it. If an object of a parent class is used to invoke the method, then the version in the parent class will be executed, but if an object of the subclass is used to invoke the method, then the version in the child class will be executed.

1. Helps in writing generic code based on parent class or interface as object resolution happens at runtime
2. Provides multiple implementations of the same method and can invoke parent class overridden method using super keyword
3. Defines what behaviour a class can have and implementation of behaviour has been taken care by a class which is going to implement.