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Difference between Abstract Classes and Interface

What is an Abstract Class?

It only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.

What is an Interface?

It is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class. 

Both Together

When we create an interface, we are basically creating a set of methods without any implementation that must be overridden by the implemented classes. The advantage is that it provides a way for a class to be a part of two classes: one from inheritance hierarchy and one from the interface.

When we create an abstract class, we are creating a base class that might have one or more completed methods but at least one or more methods are left uncompleted and declared abstract. If all the methods of an abstract class are uncompleted then it is same as an interface. The purpose of an abstract class is to provide a base class definition for how a set of derived classes will work and then allow the programmers to fill the implementation in the derived classes.

 

Abstract Classes: What is something?

Interface: How is something?

 

 

What is Polymorphism

 

The ability of a reference variable to change behavior according to what object instance it is holding.

 

This allows multiple objects of different subclasses to be treated as objects of a single super class, while automatically selecting the proper methods to apply to a particular object based on the subclass it belongs to

 

Three forms of polymorphism:

1. Method overriding

Methods of a subclass override the methods of a superclass

2. Method overriding (implementation) of the abstract methods

Methods of a subclass implement the abstract methods of an abstract class

3. Method overriding (implementation) through the Java interface

Methods of a concrete class implement the methods of the interface