Interface in Java
An interface is a blueprint of a class. It has static constants and abstract methods.
The interface is a mechanism to achieve fully abstraction in java. There can be only abstract methods in the interface. It is used to achieve fully abstraction and multiple inheritance in Java.
Interface also represents IS-A relationship.
It cannot be instantiated just like abstract class.
Why use Interface?
There are mainly three reasons to use interface. They are given below.
The java compiler adds public and abstract keywords before the interface method and public, static and final keywords before data members.
In other words, Interface fields are public, static and final bydefault, and methods are public and abstract.
Understanding relationship between classes and interfaces
As shown in the figure given below, a class extends another class, an interface extends another interface but a class implements an interface.
Simple example of Interface
Multiple inheritance in Java by interface
If a class implements multiple interfaces, or an interface extends multiple interfaces i.e. known as multiple inheritance.
Q) Multiple inheritance is not supported in case of class but it is supported in case of interface, why?
Note: A class implements interface but One interface extends another interface .
Que) What is marker or tagged interface ?
Note: An interface can have another interface i.e. known as nested interface. We will learn it in detail in the nested classes chapter. For example: