Interface in Java
An interface in java is a blueprint of a class. It has static constants and abstract methods only.
The interface in java is a mechanism to achieve fully abstraction. There can be only abstract methods in the java interface not method body. It is used to achieve fully abstraction and multiple inheritance in Java.
Java Interface also represents IS-A relationship.
It cannot be instantiated just like abstract class.
Why use Java 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 Java 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 through class in java but it is possible by interface, why?
As you can see in the above example, Printable and Showable interface have same methods but its implementation is provided by class A, so there is no ambiguity.
A class implements interface but one interface extends another interface .
Q) What is marker or tagged interface?
An interface that have no member is known as marker or tagged interface. For example: Serializable, Cloneable, Remote etc. They are used to provide some essential information to the JVM so that JVM may perform some useful operation.
Nested Interface in Java
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: