Interface in Java

Introduction 

    An interface in Java is a blueprint of a class. It has static constants and abstract methods. The interface in Java is a mechanism to achieve abstraction. There can be only abstract methods in the Java interface, not method body. It is used to achieve abstraction and multiple inheritance in Java. In other words, you can say that interfaces can have abstract methods and variables. It cannot have a method body. Java Interface also represents the IS-A relationship.

    It cannot be instantiated just like the abstract class. Since Java 8, we can have default and static methods in an interface. Since Java 9, we can have private methods in an interface.

There are mainly three reasons to use interface. They are given below

👉 It is used to achieve abstraction.

👉 By interface, we can support the functionality of multiple inheritance.

👉 It can be used to achieve loose coupling.

Declaration of interface:

An interface is declared by using the interface keyword. It provides total abstraction; means all the methods in an interface are declared with the empty body, and all the fields are public, static and final by default. A class that implements an interface must implement all the methods declared in the interface.

Syntax:

interface <interface_name>{  

      

    // declare constant fields  

    // declare methods that abstract   

    // by default.  

}  

The 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.

Java Interface Example:

interface printable{  

void print();  

}  

class A6 implements printable{  

public void print(){System.out.println("Hello");}  

  

public static void main(String args[]){  

A6 obj = new A6();  

obj.print();  

 }  

}  

OUTPUT:

Hello

Multiple inheritance in Java by interface

If a class implements multiple interfaces, or an interface extends multiple interfaces, it is known as multiple inheritance.