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 the method body. It is used to achieve abstraction and multiple inheritances in Java.

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.


Why use the Java interface?

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

  • It is used to achieve abstraction.
  • By interface, we can support the functionality of multiple inheritances.
  • It can be used to achieve loose coupling.


How to declare an 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

In this example, the Printable interface has only one method, and its implementation is provided in the A6 class.

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();  
 }  
}

Multiple inheritance in Java by interface

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

interface Printable{  
void print();  
}  
interface Showable{  
void show();  
}  
class A7 implements Printable,Showable{  
public void print(){System.out.println("Hello");}  
public void show(){System.out.println("Welcome");}  
  
public static void main(String args[]){  
A7 obj = new A7();  
obj.print();  
obj.show();  
 }  
}

Interface inheritance

A class implements an interface, but one interface extends another interface.

interface Printable{  
void print();  
}  
interface Showable extends Printable{  
void show();  
}  
class TestInterface4 implements Showable{  
public void print(){System.out.println("Hello");}  
public void show(){System.out.println("Welcome");}  
  
public static void main(String args[]){  
TestInterface4 obj = new TestInterface4();  
obj.print();  
obj.show();  
 }  
}

Nested Interface in Java

Note: An interface can have another interface which is known as a nested interface. We will learn it in detail in the nested classes chapter. For example:

interface printable{  
 void print();  
 interface MessagePrintable{  
   void msg();  
 }  
}