Interface inside an Interface

Interface can be defined in other interface. By default the inner interface is static.

A nested interface is any interface whose declaration occurs within the body of another class or interface.

Syntax

    interface Interface1{

        interface Interface2{

        }

    }

Example1

interface Sum{
 int sum(int a, int b);

 interface SumThree{
  int sum(int a, int b, int c);
 }

 interface SumFour{
  int sum(int a, int b, int c);
 }
}

class SumTest implements Sum{
  
 public int sum(int a1, int b1){
  return (a1+b1);
 }

 public static void main(String args[]){
  SumTest s1 = new SumTest();
  System.out.println(s1.sum(10,20));
 }
 
}

Output

30

Example2

class SumTest1 implements Sum.SumThree{

 public int sum(int a1, int b1, int c1){
  return (a1+b1+c1);
 }

 public static void main(String args[]){
  SumTest1 s1 = new SumTest1();
  System.out.println(s1.sum(10,20,30));
 }
 
}

Output

60

Some Points to Remember

1. Declaring class inside blocks is valid in java. Example : Local classes

class SumTest{
 {
  class ABC{
  }
 }
}

    

2. You can't declare interface inside a block

class SumTest{ 
 {
  interface ABC{
  }
 }
}

When you tries to compile the above program, compiler throws the below error

SumTest.java:4: error: interface not allowed here
interface ABC{
^
1 error

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