Interface extends Other interface

In Java, one interface can extends other interface.

Syntax

   interface Interface2 extends Interface1{

   }

Example

interface Interface1{
 int sum(int a, int b);
}

interface Interface2 extends Interface1{
 int sum(int a, int b, int c);
}

Any class implementing the interface “Interface2” must provide the implementation for both the methods

   int sum(int a, int b);

   int sum(int a, int b, int c);

class Sum implements Interface2{
 public int sum(int a, int b, int c){
  return (a+b+c);
 }
}

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

Sum.java:1: error: Sum is not abstract and does not override abstract method sum(int,int) in Interface1
class Sum implements Interface2{
^
1 error

To make the program compiles fine, implement both the methods.

class Sum implements Interface2{
 public int sum(int a, int b, int c){
  return (a+b+c);
 }

 public int sum(int a, int b){
  return (a+b);
 }
}

Some Points to Remember

1. Can an Interface abstract ?

Yes, but no point in putting abstract modifier for the interface, since all the method in interface are abstract by default

Example

    abstract interface Area{

        double PI=3.1428;

        double getArea(double r);

    }

2. Can an interface extend more than one interface ?

A single interface can extend any number of other interfaces.
  
Example

interface Interface1{
 int sum(int a, int b);
}

interface Interface2 extends Interface1{
 int sum(int a, int b, int c);
}

interface Interface3 extends Interface1, Interface2{

}

3. Is empty interface like below are valid ?

Interface Interface1{

}

Yes, It is Perfectly valid. Interfaces without methods and fields are called marker interfaces. Serializable, Cloneable and Remote interfaces are marker interfaces.

         

Implement more than one interface                                                 Class inside interface                                                 Home