Java Nested Interface

An interface, i.e., declared within another interface or class, is known as a nested interface. The nested interfaces are used to group related interfaces so that they can be easy to maintain. The nested interface must be referred to by the outer interface or class. It can't be accessed directly.

Points to remember for nested interfaces

There are given some points that should be remembered by the java programmer.

The nested interface must be public if it is declared inside the interface, but it can have any access modifier if declared within the class.
Nested interfaces are declared static

Syntax of nested interface which is declared within the interface

interface interface_name{
 ...
 interface nested_interface_name{
  ...
 }
} 

Syntax of nested interface which is declared within the class

class class_name{
 ...
 interface nested_interface_name{
  ...
 }
} 

Example of nested interface which is declared within the interface

In this example, we will learn how to declare the nested interface and how we can access it.

TestNestedInterface1.java

interface Showable{
  void show();
  interface Message{
   void msg();
  }
}
class TestNestedInterface1 implements Showable.Message{
 public void msg(){System.out.println("Hello nested interface");}

 public static void main(String args[]){
  Showable.Message message=new TestNestedInterface1();//upcasting here
  message.msg();
 }
}

Test it Now

Output:

hello nested interface

download the example of nested interface

As you can see in the above example, we are accessing the Message interface by its outer interface Showable because it cannot be accessed directly. It is just like the almirah inside the room; we cannot access the almirah directly because we must enter the room first. In the collection framework, the sun microsystem has provided a nested interface Entry. Entry is the subinterface of Map, i.e., accessed by Map.Entry.

Internal code generated by the java compiler for nested interface Message

The java compiler internally creates a public and static interface as displayed below:

  public static interface Showable$Message
  {
    public abstract void msg();
  }

Example of nested interface which is declared within the class

Let's see how we can define an interface inside the class and how we can access it.

TestNestedInterface2.java

class A{
  interface Message{
   void msg();
  }
}

class TestNestedInterface2 implements A.Message{
 public void msg(){System.out.println("Hello nested interface");}

 public static void main(String args[]){
  A.Message message=new TestNestedInterface2();//upcasting here
  message.msg();
 }
}

Test it Now

Output:

hello nested interface

Can we define a class inside the interface?

Yes, if we define a class inside the interface, the Java compiler creates a static nested class. Let's see how can we define a class within the interface: