Nested class inside an interface

Question

The java compiler allows me write a Class definition inside an Interface . Are there any specific uses of this ?

interface ClassInterface {

  void returnSomething();
  int x = 10;

  class SomeClass {
    private int y;

    private void classDoingSomething() {         
    }
  }
}

Please explain .

Answer 1

The uses are the same that a class nested in another class: it allows scoping the class to the interface. You could imagine something like this:

public interface Switch {

    public enum Status {
        ON, OFF;
    }

    void doSwitch();
    Status getStatus();
}

It avoids defining a top-level class named SwitchStatus (because Status could be a too generic name).

Answer 2

You would use it to tightly bind a certain type to an interface. Read This page for further information and an example of defining a class inside an interface.