interface with in an abstract class

Question

Is it possible to declare an interface inside an abstract class ?. I tried this and i was successfully able to do it without any compilation error?? In Practical usage is there any significance of this ??

Here is the code that i have wrote.

public abstract class X 
{
   public interface abc extends I1 
   {
        public void sum(int i,int j);
   }
}


  public class Impl extends X 
  {
    class InnerImpl implements abc
   {
        @Override
        public int sum(int i, int j)
        {
          return i+j;
        }
   }
 }



public interface I1 
{
}

Answer 1

You can declare an interface within any class, not just abstract. The interface is implicitly static, so your enclosing class is just providing a namespace scope and is otherwise unrelated to the interface.

The utility of this definitely exists and I have used it on a number of occasions. Often the interface is coupled to a method of the same class, so clients can pass implementations of that interface to the method.

With Java 8 and functional interface types the proliferation of local interfaces will only increase.

Answer 2

Yes it is possible. As you said - it compiles (and I imagine runs), so it's legal.
In fact it is legal to declare a subinterface in an interface, or an enum (but only public ones, as private ones don't make any sense in an interface, just like private methods).

As for practical uses: You can declare a private inner class in your abstract class and use it in methods of said abstract class (I hope you can think of a usage for that). Or two of those. And if you need a shared contract for both - you can use an interface. Or you would need to return a specific thing that doesn't make sense outside the parent class.

A more down-to-earth example is the interface Map and it's sub-interface Map.Entry. Some methods of Map return and Entry, yet entry is meaningless without Map, so is not a standalone interface. If Map was an abstract class rather than an interface you would have exactly the situation you described.