Inner class in interface vs in class

What is the difference between these two innerclass declarations? Also comment on advantages/disadvantages?

case A: class within a class.

public class Levels {   
  static public class Items {
    public String value;
    public String path;

    public String getValue() {
      return value;}
  }
}

and case B: class within interface.

public interface Levels{

  public class Items {
    public String value;
    public String path;

    public String getValue() {
      return value;}
  }
}

Made correction: to placement of getvalue method.

further info: I am able to instantiate Items class in both cases A and B in another class that does not implement interface AT ALL.

public class Z{//NOTE: NO INTERFACE IMPLEMENTED here!!!!
 Levels.Items items = new Levels.Items();
}

Since an interface is not instantiated, all the elements inside an interface are accessible by dot notation without LEVELS interface instantiated simply because you cannot instantiate an interface - effectively making a class defined inside an interface permeable to static reference.

So saying that Items class in case B is not static does not make sense. Since both cases A and B are instantiated the same way, I am not looking for semantics on what is static or inner or nested. Stop giving me answers on semantics. I want the compiler, runtime and behavioural differences/advantages, or if none then say so. No more answers on semantics please!!!!! An expert on JVM or .NET VM specification innards please this answer question rather than text book semanticissiests.

Edit Report

The examples you give of nested / inner classes are (IMO) bad examples. Besides the 2nd example is not valid Java since an interface can only declare (implicitly) abstract methods. Here's a better example:

public interface Worker {

    public class Response {
        private final Status status;
        private final String message;
        public Response(Status status, String message) {
            this.status = status; this.message = message;
        }
        public Status getStatus() { return status; }
        public String getMessage() { return message; }
    }

    ...

    public Response doSomeOperation(...);
}

By embedding the Response class, we are indicating that it is a fundamental part of Worker API with no other uses.

The Map.Entry class is a well-known example of this idiom.

answered 2009-12-04T09:40:23.547

Inner class in interface vs in class

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