Question

I need to define an Interface that redefines something like a hashset.

So I have methods to of the type get(byte key). As I can not use descriptive keys like String I am looking for a generic way to define the keys available within some implementation of the interface. I think of using an enum for that. But is it possible to force the definition of such Enum within the Interface?

To make clear what I mean lets look on an example class implementing the Interface:

public class ByteHashsetImpl implements ByteHashset {
    public enum Key {
        STATE_A,
        STATE_B,
        STATE_C;
    }

    public enum Value {
        VALUE_A,
        VALUE_B,
        VALUE_C;
    }

    public void get(Key k) {
        /**/
    }

}

I want to ensure that each implementation defines and uses its own Enums called Key and Value. This would make it possible to access the key/values using something like ByteHashsetImpl.Key for every implementation of the interface.

Is it possible at all or is there another option beside an informal definition like coding quideline?

Answer 1

You can’t enforce the creation of such enums, but you can force a type argument to be an enum:

public interface Frobnicator<E extends Enum<E>> {
  void frobnicate(E value);
}


enum Bar {
  X, Y;
}

class BarFrobnicator implements Frobnicator<Bar> {
  @Override
  public void frobnicate(Bar value) {
    // do stuff
  }
}

In this case a Frobnicator could simply re-use an existing enum class, but E in Frobnicator will always be an enum type.