Getting error for generic interface: The interface Observer cannot be implemented more than once with different arguments:

Question

I am getting this error in Eclipse while writing a GWT app

The interface Observer cannot be implemented more than once with different arguments: Observer<CompositeListData > and Observer<DialogBoxAuthenticate>

public class CompositeWordLists extends Composite implements Observer<DialogBoxAuthenticate>, Observer<CompositeListData>

Here is the interface

public interface Observer<T> {
    public void update(T o);
}

Is this right? How can I get around this problem without having to create a multitude of Observer classes for every possible event?

Answer 1

Because of type erasure you can't implement the same interface twice (with different type parameters). So, the eclipse error that you are receiving is correct.

You could add a base class for all possible "T", which may be limiting and not useful depending on the scope of these classes. And, you have requested a solution that prevents you from creating a multitude of Observer classes (i am assuming interfaces) for every possible event, well I can't see how else you would do that without compromising compile time safety.

I would suggest the following

interface Observer<T>{
    public void update (T o);
}

interface DialogBoxAuthenticateObserver extends Observer<DialogBoxAuthenticate>{
}

The code clutter isn't horrible and if you place them all in one file, they will be easy to reference and maintain. Hope I have helped

EDIT: After some digging around on google (which pointed me back to stackoverflow!, your question was asked in a different fashion and answered similarly here

Answer 2

Composite must already implement Observer. Is that what really intended? You want this CompositeWordLists class to observe two ways at once?

Answer 3

Not sure this can help, but I came across the same Java compile error today.

I partially solved my case by extracting all the shared implementation I could get to a parametrized abstract class:

public enum Example {
    ;
    static interface Observer<T> { public void update (T o); }
    static abstract AbstractObserver<T> implements Observer<T> {
        public void update (T o) { /* shared stuff I can put here */ }
    }
    static Composite extends AbstractObserver<Parameter1> {
        public void update (T o) {
            super(o);
            /* type-specific code here */
        }
    }
    static CompositeWordLists extends AbstractObserver<Parameter2> {
        public void update (T o) {
            super(o);
            /* type-specific code here */
        }
    }
}