Java方法中的嵌套泛型？

Question

I'm creating my own MVP framework, and I'm running into trouble with the generics.

My presenter is defined like this, with an inner class holding references to child elements which are also generic presenters:

public abstract class Presenter<TView extends View, TKey extends Key>
{
    protected final HashMap<String, StageInstances<?, ?>>   _stages;

    public <TChildView extends View, TChildKey extends Key> void addStage(Class<Presenter<TChildView, TChildKey>> stage, String name)
    {
        _stages.put(name, new StageInstances<TChildView, TChildKey>(stage));
    }

    // ...

    protected class StageInstances<TChildView extends View, TChildKey extends Key>
    {
        protected Class<Presenter<TChildView, TChildKey>>       _presenter;
        protected HashMap<Key, Presenter<TChildView, TChildKey>>    _instances;

        public StageInstances(Class<Presenter<TChildView, TChildKey>>   presenter)
        {
            _presenter = presenter;
            _instances = new HashMap<Key, Presenter<TChildView, TChildKey>>();
        }

        public Presenter<?, ?> getInstance(Key key)
        {
            if (!_instances.containsKey(key))
            {
                try
                {
                    _instances.put(key, _presenter.newInstance());
                } catch (Exception e)
                {
                    e.printStackTrace();

                    return null;
                }
            }

            return _instances.get(key);
        }
    }
}

and I have a concrete implementations of this

public class ResultsPresenter extends Presenter<ResultsView, Results>

and

public class SearchPresenter extends Presenter<SearchView, StringKey>
{
    // ...

    public void bind()
    {
        addStage(ResultsPresenter.class, "results");
    }
}

where ResultsView, SearchView extend View and Results, StringKey implement Key

The method addStage(...) throws the following compile-time error:

**The method addStage(Class<Presenter<TChildView,TChildKey>>, String) in the type 
Presenter<SearchView,StringKey> is not applicable for the arguments 
(Class<ResultsPresenter>, String)**

Any help, or better practices, would be greatly appreciated

Answer 1

Try to change the method prototype to:

public <TChildView extends View, TChildKey extends Key> void addStage(Class<? extends Presenter<TChildView, TChildKey>> stage, String name)

pay attention that I changed Class<Presenter<TChildView, TChildKey>> to Class<? extends Presenter<TChildView, TChildKey>> Class<? extends Presenter<TChildView, TChildKey>> . This will allow you to pass Class of Presenter's subclass instead of Presenter itself.

Answer 2

I've not tried it myself, but out of a hunch I'd say that

addStage(Class<Presenter<TChildView, TChildKey>> stage, String name)

should be

addStage(Class<Presenter<? extends TChildView,? extends TChildKey>> stage, String name)

Try Alex suggestion first. Keeps the code more readable and makes somewhat more sense than mine. If both fail, combine them.