Using Java Generics with Enums

Asked 2009-07-23T17:31:03.170

Update: Thanks for everyone who helped out - the answer to this one lay in what I wasn't noticing in my more complex code and what I didn't know about the Java5 covariant return types.

Original Post:

I've been playing around with something this morning. While I know that I could tackle this whole problem differently, I'm finding myself obsessed with figuring out why it isn't working the way that I was expecting. After spending some time reading around on this, I find I'm no closer to understanding, so I offer it up as a question to see if I'm just being stupid or if there is really something I don't understand going on here.

I have created a custom Event hierarchy like so:

public abstract class AbstractEvent<S, T extends Enum<T>>
{
    private S src;
    private T id;

    public AbstractEvent(S src, T id)
    {
        this.src = src;
        this.id = id;
    }

    public S getSource()
    {
        return src;
    }

    public T getId()
    {
        return id;
    }
}

With a concrete implementation like so:

public class MyEvent
extends AbstractEvent<String, MyEvent.Type>
{
    public enum Type { SELECTED, SELECTION_CLEARED };

    public MyEvent(String src, Type t)
    {
        super(src, t);
    }
}

And then I create an event like so:

fireEvent(new MyEvent("MyClass.myMethod", MyEvent.Type.SELECTED));

Where my fireEvent is defined as:

protected void fireEvent(MyEvent event)
{
    for(EventListener l : getListeners())
    {
        switch(event.getId())
        {
            case SELECTED:
                l.selected(event);
                break;
            case SELECTION_CLEARED:
                l.unselect(event);
                break;
         }
    }
}

So I thought that this would be pretty straightforward but it turns out that the ca

Edit Report

2 Answers

Yishai is right, and the magic phrase is "covariant return types" which is new as of Java 5.0 -- you can't switch on Enum, but you can switch on your Type class which extends Enum. The methods in AbstractEvent that are inherited by MyEvent are subject to type erasure. By overriding it, you're redirecting the result of getId() towards your Type class in a way that Java can handle at run-time.

answered 2009-07-23T17:52:31.113

I just tried this (copy-pasted your code) and I can't duplicate a compiler error.

public abstract class AbstractEvent<S, T extends Enum<T>>
{
    private S src;
    private T id;

    public AbstractEvent(S src, T id)
    {
        this.src = src;
        this.id = id;
    }

    public S getSource()
    {
        return src;
    }

    public T getId()
    {
        return id;
    }
}

and

public class MyEvent extends AbstractEvent<String, MyEvent.Type>
{

    public enum Type
    {
        SELECTED, SELECTION_CLEARED
    };

    public MyEvent( String src, Type t )
    {
        super( src, t );
    }


}

and

public class TestMain
{
    protected void fireEvent( MyEvent event )
    {
        switch ( event.getId() )
        {
            case SELECTED:
            break;
            case SELECTION_CLEARED:
            break;
        }
    }
}

answered 2009-07-23T18:08:01.877

Using Java Generics with Enums

2 Answers

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