Question

So I have this interface

public interface EventHandler<E extends EventObject>
{   
    public void handleEvent(E event);
}

and I want to derive classes for it that handle events, like so:

public class WorkingHandler implements EventHandler<MouseEvent>
{
    public void handleEvent(MouseEvent event)
    {
        //handle event
    }
}

Above example works fine. But compiler won’t let me implement EventHandler multiple times, due to the
extremely frustrating Type Erasure.

public class NotWorkingHandler implements EventHandler<MouseEvent>, EventHandler<KeyEvent>
{
    public void handleEvent(MouseEvent event)
    {
        //handle mouse event
    }

    public void handleEvent(KeyEvent event)
    {
        //handle key event
    }
}

Several questions…

1. Why does Java not allow Type Erasure? IIRC it is something to do with being compatible with earlier versions of Java, correct?

EDIT: I mean why does Java have Type Erasure (had it the other way around)

1. Is there any sort of “mod”/”extension” (for lack of a better word) or programming languages that allow me to get around Type Erasure? As I am not using older versions of Java I don’t care about my code being compatible with older code.

2. Are there any workarounds, within the Java language, that I can use to get around Type Erasure?

3. If no, what are some alternate ways to code event handling in my program that keeps the compiler happy?

Answer 1

I’m not sure that your understanding of Type Erasure in Java is correct. As explained here, the type of your generic class is lost at runtime.

In response to your questions:

1. I think you are right that Java implements type erasure to enable compatibility with previous versions of the langauge.
2. No I don’t think there is a “mod.”
3. No there are no workarounds (types are removed at compile time), but you can use reflection.
4. See below.

I think a simpler solution would be to so something like this:

public class WorkingHandler implements EventHandler<EventObject>
{
    public void handleEvent(EventObject event)
    {
        // handle event
        if (event instanceof MouseEvent)
            handleMouseEvent((MouseEvent) event);
        ...
    }

    ...

    private void handleMouseEvent(MouseEvent event)
    {
        // handle mice
    }
}