Using Java Generics with Enums

Question

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 call to event.getId() results in the compiler telling me that I cannot switch on Enums, only convertible int values or enum constants.

It is possible to add the following method to MyEvent:

public Type getId()
{
    return super.getId();
}

Once I do this, everything works exactly as I expected it to. I'm not just interested in finding a workaround for this (because I obviously have one), I'm interested in any insight people might have as to WHY this doesn't work as I expected it to right off the bat.

Answer 1

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.

Answer 2

This is not related to generics. Switch statement for enum in java can only use values of that particular enum, thus it's prohibited to actually specify enum name. This should work:

switch(event.getId()) {
   case SELECTED:
         l.selected(event);
         break;
   case SELECTION_CLEARED:
         l.unselect(event);
         break;
}

Update: Ok, here's an actual code (which I had to change a little bit to get it to compile with no dependencies) that I've copy / pasted, compiled and ran - no errors:

AbstractEvent.java

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;
    }
}

MyEvent.java

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

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

Test.java

public class Test {
  public static void main(String[] args) {
      fireEvent(new MyEvent("MyClass.myMethod", MyEvent.Type.SELECTED));
  }

  private static void fireEvent(MyEvent event) {
        switch(event.getId()) {
            case SELECTED:
                System.out.println("SELECTED");
                break;
            case SELECTION_CLEARED:
                System.out.println("UNSELECTED");
                break;
         }
    }
}

This compiles and runs under Java 1.5 just fine. What am I missing here?

Answer 3

Works for me.

Complete cut-out-and-paste test program:

enum MyEnum {
    A, B
}
class Abstract<E extends Enum<E>> {
    private final E e;
    public Abstract(E e) {
        this.e = e;
    }
    public E get() {
        return e;
    }
}
class Derived extends Abstract<MyEnum> {
    public Derived() {
        super(MyEnum.A);
    }
    public static int sw(Derived derived) {
        switch (derived.get()) {
            case A: return 1;
            default: return 342;
        }
    }
}

Are you using some peculiar compiler?

Answer 4

In very short, because T is erased to an Enum class, not an Enum constant, so the compiled statement looks like you are switching on the result of getID being Enum, as in this signature:

Enum getId();

When you override it with a specific type, then you are changing the return value and can switch on it.

EDIT: The resistance made me curious, so I whipped up some code:

public enum Num {
    ONE,
    TWO
}
public abstract class Abstract<T extends Enum<T>> {
    public abstract T getId();
}

public abstract class Real extends Abstract<Num> {

}

public static void main(String[] args) throws Exception {
    Method m = Real.class.getMethod("getId");
    System.out.println(m.getReturnType().getName());
}

The result is java.lang.Enum, not Num. T is erased to Enum at compile time, so you can't switch on it.

EDIT:

I tested the code samples everyone is working with, and they work for me as well, so although I suspect that this underlies the issue in the more complex real code, the sample as posted actually compiles and runs fine.