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POUsing Java Generics with Enums
 

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    <p><strong>Update:</strong> 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. </p> <p><strong>Original Post:</strong></p> <p>I've been playing around with something this morning. While I know that I <em>could</em> 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.</p> <p>I have created a custom Event hierarchy like so:</p> <pre><code>public abstract class AbstractEvent&lt;S, T extends Enum&lt;T&gt;&gt; { 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; } } </code></pre> <p>With a concrete implementation like so:</p> <pre><code>public class MyEvent extends AbstractEvent&lt;String, MyEvent.Type&gt; { public enum Type { SELECTED, SELECTION_CLEARED }; public MyEvent(String src, Type t) { super(src, t); } } </code></pre> <p>And then I create an event like so:</p> <pre><code>fireEvent(new MyEvent("MyClass.myMethod", MyEvent.Type.SELECTED)); </code></pre> <p>Where my fireEvent is defined as:</p> <pre><code>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; } } } </code></pre> <p>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. </p> <p>It is possible to add the following method to MyEvent:</p> <pre><code>public Type getId() { return super.getId(); } </code></pre> <p>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.</p>
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    1. COChris, can you show the class declaration of class containing `fireEvent`?
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    2. COThe problem was really that my AbstractEvent is much more complicated than what I posted here. I *should* have created the toy implementation and tested it **before** posting! Thank for everyone who helped out! Than answer was that my getId() method was a 'covariant return type' - I had another implementation buried in the class that was defined as Enum<T> return type. When I ditched that, the switch statement started compiling.
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    3. CO-0 for at least coming clean...
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      1. USTom Hawtin - tackline
 

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