Java中是否可能有不确定数量的类型参数？

Question

I'm making an event system in Java and I've run into a problem in writing the Listener part.

This is my current Listener class:

public interface Listener<E extends Event<?>> {
    public void handleEvent(E event);
}

I want to keep it expandable in a way that I can have one Listener class that can be flexible to any Event type, ie Listener<Foo> and Listener<Bar> instead of FooListener and BarListener , but I also want implementing classes to be able to listen to multiple events.

My problem is that a class cannot implement the Listener class with two different type parameters.

public class MultiListener implements Listener<Foo>, Listener<Bar> {
    // does not work
}

I know it's possible for a method to have an indefinite number of parameters, like this:

public void toInfinityAndBeyond(String... lotsOfStrings) {

}

But can my Listener class have an indefinite number of type parameters to listen to multiple events?

public class MultiListener implements Listener<ThisEvent, ThatEvent, AnotherEvent> {
    // hypothetical
}

Answer 1

The problem is that Listener<Foo> and Listener<Bar> are really the exact same interface, due to generic type erasure . So this isn't possible (AFAIK).

Answer 2

I don't fully understand generics, but I've found they tend to make Java, a statically typed language, a lot more static. This can be a good thing. But what you are trying to do is rather dynamic.

So in this case I would dump the generics. Make Foo and Bar extend or implement Event. (Event with no "<>".) Make sure the Event class has enough methods that the code in Listener's handleEvent() doesn't need multiple if 's using instanceof ; so it can just deal with plain Event objects. (Mind, the occasional instanceof won't hurt anything, usually.)