How can I obtain the type parameter of a generic interface from an implementing class?

Question

I have this interface:

public interface EventHandler {
    void handle(T event);
}

And this class implementing it:

Answer 1

When trying to do anything non-trivial with generics and reflection, consider Guava's TypeToken:

private interface Event {}

private interface EventHandler {
    void handle(T event);
}

TypeToken findEventType(final Class handlerClass) throws Exception {
    final TypeToken handlerTypeToken = TypeToken.of(handlerClass);
    final Invokable method = handlerTypeToken.method(EventHandler.class.getDeclaredMethod("handle", Event.class));
    return method.getParameters().get(0).getType();
}

public void testExploreGuavaTypeTokens() throws Exception {
    class MyEvent implements Event {}

    class MyEventHandler implements EventHandler {
        @Override public void handle(final MyEvent event) {}
    }

    assertEqual(MyEvent.class, findEventType(MyEventHandler.class).getRawType());
}

(Note that the TypeToken returned by findEventType() could contain much richer type information than a Class can represent; that's why it's the caller's decision whether to simplify it via getRawType().)