Generics and Class.asSubclass

I've always thought the following should work. I get an object which I know is a Class<X> where X extends some class Xyz. In order to make it type-safe I wanted to use Class.asSubclass like in the fo开发者_开发问答llowing method:

private Class<? extends Xyz> castToXyzClass(Object o) {
    final Class<?> resultClass = (Class<?>) o;
    final Class<? extends Xyz> result = Xyz.class.asSubclass(resultClass);
    return result;
}

However, in Eclipse it doesn't work, the only solution I see is an unchecked cast. I'd bet the above code must work, I've used something like this already... no idea what's wrong here.

asSubclass() operates on the object it's called on, not on its parameter - not what one is used to, but it reads quite well. You just have to do this:

final Class<? extends Xyz> result = resultClass.asSubclass(Xyz.class);

The asSubclass is a bit of a confusing name because you're not obtaining a Class object representing the subclass, you're obtaining the same class object that is retyped to reflect that it is a subclass of some parent class.

In fact, this method is fairly single purpose (and I think you've found it)...it's to take a raw or wildcarded class and get a better type parameter with a runtime check. It's not needed when you don't have gaps in your type information:

class Super {}
class Sub extends Super {}

//...
Class<Sub> subClass = Sub.class;

//both work, but the latter introduces a redundant runtime check
Class<? extends Super> subOfSuper1 = subClass;
Class<? extends Super> subOfSuper2 = subClass.asSubclass(Super.class);