Generics and Class.asSubclass

Question

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 following 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.

score 22 · Accepted Answer · answered May 09 '11 at 16:23

22

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);

answered May 09 '11 at 16:23

Michael Borgwardt

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9

I personally think it would read better if it were `resultClass.asSubclassOf(Xyz.class)`. – Mark Peters May 09 '11 at 16:39

score 10 · Answer 2 · answered May 09 '11 at 16:34

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);

Generics and Class.asSubclass

2 Answers2

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