Java APIs

Hello!

With my friends I've stumbled upon a problem in understanding how the compiler (and Java Language Specification too) treats a specific generic declaration.

public class Main {
    public static void main(String args[]) {
        ArrayList<Integer> inParam2 = new ArrayList<Integer>();
        List<Number> returnValue2 = justDoIt(inParam2);
    }

    public static <E extends Number> List<E> justDoIt(List<? super E> nums) {
        return null;
    }
}

The code compiles fine with javac and throws compile error in eclipse. The latter, I believe, is the proper behaviour - what am I missing? The method justDoIt could be possibly resolved in two ways:

public static List<Number> justDoIt(List<Number> nums)

or

public static List<Integer> justDoIt(List<Integer> nums)

both will give reasonable and well-documented compile time errors. The first is because ArrayList<Integer> can't be passed to List<Number> (and even more - Integer is not a supertype of Number, therefore <? super Number> should fail). The latter is because List<Integer> value can't be assigned to List<Number> na main() for obvious reasons. I suppose from these two the compiler would resolve the mentioned static function as the one with Number throughout the declaration... but what really happens? How come the code compiles?

I would be very grateful for any insights.


Best regards,
Mateusz