Advanced polymorphism in Java

Question

I have classes in advanced programming at my university and I have a little trouble understanding how this code works.

public final class GenericClass<T> {
    private void overloadedMethod(Collection<?> o) {
        System.out.println("Collection<?>");
    }

    private void overloadedMethod(List<Number> o) {
        System.out.println("List<Number>");
    }

    private void overloadedMethod(ArrayList<Integer> o) {
        System.out.println("ArrayList<Integer>");
    }

    public void method(List<T> l) {
        overloadedMethod(l);
    }

    public static void main(String[] args) {
        GenericClass<Integer> test = new GenericClass<Integer>();
        test.method(new ArrayList<Integer>());
    }
}

Why does this code print "Collection<?>"?

Answer 1

The declaration of method(List<T> l) does not specify any bounds on the type T. There is no guarantee that T is Number or a subclass of Number. Therefore, the compiler can only decide that this method calls overloadedMethod(Collection<?> o).

Remember: after compilation, the information about generics is not available anymore in the class files.