Why is it not possible to call List not with List even if Integer extends Number?

Question

I was wondering why is it not possible to call List not with List> even Integer is an extended class of the abstract class

Answer 1

Because you could e.g. add an instance of a different subclass of Number to List, e.g., an object of type Double, but obviously you shouldn't be allowed to add them to List:

public void myMethod(List list) {
    list.add(new Double(5.5));
}

If you were allowed to call myMethod with an instance of List this would result in a type clash when add is called.