java - Why can't add element in a upper bound generics List?

Question

I have a list with upper bound generics.

 List<? extends Number> l = new ArrayList<>();
 l.add(new Integer(3));  //ERROR
 l.add(new Double(3.3)); // ERROR

I don't understand the problem, because Integer and Double extend Number.

Answer 1

List<? extends Number> does not mean "a list that can hold all objects of subclasses of Number", it means "a list parameterized to one concrete class that extends Number". It's not the contents of the list itself you are defining, it's what the parameterized type of the actual list-object assigned to the variable can be (boy, this is harder to explain than it is to understand :) )

So, you can do:

List<? extends Number> l = new ArrayList<Integer>();
List<? extends Number> l = new ArrayList<Double>();

If you want a list that is able to hold any object of class Number or its subclasses, just do this:

List<Number> l = new ArrayList<>();

l.add(new Integer(33));
l.add(new Double(33.3d));

(The boxing of the values inserted is unnecessary, but there for clarity..)