Java How to Program, Early Objects, 11th Edition

... List<Number> would also work on a List<Integer>. Consider the fact that you can add a Double object to a List<Number> because a Double is a Number, but you cannot add a Double object to a List<Integer> because a Double is not an Integer. Thus, the subtype relationship does not hold.

How do we create a more flexible version of method sum that can total the elements of any List containing elements of any subclass of Number? This is where wildcard type arguments are important. Wildcards enable you to specify method parameters, return values, variables or fields, and so on, that act as supertypes or subtypes of parameterized types. In Fig. 20.11, method sum’s parameter is declared in line 52 with the type:

List<? extends Number>

A wildcard type argument ...