Wildcard captures/Generics

Question

Consider

class MyClass{
    List<? extends Number> nums=  new ArrayList<Integer>();
    nums.add(3.14);//Compile error
}

In the error's description we have: excepted add(int, Object), found add(int,CAP#1) . What's defenition of CAP#1 ? Why this error caused?

Answer 1

This is because nums is a List<? extends Number> List<? extends Number> , so the compiler knows that it is a List of Number or some subclass of Number, but it does not know which. Therefore, you will never be allowed to add anything to such a list. Here's an example of what this means:

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

and

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

are both valid assignments. However, if you do:

nums.add(new Integer(4));

the compiler will not accept this as it cannot be certain that nums is a List of Integer .