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?

Related: http://stackoverflow.com/q/3546745/1065197. It is a long read but it is worth. — Luiggi Mendoza, Sep 19 '13 at 04:52
Here is a sound byte version that may be illuminating also: http://stackoverflow.com/questions/2723397/java-generics-what-is-pecs — clstrfsck, Sep 19 '13 at 05:04
@LuiggiMendoza That post is about nested wildcards, which don't capture. — Paul Bellora, Sep 19 '13 at 11:19
Have a look at the Java Tutorials article on wildcard capture: http://docs.oracle.com/javase/tutorial/java/generics/capture.html — Paul Bellora, Sep 19 '13 at 11:21

score 2 · Answer 1 · answered Sep 19 '13 at 04:55

This is because nums is a 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.

@St.Antario: It is just something your compiler uses to internally represent the captured type variable. Basically, you see this when you have wildcards, and it represents the unknown type the wildcard represents. — newacct, Sep 20 '13 at 02:09

Wildcard captures/Generics

1 Answers1