Java Generics for Upper bound & lower bound wild cards

问题

I was reading java generics, I came across an interesting query. My question is as follows.

1. For an upper bounded wildcard

   public static void printList(List<? extends Number> list) {
       for (int i = 0; i < 10; i++) {
           list.add(i);// gives compilation error
       }
   }
   

2. For a lower bounded wildcard

   public static void printList(List<? super Integer> list) {
       for (int i = 0; i < 10; i++) {
           list.add(i);// successfully compiles
       }
   }
   

I am confused with this because looking at the Sun Oracle documentation I understand that the code should compile for point 1 as well

Upper Bound Wildcard Lower Bound Wildcard

Can anyone please help me to understand this?

回答1:

This is because when you are using upper bound, you cannot add elements to collection, only read them.

this means that these are some of legal assignments:

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

so you cannot guarantee that when adding object, it will hold correct types of objects. for better explatation please follow: How can I add to List<? extends Number> data structures?

回答2:

actually, fortunately the same scenario, I got the answer under next pages of Sun Oracle documentation. please find the link below. may be useful to someone who would be searching in future.

Wildcard Capture

来源：https://stackoverflow.com/questions/16189229/java-generics-for-upper-bound-lower-bound-wild-cards