Java Generics: Wildcard capture misunderstanding

Question

Reading the Java online tutorial I haven\'t understand anything about wildcard capture. For example:

    import java.util.List;
    public class WildcardErro         


        

Answer 1

According to Get-Put principle:

1. If you have extends wildcard as in List, then:

   1A. You can get from the structure using Something or its superclass reference.

   void foo(List nums) {
      Number number = nums.get(0); 
      Object number = nums.get(0); // superclass reference also works.
   }
   

   1B. You cannot add anything to the structure (except null).

   void foo(List nums) {
      nums.add(1); Compile error
      nums.add(1L); Compile error
      nums.add(null); // only null is allowed.
   }
   

2. Similarly, if you have super wildcard as in List, then:

   2A. You can add to the structure which is Something or its subclass. For eg:

   void foo(List nums) {
       nums.add(1); // Integer is a subclass of Number
       nums.add(1L); // Long is a subclass of Number
       nums.add("str"); // Compile error: String is not subclass of Number         
   }
   

   2A. You cannot get from the structure (except via Object reference). For eg:

   void foo(List nums) {
       Integer num = nums.get(0); // Compile error
       Number num = nums.get(0); // Compile error
       Object num = nums.get(0); // Only get via Object reference is allowed.        
   }
   

Coming back to OP's question, List i is just the short representation for List i. And since it's a extends wildcard, the set operation fails.

The final piece remaining is WHY the operation fails ? Or why the Get-Put principle in the first place? - This has to do with type safety as answered by Jon Skeet here.