为什么输入<K，V>中需要输入参数<K，V>？

Question

In java.util.Map (as shown below), type parameters <K, V> of Entry are over shadowing type parameters <K, V> of Map .

interface Map<K,V> {
      ......
      interface Entry<K,V> {
           .....
      }
}

class DblyLinkList from here has following innerclass DListNode that does not require overshadowing type parameter T .

public class DblyLinkList<T> implements Iterable<T> {
     .......
     class DListNode {
          private T item;
        private DListNode prev;
        private DListNode next;

        DListNode(T item, DListNode p, DListNode n) {
            this.item = item;
            this.prev = p;
            this.next = n;
        }
      }
      ......
}

Can you please help me understand the reason for overshadowing type parameters of Map ?

Answer 1

Interfaces defined inside a class or interface are static, which means they don't have access to the generic parameters of their parent class. So Entry needs the K and V from its parent.

In contrast, the DListNode is an inner-class of DblyLinkList<T> and so does have access to the generic type T - it actually contains a reference back to its containing class too.

Note, if the DListNode was defined as static class (which is how I would define it), it would no longer be an inner class and so not have access to its parents generic parameters - and so would need to be DListNode<K,V> too.