使用Java泛型與HashMap的

Question

我有一個B類和C這兩種擴展A類：

public class B extends A {...} 
public class C extends A {...} 

如何使用Java泛型有一個HashMap這種方式？

B b = new B(); 
C c = new C(); 

Map<String, ? extends A> map = new HashMap<String, A>(); 

map.put("B", b); 
map.put("C", c); 

Eclipse中始終顯示錯誤：

The method put(String, capture#1-of ? extends A) in the type Map is not applicable for the arguments (String, B)

和

The method put(String, capture#1-of ? extends A) in the type Map is not applicable for the arguments (String, C)

Answer 1

只要改變HashMap的類型<String, A>

Map<String, A> map = new HashMap<String, A>();