Java：使用接口指针转换为通用 [英] Java: Casting to Generic with Interface Pointers

问题描述

在下面的示例代码中，两个类 EventA 和 EventB 都实现 。 Java可以自动将 EventA 或 EventB 转换为历史这些对象之一作为参数传递，如下面的 examineEvent 方法中。但是，当引入一个泛型时，Java不再能够强制转换。从列表到列表< Historical> - 除非目标函数c> findClosestValidEventIndex ）使用 List< ;? extends Historical> 。

In the following sample code two classes EventA and EventB both implement the interface Historical. Java can automatically cast an EventA or EventB to Historical when one of these objects is passed as a parameter, as in the examineEvent method below. However, Java is no longer able to cast when a generic is introduced ie. from List<EventA> to List<Historical> -- Unless the target function (in this case findClosestValidEventIndex) is declared using List<? extends Historical>.

有人可以解释为什么必须这样做吗？在我看来，在一个通用的接口的使用应该自动暗示<？扩展接口> 。

Can someone explain why this must be? It seems to me that the very use of an interface in a generic should automatically imply the <? extends Interface>.

public class SampleApplication {

   public interface Historical {
      public DateTime getDate();
   }   

   public static class EventA implements Historical {
      private DateTime date;
      @Override
      public DateTime getDate() {
         return date;
      }
   }

   public static class EventB implements Historical {
      private DateTime date;
      @Override
      public DateTime getDate() {
         return date;
      }
   } 

   private static int findClosestValidEventIndex(List<Historical> history, DateTime when) {
      // do some processing
      return i;
   }

   private static int examineEvent(Historical event){
      return j;
   }

   public static void main(String[] args) {
      DateTime target = new DateTime();
      // AOK
      EventA a = new EventA(target);
      int idy = examineEvent(a);
      // Type Error --- Unless 
      List<EventA> alist = new ArrayList<EventA>();
      int idx = findClosestValidEventIndex(alist, target);
   }
}

推荐答案

列表< EventA> 不是 列表<历史> 。想象：

List<EventA> list = ...;
List<Historical> h = (List<Historical>) list;
h.add(new EventB()); //type-safety of list is compromised
for (EventA evt : list) { // ClassCastException - there's an EventB in the lsit
   ...
}

列表< ;? extends Historical> 意味着历史的一个特定子类型的列表，并且您不能添加任何东西，因为在编译时你不知道什么类型。

List<? extends Historical> means "a list of a one specific subtype of Historical", and you cannot add anything to it, because at compile time you don't know what the type is.

这篇关于Java：使用接口指针转换为通用的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！