Java泛型：无法投射List< SubClass>列出< SuperClass&gt ;? [英] Java Generics: Cannot cast List<SubClass> to List<SuperClass>?

问题描述

  List< DataNode> a1 = new ArrayList< DataNode>（）; 
列表< Tree> b1 = a1; //编译错误：不兼容类型
  

类型DataNode是Tree的子类型。

  public class DataNode implements Tree 
  

令我惊讶的是，这对数组有效：

  DataNode [] a2 = new DataNode [0]; 
 Tree [] b2 = a2; //这是好的
  

这有点奇怪。任何人都可以对此作出解释吗？

解决方案

你在第二种情况下看到的是数组协变。这是一个坏的事情，海事组织，使阵列内的任务不安全 - 他们可以在执行时失败，尽管在编译时很好。

在第一种情况下，想象一下代码完成了编译，然后是：

  b1.add（new SomeOtherTree（）） ; 
 DataNode node = a1.get（0）; 
  

您预计会发生什么？

你可以这样做：

  List< DataNode> a1 = new ArrayList< DataNode>（）; 
列表< ;?扩展树> b1 = a1; 
  

...因为您只能从 b1 ，并且它们保证与 Tree 兼容。您无法正确调用 b1.add（...），因为编译器不知道它是否安全。

查看 Angelika Langer的Java Generics常见问题的这一部分 a>获取更多信息。

Just come across with this problem:

List<DataNode> a1 = new ArrayList<DataNode>();
List<Tree> b1 = a1;  // compile error: incompatible type

Where the type DataNode is a subtype of Tree.

public class DataNode implements Tree

To my surprise, this works for array:

DataNode[] a2 = new DataNode[0];
Tree[] b2 = a2;   // this is okay

This likes a bit strange. Can anyone give an explanation on this?

解决方案

What you're seeing in the second case is array covariance. It's a bad thing IMO, which makes assignments within the array unsafe - they can fail at execution time, despite being fine at compile time.

In the first case, imagine that the code did compile, and was followed by:

b1.add(new SomeOtherTree());
DataNode node = a1.get(0);

What would you expect to happen?

You can do this:

List<DataNode> a1 = new ArrayList<DataNode>();
List<? extends Tree> b1 = a1;

... because then you can only fetch things from b1, and they're guaranteed to be compatible with Tree. You can't call b1.add(...) precisely because the compiler won't know whether it's safe or not.

Have a look at this section of Angelika Langer's Java Generics FAQ for more information.

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