Java 泛型:无法转换 List<SubClass>列出<SuperClass>? [英] Java Generics: Cannot cast List<SubClass> to List<SuperClass>?
问题描述
刚遇到这个问题:
List<DataNode> a1 = new ArrayList<DataNode>();
List<Tree> b1 = a1; // compile error: incompatible type
其中 DataNode 类型是 Tree 的子类型.
Where the type DataNode is a subtype of Tree.
public class DataNode implements Tree
令我惊讶的是，这适用于数组:
To my surprise, this works for array:
DataNode[] a2 = new DataNode[0];
Tree[] b2 = a2; // this is okay
这个好像有点奇怪.任何人都可以对此做出解释吗?
This likes a bit strange. Can anyone give an explanation on this?
推荐答案
您在第二种情况下看到的是数组协方差.这是 IMO 的一件坏事，它使数组内的赋值变得不安全——它们可能在执行时失败，尽管在编译时没问题.
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.
在第一种情况下，假设代码确实编译了，然后是:
In the first case, imagine that the code did compile, and was followed by:
b1.add(new SomeOtherTree());
DataNode node = a1.get(0);
你希望发生什么?
你可以这样做:
List<DataNode> a1 = new ArrayList<DataNode>();
List<? extends Tree> b1 = a1;
... 因为那样你只能从 b1 获取东西，而且它们保证与 Tree 兼容.你不能精确地调用 b1.add(...) 因为编译器不知道它是否安全.
... 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.
查看Angelika Langer 的 Java 泛型常见问题解答的这一部分了解更多信息.
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