instanceof中的模式匹配有时不能解决理论上可解决的范围

Question

import org.w3c.dom.Comment;
import org.w3c.dom.Document;
import org.w3c.dom.Node;

List<Node> list = new ArrayList<>();
Node x = list.get(0);
if (!(x instanceof Comment comment)) {
    if (!(x instanceof Document doc && doc.getFirstChild() instanceof Comment comment)) {
        return null;
    }
}
System.out.println(comment.ToString()); // cannot resolve comment

变量comment无法解析，但它应该解析--我们讨论了两个互斥的快乐路径，每个路径都分配comment comment-system out应该能够看到comment，否则将触发return null。

Answer 1

不能使用模式匹配在两个不同的地方声明同一个变量。它们仍然是两个不同的变量，具有两个不同的作用域。

也许，我们可以通过给它们起不同的名字来清除混乱：

if (!(x instanceof Comment comment)) {
    if (!(x instanceof Document doc && doc.getFirstChild() instanceof Comment comment2)) {
        return null;
    }
    //comment2 and doc are in scope here,
    //but their scope ends with an enclosing block
    System.out.println(comment2.toString());
}
//FAIL! You can't access comment2 outside its scope
System.out.println(comment2.toString()); 
//FAIL! You can't access comment here because 
//the compiler can't guarantee that the block above exited.
System.out.prinltn(comment.toString()

您可以使用如下简单示例来验证模式匹配变量是否仍保留在其封闭范围内：

{
    Object o = null;
    if(!(o instanceof String s)) {
          return;
    }
    System.out.println(s); //OK
}
System.out.println(s); //Doesn't work

Comment comment;
if (!(x instanceof Comment c)) {
    if (!(x instanceof Document doc && doc.getFirstChild() instanceof Comment c)) {
        return null;
    }
    comment = c;
} else {
    comment = c;
}
System.out.println(comment.toString());

private static Comment extractComment(Node x) {
    if (!(x instanceof Comment comment)) {
        if (!(x instanceof Document doc && doc.getFirstChild() instanceof Comment comment)) {
            return null;
        }
        return comment;
    } 
    return comment;
}

甚至：

private static Comment extractComment(Node x) {
    if (x instanceof Comment comment) {
        return comment;
    }     
    if (x instanceof Document doc && doc.getFirstChild() instanceof Comment comment) {
        return comment;
    }
    return null;
}