instanceof - 不兼容的条件操作数类型 [英] instanceof - incompatible conditional operand types

问题描述

以下编译正常：

  Object o = new Object();
  System.out.println(o instanceof Cloneable);

但这不是：

  String s = new String();
  System.out.println(s instanceof Cloneable);

抛出编译器错误。

问题是什么？

推荐答案

您问题的更明确的表现如下：

A more blatant incarnation of your problem is the following:

if ("foo" instanceof Number)
   // "Incompatible conditional operand types String and Number"

这在 JLS 15.20.2类型比较运算符 instanceof ：

RelationalExpression:
       RelationalExpression instanceof ReferenceType

如果将 RelationalExpression 转换为 ReferenceType 作为编译时错误而被拒绝，那么 instanceof 关系表达式同样会产生编译时错误。在这种情况下， instanceof 表达式的结果永远不会为真。

If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.

也就是说，因为这个强制转换表达式会产生编译时错误：

That is, since this cast expression generates a compile time error:

(Number) "foo"

所以必须这个表达式：

("foo" instanceof Number)

---

你的情况有点微妙，但原理是一样的：

---

Your case is a bit more subtle, but the principle is the same:

• String 是最终类


• 字符串未实现 Cloneable


• 因此你不能做（Cloneable）aString


• 所以你也做不到 aString instanceof Cloneable

• String is a final class
• String does not implement Cloneable
• Therefore you can't do (Cloneable) aString
• Therefore also you can't do aString instanceof Cloneable

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