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);

但这不会:

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

抛出编译器错误.

有什么问题?

解决方案

您的问题的一个更明显的化身如下:

if ("foo" instanceof Number)//不兼容的条件操作数类型字符串和数字"

这是在 JLS 15.20.2 中指定的类型比较运算符instanceof:

<块引用>

关系表达式:RelationalExpression 实例的 ReferenceType

如果将 RelationalExpression 强制转换为 ReferenceType 会因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.

也就是说，因为这个强制转换表达式会产生一个编译时错误:

(数字) "foo"

这个表达式也必须:

("foo" instanceof Number)

<小时>

你的情况有点微妙，但原理是一样的:

• String 是最终类
• String 没有实现 Cloneable
• 因此你不能做(Cloneable) aString
• 因此你也不能做aString instanceof Cloneable

The following compiles fine:

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

But this doesn't:

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

A compiler error is thrown.

What is the problem?

解决方案

A more blatant incarnation of your problem is the following:

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

This is specified in JLS 15.20.2 Type comparison operator instanceof:

RelationalExpression:
       RelationalExpression instanceof ReferenceType

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.

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

(Number) "foo"

so must this expression:

("foo" instanceof Number)

---

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

• 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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