Java中的instanceof - 为什么不编译？ [英] instanceof in Java - why doesn't this compile?

问题描述

class A {

    public static void main(String...args) {
        Integer var = 10; 

        if(var instanceof Character)  // Line1
            System.out.println("var is a Character");
    }
}

我知道第1行将无法编译，因为编译器发现 var 不是字符。

I know Line 1 will not compile because the compiler has found that var is not a Character.

我无法理解为什么编译器会抛出错误而不是返回 false 或 true 。

What I fail to understand is why the compiler throws an error instead of returning false or true.

如果编译器返回 false 或 true （即像处理基于if的常规验证一样处理 instanceof 操作，然后它会更有用..它不会吗？

If the compiler returns false or true (i.e treating the instanceof operation like a regular if-based validation), then it be much more useful.. would it not?

或者我错过了一些明显的东西？

Or am I missing something obvious?

推荐答案

根据JLS§15.20.2：

如果将 RelationalExpression 转换为 ReferenceType 作为编译时错误被拒绝，那么 instanceof 关系ex压力同样产生编译时错误。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.

RelationalExpression 是 instanceof 的第一个操作数， ReferenceType 是第二个。

RelationalExpression is the first operand of instanceof and ReferenceType is the second.

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