java中的instanceof运算符用于比较不同的类 [英] instanceof operator in java for comparing different classes

问题描述

我试图了解Java中的instanceof运算符是如何工作的并且面临一个非常奇怪的问题。

I was trying to see how instanceof operator in Java works and am facing a very odd issue.

public static void main(String[] args) {
    Map m = new HashMap();
    System.out.println("m instanceof Date: " + (m instanceof Date));
}

上述内容按预期返回false。但是，

The above returns false as expected. However,

public static void main(String[] args) {
    HashMap m = new HashMap();
    System.out.println("m instanceof Date: " + (m instanceof Date));
}

这甚至都没有编译。我收到错误

This does not even compile. I get an error

inconvertible types
found   : java.util.HashMap
required : java.util.Date

我在这里缺少什么？
我正在使用IntelliJ Idea 11.

What am I missing here? I am using IntelliJ Idea 11.

推荐答案

来自Java语言规范3.0，第15.20.2节：

From the Java Language Specification 3.0, section 15.20.2:

如果对于ReferenceType的RelationalExpression的强制转换将被
作为编译时错误拒绝，则关系实例为
表达式同样会产生编译时错误。在这样的
情况下，instanceof表达式的结果永远不会是
true。

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.

因为你可以不能将 HashMap 中的演员表编译成日期，你无法编译 instanceof 在两者之间进行测试。

Since you can't compile a cast from a HashMap to a Date, you can't compile an instanceof test between the two either.

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