是什么让以下代码打印错误？ [英] What makes the following code print false?

问题描述

public class Guess {
    public static void main(String[] args){
        <sometype> x = <somevalue>;
        System.out.println(x == x);
    }
}

我必须更改sometype和somevalue以便返回假？有可能吗？

i have to change sometype and somevalue so that it returns false? is it possible?

推荐答案

一：

float x = Float.NaN; 

二：

double x = 0.0/0.0;

为什么？

如前所述， NaN 永远不会等于另一个 NaN - 请参阅 http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html

As mentioned here already, NaN is never equal to another NaN - see http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html

那么为什么这不会返回false？

So why this is not returning false?

Float x = Float.NaN; 

答案是，这里有一个引用赋值，而不是原始赋值。并且在后台有一个小自动拳击。这等于：

The answer is that here, instead of a primitive assignment, there is a reference assignment. And there is a little auto boxing in the background. This is equal to:

Float x = new Float(Float.NaN); 

等于：

Float x = new Float(0.0f / 0.0f); 

这里x是对Float对象的引用，==运算符测试引用相等，而不是值。

Here x is a reference to a Float object, and the == operator tests reference equality, not value.

要看到此返回false，测试应该是：

To see this returning false as well, the test should have been:

x.doubleValue()==x.doubleValue();

确实返回错误

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