Java是如何工作的？ [英] Java isNan how it works?

问题描述

<$ p

如果指定的数字是
*非数字（NaN）值，则返回{@code true};否则返回{@code false} 。
*
* @param v要测试的值。
* @return {@code true}如果参数是NaN;
* {@code false}否则。
* /
static public boolean isNaN（float v）{
return（v！= v）;



$ b $ p
$ b

我不明白它是如何工作的，当这个方法可以返回 true ？

解决方案

对于某些操作，例如：
$ b $ pre $ System.out.println（Float.isNaN（0.0f / 0.0f））;
System.out.println（Double.isNaN（Math.sqrt（-1）））;


基本上， NaN 代表一个未定义的值。 0.0 / 0.0 的值是 NaN 和 Nan！= NaN 。这似乎是合乎逻辑的，因为 Math.sqrt（-1）也给你 NaN 。

查看 Double.NaN ：

相当于Double.longBitsToDouble（0x7ff8000000000000L）

返回的值然后 Double.longBitsToDouble（） $ b

如果参数是 0x7ff0000000000001L 到 0x7fffffffffffffffL ，或者在 0xfff0000000000001L 到 0xffffffffffffffffL ，结果是 NaN 。 Java提供的IEEE 754浮点操作不能区分具有不同位模式的相同类型的两个NaN值。





I was looking at the openjdk-1.7.0_25 source code and I have seen this method:

/**
 * Returns {@code true} if the specified number is a
 * Not-a-Number (NaN) value, {@code false} otherwise.
 *
 * @param   v   the value to be tested.
 * @return  {@code true} if the argument is NaN;
 *          {@code false} otherwise.
 */
static public boolean isNaN(float v) {
    return (v != v);
}

I can't understand how it works, when this method can return true?

解决方案

That method can return true for certain operations, for example:

System.out.println(Float.isNaN(0.0f / 0.0f));
System.out.println(Double.isNaN(Math.sqrt(-1)));

Basically, NaN represents an undefined value. The value of 0.0 / 0.0 is NaN, and Nan != NaN. It may seem logical because Math.sqrt(-1) also gives you NaN.

See the javadoc of Double.NaN:

It is equivalent to the value returned by Double.longBitsToDouble(0x7ff8000000000000L)

And then Double.longBitsToDouble():

If the argument is any value in the range 0x7ff0000000000001L through 0x7fffffffffffffffL or in the range 0xfff0000000000001L through 0xffffffffffffffffL, the result is a NaN. No IEEE 754 floating-point operation provided by Java can distinguish between two NaN values of the same type with different bit patterns.

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