介绍

在学习JavaScript时看到了Number类型中的NaN和Infinity，因为概念本身是通用的，所以趁机总结了一下Java中的NaN和Infinity。

基本

Java的Float型和Double型各有三个很特别的常量：NaN(非数），POSITIVE_INFINITY(正无穷)，NEGATIVE_INFINITY(负无穷)

Float

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/**
 * A constant holding the positive infinity of type
 * {@code float}. It is equal to the value returned by
 * {@code Float.intBitsToFloat(0x7f800000)}.
 */
public static final float POSITIVE_INFINITY = 1.0f / 0.0f;

/**
 * A constant holding the negative infinity of type
 * {@code float}. It is equal to the value returned by
 * {@code Float.intBitsToFloat(0xff800000)}.
 */
public static final float NEGATIVE_INFINITY = -1.0f / 0.0f;

/**
 * A constant holding a Not-a-Number (NaN) value of type
 * {@code float}.  It is equivalent to the value returned by
 * {@code Float.intBitsToFloat(0x7fc00000)}.
 */
public static final float NaN = 0.0f / 0.0f;

Double

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/**
 * A constant holding the positive infinity of type
 * {@code double}. It is equal to the value returned by
 * {@code Double.longBitsToDouble(0x7ff0000000000000L)}.
 */
public static final double POSITIVE_INFINITY = 1.0 / 0.0;

/**
 * A constant holding the negative infinity of type
 * {@code double}. It is equal to the value returned by
 * {@code Double.longBitsToDouble(0xfff0000000000000L)}.
 */
public static final double NEGATIVE_INFINITY = -1.0 / 0.0;

/**
 * A constant holding a Not-a-Number (NaN) value of type
 * {@code double}. It is equivalent to the value returned by
 * {@code Double.longBitsToDouble(0x7ff8000000000000L)}.
 */
public static final double NaN = 0.0d / 0.0;

NaN

NaN是Not a Number的缩写，表示未定义或者不可表示的值，NaN有一个特性，那就是它与任何数都不相等，包括它自己。判断一个数是不是NaN，只能使用Float或者Double的isNaN()方法，那么这个方法是怎么判断的呢？我们看一看(查看的是Float版，Double除了参数类型都一样，下面二者没有区别的部分都直接使用Float的内容来说明)：

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/**
 * 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.
 */
public static boolean isNaN(float v) {
    return (v != v);
}

所以，跟自己不相等的数字，就是NaN.

计算中产生的NaN

什么时候会产生NaN呢？首先，任何与NaN进行的运算，其结果都是NaN.比如下面的运算，不要去想太多，结果就是NaN

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public class Test {
	public static void main(String[] args) {
		var x = Float.NaN;
		var y = x - x + 1;
		System.out.println(y);
	}
}

其次，从数学角度讲，无法进行的运算(N/0)或者会产生复数的运算(sqrt(-N))，都会产生NaN。

(咕咕咕，后面的笔记被鸽了)