Java-关于java中的NaN定义

Question

今天研究了一下java 的排序算法，无意中发现了如下的一段

if (ak != ak)  // a[k] is NaN

Answer 1

IEEE754.

以前摘自JVM SPEC7:

关于java NaN = 0.0f / 0.0f

The IEEE 754 standard includes not only positive and negative sign-magnitude numbers, but also positive and negative zeros, positive and negative infinities, and a special Not-a-Number value (hereafter abbreviated as "NaN"). The NaN value is used to represent the result of certain invalid operations such as dividing zero by zero.

NaN的判断:

NaNs are unordered, so numerical comparisons and tests for numerical equality have the value false if either or both of their operands are NaN. In particular, a test for numerical equality of a value against itself has the value false if and only if the value is NaN.

但在java里, float只有一个值为NaN(0x7fc00000), IEEE754 里有2^24-2个.

The elements of the float value set are exactly the values that can be represented using the single floating-point format defined in the IEEE 754 standard, except that there is only one NaN value (IEEE 754 specifies 2^24-2 distinct NaN values).