java - 在哪种情况下 "a != a"可以返回 "true"？

Question

java.lang.Math#min(double, double) :

public static double min(double a, double b) {
    if (a != a) return a; // a is NaN
    if (a == 0.0d && b == 0.0d && Double.doubleToLongBits(b) == negativeZeroDoubleBits) return b;
    return (a <= b) ? a : b;
}

在什么情况下 a != a 可以返回 true？好像是 a 是 NaN 的时候，但是我想不出一个例子。你能提供一个吗？

Answer 1

一个简单的例子是

double d = Double.NaN; // or
double d = 0.0/0.0; // or
double d = Double.POSITIVE_INFINITY + Double.NEGATIVE_INFINITY;
if (Double.isNaN(a)) { // tests if a != a
   // do something

顺便说一句，Double.compare() 确实将 NaN 视为相等

if (Double.compare(d, d) == 0) // always.

对于多线程，这对于任何类型和值都是可能的。例如

if (a != /* another thread changes 'a' */ a) {
    // a thread changed a while you were looking at it.