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;
}

在哪种情况下可以a != a返回true？似乎a是NaN 的时候,但我无法想象一个例子.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

BTW Double.compare()确实看到NaN相等

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

对于任何类型和值,使用多个线程.例如

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