在哪种情况下，“ a！= a”可以返回“ true”？

一个简单的例子是

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

BTW Double.compare（）确实将NaN视为相等

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

对于多个线程，这对于任何类型和值都是可行的。例如

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