Given two double values d1, d2, what is the most reliable way to test their equality?

The most accurate way to tell whether two double values are equal to one another is to use Double.compare()and test against 0, as in:

System.out.println(Double.compare(d1, d2) == 0);

We can’t use == because of Double.NaN (literally: “Not a Number”). Consider:

final double d1 = Double.NaN;
final double d2 = Double.NaN;

System.out.println(d1 == d2);

will print false.