NaN in Double!

From source code, the definition of NaN is

/**
* A constant holding a Not-a-Number (NaN) value of type
* double. It is equivalent to the value returned by
* Double.longBitsToDouble(0x7ff8000000000000L).
*/
public static final double NaN = 0.0d / 0.0;


From JLS & API doc:

Note that in most cases, for two instances of class Double, d1 and d2,
the value of d1.equals(d2) is true if and only if
d1.doubleValue() == d2.doubleValue()also has the value true.

However, there are two exceptions:

If d1 and d2 both represent Double.NaN, then the equals method returns true, even though Double.NaN==Double.NaN has the value false.

If d1 represents +0.0 while d2 represents -0.0, or vice versa, the equal test has the value false, even though +0.0==-0.0 has the value true.

This definition allows hash tables to operate properly.