How does Double.isNaN() work?

Question

The sun jdk implementation looks like this:

return v != v;

Can anyone explain how that works?

Answer 1

NaN values are not equal to anything (if one side of an equality is NaN, the equality is false), so NaN != NaN. Obviously every normal double does equal itself

Answer 2

A nan is the only double that is not equal to itself. Thus, checking v!=v will only produce True for NaN.

Here is what the Java spec has to say:

Floating-point operators produce no exceptions (§11). An operation that overflows produces a signed infinity, an operation that underflows produces a denormalized value or a signed zero, and an operation that has no mathematically definite result produces NaN. All numeric operations with NaN as an operand produce NaN as a result. As has already been described, NaN is unordered, so a numeric comparison operation involving one or two NaNs returns false and any != comparison involving NaN returns true, including x!=x when x is NaN.