Java isNan how it works?

Question

I was looking at the openjdk-1.7.0_25 source code and I have seen this method:

/**  * Returns {@code true} if the specified number is a  * Not-a-Number (NaN) value, {@code false} otherwise.  *  * @param   v   the value to be tested.  * @return  {@code true} if the argument is NaN;  *          {@code false} otherwise.  */ static public boolean isNaN(float v) {     return (v != v); } 

I can't understand how it works, when this method can return true?

Answer 1

That method can return true for certain operations, for example:

System.out.println(Float.isNaN(0.0f / 0.0f)); System.out.println(Double.isNaN(Math.sqrt(-1))); 

Basically, NaN represents an undefined value. The value of 0.0 / 0.0 is NaN, and Nan != NaN. It may seem logical because Math.sqrt(-1) also gives you NaN.

See the javadoc of Double.NaN:

It is equivalent to the value returned by Double.longBitsToDouble(0x7ff8000000000000L)

And then Double.longBitsToDouble():

If the argument is any value in the range 0x7ff0000000000001L through 0x7fffffffffffffffL or in the range 0xfff0000000000001L through 0xffffffffffffffffL, the result is a NaN. No IEEE 754 floating-point operation provided by Java can distinguish between two NaN values of the same type with different bit patterns.