NaN

■ You get a NaN from taking the Math.sqrt(...) of a negative floating point number (except negative zero). i.e.

double d = Math.sqrt( -1 ); here d is NaN

double d = Math.sqrt( -0.0 ); here d is -0.0

■ You cannot assign NaN unqualified without its class name. Meaning you cannot just say NaN without its Wrapper name. i.e.

float f = Float.NaN; works.

double d = Double.NaN; works.

float f = Nan; will not compile.

double d = NaN; will not compile.

■ There is no such thing as Integer.NaN. NaN is only relevant to the two floating point wrappers Double and Float.

■ After casting, the integer equivalent of NaN is always zero. i.e.

double d = -1;

double nan = Math.sqrt( d );

int x = ( int ) nan; Here x becomes 0:

■ Any math operation against NaN yields NaN. i.e.

double d = -1;

float f = -1;

double dnan = Math.sqrt( d );

float fnan = ( float ) Math.sqrt( f );

double df = dnan + fnan; Here df becomes NaN.

■ You cannot detect NaN with an if == statement. The == operator will always return false for NaN. Instead you must convert to a wrapper and use the isNaN( ) method. i.e.

float f = Float.NaN;

if ( f == Float.NaN ) will not work correctly as it returns false.

if( new Float( f).isNaN( ) ) works, correctly returning true.

■ Any ( NaN == NaN ) comparison will always return false. i.e.
 
if ( Float.Nan == Float.NaN ) is false.
 

However if ( Float.POSITIVE_INFINITY == Float.POSITIVE_INFINITY ) is true. See next item.

■ NaN is treated differently than Infinity, whereby math operations against POSITIVE_INFINITY and NEGATIVE_INFINITY yield positive or negative Infinity, depending upon the sign combinations. Any identical comparisons on them yield true. i.e.

double d = Double.POSITIVE_INFINITY;

float f = Float.POSITIVE_INFINITY;

double df = d + f; Here df becomes POSITIVE_INFINITY:

Also if ( Float.POSITIVE_INFINITY == Float.POSITIVE_INFINITY ) is true.