Returns the integer double value obtained by discarding any fractional
If this is already an integer valued double, including
-0.0, or it is not
a finite value, the value is returned unmodified.
For the purpose of rounding,
-0.0 is considered to be below
d in the range
-1.0 < d < 0.0 will return
in the range
0.0 < d < 1.0 it will return 0.0.