If double a=0.0, can I compare a*b==0 directly?

Question

I know double should not be compared by == operator directly, but how about if I define an initial value as 0.0?eg:

double a=0.0;
double b=
.
.
.

If a is not modified, does a*b==0 always true?

Answer 1

I know double should not be compared by == operator direct

That is only true if you don't know how much representation or rounding error you have. A classic example of what not to do is

0.1 + 0.2 == 0.3 // false :(

However, if you use rounding like

if (round4(0.1 + 0.2) == 0.3) // true

from Chronicle Core's Maths

public static double round4(double d) {
    final double factor = 1e4;
    return d > WHOLE_NUMBER / factor || d < -WHOLE_NUMBER / factor ? d :
            (long) (d < 0 ? d * factor - 0.5 : d * factor + 0.5) / factor;
}

If a is not modified, does a*b==0 always true?

It is for finite numbers. For infinity and NaN you will get NaN and this is not equal to anything.