In Java the floating point arithmetic is not represented precisely. For example this java code:
float a = 1.2;
float b= 3.0;
float c = a * b;
if(c == 3.6){
System.out.println("c is 3.6");
}
else {
System.out.println("c is not 3.6");
}
Prints "c is not 3.6".
I'm not interested in precision beyond 3 decimals (#.###). How can I deal with this problem to multiply floats and compare them reliably?
I think it has nothing to do with Java, it happens on any IEEE 754 floating point number. It is because of the nature of floating point representation. Any languages that use the IEEE 754 format will encounter the same problem.
As suggested by David above, you should use the method abs of java.lang.Math class to get the absolute value (drop the positive/negative sign).
You can read this: http://en.wikipedia.org/wiki/IEEE_754_revision and also a good numerical methods text book will address the problem sufficiently.
public static void main(String[] args) {
float a = 1.2f;
float b = 3.0f;
float c = a * b;
final float PRECISION_LEVEL = 0.001f;
if(Math.abs(c - 3.6f) < PRECISION_LEVEL) {
System.out.println("c is 3.6");
} else {
System.out.println("c is not 3.6");
}
}