Weird Java fraction behavior

I have seen a very weird behaviour in Java's double variable, as I'm trying to simply add small fractions to a double and I see a completely bizarre results.

double test = 0;
test += 0.71;
test += 0.2;

Now I'd expect the result to be:

test = 0.91

Right? Wrong!

In reality, this is the number I get in my test double:

test = 0.9099999999999999

Now while this is very close, it's a very bizarre fraction loss, and in the long run it causes serious bugs in my program.

With a float I've gotten even a weirder result.

Any help would be greatly appreciated.

Thanks

Solution

That's the magic of binary encoding of floating point values (look for IEEE754 : http://en.wikipedia.org/wiki/IEEE_754-2008 ). If you want to be sure to never have this kind of things, you're maybe looking for BigDecimal :

http://docs.oracle.com/javase/1.5.0/docs/api/java/math/BigDecimal.html

Basic rules :

don't use equality tests when dealing with floating point numbers (you must test gaps)
round numbers you're displaying (usually using DecimalFormat)
don't use floating point numbers for financial applications
the float is generally the way to go for scientific or industrial operations, as long as you understand IEEE754