Weird Java fraction behavior

Question

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

Answer 1

There is nothing bizarre about it at all. 0.91, 0.71 and 0.2 are not representable as a IEEE754 floating point values as they would have a recurring fractional part when represented in binary. The situation is entirely analogous to trying to represent 1/3 in base 10 with a finite number of digits. You can't do it.

What you are seeing is a rounding error that is normal when doing floating point calculations. You have to code around it. So for instance, you can't reliably compare for equality, you have to see the two numbers are within some small delta of each other. For a slightly more in depth but still understandable explanation see The Floating Point Guide.