Java Double value = 0.01 changes to 0.009999999999999787 [duplicate]

Answer 1

0.01 does not have an exact representation in floating-point (and neither do 0.1 nor 0.2, for that matter).

You should probably do all your maths with integer types, representing the number of pennies.

Answer 2

Using double for currency is a bad idea, Why not use Double or Float to represent currency?. I recommend using BigDecimal or doing every calculation in cents.

Answer 3

I'm sure you know that some fractions' decimal representations terminate (e.g. .01) while some don't (e.g. 2/3=.66666...). The thing is that which fractions terminate changes depending on what base you're in; in particular, .01 doesn't terminate in binary, so even though double provides a lot of precision it can't represent .01 exactly. As others said, using BigDecimal or fixed-point integer computations (converting everything to cents) is probably best for currency; to learn more about floating point, you could start at The Floating-Point Guide- What Every Programmer Should Know About Floating-Point Arithmetic.

Answer 4

Floating point numbers are never 100% accurate (not quite true, see comments below). You should never compare them directly. Also integer rounding. The best way to do this would probably be to do it in cents and convert to dollars later (1 dollar == 100 cents). By converting to an integer you are losing precision.

Answer 5

As the others already said, do not use doubles for financial calculations.

This paper http://download.oracle.com/docs/cd/E19957-01/806-3568/ncg_goldberg.html (What Every Computer Scientist Should Know About Floating-Point Arithmetic) is a must-read to understand floating point math in computers.

Answer 6

doubles aren't kept in decimal internally, but in binary. Their storage format is equivalent to something like "100101 multiplied by 10000" (I'm simplifying, but that's the basic idea). Unfortunately, there's no combination of these binary values that works out to exactly decimal 0.01, which is what the other answers mean when they say that floating point numbers aren't 100% accurate, or that 0.01 doesn't have an exact representation in floating point.

There are various ways of dealing with this problem, some more complicated than others. The best solution in your case is probably to use ints everywhere and keep the values in cents.