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.
