because you cannot represent all decimal values exactly to 100% precision. most numbers are correct to (something like) eight significant digits. You can't actually store 0.1, but you can store 0.099999999999998590, which is usually close enough.
The reason (I think) is that java stores number by adding up powers of 2. so to get something close to 0.1, you store 1/16+ 1/32 + 1/128.... etc. (note these are not nec. the ACTUAL sum, I'm just trying to illustrate it ).
you can get pretty close most of the time, but not exact.
Never ascribe to malice that which can be adequately explained by stupidity.
Joined: Nov 03, 2006
It's actually not native to java, it's the very binary-ness (if I may invent words) of computers. The fact that they base everything off of 1s and 0s, means that they do their math in binary (base 2), which means that they have the same problems with dividing certain things by 10 as we do when we divide 10 by 3. In decimal based math that just doesn't compute very cleanly.