ItDoesntWorkIsUseless (<- click), please be more specific. The general principle proposed above should do the trick. Though, I would suggest you never have more than one JFrame per application; use JDialog instead.
Keep in mind that the division operator denotes integer division if both operands are integers, and floating-point division otherwise. Yours is the first case - both of your operands are integers (e.g. 1/2 = 0 by integer division), hence the results you get.
If they are all in the same package, there won't be a problem with that; otherwise you would use import statement for other classes.
Though it's a question for another topic, I would suggest you read a Java tutorial, specifically the Packages section, to get more details. I'm posting from phone now so I can't provide a link, but if you Google for it you'll find it easily.