How to model an amount of Money in Java

Adding to what Paul said, there's Martin Fowler's Money Pattern.

I would check the monetary module from the JScience project (by Jean Marie Dautelle).

Depending on your exact needs, Stephen Colebourne started Joda-Money ("a more focussed project" [than JScience]) some time ago. But there is no full release available yet (version 0.5 was released one year ago).