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Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction. - Ernst F. Schumacher
Here's the classic Test Infected paper that uses multiple currencies as an example to demonstrate JUnit and test-first practices. It might help you think about currency issues, and certainly has some good ideas about testing and coding. There is a Currency class in Java, but I'm not really sure what it's good for yet.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi