Even if it's "only a game", I would use Money Pattern from Martin Fowler, backed up by a long.
Why?
Localization (L10n): Using that pattern you can easily localize your game currency. Think about the old tycoon games like "Transport Tycoon". They easily allow the player to change in-game currency (i.e From British Pound to US Dollar) to meet real world currency.
And
The long data type is a 64-bit signed two's complement integer. It has a minimum value of -9,223,372,036,854,775,808 and a maximum value of 9,223,372,036,854,775,807 (inclusive) (Java Tutorials)
That means you can store 9,000 times current M2 US Money Supply (~10,000 Billion Dollars). Giving you enough room to use any other world currency, probably, even those who had/has hyperinflation (If curious, see post-WWI German inflation, where 1 pound of bread was 3,000,000,000 marks)
Long should give you enough room to do all interest calculations using integer arithmetics, eBusiness answer gives an explanation on how to do that.