public void addRate(String from, String to, int rate){
    this.rates.put(new Pair(from, to), new Integer(rate));
}

@Test
public void testreduceMoneyDifferentCurrency(){
    Bank bank = new Bank();
    bank.addRate("GBP", "USD", 2);
    Money result = bank.reduce(Money.gbpound(2), "USD");
    assertEquals(Money.dollar(1), result);
}

public Money reduce(Bank bank, String to){
    int rate = bank.rate(this.currency, to);
    return new Money(this.amount / rate, to);
}

public int rate(String from, String to){
    if (from.equals(to)) return 1;
    Integer rate = (Integer) this.rates.get(new Pair(from, to));
    return rate.intValue();
}