class ATM {
public Map<Integer, Integer> withdraw(int amount, String currency) throws Exception {
int use = Math.min(need, available);
if( use > 0) {
result.put(denomination, use);
remaining -= denomination*use;
}
if(remaining != 0) {
throw new Exception();
}
result.forEach((Integer denomination, Integer count)
-> bills.merge(denomination, count, Integer::sum));
return result;
}
}