public class ACS{
    private AC[] data;   // array of AC
    private int n;       // number of accounts in use
    private final static int MAX_NUM = 100;

    // Constructor
    public ACS(){
	n = 0;
	data = new AC[MAX_NUM];
    }
    // Mutator
    public void update (int id, double amount){
	int index = find(id);
	if (index == -1){  // new account
	    if (n<MAX_NUM){
		AC account = new AC (id, amount);
		data[n] = account;
		n++;
	    }
	}
	else // existing account at data[index]
	    data[index].update(amount);
    }
    // String form
    public String toString(){
	String ret = "\nAcctId\tBalance\n";
	for (int i=0; i<n; i++)
	    ret += data[i].toString() + "\n";
	return ret;
    }
    // Locate in array; return array index of the account 
    // numbered id (and returns -1 if the account number id
    // is not in use).
    private int find (int id){
	for (int i=0; i<n; i++)
	    if (data[i].getId() == id)
		return i;
	return -1;
    }
}