@Override
    public Double getBalance(String id) {
        if(id == null || "".equals(id.trim())) {
            return -1.0;
        }
        for(Account a : accounts) {
            if(id.equals(a.getId())) {
                if(a.getBalance() < 0) {
                    return -1.0;
                }
                System.out.println("Geted balance is " + a.getBalance() + " RMB.");
                return a.getBalance();
            }
        }
        return -1.0;
    }