public int modifyCustomer(Customer customer) {
            int result = -1;
            try {
                PreparedStatement stmt;
                if (customer.hasFirstName()) { // Has first name
                    stmt = con.prepareStatement("UPDATE Customer SET first_name = ? where customer_ID = ?");
                    stmt.setString(1, customer.getFirstName());
                    result = stmt.executeUpdate();
                }
                else if (customer.hasLastName()) { // Has last name
                    stmt = con.prepareStatement("UPDATE Customer SET last_name = ? where customer_ID = ?");
                    stmt.setString(1, customer.getLastName());
                    result = stmt.executeUpdate();
                }
                else if (customer.hasAddress1()) { // Has address1
                    stmt = con.prepareStatement("UPDATE Customer SET address1 = ? where customer_ID = ?");
                    stmt.setString(1, customer.getAddress1());
                    result = stmt.executeUpdate();
                }
                else if (customer.hasAddress2()) { // Has address2
                    stmt = con.prepareStatement("UPDATE Customer SET address2 = ? where customer_ID = ?");
                    stmt.setString(1, customer.getAddress2());
                    result = stmt.executeUpdate();
                }
                else if (customer.hasZone_ID()) { // Has zone_ID
                    stmt = con.prepareStatement("UPDATE Customer SET zone_ID = ? where customer_ID = ?");
                    stmt.setInt(1, customer.getZone_ID());
                    result = stmt.executeUpdate();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
            return result;
        }