@Override
    public Object getValueAt(int rowIndex, int columnIndex) {
        Customer customer = customers.get(rowIndex);
        switch (columnIndex) {
            case 0:
                return customer.getId();
            case 1:
                return customer.getFirstname();
            case 2:
                return customer.getLastname();
            case 3:
                return customer.getBirth();
	    case 4:
                return customer.getEmail();
            default:
                throw new IllegalArgumentException("columnIndex");
        }
    }