@Override
    public Object getValueAt(int rowIndex, int columnIndex) {
        Customer customer = customers.get(rowIndex);
        switch (COLUMNS.values()[columnIndex]) {
            case ID:
                return customer.getId();
            case FIRSTNAME:
                return customer.getFirstname();
            case LASTNAME:
                return customer.getLastname();
	    case BIRTH:
                return customer.getBirth();
            case EMAIL:
                return customer.getEmail();
            default:
                throw new IllegalArgumentException("columnIndex");
        }
    }