Display the result of a query in a JTextField's value

I want to display the new id from database in a jTextField but it dosen't.. this is my code

public void actionPerformed(ActionEvent e)  {

    try {
        DB con=new DB();
        con.connecte();

        String req = "SELECT Max(num) FROM condidat";
        ResultSet rs = stmt.executeQuery(req);
        int num = rs.getInt("num");
        int nvNum=num+1;
        txt_num.setText(valueOf(nvNum));

        } 
        catch (ClassNotFoundException ex) {
            Logger.getLogger(Ajouter.class.getName()).log(Level.SEVERE, null, ex);
        } 
        catch (SQLException ex) {
            Logger.getLogger(Ajouter.class.getName()).log(Level.SEVERE, null, ex);
        }

    }
    //To change body of generated methods, choose Tools | Templates.

}