public int CheckExistingCitizen(String s)
{
    int i = 0;
        try {
            String sql = "SELECT IDNUMBER from  CITIZENS where IDNUMBER='"+s+"'";
            ResultSet apantiseis = this.stmt.executeQuery(sql);

             while ( apantiseis.next() )
             {i++;}

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

          return i;
}