Can I implement HttpSessionListener this way?

public class my_Servlet extends HttpServlet implements HttpSessionListener
{
  String User_Id;
  static Vector<String> Valid_User_Id_Vector=new Vector<String>();
  private static int activeSessions=0;

  public void sessionCreated(HttpSessionEvent se)
  {
// associate User_Id with session Id;
// add User_Id to Valid_User_Id_Vector
    Out(" sessionCreated : "+se.getSession().getId());
    activeSessions++;
  }

  public void sessionDestroyed(HttpSessionEvent se)
  {
    if (activeSessions>0)
    {
// remove User_Id from Valid_User_Id_Vector by identifing it's session Id
      Out(" sessionDestroyed : "+se.getSession().getId());
      activeSessions--;
    }
  }

  public static int getActiveSessions()
  {
    return activeSessions;
  }

  public void init(ServletConfig config) throws ServletException
  {
  }

  public void destroy()
  {

  }

  protected void processRequest(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
  {
    User_Id=request.getParameter("User_Id");
  }

  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
  {
    processRequest(request, response);
  }

  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
  {
    processRequest(request, response);
  }

  public String getServletInfo()
  {
    return "Short description";
  }
}

public void sessionCreated(HttpSessionEvent se) {
    Vector ids = (Vector) se.getSession().getServletContext().getAttribute("currentUserIds");
    //manipulate ids
}

static String User_Id;

protected void processRequest(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
{
  HttpSession session=request.getSession(true);
  Add_User_Id(session.getId(),request.getParameter("User_Id"););
}

static void Add_User_Id()
{
  assiciate [Session_Id , User_Id] in a map
}

static void Remove_User_Id(String Session_Id)
{
  Valid_User_Id_Vector.remove(User_Id ) by looking it up in a [Session_Id , User_Id] map
}

public void sessionCreated(HttpSessionEvent se)
    {
      activeSessions++;
    }

    public void sessionDestroyed(HttpSessionEvent se)
    {
      if (activeSessions>0)
      {
        My_Servlet.Remove_User_Id(se.getSession().getId());
        activeSessions--;
      }
    }