Servlets Jsp And Null Pointers
exception

java.lang.NumberFormatException: null
    java.lang.Integer.parseInt(Unknown Source)
    java.lang.Integer.parseInt(Unknown Source)
    TutorAssign.doPost(TutorAssign.java:97)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet implementation class TutorAssign
 */
@WebServlet("/TutorAssign")
public class TutorAssign extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public TutorAssign() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        sendBack(request, response);
    }

    private void sendBack(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         HttpSession session = request.getSession(true);
            //Set data you want to send back to the request (will be forwarded to the page)
            //Can set string, int, list, array etc.
         int user_id = Integer.parseInt((String)request.getParameter("id"));
         String sql = "SELECT l.id,s.name,l.day,l.time,l.room" +
                  " FROM subject s, lab l " +
                    " WHERE s.user_id="+(Integer)session.getAttribute("id");

          try{
            Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/wae","root","");
            System.out.println("got a hj");
            System.out.println(session.getAttribute("id"));

              Statement stmt = con.createStatement();
              ResultSet res = stmt.executeQuery(sql);
              System.out.println(res);
              ArrayList<String> list1 = new ArrayList<String>();
              ArrayList<String> list2 = new ArrayList<String>();
              if (res.next()){
                  do{
                       list1.add(res.getString(1));
                       list2.add(res.getString(2)+" "+res.getString(3)+" "+res.getString(4)+" "+res.getString(5));


                  }while(res.next());
              System.out.println("Outside");
              String[] arr1 = list1.toArray(new String[list1.size()]);
              String[] arr2 = list2.toArray(new String[list2.size()]);
              System.out.println(list1);
              request.setAttribute("res1", arr1);
              request.setAttribute("res2", arr2);
              request.setAttribute("user_id", user_id);

              }



          }catch (SQLException e) {
            }
            catch (Exception e) {
            }


            //Decides what page to send the request data to
            RequestDispatcher view = request.getRequestDispatcher("TutorAssign.jsp");
            //Forward to the page and pass the request and response information
            view.forward(request, response);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {


        int user_id = Integer.parseInt((String)request.getParameter("id"));
          System.out.println(user_id);
          int lab_id = 0;
          System.out.println("I got a blow job");
          String message = null;
          try {
              Class.forName("com.mysql.jdbc.Driver");
              Connection con =
                DriverManager.getConnection("jdbc:mysql://localhost:3306/wae","root","");
            System.out.println("got connection");
            System.out.println(user_id);
            Statement s = con.createStatement();

            String sql = "INSERT INTO user_lab" +
                      " (user_id, lab_id)" +
                      " VALUES" +
                      " ('" +  user_id + "'," +
                         " '"  +  lab_id + "')";

              System.out.println(sql);
              int i = s.executeUpdate(sql);
              if (i==1) {
                message = "Successfully assigned a tutor.";
                response.sendRedirect("Lecturer_labs");
              }

              s.close();
              con.close();
            }
            catch (SQLException e) {
              message = "Error." + e.toString();
              boolean error = true;
            }
            catch (Exception e) {
              message = "Error." + e.toString();
              boolean error = true;
            }
            if (message!=null) {
              PrintWriter out = response.getWriter();
              out.println("<B>" + message + "</B><BR>");
              out.println("<HR><BR>");
            }

          }
// TODO Auto-generated method stub


}

<form name ="TutorAssign" ACTION="TutorAssign" method="post">
        <input type="hidden" name="user_id" value="user_id"/>
        <select name="lab_id">
        <%
        for(int i=0; i<list1.length;i++)
        {
        out.println("<option value="+list1[i]+"> "+list2[i]+" </option>");

        } %>
        </select>
        <input type=SUBMIT value="Submit" name="Submit" />

        </form>

<form name ="TutorAssign" ACTION="TutorAssign" method="post">
        <select name="labs">
        <%
        for(int i=0; i<list1.length;i++)
        {
        out.println("<option value="+list1[i]+"> "+list2[i]+" </option>");

        } %>
        </select>

<select name="labs">

if (request.getParameter("id") == null) {
   // handle non existance of id
}

<input type="hidden" name="id" value="your_id_here"/>