The server encountered an internal error () that prevented it from fulfilling this request.

exception

java.lang.NumberFormatException: null
    java.lang.Integer.parseInt(Unknown Source)
    java.lang.Integer.parseInt(Unknown Source)
    CreateLab.doPost(CreateLab.java:72)
    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 javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

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

    /**
     * @see HttpServlet#HttpServlet()
     */
    public CreateLab() {
        super();
        // TODO Auto-generated constructor stub
    }
     int id;
     int capacity;
     String day = "";
     String time = "";
     String room = "";
     int subject_id;
     int user_id;

    public void init() {
      try {
          Class.forName("com.mysql.jdbc.Driver");
          Connection con =
            DriverManager.getConnection("jdbc:mysql://localhost:3306/wae","root","");
        System.out.println("JDBC driver loaded");
      }
      catch (ClassNotFoundException e) {
        System.out.println(e.toString());
      } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    }

    /**Process the HTTP Get request*/
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws
  ServletException,IOException {
      sendPageHeader(response);
      sendRegistrationForm(request, response, false);
      sendPageFooter(response);
    }

    /**Process the HTTP Post request*/
    public void doPost(HttpServletRequest request,
      HttpServletResponse response)
      throws ServletException, IOException {
      sendPageHeader(response);

      capacity = Integer.parseInt(request.getParameter("capacity"));
      id = Integer.parseInt(request.getParameter("id"));
      day = request.getParameter("day");
      time = request.getParameter("time");
      room = request.getParameter("room");
      user_id = Integer.parseInt(request.getParameter("user_id"));
      subject_id = Integer.parseInt(request.getParameter("subject_id"));


      boolean error = false;
      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(id);
        Statement s = con.createStatement();

        String sql = "SELECT id FROM user" +
                " WHERE id='" + user_id + "'";
        ResultSet rs = s.executeQuery(sql);
        if (rs.next()) {
          rs.close();
           sql = "INSERT INTO lab" +
                  " (capacity, day, time, room, subject_id, user_id)" +
                  " VALUES" +
                  " (" +  capacity + "','" +
                     " '"  +  day + "'," +
                     " '"  +  time + "'," +
                     " '"  + room + "','" + subject_id + "','" + user_id + "')";

          System.out.println(sql);
          int i = s.executeUpdate(sql);
          if (i==1) {
            message = "Successfully a new lab class.";
          }
        }
          s.close();
          con.close();
        }
        catch (SQLException e) {
          message = "Error." + e.toString();
          error = true;
        }
        catch (Exception e) {
          message = "Error." + e.toString();
          error = true;
        }
        if (message!=null) {
          PrintWriter out = response.getWriter();
          out.println("<B>" + message + "</B><BR>");
          out.println("<HR><BR>");
        }
        if (error==true)
          sendRegistrationForm(request, response, true);
        else
          sendRegistrationForm(request, response, false);
        sendPageFooter(response);
      }

      /**
       * Send the HTML page header, including the title
       * and the <BODY> tag
       */
      private void sendPageHeader(HttpServletResponse response)
        throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<HTML>");
        out.println("<HEAD>");
        out.println("<TITLE>Create Lab Page</TITLE>");
        out.println("</HEAD>");
        out.println("<BODY>");
        out.println("<CENTER>");
      }

      /**
       * Send the HTML page footer, i.e. the </BODY>
       * and the </HTML>
       */
      private void sendPageFooter(HttpServletResponse response)
        throws ServletException, IOException {
        PrintWriter out = response.getWriter();
        out.println("</CENTER>");
        out.println("</BODY>");
        out.println("</HTML>");
      }
      /**Send the form where the user can type in
       * the details for a new user
       */
      private void sendRegistrationForm(HttpServletRequest request,
        HttpServletResponse response, boolean displayPreviousValues)
        throws ServletException, IOException {

        PrintWriter out = response.getWriter();
        out.println("<BR><H2>Create A Lab</H2>");
        out.println("<BR>Please enter the lab details.");
        out.println("<BR>");
        out.println("<BR><FORM METHOD=POST>");
        out.println("<TABLE>");
        out.println("<TR>");
        out.println("<TD>Class Capacity</TD>");
        out.print("<TD><INPUT TYPE=TEXT Name=capacity");

        if (displayPreviousValues)
          out.print(" VALUE="" + capacity + """);

        out.println("></TD>");
        out.println("</TR>");
        out.println("<TR>");
        out.println("<TD>Day</TD>");
        out.print("<TD><INPUT TYPE=TEXT Name=day");

        if (displayPreviousValues)
          out.print(" VALUE="" + day + """);

        out.println("></TD>");
        out.println("</TR>");
        out.println("<TR>");
        out.println("<TD>Time</TD>");
        out.print("<TD><INPUT TYPE=TEXT Name=time");

        if (displayPreviousValues)
          out.print(" VALUE="" + time + """);

        out.println("></TD>");
        out.println("</TR>");
        out.println("<TR>");
        out.println("<TD>Room</TD>");
        out.print("<TD><INPUT TYPE=TEXT Name=room");
        if (displayPreviousValues)
            out.print(" VALUE="" + room + """);
        out.println("></TD>");
        out.println("</TR>");
        out.println("<TR>");
        out.println("<TD>subject_id</TD>");
        out.print("<TD><INPUT TYPE=TEXT Name=subject_id");
        if (displayPreviousValues)
            out.print(" VALUE="" + subject_id + """);
        out.println("></TD>");
        out.println("</TR>");
        out.println("<TR>");
        out.println("<TD>user_id</TD>");
        out.print("<TD><INPUT TYPE=TEXT Name=user_id");
        out.println("></TD>");
        out.println("</TR>");

        if (displayPreviousValues)
        out.print(" VALUE="" + user_id + """);
        out.println("</TD>");
        out.println("</TR>");

        out.println("<TR>");
        out.println("<TD><INPUT TYPE=RESET></TD>");
        out.println("<TD><INPUT TYPE=SUBMIT></TD>");
        out.println("</TR>");
        out.println("</TABLE>");
        out.println("</FORM>");
        out.println("<BR>");
        out.println("<BR>");
      }
      }

public int getIntParameter(String strParamval){

        int nRetVal=0;
        if((strParamval != null) && (!strParamval.matches("\d*")))
            return nRetVal;
        try{
            nRetVal = Integer.parseInt(strParamval);
        }catch(NumberFormatException nfe){
        }
        return nRetVal;
    }

capacity = getIntParameter(request.getParameter("capacity"));
id = getIntParameter(request.getParameter("id"));
user_id = getIntParameter(request.getParameter("user_id"));
subject_id = getIntParameter(request.getParameter("subject_id"));

capacity = Integer.parseInt(request.getParameter("capacity"));
id = Integer.parseInt(request.getParameter("id"));
user_id = Integer.parseInt(request.getParameter("user_id"));
subject_id = Integer.parseInt(request.getParameter("subject_id"));

capacity = Integer.parseInt(request.getParameter("capacity"));

id = Integer.parseInt(request.getParameter("id"));

String str = request.getParameter("capacity");
if(str!=null && str.length > 0){
   capacity = Integer.parseInt(str);
}
str = request.getParameter("id");
if(str!=null && str.length > 0){
   id = Integer.parseInt(str);
}
str = request.getParameter("user_id");
if(str!=null && str.length > 0){
   user_id = Integer.parseInt(str);
}
str = request.getParameter("subject_id");
if(str!=null && str.length > 0){
   subject_id = Integer.parseInt(str);
}

public int getIntParameter(String strParamval){
    int nRetVal=0;
    try{
            nRetVal=Integer.parseInt(strParamVal);
        }catch(NumberFormatException nfe){
            return 0;
        }
        return nRetVal;
}

String id =request.getParameter("id");
       try {
    if(!id.isEmpty()) {
      Integer.parseInt(id);
     }
         }
       catch(NFE ex) {
         ex.printStrackTrace();
      }

String id= request.getParameter("id")
    try {
    if(!id.isEmpty() && id!=null) {
      Integer.parseInt();
     }
         }
       catch(NFE ex) {
         ex.printStrackTrace();
      }

user_id = Integer.parseInt(request.getParameter("user_id"));

request.getParameter("user_id")