servlet mysql not in statements
user - id, name, password

subject - id, name, user_id

lab - id, time, room, capacity, subject_id

user_lab - id, user_id, lab_id

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.

     String id = request.getParameter("id");
     System.out.println("beej");
     int user_id = 0;

     if(id != null && (!id.equals("")) ) {
     user_id= Integer.parseInt(id);
     }

     String sql = "SELECT l.id,s.name,l.day,l.time,l.room" +
              " FROM subject s, lab l " +
                " WHERE" +
                " s.id = l.subject_id"+
                            " AND l.id "+"NOT IN"+
                "SELECT l.id"+
                " FROM lab ll, user_lab ul" +
                "WHERE"+"l.id ="+"ul.lab_id" + "AND ul.user_id="+(Integer)session.getAttribute("id");


      try{
        Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/wae","root","");

        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());

          String[] arr1 = list1.toArray(new String[list1.size()]);
          String[] arr2 = list2.toArray(new String[list2.size()]);
          System.out.println(list2);
          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("Student_enrol.jsp");
        //Forward to the page and pass the request and response information
        view.forward(request, response);




org.apache.jasper.JasperException: java.lang.NullPointerException
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:534)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:457)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:391)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    Student_enrol.sendBack(Student_enrol.java:104)
    Student_enrol.doGet(Student_enrol.java:40)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)


root cause

java.lang.NullPointerException
    org.apache.jsp.Student_005fenrol_jsp._jspService(Student_005fenrol_jsp.java:95)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:419)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:391)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    Student_enrol.sendBack(Student_enrol.java:104)
    Student_enrol.doGet(Student_enrol.java:40)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

" s.id = l.subject_id"+
" AND l.id "+"NOT IN  " +     // <== here
"SELECT l.id"+

SELECT..
FROM...
WHERE l.id NOT IN
(
   SELECT..
   FROM...
)

"WHERE"+"l.id ="+"ul.lab_id" + "AND ul.user_id="+(Integer)session.getAttribute("id");

" WHERE "+"l.id ="+"ul.lab_id" + " AND ul.user_id="+(Integer)session.getAttribute("id");