Cannot get over the null pointer exception in exam.jsp

Question

I am making an online examination system in java using oracle database and html... pages are in .jsp. I'm running the code in eclipse..everything seems to work fine except the null pointer exception which i get as an error when i try to run exam.jsp. I am uploading the code and the screenshot of the error:

 <%@ page language="java" import="co.etest.quiz.Exam" 
 contentType="text/html;     charset=ISO-8859-1"
 pageEncoding="ISO-8859-1"%>

 <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"     

 <html>
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
 <title>Quiz</title>
 <style type="text/css">
 body {
 background:     
 url("${pageContext.request.contextPath}/images/background.jpg");
 }
 </style>

 </head><br/>
 <body>
 <div style="position:absolute;left:50px;top:20px">

 <%
 int currentQuestion=       
(`enter code here`   (Exam)request.getSession().getAttribute("currentExam")).getCurrentQuestion();
// System.out.println("Question Number "+currentQuestion+ " retrieved ");
 %>
Current Question ${sessionScope.quest.questionNumber+1} / 10
</div>


<div style="position:absolute;width:1000px;padding:25px;
height: 200px;border: 1px solid green ;left:100px;top:60px">
<span>${sessionScope.quest.question}</span><br/><br/>
<form action="exam" method="post" >
<c:forEach var="choice" items="${sessionScope.quest.questionOptions}" 
 varStatus="counter">
<input type="radio" name="answer" value="${counter.count}" >${choice}  <br/>
</c:forEach> <br/> 

 <%
 if(currentQuestion > 0)
{
%>
<input type="submit" name="action" value="Previous" />
<%} %>

<%
if(currentQuestion < 9)
{
%>
<input type="submit" name="action" value="Next" />
<%} %>
<input type="submit" name="action" value="Finish Exam" />

</form>
</div>


</body>
</html>

score 0 · Answer 1 · edited May 23 '17 at 12:07

A NullPointerException is thrown when you try to access a null object. In the stacktrace in the picture you obviously can see it's pointing you to line 21:

int currentQuestion = ((Exam)request.getSession().getAttribute("currentExam")).getCurrentQuestion();

Which means that something in this line is null and you're still trying to access it. You're only trying to access request, request.getSession(), and request.getSession().getAttribute("currentExam")).

Most likely what's causing the problem is that request.getSession().getAttribute("currentExam")) is returning null. Try checking if it's not null before running the code.

You might want to take a look here and here.

score 0 · Answer 2 · answered Apr 14 '16 at 13:36

 can u please tell me about it in a bit of detail...
 i am uploading another code ExamController.java which contains the        
 request.getSession().getAttribute("currentExam) method

package co.etest.quiz.controller;

import java.io.IOException;


import java.text.SimpleDateFormat;
import java.util.Date;

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;

import co.etest.quiz.Exam;
import co.etest.quiz.QuizQuestion;

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

protected void doGet(HttpServletRequest request, HttpServletResponse     
response) throws ServletException, IOException {
// TODO Auto-generated method stub
doPost(request,response);
}

protected void doPost(HttpServletRequest request, HttpServletResponse       
response) throws ServletException, IOException {
    // TODO Auto-generated method stub

    boolean finish=false;

    HttpSession session=request.getSession();       
    try
    {
    if(session.getAttribute("currentExam")==null)
    {  session=request.getSession();    
    String selectedExam= 
    (String)request.getSession().getAttribute("exam"); 
    System.out.println("Setting Exam "+selectedExam);
    Exam newExam=new Exam(selectedExam);          
    session.setAttribute("currentExam",newExam);
    SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd    
    HH:mm:ss a");
    Date date = new Date();
    String started=dateFormat.format(date);
    session.setAttribute("started",started);
    }

    }catch(Exception e){e.printStackTrace();}

    Exam exam=(Exam)request.getSession().getAttribute("currentExam");       

    if(exam.currentQuestion==0){    
    exam.setQuestion(exam.currentQuestion);
    QuizQuestion q=exam.questionList.get(exam.currentQuestion); 
    session.setAttribute("quest",q);
    }
    String action=request.getParameter("action");

    String radio=request.getParameter("answer");
    int selectedRadio=-1;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    if("1".equals(radio))
    {
    selectedRadio=1;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    System.out.println("You selected "+selectedRadio);
    }
    else if("2".equals(radio))
    {
    selectedRadio=2;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    System.out.println("You selected "+selectedRadio);
    }
    else if("3".equals(radio))
    {
    selectedRadio=3;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    System.out.println("You selected "+selectedRadio);
    }
    else if("4".equals(radio))
    {
    selectedRadio=4;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    System.out.println("You selected "+selectedRadio);
    }


    if("Next".equals(action)){
    exam.currentQuestion++;
    exam.setQuestion(exam.currentQuestion);
    QuizQuestion q=exam.questionList.get(exam.currentQuestion); 
    session.setAttribute("quest",q);
    }
    else if("Previous".equals(action))
    {   System.out.println("You clicked Previous Button");
    exam.currentQuestion--;
    exam.setQuestion(exam.currentQuestion);
    QuizQuestion q=exam.questionList.get(exam.currentQuestion); 
    session.setAttribute("quest",q);
    }
    else if("Finish Exam".equals(action))
    {   finish=true;
    int result=exam.calculateResult(exam);              
    request.setAttribute("result",result);
    request.getSession().setAttribute("currentExam",null);
    request.getRequestDispatcher("result.jsp").forward(request,response);       

    }

    if(finish!=true){
    request.getRequestDispatcher("exam.jsp").forward(request,response);
    }

    }

    }

Cannot get over the null pointer exception in exam.jsp

2 Answers2