Why am i getting NullPointerException but runs properly

What i am trying to make is a quiz. For Fetching the records from the same location from where the user quits, I need to maintain a session. So i am using a attribute "nq" which increments each time a question is answered. However, the initial value of nq will be null so i change it to 1 to fetch the first record. And increment it then on.

I get NullPointerException. If i get it only the first time I execute its understandable because of the above reason but If I close and start again even before session expires, i get it again. Why so?
There should be no NullPointerException again as it can fetch the value of nq during the session.
Also am i doing this the right way?

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.sql.*;

public class Quiz extends HttpServlet {
   Connection con;
   Statement ps;
   ResultSet rs;

static int correct=1;

@Override
    public void init() throws ServletException
    {
        try{
    Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
    con = DriverManager.getConnection("jdbc:odbc:gdsn","","");
        }
        catch (Exception e)
        {
        e.printStackTrace();
        System.out.println("Driver not found ");
        }

}

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException 
    
    
    {
       
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

HttpSession session = request.getSession(true);

session.setMaxInactiveInterval(20);
        //session.setAttribute("nq", "1");

try {

ps = con.createStatement();

int nqvalue;
                 //  nq=Integer.parseInt(request.getParameter("nq"));

try
                {
                 nqvalue = Integer.parseInt((String)session.getAttribute("nq"));
                }
                catch(NullPointerException f)
                {
                nqvalue=1;
                }
                catch(NumberFormatException f)
                {
                nqvalue=1;
                }

rs=ps.executeQuery("select * from Qtable where Number=" + nqvalue);
                  rs.next();
                 
                
              
                  
                  String number = Integer.toString(rs.getInt(1));
                  String ques = rs.getString("Question");
                  String op1 = rs.getString("opt1");
                  String op2 = rs.getString("opt2");
                  String op3 = rs.getString("opt3");
                  String op4 = rs.getString("opt4");
                  String ans = rs.getString("ans");

out.println("<html>");
            out.println("<head>");
            out.println("<title>Servlet Quiz</title>");  
            out.println("</head>");
            out.println("<body>");
            out.println("<form name=\"ques\" action=\"Quiz\">");
            out.println("<h3> <center> Question Number " + number + "</center></h3>");
            out.println("<br><br>");
            out.println("<h4>"+ ques + "</h4>");
            out.println("<br><br>");
            out.println("<input type=\"radio\" name = \"option\" value = \"" + op1 + "\">");
            out.println("<b>"  + op1 );
            out.println("<br>");
            out.println("<input type=\"radio\" name = \"question="+nqvalue+"&option\" value = \"" + op2+ "\">");
            out.println(op2);
            out.println("<br>");
            out.println("<input type=\"radio\" name = \"option\" value = \"" + op3 + "\">");
            out.println(op3);
            out.println("<br>");
            out.println("<input type=\"radio\" name = \"option\" value = \"" + op4 + "\">");
            out.println(op4 +"</b>");
            out.println("<br><br><br><br>");
            out.println(request.getParameter("option"));
            out.println(ans);
            out.println(correct);
            out.println("<input type = \"submit\" value=\"Submit\">        <input type = \"reset\"");

out.println("</form>");
            out.println("</body>");
            out.println("</html>");

session.setAttribute( "nq", String.valueOf(nqvalue+1) );

if (request.getParameter("option").equals(ans))
            {
            correct++;
                  }
            
            
                  
        }

catch(SQLException m)
                     {
                        out.println("exception in Sql REquest ");
                         m.printStackTrace();
                      }
        finally {
            out.close();

}
    }

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

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

@Override
    public void destroy ()
    {
        try {
            rs.close();
            ps.close();
            con.close();
        }
        catch (Exception x)
        {
            x.toString();
        }
    }

@Override
    public String getServletInfo() {
        return "Quiz Servlet";
    }
}

You could test it for null, and set it to some initial value if so.

P.S. Do you really think that "nq" is a good name for anything?

P.P.S. Instance variables in a servlet? Generally bad news!

Hi Gaurav,

If you are using any IDE like eclipse or NetBeans you can debug your program by adding NullPointerException breakpoint and see where the problem is.

Gaurav Wadhwani wrote:If i get it only the first time I execute its understandable because of the above reason
but If I close and start again even before session expires, i get it again. Why so?

What defines a session?
Most often it is stored in a session cookie which is lost when you finish the browser session.

So closing the window and starting again issues you a new session, even if the old one is still active, because it knows nothing about the old one.

Sorry for the very late reply everyone.

@Bear Bibeault
Do you really think that "nq" is a good name for anything?
Yes its a dumb name...only for testing purposes.

Instance variables in a servlet? Generally bad news!
Yes, and what i want in my application is to give one DB connection to every user but i dont know how to give a DB connection in a session. Please help

@Nikhil Pasupukuntla
I learnt how to add a breakpoint but not how to test it. Please help.

@Stefan Evans
If the session cookie is lost on closing and restarting then how is it possible that i get to continue with the same question i left at. If i close on question 4 -> close and start it again i get question 5 . So it has session and the cookie available why then a null pointer Exception?

Bear said something about checking it for null.

hmm, That what i did now and found the error.

It was line 95 , it had to be

What a time waste, kept thinking there is some logical error and it was some typing mistake. Anyway finally found and learned a lesson

Thank you guys

Gaurav Wadhwani wrote: What a time waste, kept thinking there is some logical error and it was some typing mistake. Anyway finally found and learned a lesson Thank you guys

This happens many of the times..and it shooks our head..by thinking where is the mistake...