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Why am i getting NullPointerException but runs properly

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posted 13 years ago

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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";
    }
}

Bear Bibeault

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posted 13 years ago

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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!

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Nikhil Pasupukuntla

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posted 13 years ago

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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.

Nikhil Pasupukuntla
SCJP 1.5 | OCPJWCD 1.5

Stefan Evans

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posted 13 years ago

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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.

Gaurav Wadhwani

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posted 13 years ago

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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?

Amit Ghorpade

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posted 13 years ago

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Bear said something about checking it for null.

SCJP, SCWCD.
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Gaurav Wadhwani

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posted 13 years ago

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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

Shanky Sohar

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posted 13 years ago

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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...

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