Null pointer exception sometimes

[ankur tyagi]

hey i am writing a class file which takes values from database and adds them to arraylist and then a servlet uses those arraylist
to generate a web page..
the problem is that the servlet returns NULL POINTER EXCEPTION sometimes and sometimes the values are returned and are shown on the page.
usually this happens when i start the database and tomcat everything works fine but after refreshing the page 4-5 times or loading similar pages ..it starts to return NULL POINTER EXCEPTION ....i think this might be because of the fact that i am not closing the connections in the class file..
i am showing the code, can anyone suggest how to close connections according to this code.
Servlet

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

public class AirportSelectDD extends HttpServlet
{
	public void doPost(HttpServletRequest request,
						HttpServletResponse response)
						throws IOException, ServletException
		{
			String param=request.getParameter("check");
			String c= request.getParameter("airport");



				AirportDD ad = new AirportDD();
				ad.getFlights(c);
				request.setAttribute("styles1",ad.code);
				request.setAttribute("styles2",ad.from);
				request.setAttribute("styles3",ad.to);
				request.setAttribute("styles4",ad.Dtime);
				request.setAttribute("styles5",ad.ARtime);
				request.setAttribute("styles6",ad.Airline);
				request.setAttribute("styles7",ad.Ter);


			RequestDispatcher view= request.getRequestDispatcher("result.jsp");
			view.forward(request,response);
		}
	}

  

class file

  import java.sql.*;
import java.util.*;
import java.io.*;
import javax.swing.*;
public class AirportDD
{

	Statement stmt;
	ResultSet rset;
	PreparedStatement p;
	Connection con;
	List code,from,to,Dtime,ARtime,Airline,Ter;


	public void getFlights(String airport)
		{
			String fcode,from_d,to_d,day,arline,ter;
			int artime,dtime;
			code= new ArrayList();
			from= new ArrayList();
			to= new ArrayList();
			Dtime= new ArrayList();
			ARtime= new ArrayList();
			Airline= new ArrayList();
			Ter= new ArrayList();
			String q1= "select * from FlightCD ,FlightDD where fromD = "+ "'" +airport +"' and FlightCD.FCode= FlightDD.FCode"
			              + " and day = initcap(To_Char(Sysdate,'DY'))";
			ResultSet rs1=View(q1);

			try
					{
						System.out.println("resulting flights are");

						while(rs1.next())
						{
							fcode=rs1.getString(1);
							from_d=rs1.getString(2);
							to_d=rs1.getString(3);
							dtime=rs1.getInt(5);
							day=rs1.getString(6);
							String q3= "select * from FlightD , DAirplane where FlightD.Airline=DAirplane.Airline and FCode="+ "'" + fcode + "'" + "and  DTime="+
								         "'" + dtime + "'"+ " order by DTime asc";
							ResultSet rs3= View(q3);

							while(rs3.next())
								{

									artime=rs3.getInt(3);
									arline=rs3.getString(4);
									ter=rs3.getString(6);
									System.out.println("flights for today are " + fcode+" "+ from_d+" "+to_d+" "+ dtime+" "+ artime +" "+arline+ " "+ ter);
									code.add(fcode);from.add(from_d);to.add(to_d);Dtime.add(dtime);ARtime.add(artime);Airline.add(arline);Ter.add(ter);
								}

							}
						}
			catch(SQLException e)
					{
						System.out.println("problem");
					}




		}

	public Connection conn() throws SQLException
		{

			try
				{
					Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
				}
			catch(ClassNotFoundException e)
				{
					System.out.println("Database Driver not found");
					System.out.println(e.toString());
				}
			catch(Exception e)
				{
					System.out.println(e.toString());
				}
			return(DriverManager.getConnection("jdbc:odbc:link","system","oracle"));
			}
	public  ResultSet View(String qry)
	{
		try
		{
			 con=conn();
			qry=qry;
			stmt=con.createStatement();
			rset=stmt.executeQuery(qry);


		}
		catch(Exception e)
		{
			System.out.println(e.toString());
		}

	return rset;
	}
}  

thanks in advance

[manoj r patil]

Can you also post the stack trace of the NullPointerException containing line number?

[ankur tyagi]

HTTP Status 500 -

type Exception report

message

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

exception

java.lang.NullPointerException
AirportDD.getFlights(AirportDD.java:34)
AirportSelectDD.doPost(AirportSelectDD.java:18)
javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)

note The full stack trace of the root cause is available in the Apache Tomcat/5.5.28 logs.

[Ben Souther]

Ankur Tyagi,

It looks like a lot of people have already spent time trying to help you with this issue in another thread.
https://coderanch.com/t/493572/Servlets/java/Null-Pointer-Exception

By starting a new one, you've not only wasted the time of the people who've tried to help you, your wasting the time of the people who might try to help you here because they won't know what ideas have already been suggested.

If you still haven't solved this, post what you've changed to the original.