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S_Grimm

Thu Oct 30, 2008 10:35 am


Attempt to use Null Pointer (java.lang.NullPointerException)

-----------------------------------

When i try to run the code it tells me that there is a null pointer exception. what does that mean? and how do i fix it?

//Chat Client Version 2.2 works with any chatServer
import java.net.*;
import java.io.*;
public class chatClientv2p2
{
    //variable declarations
    public static Socket mySocket;
    public static String datavar;
    public static String name;
    public static InputStreamReader isr;
    public static BufferedReader br;
    public static InputStreamReader reader = new InputStreamReader (System.in);
    public static BufferedReader bread = new BufferedReader (reader);
    public static PrintWriter pw;

    //thread declarations
    /*
    public static class Connect extends Thread
    {
        public void run ()
        {
            try
            {
                synchronized (mySocket)
                {

                }
                //synchronized (pw)
                //{
                 //
                //}
            }
            catch (Exception e)
            {
            System.out.println ("Error connecting to host");
            }
        }
    }
    */

    public static class Write extends Thread
    {
        public void run ()
        {
            try
            {
                synchronized (datavar)
                {
                    datavar = br.readLine ();
                    System.out.println (datavar);
                }
            }
            catch (Exception e)
            {
                System.out.println ("Unable to recieve message");
            }

        }
    }


    public static class Read extends Thread
    {
        public void run ()
        {
            try
            {
                pw.println (name + bread.readLine ());
            }
            catch (Exception e)
            {
            }
        }
    }


    //main method

    public static void main (String 


-----------------------------------

btiffin

Thu Oct 30, 2008 12:06 pm


RE:Attempt to use Null Pointer (java.lang.NullPointerException)

-----------------------------------

Well, I can tell you what it means, but not how to fix it other than this just doesn't look right.

A NullPointerException is usually caused by access to uninitialized data.  In this case you are asking Java to check the object monitor for mySocket before mySocket is initialized.

I'm not a Java head by any means, but you usually surround sychronized around critical code sections that may use/modify an object, but I don't think that can include the instantiation.

Cheers


-----------------------------------

Tony

Thu Oct 30, 2008 12:23 pm


RE:Attempt to use Null Pointer (java.lang.NullPointerException)

-----------------------------------

In Java's case, it's likely that you are calling a method on an object that is actually NULL (as btiffin says -- has not been initialized).

Look through the stack trace to see what line the problem originates at.


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S_Grimm

Thu Oct 30, 2008 3:03 pm


Re: Attempt to use Null Pointer (java.lang.NullPointerException)

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public static void main (String  thanks biffin. i removed the snyc and it worked.


-----------------------------------

btiffin

Thu Oct 30, 2008 5:36 pm


RE:Attempt to use Null Pointer (java.lang.NullPointerException)

-----------------------------------

No problem A\V

Instead of synchronized and wrapping all the snippets, you may want to look into java.util.concurrent (Assuming Java > 1.5) and use a Semaphore.  Higher level abstraction and a little less finicky. 

synchronized enclosures have quite a few "holes" that need care and attention.  Semaphores are external to the Objects and methods needing special protection measures and will usually be easier to manage/debug.

Multi-threading gotchas can be a real bitch to catch, when they may only happen every 8,003 runs or on full moon Tuesdays when someone in Norway sneezes.  ;)

Cheers


-----------------------------------

Dion van Overdijk

Sun Dec 06, 2009 7:37 am


Re: Attempt to use Null Pointer (java.lang.NullPointerException)

-----------------------------------

Hi guys I've a similar problem where I get a NullPointerException

The exception points to a line of code in a XMLRPCClient file where it asks for the execution of a server procedure

the line in the client: object result = client.execute("serverHandler.getTelNo", argument);

The server has a server object with a handler as follows: server.addHandler("serverHandler", new TelNos());

TelNos is a separate java class on the server for which the code is below:

What am I doing wrong?

package Server;


import java.util.*;


public class TelNos {
    
    	//database to hold names and telephone numbers
	private Hashtable hashTab;   
  
        public TelNos() {
        Hashtable hashTab = new Hashtable();
        hashTab.put("Thomas", 9088);
        hashTab.put("Wilkinson",7856);
        hashTab.put("Davies", 2234);
        hashTab.put("Mabbut", 2267);
        hashTab.put("Tomkins", 4456);
        hashTab.put("Price", 3377);
        hashTab.put("Wilson", 2300);
    }
   /*I did try this as well with a main method but the result is the same*/   
     /*public static void main(String       
    public int noStaff(){  
        //Return the number of staff in the database
        return hashTab.size();
    }
    
    public int getTelNo(String staffName){  
        //Return the tel no  of the argument
        Integer n = (Integer)hashTab.get(staffName);
        return n;
    }
        
    public Vector getNames(){  
        //Returns with a vector containing the names of all the staff
         Vector names = (Vector)hashTab.keys(); 
         return names;
      }        
    
}