Computer Science Canada :: View topic - Attempt to use Null Pointer (java.lang.NullPointerException)

//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 [] args) throws IOException
{
synchronized (mySocket) //null pointer exception here.
{
try
{
mySocket = new Socket ("10.217.105.40", 5050); //input your server or host ip address
}
catch (Exception e)
{
}
}
//new Connect ().start();
InetAddress lclAdr = InetAddress.getLocalHost ();
System.out.println ("Your IP address is " + lclAdr.getHostAddress ());
// = new InputStreamReader (mySocket.getInputStream ());
//= new BufferedReader (isr);
synchronized (pw)
{
pw = new PrintWriter (mySocket.getOutputStream (), true);
}
System.out.print ("Input Screen Name : ");
synchronized (name)
{
name = bread.readLine ();
name = (name + " says : ");
}
new Write ().setPriority (1);
new Read ().setPriority (2);
for (;;)
{
new Write ().run ();
new Read ().run ();
}
}
}

Author:	btiffin [ Thu Oct 30, 2008 12:06 pm ]
Post subject:	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

Author:	Tony [ Thu Oct 30, 2008 12:23 pm ]
Post subject:	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.

Author:	btiffin [ Thu Oct 30, 2008 5:36 pm ]
Post subject:	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

Author:	Dion van Overdijk [ Sun Dec 06, 2009 7:37 am ]
Post subject:	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<String, Integer> hashTab = new Hashtable<String, Integer>(); 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 [] args){ Hashtable<String, Integer> hashTab = new Hashtable<String, Integer>(); try{ 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); } catch(Exception e){ e.printStackTrace(); } }*/ 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; } }