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

[S_Grimm]

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?

Java:

//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 ();         }     } }

[btiffin]

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]

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.

[S_Grimm]

Quote:

Java:

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)             {             }         }

thanks biffin. i removed the snyc and it worked.

[btiffin]

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]

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;
}

}