Hi there I was wondering if anyone can help me please.

I ve been working on a simple chat server program and it works when the
server handles one client at the time.  I ve been trying to make the
server able to answer clients requests at the same time with threads.

I always get the same error message when I run the server:

java.lang.NullPointerException

at chatserverq3.ServerGUI.processClientRequests(ServerGUI.java:113)

    at chatserverq3.ServerGUI.run(ServerGUI.java:73)

    at java.lang.Thread.run(Thread.java:484)

This is the client code:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;

public class ClientGUI extends JFrame implements ActionListener
{
 private JTextField nameField;
 private JLabel nameLabel;
 private JLabel areaLabel;
 private JPanel topPanel, botPanel;
 private JButton connectButton;
 private JTextArea listArea;

 private InputStream is;
 private OutputStream os;
 private BufferedReader fromServer;
 private Socket socket;
 private PrintWriter toServer;

 static final String SERVER_ADDRESS = "127.0.0.1";
 static final int SERVER_PORT_NUMBER = 4000;
 static final String CLIENT_LOGGINGOFF = "Exit";

//constructor for the client GUI
 public ClientGUI(String title)
 {
  setSize(400, 400);
  setTitle(title);
  setLocation(100, 100);
  nameLabel = new JLabel("Name");
  nameField = new JTextField(10);
  areaLabel = new JLabel("Connected Users");
  topPanel = new JPanel();
  topPanel.setLayout(new BorderLayout());
  botPanel = new JPanel();
  connectButton = new JButton("Connect");
  connectButton.addActionListener(this);
  listArea = new JTextArea(10, 30);
  JScrollPane scr = new JScrollPane(listArea);

  topPanel.add(nameField, "West");
  topPanel.add(nameLabel, "Center");
  topPanel.add(connectButton, "East");

  botPanel.add(areaLabel);
  botPanel.add(listArea);

  getContentPane().add(topPanel, "North");
  getContentPane().add(botPanel, "Center");

  setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

 }

 //attempts to connect the client to the server
 private void connectServer()
 {
  try
   {
    socket = new Socket(SERVER_ADDRESS, SERVER_PORT_NUMBER);
    openStreams();
   }
   catch(IOException e)
     {
       System.out.println("Trouble contacting the server " + e);
     }
  }

 private void open()throws IOException
 {
  final boolean AUTO_FLUSH = true;
  is = socket.getInputStream();
  fromServer = new BufferedReader(new InputStreamReader(is));
  os = socket.getOutputStream();
  toServer = new PrintWriter(os, AUTO_FLUSH);
 }

//action performed when the client GUI button is pressed
public void actionPerformed(ActionEvent a)
 {
  String buttonLabel = connectButton.getText();
  String userName = nameField.getText();
  try
  {
   if((buttonLabel == "Connect")&&(userName.length()> 0))
     {
      connectToServer();
      sendUserName(userName);
      connectButton.setText("Disconnect");
     }
     else
     {
       logOff(userName);
       connectButton.setText("Connect");
     }

   }
   catch(IOException e)
     {
       System.out.println("Problem with the server " + e);
     }
  }

 private void close()throws IOException
 {
  toServer.close();
  os.close();
  fromServer.close();
  is.close();
 }

 //sends the user name to the server
 private void sendUserName(String aName)throws IOException
 {
  String reply;
  String uName = aName;
  toServer.println(uName + " logged on");

  reply = fromServer.readLine();
  listArea.setText(reply);
 }

//notifies the server a client has logged off
private void logOff(String aName)throws IOException
{
 String uName = aName;
 toServer.println(uName + " logged off");
 listArea.setText("");
 toServer.println(CLIENT_LOGGINGOFF);
 closeStreams();
 socket.close();
}
}

public class Main {

 public static void main(String[] args)
 {
   ClientGUI cg = new ClientGUI("ChatClient");
   cg.setVisible(true);
 }

}

The server code is:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import java.util.*;

public class ServerGUI extends JFrame implements Runnable
{
 private JTextArea logArea;
 private JLabel logLabel;
 private JPanel logPanel;

 private ServerSocket ss;
 private Socket socket;

 private InputStream is;
 private OutputStream os;

 private PrintWriter toClient;
 private BufferedReader fromClient;

 static final int PORT_NUMBER = 4000;
 static final String CLIENT_LOGGINGOFF = "Exit";

 //constructor for the Server GUI
 public ServerGUI(String title)
 {
  setSize(400, 400);
  setTitle(title);
  setLocation(100, 100);
  logArea = new JTextArea(15, 30);
  JScrollPane scr = new JScrollPane(logArea);
  logLabel = new JLabel("Logging history");

  logPanel = new JPanel();
  logPanel.add(logLabel);
  logPanel.add(logArea);

  getContentPane().add(logPanel);

  setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 }

//constructor for the server to set the socket to handle a new thread
 public ServerGUI(Socket s)
 {
  socket = s;
 }

public void run()
{
 try
 {
  while(true)
   {

     openStreams();
     processClientRequests();
     closeStreams();
     socket.close();
   }
 }
 catch(IOException e)
     {
       System.out.println("Trouble with a connection " + e);
     }

}

//helper method for the class to set up the streams
private void open() throws IOException
{
 final boolean AUTO_FLUSH = true;
 is = socket.getInputStream();
 fromClient = new BufferedReader(new InputStreamReader(is));
 os = socket.getOutputStream();
 toClient = new PrintWriter(os, AUTO_FLUSH);
}

//process the string from received from each client
private void processRequests() throws IOException
{
 String userName;
 String reply;
 int numberOfFields = 3;
 String [] fieldContents =  new String[numberOfFields];
 userName = fromClient.readLine();
 while(!(userName.equals(CLIENT_LOGGINGOFF)))
 {
    StringTokenizer tokensIn = new StringTokenizer(userName, " ");
    int i = 0;
    while(tokensIn.hasMoreTokens())
     {
      fieldContents[i] = tokensIn.nextToken();
      i++;
     }

 reply = fieldContents[0];
 toClient.println(reply);
 logArea.append("[ " + reply + " ] "+ fieldContents[1]+"
"+fieldContents[2]+"\n");
 userName = fromClient.readLine();
 }
}

//helper method to close the streams with each client
private void close() throws IOException
{
 toClient.close();
 os.close();
 fromClient.close();
 is.close();
}
}

import java.net.*;
import java.io.*;

public class ChatHandler
{
static final int PORT_NUMBER = 4000;

 public ChatHandler()
 {
 }

 public void run()
 {
   try
   {
   ServerSocket server = new ServerSocket(PORT_NUMBER);
   while(true)
     {
     Socket client = server.accept ();
     System.out.println ("Accepted from " + client.getInetAddress ());
     Thread aThread = new Thread(new ServerGUI(client));
     aThread.start ();
     }
   }
   catch(IOException e)
     {
       System.out.println("Trouble with ServerSocket, port
"+PORT_NUMBER +": " + e);
     }
 }
}

public class Main {

 public static void main(String[] args)
 {
   ServerGUI sg = new ServerGUI("ChatServer");
   sg.setVisible(true);
   ChatHandler cH = new ChatHandler();
   cH.run();
 }

}

It is the first time I use threads and maybe there is something I dont
understand.  Can anyone point me to the right direction please? I think
the problem may be with the 2 server constructor but I am not sure how
to solve it.

Cheers Pat