Java APIs

Hi
I want to develop a lan messenger using java.
I have written a client code to interact with a server
it connects properly but as soon as i start a thread for listening
connection is ended. please suggest me on this....
below i m pasting my code and the error message

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.net.*;
import java.lang.*;
 class interfaceclient
{
    public static void main(String args[])
    {
        TextInputFrame frame=new TextInputFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.show();
    }

    static class TextInputFrame extends JFrame
    {
         PrintWriter dout;
         BufferedReader din;
         JPanel panel=new JPanel();
         JButton SendButton=new JButton("send");
         JButton ExitButton=new JButton("exit");
         JTextArea ta=new JTextArea(10,50);
         JScrollPane scrollPane=new JScrollPane(ta);
           
         JTextField tf=new JTextField(40);
         public TextInputFrame()
        {
            setTitle("messenger");
            setSize(WIDTH,HEIGHT);
            Container contentPane=getContentPane();
            panel.add(SendButton);
            panel.add(ExitButton);
            panel.add(tf);
            contentPane.add(panel,BorderLayout.SOUTH);
            panel.add(ta);
            contentPane.add(scrollPane,BorderLayout.CENTER);
            
         try
         {
            Socket cs=new Socket("localhost",7822);
            BufferedReader din = new BufferedReader(new InputStreamReader(cs.getInputStream()));
            PrintWriter dout= new PrintWriter(cs.getOutputStream(),true);
            dout.println("hi server");
            ta.setText("connected to server");
            sockthread thread=new sockthread(cs);
            thread.start();
           SendButton.addActionListener(new action());
        }catch(IOException ie){System.out.println("cant find server");}}
    
    
    class action implements ActionListener
    {
     
            public void actionPerformed(ActionEvent event)
            {
                try
                {
                    String message=(tf.getText());
                    dout.println(message);
                    tf.setText("");
                }
                catch(Exception e)
                {
                    System.out.println(e);
                }
            }
    }
     class sockthread extends Thread
     {private Socket cs;
         public sockthread(Socket acs){cs=acs;}
            public void run()
        {
        try
            {
            dout.println("before while");
                while(true)
                {
                dout.println("after while");
                    String mess=din.readLine();
                    ta.append("Server:"+mess+"\n");
                }
            }catch(IOException ie){System.out.println(ie);}
         }
     }
     public static final int WIDTH=600;
     public static final int HEIGHT=400;
   }
 }
the following error message is generated

Exception in thread "Thread-2" java.lang.NullPointerException
        at interfaceclient$TextInputFrame$sockthread.run(Main.java:77)
java.lang.NullPointerException
java.lang.NullPointerException
java.lang.NullPointerException
BUILD SUCCESSFUL (total time: 23 seconds)