help with java. lang. NullPointer Exception

[Amit Ellana]

please help i have been getting this error and i cant fix it
a lil background info:
This is my first major java project after HelloWorld, its suppose to be a Client Server program. I got the Client Server part to work with both echoing a string back and forth...Then i wanted to add something that would let the server know when a client is no longer available....so i add timers to both the server and the client...for the client timer i wanted it to send a "*" to server every 10 seconds.but when the client timer tries to send the "*" java.lang.NullPointerException.....please take a look at my code and tell me what i am doing wrong
im posting only the client codes cuz i feel that is what is causeing the problem CThreaded a threaded client, CScon connection class for the client, and CTimer a client timer.
//CThreaded

package nwpj;

import java.net.*;
import java.io.*;
import java.util.*;
import java.awt.*;
public class CThreaded extends Frame  {
//*****  Mult-thread client

CScon connection;
public final String DEFAULT_HOST = "apc";
public final int DEFAULT_PORT = 8888;
public final int DEFAULT_TIME=5;
MyFrame outerBox;
public TextField hostDisplay, portDisplay, timeDisplay;
public TextArea logDisplay, msgDisplay;
Panel topPanel;
Panel middlePanel;
Panel buttonPanel;
Button connectButton, sendButton, cancelButton, quitButton;
// **************
  public CThreaded()  {  
    super ( "CThreaded " );
  }  // end CThreaded constructor
// **************
  public static void main ( String [ ] args )  {
    CThreaded client = new CThreaded ();
    client.buildUI ();
  }  // end main
// **************
  public boolean action (Event event, Object arg) {
    if (event.target == connectButton )  {
      connection = new CScon ( this );
      connection.start();
      connection.connectServer ();
      return true;
    }  // end connectButton 
    if (event.target == sendButton )  {
      connection.sendReceive ();
      return true;
    }  // end sendButton
    if (event.target == cancelButton )  {
      msgDisplay.setText ( "" );
      return true;
    }  // end cancelButton 
    if (event.target == quitButton )  {
      connection.quitServer();
      hide ();
      dispose ();
      System.exit ( 0 );
      return true;
    }  // end quitButton 
    return false;
  }  // end action
// **************
  public boolean handleEvent ( Event event )  {
    if ( event.id == Event.WINDOW_ICONIFY )  {
      hide ();
      dispose ();
      System.exit ( 0 );
    }  // end if
    return super.handleEvent ( event );
  }  // end handleEvent 
// **************
  public void buildUI ()  {
  hostDisplay = new TextField ( DEFAULT_HOST, 30 );
  portDisplay = new TextField ( Integer.toString ( DEFAULT_PORT ), 4 );
  timeDisplay = new TextField ( Integer.toString (DEFAULT_TIME), 5);
  topPanel = new Panel ();
  topPanel.setLayout ( new GridLayout(3,1) );
  topPanel.add (  hostDisplay );
  topPanel.add ( portDisplay );
  topPanel.add( timeDisplay );
  logDisplay = new TextArea ( 40, 10 );
  msgDisplay = new TextArea ( 40, 10 );
  msgDisplay .setText ( "Default message." );
  middlePanel = new Panel ();
  middlePanel.setLayout ( new GridLayout ( 2, 1 ) );
  middlePanel.add ( logDisplay );
  middlePanel.add ( msgDisplay );
  connectButton = new Button ( "Connect" );
  sendButton = new Button ( "Send" );
  cancelButton = new Button ( "Cancel" );
  quitButton = new Button ( "Quit" );
  buttonPanel = new Panel ( );
  buttonPanel.add ( connectButton );
  buttonPanel.add ( sendButton );
  buttonPanel.add ( cancelButton );
  buttonPanel.add ( quitButton );
  outerBox = new MyFrame ( this, "CThreaded" );
  outerBox.add ( "North", topPanel );
  outerBox.add ( "Center", middlePanel );
  outerBox.add ( "South", buttonPanel );
  outerBox.resize ( 350, 400 );
  outerBox.show ();
  }  // end buildUI 
}  // end CThreaded 
// ************************  SCon Class ************************
  // end SConn
// ************************  MyFrame Class ************************
class MyFrame extends Frame  {
  Frame source;
  MyFrame ( Frame a )  {
    super ( );
    source = a;
  }  // end constructor
  MyFrame ( Frame a, String title )  {
    super ( title );
    source = a;
  }  // end constructor
  public boolean action ( Event event, Object object )  {
    if ( source.action ( event, object ) ) return true;
    else return false;
  }  // end action
  public boolean handleEvent ( Event event )  {
    if ( source.handleEvent ( event ) ) return true;
    else return super.handleEvent ( event );
  }  // end handleEvent
}  // end MyFrame

CScon

/*
 * CScon.java
 *
 * Created on November 6, 2003, 9:38 PM
 */
package nwpj;
import java.net.*;
import java.io.*;
import java.util.*;
import java.awt.*;

/**
 *This class is used by CThreaded to make the connection with the server
 * @author  amit
 */
public class CScon extends Thread{
    
    /** Creates a new instance of CScon */
    
  CThreaded source;
  Socket socket=null;
  InputStream input=null;
  OutputStream output=null;
  char c;
  String host="";
  int port=0;
  boolean again = true;
  String string="";
  
// **************
  CScon (Frame s)  {
    super ();
    source = (CThreaded) s;
  }// end constructor
// **************
  public void run ()  {
  }  // end run
// **************
  public void connectServer (  )  {
    host = (source.hostDisplay).getText ();
    port = Integer.parseInt ( (source.portDisplay).getText () );
    try  {
      socket = new Socket ( InetAddress.getByName ( host), port );
      output = socket.getOutputStream ();
      input = socket.getInputStream ();
      source.logDisplay.setText ( "Socket created:  connecting to server\n" );
      
     CTimer ctimer = new CTimer(5000);
     ctimer.start();
     source.logDisplay.appendText("CTimer Started");
     
               
     }
    catch ( IOException except)  {
      except.printStackTrace ();
    }  // end catch
  }  // end connectServer 
// **************
  public void  sendReceive () {
      source.logDisplay.appendText ( "Message, below, sent to Server\n" );
      putMsg ( output, source.msgDisplay.getText () );
      
      source.logDisplay.appendText ( "Message, below, received from server\n" );
      source.msgDisplay.setForeground ( Color.red );
      String stringIn = getMsg ( input );
      source.msgDisplay.setText ( stringIn);
}  // end sendReceive  
// **************
  public void quitServer () {
    putMsg ( output, "$" );
    try {
      socket.close ();
     }  catch ( IOException except )  {
       except.printStackTrace ();
     }
    (source.logDisplay).appendText ( "Connection closed\n" );
  }  // end quitServer
// **************
  public String getMsg ( InputStream input )  {
    char[] charArray = new char [ 1024 ];
  char c;
        try {
           int i = 0;
            while ( ( (c = (char) input.read () ) != '#' ) && (i < 1024) ) {
             charArray [ i ] =  c ;
             i++;
           }  // end while
        }  catch ( IOException except )  {
           except.printStackTrace ();
        }  // end catch
         String string = new String ( String.valueOf ( charArray) );
          return string;
  }  // end getMsg
// **************
  public void putMsg ( OutputStream output, String string )  {
    try {
      for ( int i = 0; i < string.length(); i++ ) {
        output.write ( (int) string.charAt ( i ) );
      }  // end for
      output.write ( (int)'#' );  // end of message mark
    }  catch ( IOException except )  {
       except.printStackTrace ();
    }  // end catch
  }  // end putMsg

public void pstar() {
    
 putMsg(output,"*");   
    
    }
}

CTimer

package nwpj;

import java.lang.*;
import java.net.*;
import java.io.*;
import java.util.*;
import java.awt.*;
import nwpj.CThreaded.*;
import nwpj.CScon.*;
/** 
  * The Timer class allows a graceful exit when an application
  * is stalled due to a networking timeout. Once the timer is
  * set, it must be cleared via the reset() method, or the
  * timeout() method is called.
  */
public class CTimer extends Thread
{
CScon silfa;
      

    /** Rate at which timer is checked */
protected int m_rate = 100;

/** Length of timeout */
private int m_length;
/** Time elapsed */
private int m_elapsed;
/**
  * Creates a timer of a specified length
  * @paramlengthLength of time before timeout occurs
  */
public CTimer(int length)
{
// Assign to member variable
m_length = length;
// Set time elapsed
m_elapsed = 0;
}

/** Resets the timer back to zero */
public synchronized void reset()
{
m_elapsed = 0;
                
                System.out.println("reset");
}
/** Performs timer specific code */
public void run()
{
         
         
            boolean test=true;
// Keep looping
while(true)
{
// Put the timer to sleep
try
{ 
Thread.sleep(m_rate);
}
catch (InterruptedException ioe) 
{
continue;
}
// Use 'synchronized' to prevent conflicts
synchronized ( this )
{
// Increment time remaining
m_elapsed += m_rate;
// Check to see if the time has been exceeded
if (m_elapsed > m_length)
{
// Trigger a timeout
                                      
                                        reset();
                                    silfa.pstar();
                                        
}
}
System.out.println("While");}
}
// Override this to provide custom functionality
public void timeout()
{

            System.err.println ("Network timeout occurred.... terminating");
                System.out.println("inside timeout");

}
}

the error message is:
java.lang.NullPointerException
at nwpj.CTimer.run(CTimer.java:97)

if you could please tell me what is causing this error or if you could atleaset explain the error to me i would be greatful..
Amit

[Ernest Friedman-Hill]

No value is ever assigned to the member variable "silfa" in CTimer, so in CTimer.run, when you call a method on silfa, you get a null pointer exception.
silfa has to get initialized to point to a CScon object; you should probably do this in a constructor for CTimer, which would takethe CScon as a constructor argument.

[Amit Ellana]

Thanks for the reply Ernest Friedman-Hill but im not sure what the answer is? could you explain it a bit more or point me to a place where I could find that answer....again thanks for the help
amit

[Ernest Friedman-Hill]

You would definitely benefit from reading some of the JavaRanch "campfire stories", here and especially here. If you read and understand both of these, then the answer to this problem should be obvious.
I'm going to move this thread to "Java in General (Beginner)" because the problem has nothing to do with Sockets, per se. Followups there, please.
[ November 08, 2003: Message edited by: Ernest Friedman-Hill ]

[Amit Ellana]

I have read the two articles above, but still can't resolve my problem. I need to create a constutor in CTimer but im still not sure what value to put into 'silfa'?

[Ernest Friedman-Hill]

CTimers are being created by code inside a method of CScon, so it's clear that a CTimer should have a reference to the particular CScon object that created it, right? So the CScon code should pass "this" to the CTimer constructor, which should store that argument into the member variable.
Inside any member method, the keyword "this" refers to the object on which the method was invoked.

[Amit Ellana]

thanks for the help....i think im a bit in over my head with this issue and i need to take a couple steps back and review.....