SOURCE CODE for TicTacToe Clientapplet which allows the user to connect to a server and play TicTacToe

import java.applet.Applet;
import java.net.*;
import java.io.*;
import java.awt.*;

// set up a TicTacToe client applet

public class TicTacToeClient extends Applet
				implements Runnable {
   TextField id;
   TextArea display;
   Panel boardPanel, panel2;	
   Square board[][], currentSquare;
   Socket connection;
   DataInputStream input;
   DataOutputStream output;
   Thread outputThread;
   char myMark;
  
   // set up user-interface and board
   public void init()
   {
	setLayout ( new BorderLayout() );
	display = new TextArea( 4, 30);
	display.setEditable(false);
	add ( "South", display );

	boardPanel = new Panel();
	boardPanel.setLayout( new GridLayout( 3, 3, 0, 0 ) );
	board = new Square[3][3];

	for ( int row = 0; row < board.length; row++ )
		for ( int col = 0; col < board[row].length; col++)
		{
			board[row][col] = new Square();
			boardPanel.add(board[row][col]);
		}

	id = new TextField();
	id.setEditable(false);
	add ( "North", id );

	panel2 = new Panel();
	panel2.add (boardPanel);
	add ( "Center", panel2 );
   }

   // Make connection to server and get associated streams.
   // Start separate thread to allow this applet to continually update its output in display
   public void start()
   {
	try
	{	
		connection = new Socket (InetAddress.getLocalHost(), 5000);
		input = new DataInputStream( connection.getInputStream() );
		output = new DataOutputStream (connection.getOutputStream() );
	}
	catch ( IOException e)
	{	e.printStackTrace();	}

	outputThread = new Thread ( this );
	outputThread.start();
   }
 
   public boolean mouseUp( Event e, int x, int y)
	// when user clicks mouse and releases in a square, an integer [0-8] representing 
	// the clicked square is sent to the server 
   {
	for ( int row = 0; row < board.length; row++ )
		for ( int col = 0; col < board[row].length; col++)
			try {
				if ( e.target == board[row][col] )
				{ 
					currentSquare = board[row][col];
					output.writeInt ( row * 3 + col );
				}
			}
			catch ( IOException exc)
			{	exc.printStackTrace();	}
	return true;
   }

   public void run()
   // control thread that allows the continuous update of the display
   {
	// first get player's mark (X or O)
	try	{
		myMark = input.readChar();
		id.setText ( "You are player \"" + myMark + "\"" );
	}
	catch ( IOException e)
	{	e.printStackTrace();	}

	// Receive messages sent to client
	while (true)
		try {
			String str = input.readUTF();
			processMessage (str);
		}
		catch ( IOException exc)
		{	exc.printStackTrace();	}
   }

   public void processMessage (String str)
   // process message sent to client
   {
	if ( str.equals( "Valid move." ) )
	{
		display.appendText( "Valid move, please wait.\n" );
		currentSquare.setMark (myMark);
		currentSquare.repaint();
	}
	else if ( str.equals( "Opponent moved" ) )
	{
		try {
			int loc = input.readInt();

			done:
				for ( int row = 0; row < board.length; row++ )
					for ( int col = 0; col < board[row].length; col++)
						if ( row * 3 + col == loc)
						{
							board[row][col].setMark (
								( myMark == 'X' ? 'O' : 'X' ));
							board[row][col].repaint();
							break done;
						}
			display.appendText( "Opponent moved. Your turn.\n");
		}
		catch ( IOException e)
		{	e.printStackTrace();	}
	}
	else
		display.appendText( str + "\n");
   }
}

class Square extends Canvas {
    // maintains one square of the board

    char mark;

    public Square()
    {
	resize( 30, 30 );
    }

    public void setMark ( char ch ) { mark = ch; }

    public void paint ( Graphics g)
    {
	g.drawRect ( 0, 0, 29, 29 );
	g.drawString( String.valueOf ( mark ), 11, 20 );
    }
}