/** When receive a message, analyze message content and then execute the command: Draw or Clear */
  public void receive(Message msg) {
    byte[] buf = msg.getRawBuffer();
    if (buf == null) {
      System.err.println(
          "["
              + channel.getAddress()
              + "] received null buffer from "
              + msg.getSrc()
              + ", headers: "
              + msg.printHeaders());
      return;
    }

    try {
      DrawCommand comm =
          (DrawCommand)
              Util.streamableFromByteBuffer(
                  DrawCommand.class, buf, msg.getOffset(), msg.getLength());
      switch (comm.mode) {
        case DrawCommand.DRAW:
          if (drawPanel != null) drawPanel.drawPoint(comm);
          break;
        case DrawCommand.CLEAR:
          clearPanel();
        default:
          System.err.println("***** received invalid draw command " + comm.mode);
          break;
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
  /**
   * Init JWhiteBoard interface
   *
   * @throws Exception
   */
  public void go() throws Exception {
    if (!noChannel && !useState) channel.connect(groupName);
    mainFrame = new JFrame();
    mainFrame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    drawPanel = new DrawPanel(useState);
    drawPanel.setBackground(backgroundColor);
    subPanel = new JPanel();
    mainFrame.getContentPane().add("Center", drawPanel);
    clearButton = new JButton("Clean");
    clearButton.setFont(defaultFont);
    clearButton.addActionListener(this);
    leaveButton = new JButton("Exit");
    leaveButton.setFont(defaultFont);
    leaveButton.addActionListener(this);
    subPanel.add("South", clearButton);
    subPanel.add("South", leaveButton);
    mainFrame.getContentPane().add("South", subPanel);
    mainFrame.setBackground(backgroundColor);
    clearButton.setForeground(Color.blue);
    leaveButton.setForeground(Color.blue);
    mainFrame.pack();
    mainFrame.setLocation(15, 25);
    mainFrame.setBounds(new Rectangle(250, 250));

    if (!noChannel && useState) {
      channel.connect(groupName, null, stateTimeout);
    }
    mainFrame.setVisible(true);
  }
 /** Clear all the content on a whiteboard */
 public void clearPanel() {
   if (drawPanel != null) drawPanel.clear();
 }
 /** Set state of the Group */
 public void setState(InputStream istream) throws Exception {
   drawPanel.readState(istream);
 }
 /** Get state of the Group */
 public void getState(OutputStream ostream) throws Exception {
   drawPanel.writeState(ostream);
 }