Learning Java - Chapter 7 : Java

Here we use a thread to control the display of a clock. We use the Runnable interface with the run() method to control a call to repaint() for the panel that displays the clock value.

Note that repaint() does not immediately initiate a change in the display but rather puts a request on a queue in the AWT graphics system. This approach avoids interference with other graphics tasks occurring.

ClockApplet.java
DataFormatPanel.java

import javax.swing.*;
import java.awt.*;

/**
  * This applet implements Runnable and uses a thread
  *   to create a digital clock.
**/
public class ClockApplet extends JApplet
                       implements Runnable {
  // Will use thread reference as a flag
  Thread fThread = null;

  // Need panel reference in run ().
  DateFormatPanel fClockPanel = null;

  public void init () {
    Container content_pane = getContentPane ();

    // Create an instance of the time panel
    fClockPanel = new DateFormatPanel ();

    // Add the time panel to the applet's panel.
    content_pane.add (fClockPanel);
  } // init

  // This method called by browser when web page and
  // applet loaded.
  public void start () {
    // If the thread reference not null then a
    // thread is already running. Otherwise, create
    // a thread and start it.
    if (fThread == null){
        fThread = new Thread (this);
        fThread.start ();
    }
  } // start

  // This method called by browser when web page and
  // applet loaded.
  public void stop () {
    // Setting thread to null will cause loop in
    // run () to finish and kill the thread.
    fThread = null;
  } // stop

  public void run () {
    // Loop through sleep periods until
    // thread stopped by setting thread
    // reference to null.
    while (fThread != null) {
      try{
        Thread.sleep (1000);
      } catch  (InterruptedException e) { }

      // Repaint clock
        fClockPanel.repaint ();
    }
  } // run

} // class ClockApplet

import javax.swing.*;
import java.awt.*;
import java.text.*;
import java.util.*;

/**
  * This JPanel subclass uses the DateFormat class
  * to display the current time.
**/
class DateFormatPanel extends JPanel {
  DateFormat fDateFormat = null;
  boolean fFirstPass = true;
  int fMsgX = 0, fMsgY = 0;
  Font fFont = new Font ("Serif", Font.BOLD, 24);

  /** Get the DateFormat object with the default time style. **/
  DateFormatPanel () {
    fDateFormat =
      DateFormat.getTimeInstance (DateFormat.DEFAULT);
  } // ctor

  /** Draw the time string on the panel center. **/
  public void paintComponent (Graphics g) {
    super.paintComponent (g);

    // Get current date object
    Date now = new Date ();

    // Format the time string.
    String date_out = fDateFormat.format (now);

    // Use our choice for the font.
    g.setFont (fFont);

    // Do the size and placement calculations only for the
    // first paint  (assumes the applet window is never resized.)
    if (fFirstPass) {

       // Get measures needed to center the message
      FontMetrics fm = g.getFontMetrics ();

      // How many pixels wide is the string
      int msg_width = fm.stringWidth (date_out);

      // Use the string width to find the starting point
      fMsgX = getSize ().width/2 - msg_width/2;

      // How far above the baseline can the font go?
      int ascent = fm.getMaxAscent ();

      // How far below the baseline?
      int descent= fm.getMaxDescent ();

      // Use the vertical height of this font to find
      // the vertical starting coordinate
      fMsgY = getSize ().height/2 - descent/2 + ascent/2;
    }
    g.drawString (date_out,fMsgX,fMsgY);
    fFirstPass = false;
  } // paintComponent

} // class DateFormatPanel