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16.4 A Timer Applet

Example 16-3 is a full-featured applet named Countdown. As illustrated in Figure 16-3, it displays an amount of remaining time in minutes and seconds and counts backward down to zero. When it reaches zero, it can play an optional sound and also optionally make the web browser load a new page. The applet demonstrates all of the applet methods listed at the beginning of this chapter, including getParameter( ), getImage( ), getAudioClip( ), showStatus( ), and AppletContext.showDocument( ). It also includes event-handling methods to pause the countdown while the mouse is over the timer. Finally, it demonstrates how to write a Swing-based applet by extending JApplet (you may notice an increase in applet start-up time with the switch to Swing and its attendant overhead). Instead of using a Thread as the Clock applet did, this Countdown applet uses the javax.swing.Timer class.

Figure 16-3. The Countdown applet

An important feature of this applet is its configurability: it uses getParameter( ) to read configuration parameters specified in the <param> tags inside the <applet> tag. Invoke the applet with HTML code like this:

<applet code="je3/applet/Countdown.class"
        codebase="../../" width=400 height=100>
  <param name="minutes" value="5">              <!-- Count down from 05:00 -->
  <param name="font" value="helvetica-bold-48"> <!-- Use a big bold font -->
  <param name="foreground" value="#0000ff">     <!-- and blue text-->
  <param name="background" value="#ffffff">     <!-- on white --> 
  <param name="image" value="clock.gif">        <!-- with an image -->
  <param name="sound" value="beep.au">          <!-- and an alarm bell. -->
  <!-- Where to go when time is up.  Here we just display a dialog box -->
  <param name="newpage" value="javascript:void alert('times up!');">
Example 16-3. Countdown.java
package je3.applet;
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.Timer;  // not java.util.Timer
import java.text.NumberFormat;
import java.net.*;

 * An applet that counts down from a specified time. When it reaches 00:00,
 * it optionally plays a sound and optionally moves the browser to a new page.
 * Place the mouse over the applet to pause the count; move it off to resume.
 * This class demonstrates most applet methods and features.
public class Countdown extends JApplet implements ActionListener, MouseListener
    long remaining;       // How many milliseconds remain in the countdown.
    long lastUpdate;      // When count was last updated
    JLabel label;         // Displays the count
    Timer timer;          // Updates the count every second
    NumberFormat format;  // Format minutes:seconds with leading zeros
    Image image;          // Image to display along with the time
    AudioClip sound;      // Sound to play when we reach 00:00

    // Called when the applet is first loaded
    public void init( ) {
        // Figure out how long to count for by reading the "minutes" parameter
        // defined in a <param> tag inside the <applet> tag.  Convert to ms.
        String minutes = getParameter("minutes");
        if (minutes != null) remaining = Integer.parseInt(minutes) * 60000;
        else remaining = 600000; // 10 minutes by default

        // Create a JLabel to display remaining time, and set some properties.
        label = new JLabel( );
        label.setOpaque(true);  // So label draws the background color

        // Read some parameters for this JLabel object
        String font = getParameter("font");
        String foreground = getParameter("foreground");
        String background = getParameter("background");
        String imageURL = getParameter("image");

        // Set label properties based on those parameters
        if (font != null) label.setFont(Font.decode(font));
        if (foreground != null) label.setForeground(Color.decode(foreground));
        if (background != null) label.setBackground(Color.decode(background));
        if (imageURL != null) {
            // Load the image, and save it so we can release it later
            image = getImage(getDocumentBase( ), imageURL);
            // Now display the image in the JLabel.
            label.setIcon(new ImageIcon(image));

        // Now add the label to the applet. Like JFrame and JDialog, JApplet
        // has a content pane that you add children to
        getContentPane( ).add(label, BorderLayout.CENTER);

        // Get an optional AudioClip to play when the count expires
        String soundURL = getParameter("sound");
        if (soundURL != null) sound=getAudioClip(getDocumentBase( ), soundURL);

        // Obtain a NumberFormat object to convert number of minutes and
        // seconds to strings.  Set it up to produce a leading 0 if necessary
        format = NumberFormat.getNumberInstance( );
        format.setMinimumIntegerDigits(2); // pad with 0 if necessary

        // Specify a MouseListener to handle mouse events in the applet.
        // Note that the applet implements this interface itself

        // Create a timer to call the actionPerformed( ) method immediately,
        // and then every 1000 milliseconds. Note we don't start the timer yet.
        timer = new Timer(1000, this);
        timer.setInitialDelay(0);  // First timer is immediate.

    // Free up any resources we hold; called when the applet is done
    public void destroy( ) { if (image != null) image.flush( ); }

    // The browser calls this to start the applet running
    // The resume( ) method is defined below.
    public void start( ) { resume( ); } // Start displaying updates

    // The browser calls this to stop the applet.  It may be restarted later.
    // The pause( ) method is defined below
    public void stop( ) { pause( ); }   // Stop displaying updates

    // Return information about the applet
    public String getAppletInfo( ) {
        return "Countdown applet Copyright (c) 2003 by David Flanagan";

    // Return information about the applet parameters
    public String[  ][  ] getParameterInfo( ) { return parameterInfo; }
    // This is the parameter information.  One array of strings for each
    // parameter.  The elements are parameter name, type, and description.
    static String[  ][  ] parameterInfo = {
        {"minutes", "number", "time, in minutes, to countdown from"},
        {"font", "font", "optional font for the time display"},
        {"foreground", "color", "optional foreground color for the time"},
        {"background", "color", "optional background color"},
        {"image", "image URL", "optional image to display next to countdown"},
        {"sound", "sound URL", "optional sound to play when we reach 00:00"},
        {"newpage", "document URL", "URL to load when timer expires"},

    // Start or resume the countdown
    void resume( ) {
        // Restore the time we're counting down from and restart the timer.
        lastUpdate = System.currentTimeMillis( );
        timer.start( );  // Start the timer

    // Pause the countdown
    void pause( ) {
        // Subtract elapsed time from the remaining time and stop timing
        long now = System.currentTimeMillis( );  
        remaining -= (now - lastUpdate);
        timer.stop( );   // Stop the timer

    // Update the displayed time.  This method is called from actionPerformed( )
    // which is itself invoked by the timer.
    void updateDisplay( ) {
        long now = System.currentTimeMillis( );  // current time in ms
        long elapsed = now - lastUpdate;        // ms elapsed since last update
        remaining -= elapsed;                   // adjust remaining time
        lastUpdate = now;                       // remember this update time

        // Convert remaining milliseconds to mm:ss format and display 
        if (remaining < 0) remaining = 0; 
        int minutes = (int)(remaining/60000);
        int seconds = (int)((remaining%60000)/1000);
        label.setText(format.format(minutes) + ":" + format.format(seconds));

        // If we've completed the countdown, beep and display new page
        if (remaining == 0) {
            // Stop updating now.
            timer.stop( );
            // If we have an alarm sound clip, play it now.
            if (sound != null) sound.play( );
            // If there is a newpage URL specified, make the browser
            // load that page now.
            String newpage = getParameter("newpage");
            if (newpage != null) {
                try {
                    URL url = new URL(getDocumentBase( ), newpage);
                    getAppletContext( ).showDocument(url);
                catch(MalformedURLException ex) { showStatus(ex.toString( )); }

    // This method implements the ActionListener interface.
    // It is invoked once a second by the Timer object
    // and updates the JLabel to display minutes and seconds remaining.
    public void actionPerformed(ActionEvent e) { updateDisplay( ); }

    // The methods below implement the MouseListener interface.  We use 
    // two of them to pause the countdown when the mouse hovers over the timer.
    // Note that we also display a message in the statusline
    public void mouseEntered(MouseEvent e) {
        pause( );                // pause countdown
        showStatus("Paused");   // display statusline message
    public void mouseExited(MouseEvent e) {
        resume( );               // resume countdown               
        showStatus("");         // clear statusline
    // These MouseListener methods are unused.
    public void mouseClicked(MouseEvent e) {  }
    public void mousePressed(MouseEvent e) {  }
    public void mouseReleased(MouseEvent e) {  }
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