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.
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!');">
</applet>
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.setHorizontalAlignment(SwingConstants.CENTER);
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
addMouseListener(this);
// 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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