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16.3 A Clock Applet
Example 16-2 is an applet
that displays the current time, as shown in Figure 16-2, and updates it once every second. Unlike
Example 16-1, which defines a paint(
) method and does its own text drawing with
Graphics.drawString( ), this example uses a
java.awt.Label component to do the drawing. While
it is common for applets to do their own drawing with a
paint( ) method, it is also important to remember
that applets extend java.awt.Panel and can contain
any type of GUI components. Clock defines an
init( ) method that creates and configures the
Label component.
Figure 16-2. A clock applet
In order to update the time every
second, Clock implements the
Runnable interface and creates a
Thread that runs the run( )
method. The applet's start( ) and
stop( ) methods are invoked by the browser when
the applet becomes visible or is hidden; they start and stop the
thread. (Although the example is written to use Java 1.1, it does not
rely on the Thread.stop( ) method, which was
deprecated in Java 1.2.)Finally, the Clock
applet implements getAppletInfo( ) to provide
information about the applet. Sun's
appletviewer tool is able to display this
information, but most web browsers don't.
Example 16-2. Clock.java
package je3.applet;
import java.applet.*; // Don't forget this import statement!
import java.awt.*; // Or this one for the graphics!
import java.util.Date; // To obtain the current time
import java.text.DateFormat; // For displaying the time
/**
* This applet displays the time, and updates it every second
**/
public class Clock extends Applet implements Runnable {
Label time; // A component to display the time in
DateFormat timeFormat; // This object converts the time to a string
Thread timer; // The thread that updates the time
volatile boolean running; // A flag used to stop the thread
/**
* The init method is called when the browser first starts the applet.
* It sets up the Label component and obtains a DateFormat object
**/
public void init( ) {
time = new Label( );
time.setFont(new Font("helvetica", Font.BOLD, 12));
time.setAlignment(Label.CENTER);
setLayout(new BorderLayout( ));
add(time, BorderLayout.CENTER);
timeFormat = DateFormat.getTimeInstance(DateFormat.MEDIUM);
}
/**
* This browser calls this method to tell the applet to start running.
* Here, we create and start a thread that will update the time each
* second. Note that we take care never to have more than one thread
**/
public void start( ) {
running = true; // Set the flag
if (timer == null) { // If we don't already have a thread
timer = new Thread(this); // Then create one
timer.start( ); // And start it running
}
}
/**
* This method implements Runnable. It is the body of the thread. Once a
* second, it updates the text of the Label to display the current time.
* AWT and Swing components are not, in general, thread-safe, and should
* typically only be updated from the event-handling thread. We can get
* away with using a separate thread here because there is no event
* handling in this applet, and this component will never be modified by
* any other thread.
**/
public void run( ) {
while(running) { // Loop until we're stopped
// Get current time, convert to a String, and display in the Label
time.setText(timeFormat.format(new Date( )));
// Now wait 1000 milliseconds
try { Thread.sleep(1000); }
catch (InterruptedException e) { }
}
// If the thread exits, set it to null so we can create a new one
// if start( ) is called again.
timer = null;
}
/**
* The browser calls this method to tell the applet that it is not visible
* and should not run. It sets a flag that tells the run( ) method to exit
**/
public void stop( ) { running = false; }
/**
* Returns information about the applet for display by the applet viewer
**/
public String getAppletInfo( ) {
return "Clock applet Copyright (c) 2000 by David Flanagan";
}
}
