Beginning Java 2- JDK 1.3 Version
Images and Animation
Try It Out – Using a Timer
Much of the code will be the same so we will only repeat the essentials
here. The class no longer needs to implement the Runnable
interface so the run() method is no longer required in the
applet class.
import java.awt.*;
import java.awt.image.*;
import javax.swing.*;
import java.net.*;
import java.awt.geom.*; // For AffineTransform
public class TimedWhirlingLogo extends JApplet
{
// This method is called when the applet is loaded
public void init()
{
// Code exactly as before...
}
// This method is called when the browser starts the applet
public void start()
{
if(tracker.isErrorAny()) // If any image errors
return; // don't create the thread
timer = new java.util.Timer(true);
timer.schedule(new java.util.TimerTask()
{
public void run()
{
imagePanel.repaint(); // Repaint the image
angle = 2.0*Math.PI*stepCount++/STEPS_PER_ROTATION;
stepCount = ++stepCount%STEPS_PER_ROTATION;
}
},
0, INTERVAL);
}
// This method is called when the browser wants to stop the applet
// - when is it not visible for example
public void stop()
{
timer.cancel();
}
// Class representing a panel displaying an image
class ImagePanel extends JPanel
{
// Code exactly as before...
)
java.util.Timer timer; // Animation timer
MediaTracker tracker; // Tracks image loading
ImagePanel imagePanel;
AffineTransform at = new AffineTransform();
int imageWidth, imageHeight; // Image dimensions
double angle; // Rotation angle
final int INTERVAL = 50; // Time interval msec
final int ROTATION_TIME = 2000; // Complete rotation time msec
final int STEPS_PER_ROTATION = ROTATION_TIME/ INTERVAL;
int stepCount; // Total number of steps
}
If you compile and run this applet, it should run just as well as the
previous version.
How It Works
The code is a lot shorter because the Timer object does all
the scheduling work. The start() method in our applet class
creates the Timer object we will use to schedule the animation
with the statement:
timer = new java.util.Timer(true);
The variable, timer, is a member of the
TimedWhirlingLogo class rather than a variable local to the
start() method because we also need to reference it in the
stop() method. Note how we have used the fully qualified name
for the Timer class here, and in the declaration of timer as a
member of the applet class. This is essential in this case. Importing the
package, java.util, containing the Timer class
would not be sufficient. As we said earlier, the javax.swing
package also defines a class with the name, Timer, so without
qualification of the name, the compiler would be unable to decide which
class we wanted to use.
We then use the Timer object to schedule the animation with
the statement:
timer.schedule(new java.util.TimerTask()
{
public void run()
{
imagePanel.repaint(); // Repaint the image
angle = 2.0*Math.PI*stepCount++/STEPS_PER_ROTATION;
stepCount = ++stepCount%STEPS_PER_ROTATION;
}
},
0, INTERVAL);
We use the schedule() method here because we need the task to
be executed at evenly distributed intervals to get a smooth animation. The
first argument to the schedule() method is defined by an
anonymous class derived from TimerTask. We again use a fully
qualified class name here – not because there is a duplicate use of the
name, but because we have not imported the java.util package
into our source file. The run() method in our anonymous class
specifies the task to be executed. This consists of two steps: redrawing
imagePanel containing the logo, and updating
angle that determines the orientation of the logo next time
around. The second argument to schedule() specifies a delay of
zero milliseconds before the first execution of the task, so the animation
will begin immediately. The third argument specifies the interval between
successive frames of the animation. Defining the constant,
INTERVAL, as a member of the applet class enables it to be
referenced as an argument to the schedule() method and within
the run() method of the anonymous class.
That's it. Using Timer objects makes scheduling animations a
lot simpler. Let's try one more example to get a feel for using the
scheduleAtFixedRate() method.
New on the Java Boutique:
New Review:
Time Management Made Easy with the Quartz Enterprise Job Scheduler
Why not just use the Java timer API? This open source scheduling
API boasts simplicity, ease-of-integration, a well-rounded feature
set, and it's free!
New Applet:
Reverse Complement
Reverse Complement is a simple applet that converts DNA or RNA
sequences into three useful formats.
Elsewhere on internet.com:
WebDeveloper Java
Lots of Java information on webdeveloper.com
WDVL Java
Thorough Java resource at the Web Developer's Virtual Library.
ScriptSearch Java
Hundreds of free Java code files to download.
jGuru: Your View of the Java Universe
Customizable portal with online training, FAQs, regular news updates, and tutorials.
