Beginning Java 2- JDK 1.3 Version
Images and Animation
An Animated Applet
Just so that you know where we are heading, our first program illustrating
animation will be an applet that drops the Wrox logo from a great height to see
what happens. Since Wrox Press is an immensely resilient company, the logo will
bounce.
To implement this we just need to draw the logo at its new position at fixed
intervals of time – the new position being determined by how far the logo has
fallen during the time interval. We can store the coordinates of the current
location of the image in data members of the applet class, imageX
and imageY and the paint()ImagePanel will
draw the image at that position. The animation code in the run()
method will need access to the height of the image so it can work out when the
image hits the ground, so we should store the image dimensions,
imageWidth imageHeight, as data members too.
We will call the applet class LogoBounce, so the outline contents
of the source file will be:
import java.awt.*;
import java.awt.image.*;
import javax.swing.*;
import java.net.*;
public class LogoBounce extends JApplet
implements Runnable
{
// This method is called when the applet is loaded
public void init()
{
// Code to initialize the applet...
}
// This method is called when the browser starts the applet
public void start()
{
bouncer = new Thread(this); // Create animation thread
bouncing = true;
bouncer.start(); // and start it
}
// This method is called when the browser wants to stop the applet
// - when is it not visible for example
public void stop()
{
bouncing = false; // Stop the animation loop
bouncer = null; // Discard the thread
}
// This method is called when the animation thread is started
public void run()
{
// Code for the animation thread...
}
class ImagePanel extends JPanel
{
public ImagePanel(Image image)
{
this.image = image;
}
public void paint(Graphics g)
{
// Initialize the animation...
while(bouncing)
{
// Code for the animation loop
}
}
Image image; // The image
}
Thread bouncer; // The animation thread
boolean bouncing = false; // Controls animation thread
ImagePanel imagePanel; // Panel for the image
int imageWidth, imageHeight; // Image dimensions
int imageX, imageY; // Current image position
}
All the basic things we need are here. In the init() method we will
set up a component on which we can draw an image and add this to the content
pane for the applet, just as we did in the previous example. The
start() method creates the animation thread, bouncer,
sets the variable, bouncing, that will control the animation loop
to true, and starts the thread. The run() method will
contain the animation loop that will continue to run as long as
bouncing is true. In the stop() method we
just set bouncing to false to stop the animation loop in the
run() method, then discard the animation thread object by setting
bouncer to null. The start() method will
create a new thread if the animation needs to be restarted.
We will fetch the image from the file in the init() method using
the getImage() method as we did in the previous example. There's a
complication here though. The browser will call start() as soon as
the init() method returns to start the applet, and this will start
the thread to do the animation. Since this will involve calculations involving
the height of the image, we don't want this to go ahead until we are sure the
height is available. One way of determining when an image has been loaded is to
make use of something called a media tracker.
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