Hour 24.
Creating Animation
Whether you are reading this book in 24 one-hour sessions or in a single 24-hour-long-bring-me-
more-coffee-can't-feel-my-hand-are-you-going-to-finish-that-donut marathon, you deserve something
for making it all this way. Unfortunately, Sams Publishing declined my request to buy you a pony, so
the best I can offer as a reward is the most entertaining subject in the book: animation.
At this point, you have learned how to use text, fonts, color, lines, polygons, and sound in your Java
programs. For the last hour on Java's multimedia capabilities, and the last hour of the book, you will
learn how to display image files in GIF and JPEG formats in your programs and present them in
animated sequences. The following topics will be covered:
• Using Image objects to hold image files
• Putting a series of images into an array
• Cycling through an image array to produce animation
• Using the update() method to reduce flickering problems
• Using the drawImage() command
• Establishing rules for the movement of an image
• Animating a Sequence of Images
• Computer animation at its most basic consists of drawing an image at a specific place, moving the
location of the image, and telling the computer to redraw the image at its new location. Many
animations on Web pages are a series of image files, usually .GIF or .JPG files that are displayed
in the same place in a certain order. You can do this to simulate motion or to create some other
effect.
• The first program you will be writing today uses a series of image files to create an animated
picture of the Anastasia Island Lighthouse in St. Augustine, Florida. Several details about the
animation will be customizable with parameters, so you can replace any images of your own for
those provided for this example.
• Create a new file in your word processor called Animate.java. Enter Listing 24.1 into the file, and
remember to save the file when you're done entering the text.
• Example 24.1. The Full Text of Animate.java
• 1: import java.awt.*;
• 2:
• 3: public class Animate extends javax.swing.JApplet
• 4: implements Runnable {
• 5:
• 6: Image[] picture = new Image[6];
• 7: int totalPictures = 0;
• 8: int current = 0;
• 9: Thread runner;
• 10: int pause = 500;
• 11:
• 12: public void init() {
• 13: for (int i = 0; i < 6; i++) {
• 14: String imageText = null;
• 15: imageText = getParameter("image"+i);
• 16: if (imageText != null) {
• 17: totalPictures++;
• 18: picture[i] = getImage(getCodeBase(), imageText);
• 19: } else
• 20: break;
• 21: }
• 22: String pauseText = null;
• 23: pauseText = getParameter("pause");
• 24: if (pauseText != null) {
• 25: pause = Integer.parseInt(pauseText);
• 26: }
• 27: }
• 28:
• 29: public void paint(Graphics screen) {
• 30: super.paint(screen);
• 31: Graphics2D screen2D = (Graphics2D) screen;
• 32: if (picture[current] != null)
• 33: screen2D.drawImage(picture[current], 0, 0, this);
• 34: }
• 35:
• 36: public void start() {
• 37: if (runner == null) {
• 38: runner = new Thread(this);
• 39: runner.start();
• 40: }
• 41: }
• 42:
• 43: public void run() {
• 44: Thread thisThread = Thread.currentThread();
• 45: while (runner == thisThread) {
• 46: repaint();
• 47: current++;
• 48: if (current >= totalPictures)
• 49: current = 0;
• 50: try {
• 51: Thread.sleep(pause);
• 52: } catch (InterruptedException e) { }
• 53: }
• 54: }
• 55:
• 56: public void stop() {
• 57: if (runner != null) {
• 58: runner = null;
• 59: }
• 60: }
• 61: }
• Because animation is usually a process that continues over a period of time, the portion of the
program that manipulates and animates images should be designed to run in its own thread. This
becomes especially important in a Swing program that must be able to respond to user input while
an animation is taking place. Without threads, animation often takes up so much of the Java
interpreter's time that the rest of a program's graphical user interface is sluggish to respond.
• The Animate program uses the same threaded applet structure that you used during Hour 19,
"Creating a Threaded Program." Threads are also useful in animation programming because they
give you the ability to control the timing of the animation. The Thread.sleep() method is an
effective way to determine how long each image should be displayed before the next image is
shown.
• The Animate applet retrieves images as parameters on a Web page. The parameters should have
names starting at "image0" and ending at the last image of the animation, such as "image3" in this
hour's example. The maximum number of images that can be displayed by this applet is six, but
you could raise this number by making changes to Lines 6 and 13.
• The totalPicture integer variable determines how many different images will be displayed in an
animation. If less than six image files have been specified by parameters, the Animate applet will
determine this during the init() method when imageText equals null after Line 15.
• The speed of the animation is specified by a "pause" parameter. Because all parameters from a
Web page are received as strings, the Integer.parseInt() method is needed to convert the text into
an integer. The pause variable keeps track of the number of milliseconds to pause after displaying
each image in an animation.
• Loading and Displaying Images
• As with most threaded programs, the run() method contains the main part of the program. A while
(runner == thisThread) statement in Line 44 causes Lines 45–51 to loop until something causes
these two Thread objects to have different values.
• The first thing that happens in the while loop is a call to the applet's repaint() method. This
statement requests that the applet's paint() method be called so that the screen can be updated. Use
repaint() any time you know something has changed and the display needs to be changed to bring
it up to date. In this case, every time the Animate loop goes around once, a different image should
be shown.
• Note
• In Java, you can never be sure that calling repaint() will result in the component or applet window
being repainted. The interpreter will ignore calls to repaint() if it can't process them as quickly as
they are being called, or if some other task is taking up most of its time.
• The paint() method in Lines 29–34 contains the following statements:
• Graphics2D screen2D = (Graphics2D) screen;
• if (picture[current] != null)
• screen2D.drawImage(picture[current], 0, 0, this);
• First, a Graphics2D object is cast so that it can be used when drawing to the applet window. Next,
an if statement determines whether the Image object stored in picture[current] has a null value.
When it does not equal null, this indicates that an image is ready to be displayed. The
drawImage() method of the screen2D object displays the current Image object at the (x,y) position
specified.
• Note
• The paint() method of this applet does not call the paint() method of its superclass, unlike some of
the other graphical programs in the book, because it makes the animated sequence look terrible.
The applet's paint() method clears the window each time it is called, which is OK when you're
drawing a graphical user interface or some other graphics that don't change. However, clearing it
again and again in a short time causes an animation to flicker.
• The this statement sent as the fourth argument to drawImage() enables the program to use a class
called ImageObserver. This class tracks when an image is being loaded and when it is finished.
The JApplet class contains behavior that works behind the scenes to take care of this process, so
all you have to do is specify this as an argument to drawImage() and some other methods related
to image display. The rest is taken care of for you.
• An Image object must be created and loaded with a valid image before you can use the
drawImage() method. The way to load an image in an applet is to use the getImage() method. This
method takes two arguments: the Web address or folder that contains the image file and the file
name of the image.
• The first argument is taken care of with the getCodeBase() method, which is part of the JApplet
class. This method returns the location of the applet itself, so if you put your images in the same
folder as the applet's class file, you can use getCodeBase(). The second argument should be a
.GIF file or .JPG file to load. In the following example, a turtlePicture object is created and an
image file called Mertle.gif is loaded into it:
• Image turtlePicture = getImage(getCodeBase(), "Mertle.gif");
• Note
• As you look over the source code to the Animate applet, you might wonder why the test for a null
value in Line 31 is necessary. This check is required because the paint() method may be called
before an image file has been fully loaded into a picture[] element. Calling getImage() begins the
process of loading an image. To prevent a slowdown, the Java interpreter continues to run the rest
of the program while images are being loaded.
• Storing a Group of Related Images
• In the Animate applet, images are loaded into an array of Image objects called pictures. The
pictures array is set up to handle six elements in Line 6 of the program, so you can have Image
objects ranging from picture[0] to picture[5]. The following statement in the applet's paint()
method displays the current image:
• screen.drawImage(picture[current], 0, 0, this);
• The current variable is used in the applet to keep track of which image to display in the paint()
method. It has an initial value of 0, so the first image to be displayed is the one stored in
picture[0]. After each call to the repaint() statement in Line 45 of the run() method, the current
variable is incremented by one in Line 46.
• The totalPictures variable is an integer that keeps track of how many images should be displayed.
It is set when images are loaded from parameters off the Web page. When current equals
totalPictures, it is set back to 0. As a result, current cycles through each image of the animation,
and then begins again at the first image.
• Sending Parameters to the Applet
• Because the Animate applet relies on parameters to specify the image files it should display, you
need to create a Web page containing these file names before you can test the program. After
saving and compiling the Animate.java file, open up a new file in your word processor and call it
Animate.html. Enter Listing 24.2 into that file and save it when you're done.
• Example 24.2. The Full Text of Animate.html
• 1: <applet code="Animate.class" width=215 height=298>
• 2: <param name="image0" value="lh0.gif">
• 3: <param name="image1" value="lh1.gif">
• 4: <param name="image2" value="lh2.gif">
• 5: <param name="image3" value="lh3.gif">
• 6: <param name="pause" value="800">
• 7: </applet>
• This file specifies four image files: lh0.gif, lh1.gif, lh2.gif, and lh3.gif. These files are listed as the
values for the parameters image0 through image3. You can find the files used in this example on
the Hour 24 page of the book's Web site at http://www.java24hours.com.
• You can also specify any of your own .GIF or .JPG files, if desired. Whichever files you choose
should be placed in the same folder as the Animate.class and Animate.html files. With the "pause"
parameter, you can specify how long the program should pause after each image is displayed.
• Note
• You might be wondering why the files and the parameters are given names that start numbering
with 0 instead of 1. This is done because the first element of an array in a Java program is
numbered 0. Putting an image0 called lh0.gif into pictures[0] makes it easier to know where these
images are being stored.
• Once the files have been put in the right place, you're ready to try out the Animate applet. Type
the following command to open the page in your Web browser and view the applet:
• Animate.html
• Figure 24.1 shows one of the four images being displayed as the applet runs.
•
• Figure 24.1 Animating GIF files in an applet.
• Although this is a simple animation program, hundreds of applets on the Web use similar
functionality to present a series of image files as an animation. Presenting a sequence of image
files through Java is similar to the animated .GIF files that are becoming more commonplace on
Web pages. Although Java applets are often slower to load than these .GIF files, they can provide
more control of the animation and allow for more complicated effects.
• Summary
• The purpose of Sams Teach Yourself Java 2 Programming in 24 Hours, Third Edition is to help
you become comfortable with the concepts of programming and confident in your ability to write
your own applications and applets. Java has an approach that is somewhat difficult to master.
(Feel free to scratch out the word "somewhat" in the previous sentence if it's a gross misstatement
of the truth.)
• As you build experience in Java, you're building experience that will be increasingly relevant in
the coming years, because concepts such as object-oriented programming, virtual machines, and
secure environments are on the leading edge of software development.
• If you haven't already, you should read the appendixes to find out about this book's Web site,
additional Java books from Sams Publishing, and other useful information.
• At the conclusion of this hour, you can explore Java in several different places. Programmers are
discussing the language in comp.lang.java.programmer and other Usenet discussion groups.
Several hundred Java User Groups meet regularly, according to the list published at Sun's Java
site (http://servlet.java.sun.com/jugs). Numerous Java job openings are displayed in the database
of employment World Wide Web sites such as http://www.careerbuilder.com. There's also a Web
site for this book at http://www.java24hours.com where you can send electronic mail to the author
and read answers to reader questions, clarifications to the book, and (gulp) corrections.
• Of course, my favorite way for you to continue building your skills as a Java programmer is to
read Sams Teach Yourself Java 2 in 21 Days, Third Edition. I coauthored the book with Laura
Lemay, and it expands on the subjects covered here and introduces new ones, such as JavaSound,
JavaBeans, and network programming.
• My apologies for this brazen attempt to hawk another book…if I sell enough copies, my name is
entered into a drawing with other top Sams authors to win special prizes.
• My heart's set on a Kool Orange Schwinn Sting-Ray bicycle with a banana seat.
• —Rogers Cadenhead
