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<META name=vsisbn content="1571690433"><META name=vstitle content="Black Art of Java Game Programming"><META name=vsauthor content="Joel Fan"><META name=vsimprint content="Sams"><META name=vspublisher content="Macmillan Computer Publishing"><META name=vspubdate content="11/01/96"><META name=vscategory content="Web and Software Development: Programming, Scripting, and Markup Languages: Java"><TITLE>Black Art of Java Game Programming:Advanced Techniques</TITLE>
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<font face="arial, helvetica" size="-1" color="#336633"><b>Black Art of Java Game Programming</b></font>
<br>
<font face="arial, helvetica" size="-1"><i>by Joel Fan</i>
<br>
Sams, Macmillan Computer Publishing
<br>
<b>ISBN:</b> 1571690433<b> Pub Date:</b> 11/01/96</font>
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<P><BR></P>
<H4 ALIGN="LEFT"><A NAME="Heading12"></A><FONT COLOR="#000077">Creating a Simple Multithreaded Animation</FONT></H4>
<P>Let’s create a multithreaded animation applet. Although this example is simple, it’ll highlight the differences of programming with threads.
</P>
<P>First of all, let’s define an abstract class, Actor, that specifies the essential methods and variables of objects that move independently on the screen. This class is shown in Listing 10-1.</P>
<P><B>Listing 10-1</B> Actor class</P>
<!-- CODE SNIP //-->
<PRE>
abstract class Actor extends Thread {
Applet applet;
protected boolean drawNow;
abstract void paint(Graphics g);
}
</PRE>
<!-- END CODE SNIP //-->
<P>Each Actor object is a separate thread that runs independently. When it needs to paint itself (for example, after a change in color or position), it will set <I>drawNow</I> to <I>true</I>, and notify the mother applet. For example, here is the run() method of an Actor subclass, called CycleColorActor, which paints a rectangle that changes colors every few milliseconds:</P>
<!-- CODE //-->
<PRE>
public void run() {
while (true) {
drawNow = true;
applet.repaint();
updateColor();
try {
sleep(delay);
}
catch (InterruptedException e) {
}
}
}
</PRE>
<!-- END CODE //-->
<P>The repaint() method of the applet eventually triggers a call to each Actor’s individual paint() method. Here’s the paint() of the CycleColorActor:
</P>
<!-- CODE SNIP //-->
<PRE>
public void paint(Graphics g) {
if (drawNow) {
g.setColor(new Color(red,green,blue));
g.fillRect(locx,locy,73,73);
drawNow = false;
}
}
</PRE>
<!-- END CODE SNIP //-->
<P>As you see, only those Actors that need to paint will draw themselves.
</P>
<P>The full definition of the CycleColorActor is shown in Listing 10-2. The constructor allows you to specify the location of the rectangle, the delay, and the starting (<I>r,g,b</I>) triple.</P>
<P><B>Listing 10-2</B> CycleColorActor class</P>
<!-- CODE //-->
<PRE>
class CycleColorActor extends Actor {
protected int locx,locy; // location
protected int red,green,blue; // color
protected int delay; // delay in ms
public CycleColorActor(Applet a,int x,int y,
int r,int g,int b,int d) {
// initialize inherited variables
applet = a;
drawNow = false;
// initialize other variables
locx = x;
locy = y;
red = r;
green = g;
blue = b;
delay = d;
start(); // start thread running!
}
// override run() -- each thread executes this
// when it's alive
public void run() {
while (true) {
drawNow = true;
applet.repaint();
updateColor();
try {
sleep(delay);
}
catch (InterruptedException e) {
}
}
}
public void updateColor() {
red = (red + 4) % 256;
green = (green + 4) % 256;
blue = (blue + 4) % 256;
}
// provide implementation of abstract methods:
public void paint(Graphics g) {
if (drawNow) {
g.setColor(new Color(red,green,blue));
g.fillRect(locx,locy,73,73);
drawNow = false;
}
}
}
</PRE>
<!-- END CODE //-->
<P>Finally, Listing 10-3 shows the applet (ActorTest) that will define three Actors and set them running. Figure 10-5 shows how this applet works in conjunction with the Actors.
</P>
<P><A NAME="Fig5"></A><A HREF="javascript:displayWindow('images/10-05.jpg',600,408 )"><IMG SRC="images/10-05t.jpg"></A>
<BR><A HREF="javascript:displayWindow('images/10-05.jpg',600,408)"><FONT COLOR="#000077"><B>Figure 10-5</B></FONT></A> ActorTest applet execution</P>
<P>ActorTest doesn’t implement Runnable, as our previous animation applets did. Study the following code so you understand how things work!
</P>
<P><B>Listing 10-3</B> ActorTest applet</P>
<!-- CODE //-->
<PRE>
import java.applet.*;
import java.awt.*;
public class ActorTest extends Applet {
Actor actors[] = new Actor[4];
public void init() {
int width = size().width;
int height = size().height;
setBackground(Color.black);
// define 4 Actors:
// this changes color every 50 ms
actors[0] = new CycleColorActor(this,0,0,
90,150,0,50);
// this changes color every 100 ms
actors[1] = new CycleColorActor(this,width-73,0,
90,150,0,100);
// this changes color every 200 ms
actors[2] = new CycleColorActor(this,0,height-73,
90,150,0,200);
// this changes color every 400 ms
actors[3] = new CycleColorActor(this,width-73,height-73,
90,150,0,400);
}
// override update so it doesn't erase screen
public void update(Graphics g) {
paint(g);
}
public void paint(Graphics g) {
for (int i=0; i<actors.length; i++) {
actors[i].paint(g); // paint each rectangle
}
}
}
</PRE>
<!-- END CODE //-->
<P>Next, let’s see how you can structure games with threads. The ActorTest applet contains the basic idea.
</P>
<H3><A NAME="Heading13"></A><FONT COLOR="#000077">Structuring Games with Multiple Threads</FONT></H3>
<P>First, let’s review how we’ve written games without using multiple threads. The key to our previous approach is the Video Game Loop, diagrammed in Figure 10-6.
</P>
<P><A NAME="Fig6"></A><A HREF="javascript:displayWindow('images/10-06.jpg',497,496 )"><IMG SRC="images/10-06t.jpg"></A>
<BR><A HREF="javascript:displayWindow('images/10-06.jpg',497,496)"><FONT COLOR="#000077"><B>Figure 10-6</B></FONT></A> The Video Game Loop revisited</P>
<P>In this scheme, the run() loop contained in the applet acts as the master clock, telling each of the classes used in the game when to update and paint.
</P>
<P>With multiple threads, a different approach is possible. Each Actor in the game will update at an individual rate, based on its own internal clock. When it’s ready to paint, it notifies the mother applet. The mother applet passes the graphics context to the individual Actors, which paint if necessary. This scheme is diagrammed in Figure 10-7.</P>
<P><A NAME="Fig7"></A><A HREF="javascript:displayWindow('images/10-07.jpg',600,747 )"><IMG SRC="images/10-07t.jpg"></A>
<BR><A HREF="javascript:displayWindow('images/10-07.jpg',600,747)"><FONT COLOR="#000077"><B>Figure 10-7</B></FONT></A> Multithreaded Video Game structure<P><BR></P>
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