Objects and Classes
Introducing Classes and Objects
You might wonder why you should need a separate class at all for the last program we discussed, but the advantages might come clearer to you in a more complicated one. The next applet uses a thread to make the bubbles you produce by clicking grow. The applet calles the grow() method of the GrowingBubbles class from within the run() method, which does all there is to do to make the
bubbles grow: increment its diameter. Making the objects of your programs also objects of a separate class, allows for more logical and simple programming, especially for big projects, as you will soon discover.
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//Sourcecode
import java.awt.*;
import java.applet.*;
class GrowingBubble
{
//instance variables
int x, y, diameter, startDiameter, endDiameter;
Color randomColor;
Graphics g;
//constructor
public GrowingBubble()
{
int red=(int)(Math.random()*255);
int green=(int)(Math.random()*255);
int blue=(int)(Math.random()*255);
//each bubble object has one random color
randomColor=new Color(red, green, blue);
//start diameter between 10 and 30
startDiameter=(int)(Math.random()*20)+10;
//end diameter between 50 and 150
endDiameter=(int)(Math.random()*100)+50;
diameter=startDiameter;
}
public void setPosition(int x, int y)
{
this.x=x;
this.y=y;
}
public void drawBubble()
{
g.setColor(randomColor);
g.fillOval(x-diameter/2,y-diameter/2,diameter,diameter);
}
public void grow()
{
//quickly grow, till end diameter is reached
if(diameter<endDiameter)
diameter+=4;
}
public void paint(Graphics gr)
{
g=gr;
drawBubble();
}
}
public class Project24 extends Applet implements Runnable
{
Image Buffer;
Graphics gBuffer;
GrowingBubble myBubble;
Thread runner;
boolean clicked;
public void init()
{
Buffer=createImage(size().width,size().height);
gBuffer=Buffer.getGraphics();
}
public void start()
{
if (runner == null)
{
runner = new Thread (this);
runner.start();
}
}
public void stop()
{
if (runner != null)
{
runner.stop();
runner = null;
}
}
public void run()
{
while(true)
{
//halt the thread for 15 ms here
try {runner.sleep(15);}
catch (Exception e) { }
//bubble methods may only be accessed, if we have
//created the object in the mouseDown method!
if(clicked)
{
myBubble.grow();
myBubble.paint(gBuffer);
}
repaint();
}
}
public boolean mouseDown(Event evt,int x,int y)
{
myBubble = new GrowingBubble();
myBubble.setPosition(x,y);
clicked=true;
return true;
}
public void update(Graphics g)
{
paint(g);
}
public void paint(Graphics g)
{
gBuffer.setColor(Color.black);
gBuffer.drawString("Click the applet to paint growing bubbles!", 30,20);
g.drawImage (Buffer,0,0, this);
}
}
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