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Java by Example - using the Vector class
In the last applet we used an array, to store a fixed number of objects of our "BreathingBubbles"-class. We created an array of 100 objects. This number was arbitrary and could of course be increased. Whenever we create an array though, the number of objects (or simple data types) has to be set when the program is created, we cannot change it at program runtime. For every item in the array the appropriate amount of space is reserved in memory, so if you don't use it up, it is wasted and makes your program slower. Wouldn't it be nice to have a kind of container, which can grow and shrink just as we need it? Such a container exists, it is the vector, defined in the java.util package. It can dynamically adjust, just as required. This is highly convenient for many programming tasks. Be aware though, that due to the complexity of the Vector class, using vectors is slower than using arrays! This program uses only the most common methods of this class, you should explore the other possibilities on your own with the API documentation.
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//Sourcecode
import java.awt.*;
import java.applet.*;
//for the Vector class
import java.util.*;
class BreathingBubble
{
//instance variables
int x, y, diameter, startDiameter, endDiameter;
boolean growing=true;
Color randomColor;
Graphics g;
//constructor
public BreathingBubble()
{
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 breathe()
{
//grow till end diameter is reached
if(growing)
{
if(diameter<endDiameter)
diameter++;
else
growing=false;
}
//shrink till start diameter is reached
else
{
if(diameter>startDiameter)
diameter--;
else
growing=true;
}
}
public void paint(Graphics gr)
{
g=gr;
drawBubble();
}
}
public class Project26 extends Applet implements Runnable
{
Image Buffer;
Graphics gBuffer;
BreathingBubble myBubble;
Thread runner;
boolean clicked;
//stores the number of bubbles we created
int index;
//our vector variable, can store unlimited number of bubbles
Vector v;
public void init()
{
Buffer=createImage(size().width,size().height);
gBuffer=Buffer.getGraphics();
v = new Vector();
}
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()
{
Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
while(true)
{
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)
{
for(int i=0;i<index;i++)
{
//the element stored in the vector at index position i
//is casted to a BreatingBubble object and assigned to
//the variable myBubble, to access it's methods!
myBubble=(BreathingBubble)v.elementAt(i);
myBubble.breathe();
myBubble.paint(gBuffer);
}
}
repaint();
}
}
public boolean mouseDown(Event evt,int x,int y)
{
myBubble = new BreathingBubble();
myBubble.setPosition(x,y);
//we add a new element to the vector
v.addElement(myBubble);
//our index number is incremented, to keep track of the number
//of created bubble objects
index++;
//flag "clicked" is set, now we may access myBubbles methods
clicked=true;
return true;
}
public void update(Graphics g)
{
paint(g);
}
public void paint(Graphics g)
{
gBuffer.setColor(Color.black);
gBuffer.drawString("Click for unlimited number of breathing bubbles!", 15,20);
g.drawImage (Buffer,0,0, this);
gBuffer.setColor(Color.white);
gBuffer.fillRect(0,0,size().width,size().height);
}
}
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