GAMES LESSON SEVENTEEN: Gravity
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This sample applet shows a ball bouncing. As with a real ball, the ball
bounces a little less high after each bounce until it more or less stops
bouncing. This effect is achieved as a result of the use of several
variables which keep track of the following things:
- y-coordinate (y)
- speed (speed)
- rate of acceleration (acc)
- rate of deceleration (dec)
- direction of ball bounce (up)
This applet allows you to re-drop a ball at any time by pressing the ENTER
key.
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Two important tasks are accomplished in the run method: 1) the value of y is
changed, 2) the value of up is changed when necessary. Here is the entire
run method:
<![CDATA[
1 public void run(){
2 while(true){
3 if(up){
4 y-=(int)speed;
5 speed-=dec;
6 }
7 else{
8 y+=(int)speed;
9 speed+=acc;
10 }
11 if(!up && y>floor-5) up=true;
12 else if(up && y<0) up=false;
13 else if(speed<=0) up=false;
14 repaint();
15 try{
16 Thread.sleep(30);
17 }catch(InterruptedException ie){ }
18 }
19 }
]]>
Lines three through ten are responsible for changing the value of y. Lines
eleven through thirteen change the value of up when necessary.
GRAVITY.java
<![CDATA[
import java.awt.*;
import java.applet.*;
import java.util.*;
import java.awt.event.*;
public class GRAVITY extends Applet implements Runnable{
Dimension d;
Image offI;
Thread tt;
int y;
double speed;
final static double dec=1.2;
final static double acc=.9;
boolean up;
int floor;
public void init(){
d = getSize();
floor = d.height-20;
reset();
offI=createImage(d.width,d.height);
tt = new Thread(this);
tt.start();
requestFocus();
this.addKeyListener(new KeyAdapter(){
public void keyPressed(KeyEvent k) {
if(k.getKeyCode()==KeyEvent.VK_ENTER){
reset();
}
}
});
}
public void reset(){
y = 0;
speed=1;
}
public void run(){
while(true){
if(up){
y-=(int)speed;
speed-=dec;
}
else{
y+=(int)speed;
speed+=acc;
}
if(!up && y>floor-5) up=true;
else if(up && y<0) up=false;
else if(speed<=0) up=false;
repaint();
try{
Thread.sleep(30);
}catch(InterruptedException ie){ }
}
}
public void update(Graphics g){
paint(g);
}
public void paint(Graphics g){
Graphics offG = offI.getGraphics();
offG.setColor(Color.yellow);
offG.fillRect(0,0,d.width,d.height);
offG.setColor(Color.blue);
offG.fillOval(40,y,10,10);
offG.drawLine(0,floor,d.width,floor);
g.drawImage(offI, 0,0, this);
}
}
]]>
ASSIGNMENT: