Java Games
Another one: Thief and Policemen
In this java applet, you can design and add upto 20 balls in the square, watch how they
collide each other.
The kernel of this applet is a HideBall.java, in which the methods and properties of a ball
is defined. The most important method "public boolean CheckCollision(HideBall ballAnother)".
In this method, this ball check the distance between itself and another ball,
if the distance is less than the sum of these two balls' radiums, collision occured.
According to Conservation of Moment and Conservation of Energy,
these two balls would have new speed:
v1' = (m1*v1+2*m2*v2-m2*v1)/(m1+m2);
v2' = (m2*v2+2*m1*v1-m1*v2)/(m1+m2);
import java.awt.*;
import java.math.*;
/* Kernel of this program
define the attributes and methods of a ball.
result: http://fadshop.net/resume/javademo/ball.htm
Author: Ben (ben@fadshop.net)
16, April, 2002
18, April: changed: add a DrawBall in this class, and delete "SavedColor".
*/
class HideBall
{
private double PositionX;
private double PositionY;
private int DeltaX; // +1 or -1
private double DeltaY; //tg of the direction. Delta Y / Delta x
private int Radius;
private int Radius3; //Radius3 = Radius^3, ~ mass.
private Color BallColor;
// private Color SavedColor;
private double Speed;
private boolean ready;
private boolean colliding; // if colliding, use Color.white to show it.
private int BorderStartX;
private int BorderStartY;
private int BorderEndX;
private int BorderEndY;
public void HideBall()
{
colliding = false;
ready = false; //Not ready before Border, Radius,
Delta are set.
}
public void setPositionX(double x){ PositionX = x; }
public double getPositionX(){ return PositionX; }
public void setPositionY(double x){ PositionY = x; }
public double getPositionY(){ return PositionY; }
public void setDeltaX(int x){
if (x>0) DeltaX = 1;
else DeltaX = -1;
}
public int getDeltaX(){ return DeltaX; }
public void setDeltaY(double x){ DeltaY = x; }
public double getDeltaY(){ return DeltaY; }
public void setSpeed(double x){ Speed = x; }
public double getSpeed(){ return Speed; }
public void setRadius(int x){ Radius = x;
Radius3 = Radius*Radius*Radius; }
public int getRadius(){ return Radius; }
public int getRadius3(){ return Radius3; }
public void setBallColor(Color x){ BallColor = x; }
public void setColliding(boolean x){ colliding = x; }
public void setBorder(int StartX, int StartY, int EndX, int EndY){
if (StartX < EndX)
{
BorderStartX = StartX;
BorderEndX = EndX;
}
else
{
BorderStartX = EndX;
BorderEndX = StartX;
}
if (StartY < EndY)
{
BorderStartY = StartY;
BorderEndY = EndY;
}
else
{
BorderStartY = EndY;
BorderEndY = StartY;
}
}
public boolean Enable(boolean bEnable)
{ //Not ready before Border, Radius, DeltaX are set.
//maybe I should check if (PositionX, PositionY)
is located between Border.
if (bEnable)
{
if ( (BorderStartX < BorderEndX) &&
(BorderStartY < BorderEndY) &&
(Radius > 0) &&
( (DeltaX == 1) || (DeltaX == -1) ) )
{
ready = true;
}
return ready;
}
else
{
ready = false;
return true; // success.
}
}
public boolean IsReady()
{
return ready;
}
public boolean step()
{
if (ready == false)
{
return false;
}
PositionX = PositionX + Speed * DeltaX;
if (PositionX+Radius > BorderEndX)
{
DeltaX = (-1) * DeltaX;
PositionX = BorderEndX - (PositionX - BorderEndX) - 2*Radius;
colliding = true;
}
if (PositionX-Radius < BorderStartX)
{
DeltaX = (-1) * DeltaX;
PositionX = BorderStartX + (BorderStartX - PositionX) + 2*Radius;
colliding = true;
}
PositionY = PositionY + (Speed * DeltaY);
if (PositionY+Radius > BorderEndY)
{
DeltaY = (-1) * DeltaY;
PositionY = BorderEndY - (PositionY - BorderEndY) - 2*Radius;
colliding = true;
}
if (PositionY-Radius < BorderStartY)
{
DeltaY = (-1) * DeltaY;
PositionY = BorderStartY + (BorderStartY - PositionY) + 2*Radius;
colliding = true;
}
return true;
}
//After collict, x2 = (m2*v2+2*m1*v1-m1*v2)/(m1+m2);
// x1 = (m1*v1+2*m2*v2-m2*v1)/(m1+m2)
// m ~ r^3;
public boolean CheckCollision(HideBall ballAnother)
{
if (ready == false)
{
return false;
}
double DistanceSquare ;
double vOriginalX1, vOriginalY1, vOriginalX2, vOriginalY2;
double vCurrentX1, vCurrentY1, vCurrentX2, vCurrentY2;
DistanceSquare = (PositionX-ballAnother.getPositionX())*(
PositionX-ballAnother.getPositionX()) +
(PositionY-ballAnother.getPositionY())*
(PositionY-ballAnother.getPositionY());
if (DistanceSquare < (Radius+ballAnother.getRadius())*
(Radius+ballAnother.getRadius())) //collision occured.
{
// System.out.println("Clision! Data:");
// System.out.println("x=" + (PositionX-ballAnother.getPositionX) +
"y=" + (PositionY-ballAnother.getPositionY));
// System.out.println(" Radius=" + Radius +
" AnotherRadius=" + ballAnother.getRadius);
// System.out.println(" Speed=" + Speed +
" AnotherSpeed=" + ballAnother.getSpeed);
vOriginalX1 = Speed * DeltaX;
vOriginalY1 = Speed * DeltaY;
vOriginalX2 = ballAnother.getSpeed() * ballAnother.getDeltaX();
vOriginalY2 = ballAnother.getSpeed() * ballAnother.getDeltaY();
vCurrentX2 = (ballAnother.getRadius3() * vOriginalX2 +
2 * Radius3 * vOriginalX1 -
Radius3*vOriginalX2)
/ (Radius3 + ballAnother.getRadius3());
vCurrentX1 = (Radius3 * vOriginalX1 +
2 * ballAnother.getRadius3() * vOriginalX2 -
ballAnother.getRadius3() * vOriginalX1)
/ (Radius3 + ballAnother.getRadius3());
vCurrentY2 = (ballAnother.getRadius3() * vOriginalY2 +
2 * Radius3 * vOriginalY1 -
Radius3*vOriginalY2)
/ (Radius3 + ballAnother.getRadius3());
vCurrentY1 = (Radius3 * vOriginalY1 +
2 * ballAnother.getRadius3() * vOriginalY2 -
ballAnother.getRadius3() * vOriginalY1)
/ (Radius3 + ballAnother.getRadius3());
if (vCurrentX1 > 0)
{
DeltaX = 1;
Speed = vCurrentX1;
}
else
{
DeltaX = -1;
Speed = (-1) * vCurrentX1;
}
DeltaY = vCurrentY1 / Speed;
if (vCurrentX2 > 0)
{
ballAnother.setDeltaX( 1 );
ballAnother.setSpeed( vCurrentX2 );
}
else
{
ballAnother.setDeltaX( -1 );
ballAnother.setSpeed( (-1) * vCurrentX2 );
}
ballAnother.setDeltaY( vCurrentY2 / ballAnother.getSpeed());
colliding = true;
ballAnother.setColliding( true );
int iCalNum = 0;
while(DistanceSquare < (Radius+ballAnother.getRadius())*
(Radius+ballAnother.getRadius()) && (iCalNum<10))
{ iCalNum++;
step();
ballAnother.step();
DistanceSquare = (int)((PositionX-ballAnother.getPositionX())*
(PositionX-ballAnother.getPositionX()) +
(PositionY-ballAnother.getPositionY())*
(PositionY-ballAnother.getPositionY()));
}
if (iCalNum > 3)
{
System.out.println("iCalNum="+iCalNum);
}
}
return true; //ok
}
public void drawBall (Graphics g)
{
if (colliding == true)
{
g.setColor (Color.white);
colliding = false;
}
else
{
g.setColor (BallColor);
}
g.fillOval ((int)PositionX - Radius, (int)PositionY - Radius,
2 * Radius, 2 * Radius);
}
}
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