Tutorials : Java by Example : Section 6 :

Input Devices

Detecting Collisions and Intersections

To compute the distance between two points (for example in order to tell the score in a shooting game), you have to follow a different approach. A simple linear comparison between the x- and y-coordinates of two points will fail, when the two points are not aligned vertically or horizontally. We use the Theorem of Pythagoras here: Squareroot of((x1-x2)²*(y1-y2)²). That way we can tell exactly, which distance lies between two given points, regardless of their orientation on the screen. We use the Point class here, which is only a simple encapsulation for two variables, representing the x and y coordinates of a given point. To tell when the two circles collide, you just have to take the diameters into account:

//Sourcecode import java.awt.*; import java.applet.*; public class Project36 extends Applet { Image Buffer; Graphics gBuffer; int x, y, dist; boolean mouseInside; //middle points of the two circles Point p1, p2; public void init() { Buffer=createImage(size().width,size().height); gBuffer=Buffer.getGraphics(); //center of red circle p1=new Point(150,150); //center of blue circle p2=new Point(0,0); } public void drawStuff() { gBuffer.setColor(Color.white); gBuffer.fillRect(0,0,size().width,size().height); gBuffer.setColor(Color.red); gBuffer.fillOval(p1.x-20,p1.y-20,40,40); gBuffer.setFont(new Font("Helvetica",Font.BOLD,16)); gBuffer.setColor(Color.blue); if(mouseInside) gBuffer.fillOval(p2.x-20,p2.y-20,40,40); else gBuffer.drawString("Move the mouse around!", 10, 20); if(mouseInside) { if(dist<=40) { gBuffer.setColor(Color.red); gBuffer.drawString("Collision!", 10, 20); } gBuffer.setColor(Color.black); gBuffer.drawString("Distance: "+dist, 10, 40); } } public boolean mouseEnter(Event evt,int x,int y) { mouseInside=true; repaint(); return true; } public boolean mouseExit(Event evt,int x,int y) { mouseInside=false; repaint(); return true; } public boolean mouseMove(Event evt,int x,int y) { //submit the mouse coordinates to our instance variables this.x=x; this.y=y; p2.move(x,y); //calculate the distance of the centers (Theorem of Pythagoras) dist=(int)Math.sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y)); drawStuff(); repaint(); return true; } public void update(Graphics g) { paint(g); } public void paint(Graphics g) { drawStuff(); g.drawImage (Buffer,0,0, this); } }

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