Tutorials : Java by Example : Section 6 :
Section Six Contents
Using Mousemove and Mousedrag
Keyboard Commands and Playing Sound
Detecting Collisions and Intersections
A Bouncing Balls Applet

Input Devices

Detecting Collisions and Intersections

The simplest method to detect collisions of two shapes is the bounding rectangle detection. Whenever the rectangles that surround the shapes intersect, a collision is detected. To accomplish this, we can use the convenient method intersects of the Rectangle class, Java provides us with: collide=r1.intersects(r2); The boolean instance variable collide receives the value of an intersection taken place, between the Rectangle objects r1 and r2. Whenever the two rectangles collide, it it reported and signaled:

//Sourcecode 

import java.awt.*;
import java.applet.*;

public class Project34 extends Applet
{
   Image Buffer;
   Graphics gBuffer;
   int x, y;
   boolean mouseInside, collide;
        Rectangle r1, r2;

        public void init()
        {
             Buffer=createImage(size().width,size().height);
             gBuffer=Buffer.getGraphics();

             //create 2 rectangle objects
             r1=new Rectangle(110,115,60,50);
             //we don't know the coordinates yet, so we create it
             //with a different constructor, passing only the size
             r2=new Rectangle(40,40);
        }

        public void drawStuff()
        {
             gBuffer.setColor(Color.white);
             gBuffer.fillRect(0,0,size().width,size().height);

             gBuffer.setColor(Color.red);
             gBuffer.fillRect(r1.x, r1.y, r1.width, r1.height);

             gBuffer.setColor(Color.blue);

             gBuffer.setFont(new Font("Helvetica",Font.BOLD,16));

             if(mouseInside)
                   gBuffer.fillRect(r2.x, r2.y, r2.width, r2.height);
              else
                   gBuffer.drawString("Move the mouse around!", 10, 20);

              gBuffer.setColor(Color.black);

              if(collide)
                   gBuffer.drawString("Collision!", 10, 20);
        }

        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;

             //move the blue square according to mouse position
             //we use the Rectangle method "move" here
             r2.move(x-r2.width/2, y-r2.height/2);

             //check for collision here, use the "intersects" method
             //of the Rectangle class
             collide=r1.intersects(r2);

             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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