Generate Bouncing Lines Using Applet

package org.best.example;

    /*
            Generate Bouncing Lines Using Applet Example
            This Java example shows how to create bouncing lines using java Applet
            example.
    */
    
    import java.awt.*;
    import java.applet.Applet;
    
    public class BouncingLines extends Applet implements Runnable {
    
        Thread runner = null;
    
        final static int WIDTH  = 200;
        final static int HEIGHT = 100;
    
        Image    image;
        Graphics graphics;
    
        // bouncing lines member variables
        int[] x1;
        int[] y1;
        int[] x2;
        int[] y2;
    
        int dx1 = 2 + (int)( 3 * Math.random() );
        int dy1 = 2 + (int)( 3 * Math.random() );
        int dx2 = 2 + (int)( 3 * Math.random() );
        int dy2 = 2 + (int)( 3 * Math.random() );
        static int first = 0;
        final static int LINES = 50;
    
        public void init() {
    
            // create arrays to hold the line coordinates
            x1 = new int[LINES];
            y1 = new int[LINES];
            x2 = new int[LINES];
            y2 = new int[LINES];
    
            // initialise the first line
            x1[0] = (int)( WIDTH  * Math.random() );
            y1[0] = (int)( HEIGHT * Math.random() );
            x2[0] = (int)( WIDTH  * Math.random() );
            y2[0] = (int)( HEIGHT * Math.random() );
    
            // initialise all the other lines
           for ( int i = 1; i < LINES; i++ ) {
    
                x1[i] = x1[0];
                y1[i] = y1[0];
                x2[i] = x2[0];
                y2[i] = y2[0];
            }
            image    = createImage( WIDTH, HEIGHT );
            graphics = image.getGraphics();
        }
    
    
        public void start() {
    
            // user visits the page, create a new thread
           if ( runner == null ) {
    
                runner = new Thread( this );
                runner.start();
            }
        }
    
    
        public void stop() {
    
            // user leaves the page, stop the thread
           if ( runner != null && runner.isAlive() )
                runner.stop();
    
            runner = null;
        }
    
    
        public void run() {
    
            while (runner != null) {
    
                repaint();
    
                try {
    
                    Thread.sleep( 20 );
    
                } catch ( InterruptedException e ) {
    
                    // do nothing
                }
            }
        }
    
    
        public void paint( Graphics g ) {
    
            update( g );
        }
    
    
        public void update( Graphics g ) {
    
            // clear the background to white
    
            graphics.setColor( Color.black );
            graphics.fillRect( 0, 0, WIDTH, HEIGHT );
    
            // draw the lines
            for(int r=4;r<=9;r++)
            {
                  
                    graphics.setColor( Color.green );
          
                    int line = first;
          
                    for ( int i = 0; i < LINES; i++ ) {
          
                        graphics.drawLine( x1[line], y1[line],
                                           x2[line], y2[line] );
                        line++;
                        if ( line == LINES ) line = 0;
                    }
          
                    line = first;
          
                    first--;
          
                    if ( first < 0 ) first = LINES - 1;
          
                    x1[first] = x1[line];
                    y1[first] = y1[line];
                    x2[first] = x2[line];
                    y2[first] = y2[line];
          
                    // move the "first" line
          
                    if (x1[first] + dx2 < WIDTH)
                        x1[first] += dx1;
                    else
                        dx1 = -(2 + (int)( 3 * Math.random() ));
          
                    if (x1[first] + dx1 >= 0)
                        x1[first] += dx1;
                    else
                        dx1 = 2 + (int)( 3 * Math.random() );
          
                    if (y1[first] + dy1 < HEIGHT)
                        y1[first] += dy1;
                    else
                        dy1 = -(2 + (int)( 3 * Math.random() ));
          
                    if (y1[first] + dy1 >= 0)
                        y1[first] += dy1;
                    else
                        dy1 = 2 + (int)( 3 * Math.random() );
          
                    if (x2[first] + dx2 < WIDTH)
                        x2[first] += dx2;
                    else
                        dx2 = -(2 + (int)( 3 * Math.random() ));
          
                    if (x2[first] + dx2 >= 0)
                        x2[first] += dx2;
                    else
                        dx2 = 2 + (int)( 3 * Math.random() );
          
                    if (y2[first] + dy2 < HEIGHT)
                        y2[first] += dy2;
                    else
                        dy2 = -(2 + (int)( 3 * Math.random() ));
          
                    if (y2[first] + dy2 >= 0)
                        y2[first] += dy2;
                    else
                        dy2 = 2 + (int)( 3 * Math.random() );
          
                    // copy buffer to screen
                    g.drawImage( image, 0, 0, this );
                }
        }
    }