Flashing Graphics animations with threads

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/*
 * 1.0 code.
 */

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

/* 
 * This applet animates graphics that it generates.  This example
 * isn't a good one to copy -- it flashes.  The next couple of examples 
 * will show how to eliminate the flashing.
 */
 
public class FlashingGraphics extends Applet implements Runnable {
    int frameNumber = -1;
    int delay;
    Thread animatorThread;
    boolean frozen = false;

    int squareSize = 20;
    boolean fillColumnTop = true;

    public void init() {
        String str;
        int fps = 10;

        //How many milliseconds between frames?
        str = getParameter("fps");
        try {
            if (str != null) {
                fps = Integer.parseInt(str);
            }
        } catch (Exception e) {}
        delay = (fps > 0) ? (1000 / fps) : 100;

        //How many pixels wide is each square?
        str = getParameter("squareWidth");
        try {
            if (str != null) {
                squareSize = Integer.parseInt(str);
            }
        } catch (Exception e) {}
    }

    public void start() {
        if (frozen) {
            //Do nothing.  The user has requested that we
            //stop changing the image.
        } else {
            //Start animating!
            if (animatorThread == null) {
                animatorThread = new Thread(this);
            }
            animatorThread.start();
        }
    }

    public void stop() {
        //Stop the animating thread.
        animatorThread = null;
    }

    public boolean mouseDown(Event e, int x, int y) {
        if (frozen) {
            frozen = false;
            start();
        } else {
            frozen = true;
            stop();
        }
        return true;
    }

    public void run() {
        //Just to be nice, lower this thread's priority
        //so it can't interfere with other processing going on.
        Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
 
        //Remember the starting time.
        long startTime = System.currentTimeMillis();

        //This is the animation loop.
        while (Thread.currentThread() == animatorThread) {
            //Advance the animation frame.
            frameNumber++;

            //Display it.
            repaint();

            //Delay depending on how far we are behind.
            try {
                startTime += delay;
                Thread.sleep(Math.max(0, 
                                      startTime-System.currentTimeMillis()));
            } catch (InterruptedException e) {
                break;
            }
        }
    }

    public void paint(Graphics g) {
        Dimension d = size();
        boolean fillSquare;
        boolean fillNextFrame;
        int rowWidth = 0;
        int x = 0, y = 0;
        int w, h;
        int tmp;

        //Set width of first "square". Decide whether to fill it.
        fillSquare = fillColumnTop;
        fillColumnTop = !fillColumnTop;
        tmp = frameNumber % squareSize;
        if (tmp == 0) {
            w = squareSize;
            fillNextFrame = !fillSquare;
        } else {
            w = tmp;
            fillNextFrame = fillSquare;
        }

        //Draw from left to right.
        while (x < d.width) {
            int colHeight = 0;

            //Draw the column.
            while (y < d.height) {
                colHeight += squareSize;

                //If we don't have room for a full square, cut if off.
                if (colHeight > d.height) {
                    h = d.height - y;
                } else {
                    h = squareSize;
                }

                //Draw the rectangle if necessary.
                if (fillSquare) {
                    g.fillRect(x, y, w, h);
                    fillSquare = false;
                } else {
                    fillSquare = true;
                } 

                y += h;
            } //while y

            //Determine x, y, and w for the next go around.
            x += w;
            y = 0;
            w = squareSize;
            rowWidth += w;
            if (rowWidth > d.width) {
                w = d.width - x;
            }
            fillSquare = fillColumnTop;
            fillColumnTop = !fillColumnTop;
        } //while x
        fillColumnTop = fillNextFrame;
    }
}