The paint loops in both Mondrian and FlyingLines are ideal for a thread , a separate stream of execution that takes place simultaneously and independently of everything else that might be happening (like responding to the programmer's insistence to "Quit!, Damnit!). Without threads an entire program can be held up by one CPU intensive task or, as in Flying Lines, one infinite loop, intentional or otherwise.

As a general rule all CPU intensive tasks should be placed in their own threads. Here's one way to do it.

//Draw infinitely many random rectangles import java.applet.Applet; import java.awt.*; public class ThreadedMondrian extends Applet implements Runnable { int Rect, Rect, RectTop, RectLeft, Applet, Applet; Color RectColor; Thread kicker = null; int pause; public void init() { Dimension d = size(); Applet = d.; Applet = d.; repaint(); } public void paint(Graphics g) { g.setColor(Color.black); g.drawRect(0, 0, Applet-1, Applet-1); for (int i=0; i < 10; i++) { RandomRect(); g.setColor(RectColor); g.fillRect(RectLeft, RectTop, Rect-1, Rect-1); } } public void run() { Thread.currentThread().setPriority(Thread.MIN_PRIORITY); while (true) { // infinite loop repaint(); try { Thread.sleep(100); } catch (Exception e) { } } } public void start() { if (kicker == null) { kicker = new Thread(this); kicker.start(); } } public void stop() { kicker = null; } public void RandomRect() { RectTop = Randomize(Applet); RectLeft = Randomize(Applet); Rect= Randomize(Applet - RectTop); Rect = Randomize(Applet - RectLeft); RectColor = new Color(Randomize(255),Randomize(255),Randomize(255)); } private int Randomize(int range) { double rawResult; rawResult = Math.random(); return (int) (rawResult * range); } }