Find the bug - part two.

Nico, a wonderful idea! Here's my contribution:

The most complicated thing in the world to understand is concurrency. Not even Einstein's General Theory of Relativity comes close. Here is an interesting gem that I got from a newsletter quite some time ago. I found the puzzle incredibly interesting and it convinced me to NEVER EVER EVER EVER:

1. Assume I understand concurrency.
2. Use the old core Threading stuff rather than the new modern concurrency frameworks (i.e. Threading pools, etc.).
3. Assume I understand concurrency.
4. Skip testing threaded applications - because I'm clever, it makes sense... and besides I understand concurrency.
5. Assume I understand concurrency.

Oh, I almost forgot, the most important rule is to never assume I understand concurrency (not even at a job interview at google)!

Look at the following Rorotika-inspired class and tell me what happens. Keep careful track of all the delays.

	public class Worker extends Thread {
	private volatile boolean quittingTime = false;
	public void run() {
	while (!quittingTime) {
	pretendToWork();
	}
	System.out.println("Beer is good");
	}
	private void pretendToWork() {
	try {
	System.out.println("..working!");
	Thread.sleep(300); // Sleeping on the job?
	} catch (InterruptedException e) {
	}
	}
	// It's quitting time, wait for worker -
	// Called by good boss
	synchronized void quit() throws InterruptedException {
	System.out.println("Ok guys! Have a beer!");
	quittingTime = true;
	join();
	}
	// Rescind quitting time - Called by evil boss
	synchronized void keepWorking() {
	System.out.println("Work! Dammit! I don't "+
	"pay you to drink beer!");
	quittingTime = false;
	}
	public static void main(String[] args)
	throws InterruptedException {
	final Worker worker = new Worker();
	worker.start();
	Timer t = new Timer(true); // Daemon thread
	t.schedule(new TimerTask() {
	public void run() {
	worker.keepWorking();
	}
	}, 500);
	Thread.sleep(400);
	worker.quit();
	}
	}

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