I spend most of my free time on the computer doing homework (free time defined as not at work, not spending quality time with family).
Anyway, this will be less than interesting to you, but I got so excited I had to share. I had a breakthrough last night--I'm not going to get nominated for a Nobel prize anytime soon because of it, but I was excited since this has been vexing me for days.
I'm writing a program that simulates a barber shop. Why a barber shop? Because it demonstrates some basic computer operating system concepts (Windows, Mac OS, Linux, and all the others all have to deal with this particular issue). The problem is that there is only one barber and he can only serve one person at a time. If the barber has several people come in at roughly the same time there has to be a mechanism to control who gets to sit down in the chair.
In a computer system the barber is a shared resource, usually thought of as a piece of data that resides on the system, but could be any other system resource. The customers are programs that need to use the piece of data--the task is to write an underlying operating system (like Windows) in order to match the needs of the customers to the availability of the barber. It’s both complicated and simple at the same time.
Basically what you have to do is have the customer look to see if the barber is already busy--if he is the customer sits in the waiting room and waits until the barber is free. If he's free the customer sits down and gets a haircut. Oversimplified of course, but that's the general concept.
Where my breakthrough came in though was in the timing of it all. Forcing pauses (each haircut is supposed to take a random time between 1 and 3 seconds and customers come in at random intervals, between 1 and 3 seconds), keeping the barber busy, and making sure the waiting room doesn't get overcrowded, are all very manageable actually. But how to continuously generate customers over a two minute period? Ah, that's what has been plaguing me for days. Now that I've solved it, the solution looks obvious.
I was trying to figure out how to make my program to run for two minutes and then stop, while continuously generating new "customers" every 1 to 3 seconds. I was using a for-loop, nested inside either a do- or while-loop, but I couldn't figure out why it was ignoring the do and while loops and only exiting when the for-loop completed. It also wasn't very elegant since I had to predefine an upper limit for the for-loop.
My first solution looked like the following. One other note is that the method call to System.currentTimeMillis() gives the time since January 1, 1970 in milliseconds (a very, very huge number)--120,000 milliseconds is two minutes. So basically, it is supposed to finish when the start time equals the current time minus two minutes (clever, actually--well, I think so anyway). Anyway, here's the code I tried first:
long startTime = System.currentTimeMillis();
while (startTime >= (System.currentTimeMillis() – 120000) ) {
for (int customerID = 1; customerID < 100 ; customerID++) {
threadWait();
customerID++;
}
}
It seemed like it would work, but runtime would take over three minutes and it wouldn't stop until customerID equaled 100! I know, right? Annoying.
Finally, late last night, around 12:30 AM, I realized that there is a much easier (and like I said, obvious) solution. Junk the for-loop since I don't really want to have a limit on the number of customers in the two minutes anyway. Reality is that I wouldn't approach one hundred in two minutes, but once the JVM enters a for-loop I guess it ignores whatever other instructions there might be. And voila! It becomes a simple do-loop that increases the customerID each time it generates a new customer.
long startTime = System.currentTimeMillis();
int customerID= 1;
do {
new customer(customerID);
threadWait();
customerID++;
} while (startTime >= (System.currentTimeMillis() -120000));
Like I said, this will be less than interesting for most folks, so if you've read this far then you have my congratulations. But, if you've read this far AND you've understood everything then we need to talk because I might have some questions for you... :)
Yay for me.
