Thread: Queue Simulation

This thread has a good description: http://www.javaprogrammingforums.com...on-theory.html

I don't remember seeing any assignments like this one you are working on.

What techniques have you been shown in class that could be useful in doing this assignment?
The clock time model uses threads, timers and synchronized queues.
The event time model uses none of the above.
They both use a random number generator.

In the clock time model, the timers would be for the cashier processing.

shown in class is simple loops, if statements and how use methods

I can't see that you have the knowledge yet to write a clock time model.
The event time model requires random numbers which you say your comfortable with. There is one thing you'll also need:

Have an event list (sorted according according to time, smallest time value is at the top of the list).

This would require a PriorityQueue and classes that implement the Comparable interface.

Norm I've done alot of the coding I didnt use real time but for now just attached the name and time in a separate class as methods. What the problem is im trying to catch the exception when the user enters a letter instead of a integer. However the Scanner object and is within the curly brackets for that "for" statement. Because of this im not able to use the Scanner user input of the cashier in my loop, its in the part where im dividing the

Thread.sleep((wait * 1000)[]

with the cashier input of the user hence simulating that there are more cashiers and the queue get processed faster.. How can I over come this problem ?

public class Station 
{
	public static Queue<Customer> line = new  LinkedList<Customer> ();
	static Scanner input = new Scanner (System.in);
	public static void main(String[] args) 
	{	
		String[] list = {"Customer 1", "Customer 2", "Customer 3", "Customer 4", "Customer 5", "Customer 6", "Customer 7", "Customer 8", "Customer 9", "Customer 10", };
		int customerlist = list.length;
 
		for (int x = 0; x < customerlist; x++ )
		{
		int cus = (int) (Math.random() * customerlist);
		line.add(new Customer(list[cus]));
		}
 
		try
		{
			System.out.println("Enter the number of cashiers available HERE: " + "\n");
			int cashier = input.nextInt();
		}
		catch (Exception ex)
		{
			System.out.println("Enter a whole number you douche. " + "\n");
			System.exit(0);
		}
		Random ran = new Random();
		while (!line.isEmpty())
		{
			System.out.println(line  + "\n");
			System.out.println("The queue has " + line.size() + " customers left");
			Customer cus = line.remove();
			System.out.println(cus.name + " queued at " + cus.getTime() + " <=== SERVED" + "\n");
			// you will have to sleep a random number of seconds here
			int wait = ran.nextInt(2) + 1;  // will generate 1 or 2
			try 
			{
			     Thread.sleep((wait * 1000) / cashier);
			}
			catch(Exception e) 
			{
				System.out.println("Sleep error: " + e);
			}
 
		}
	}
}

trying to catch the exception when the user enters a letter instead of a intege

That's a completely separate problem. Don't mix it in with the rest of the code for now. Come back to it later.

What is the array: list for? The customers should NOT be named. Give them numbers.

Can you explain the design for the program you are trying to write? The use of the sleep() method looks like you are trying to use the clock time model. You will NOT want to sleep full seconds (*1000). Testing with 200 customers could take hours. Use ms vs sec.

each customer be represented by

Create a Customer class that holds all the data needed.

Can you get more information from your tutor about this assignment? Both solutions I've been talking about involve classes and techniques that you have not studied yet.

instead of sleeping for 1 seconds how about 1/4 of second

I'd scale it down farther. Try 1ms for 1 sec first. If there are problems with that try 5ms and then 10ms for 1 sec

For the clock time model, I used the System class's method currentTimeMillis() to get the current time.

Using the clock model: There would be one main class and a class for the customers, a class to handle the timer events. The main class would start a thread that would have a loop to generate x customers at the customer arrival rate and add them to the wait queue as they were generated, except the first n (n = number of cashiers) customers would enter a cashier processing station. When a customer enter a cashier station, start a timer to represent the time the cashier takes to process the customer. When the timer expires, the customer is removed from the cashier station and the next customer from the wait queue is move to the cashier and the customer's waiting time in queue is recorded. The processing continues until the wait queue is empty.

Customer class contains the actual empty priority Queue where the generated customer

The queue would be in the main class. It would hold Customer class objects.

What is the third class?

what is the actual cashier where the customer will be placed

The cashiers could be an array of Customer objects. The Customer in a slot would be the one being processed by that cashier.

record the time the customer enters/exits the queue

When the customer is added to the wait queue, save the current time in the Customer object.
When the Customer is removed from the wait queue, the time in queue would be the current time minus the time when the Customer was added to the wait queue.

the cashier could be Array of Customer objects.

You can create an array of any datatype: primitives like int or objects like String or Customer.
Customer[] cashiers; // define array for cashiers

Are you saying that I first create ten new customer objects

No. See post#22. The Customers are generated at random time intervals inside a loop in a separate Thread.

have multiple threads within my Customer class

What code would you put in a Customer? I don't have any code in mine.

while loop to check if the cashier slot is empty

No. The cashier slots are empty at the start of the simulation and then become available when the processing for a Customer is finished. The timer listener method is called when the processing is done. At that time in the listener method, the next Customer could be moved from the wait queue to the empty cashier.

syntax to move customer from the queue

Read the API doc for the class to see what method can be used to remove an element from the list.

I suggested in post#22 that the code be written in steps. The first step would be the thread with a loop to generate n Customer objects at random times, put them in a queue and then when the loop exits, print out the contents of the queue to see what values for times were generated.

ive generated maximum of 10 customer

I thought it was supposed to be a minimum of 10.

Where is the class to hold the Customer's data? Things like time of arrival in queue and what cashier is doing the processsing and how much time was spent in the wait queue?

The posted code looks like what would go in the Main class. Where are the Customer objects created and added to the wait queue? This code is adding Strings??? The Customer objects saved on the queue need the data mentioned in the first part of this post.

You do NOT want to wait seconds. Use scaled down values for the times: 1ms for 1 sec (See earlier posts on this)

you do NOT need the String array: list

I need to record the time the customers enter the queue and exit the queue

You need the fields in the Customer class to do that.

Define a Customer class and work on filling the wait queue with Customer objects with the time they entered the queue saved in them. Make a loop that creates n Customer objects, and puts them in the queue with the time set.
After n are generated, print out the queue's contents so you can verify that the generate step worked correctly.


Timing values seem way to quick:
arriving randomly in the queue between 1-4 sec. <<<<< seems very short times
take 3 seconds for the customer to get processed <<< WAY TOO SHORT

Now you need to add code to save the time the Customer object is added to the queue.

And add a toString() method to the Customer class that returns the id number for the customer and the time the object was added to the queue. The String that the toString() method returns will replace this: Customer@69ddad02


Don't bother to print out the contents of the queue until all Customers have been added to it.

There should be some variables at the very beginning of the class that define:
the number of customers to generate (don't hardcode a 10 in the for loop)
the max time between customer arrivals
the number of cashiers
and some others

The class's name should be something besides Customer. There should be a small inner class named Customer that has several data fields such as an id, the time entered queue, the cashier doing the processing and the time spent waiting in queue. Some of those will be given values later.

The toString() method's DEFINITION is added to the Customer class. You don't call the method.
The println() of the line object will call the toString() method and change this: [Customer@8955b34]
to the String the toString() method returns:

	private class Customer
	{
		int id;
		int arrival_time;
		String cashier;
		int queue_time;
 
               public String toString() {
                   return "Customer id="+id + " arvTm="+arrival_time;
               }
	}