Question

I am trying to write a simulation for queues although I cannot see why I always get 0.0 at the end for all results minus the requests. It makes logical sense to me but I dont quite understand why it always ends up subtracting itself in the waitTime variable modification
Here is my results:
Numbers Served 322
Total wait time 0.0
Average wait time: 0.0

I was wondering as to why each time I generate and run the program, I dont get real values. In other words, it compiles just fine, I just dont see the mistake I made

import java.util.*;


public class IndividualQueues {

final static int simulationTimeLimit = 3600;
final static double arrivalChance = 1.0/10;
final static int numberOfServers = 5;

public static void main(String[] args) {
    Queue queueArray[] = new Queue[numberOfServers];
    int[] serviceTime = new int[numberOfServers];
    Random checkArrival = new Random();
    double totalWaitTime = 0;
    int requests = 0;
    int serviceTimer = 0;
    int waitTime = 0;
    Queue <Integer> tempQueue = new Queue<Integer>();

      for (int i=0;i<numberOfServers;i++){
       queueArray[i]= tempQueue;
      }

   for(int seconds = 1;seconds<= simulationTimeLimit;seconds++) {
       if(seconds <= simulationTimeLimit) {
           if(checkArrival.nextDouble() <= arrivalChance) {
               requests++;
                       serviceTimer = checkArrival.nextInt(49) + 10;
                       int min = queueArray[0].size();
                       for(int i = 1;i<queueArray.length;i++) {
                           int temp = queueArray[i].size();
                           if(min >temp)
                           min = temp;
                       }
                       queueArray[min].enqueue(seconds);
                       serviceTime[min] = serviceTimer;
               }


               for(int j = 0;j<serviceTime.length;j++) {
                   if(serviceTime[j] >0) {
                       serviceTime[j]--;
                     //  System.out.println(serviceTime[j]);
                   }
                   else if(serviceTime[j]== 0 && queueArray[j].size() >0) {
                       System.out.println("You got here");
                       int dequeue = (Integer)queueArray[j].dequeue();
                       waitTime = seconds - dequeue;
                       totalWaitTime += waitTime;
                   }
               }

           }
   }
   System.out.println("Numbers Served " +requests);
   System.out.println("Total wait time " +totalWaitTime);
   System.out.println("Average wait time: " +(double)totalWaitTime/requests);
}

}


public class Queue<Item> {
private int N;         // number of elements on queue
private Node first;    // beginning of queue
private Node last;     // end of queue

// helper linked list class
private class Node {
    private Item item;
    private Node next;
}

/**
  * Create an empty queue.
  */
 public Queue() {
     first = null;
     last  = null;
     N = 0;
 }

/**
  * Is the queue empty?
  */
  public boolean isEmpty() {
      return first == null;
  }

 /**
  * Return the number of items in the queue.
  */
  public int size() {
     return N;
  }

 /**
   * Return the item least recently added to the queue.
   * @throws java.util.NoSuchElementException if queue is empty.
   */
  public Item peek() {
     if (isEmpty()) throw new NoSuchElementException("Queue underflow");
     return first.item;
   }

 /**
   * Add the item to the queue.
   */
  public void enqueue(Item item) {
      Node oldlast = last;
      last = new Node();
      last.item = item;
      last.next = null;
      if (isEmpty()) first = last;
      else           oldlast.next = last;
      N++;
  }

 /**
   * Remove and return the item on the queue least recently added.
   * @throws java.util.NoSuchElementException if queue is empty.
   */
  public Item dequeue() {
      if (isEmpty()) throw new NoSuchElementException("Queue underflow");
      Item item = first.item;
      first = first.next;
      N--;
      if (isEmpty()) last = null;   // to avoid loitering
      return item;
  }    

 /* *
   * Return string representation.
   */
  public String toString() {
      StringBuilder s = new StringBuilder();
      Node current = first;
      while (current != null)
      {
          s.append(current.item + " ");
          current = current.next;
    }
      return s.toString();
  }
  }  

Answer 1

You’re using the same queue for all the queues in the array

Queue <Integer> tempQueue = new Queue<Integer>();
for (int i=0;i<numberOfServers;i++){
    queueArray[i]= tempQueue;
}

You should create each queue in the for loop:

for (int i=0;i<numberOfServers;i++){
    //I don't know if this works
    //queueArray[i]= new Queue<Integer>();
    //LinkedList implements the Queue interface
    queueArray[i]= new LinkedList<Integer>();
}