Question

 // Queue.java
 // demonstrates queue
 // to run this program: C>java QueueApp

 class Queue
 {
private int maxSize;
private long[] queArray;
private int front;
private int rear;
private int nItems;

public Queue(int s)          // constructor
  {
  maxSize = s;
  queArray = new long[maxSize];
  front = 0;
  rear = -1;
  nItems = 0;
  }

public void insert(long j)   
  {
  if(rear == maxSize-1)         
     rear = -1;
  queArray[++rear] = j;        
  nItems++;                    
  }


public long remove()         
  {
  long temp = queArray[front++];
  if(front == maxSize)           
     front = 0;
  nItems--;                     
  return temp;
  }

 public long peekFront()     
  {
  return queArray[front];
  }

 public boolean isEmpty()    // true if queue is empty
  {
  return (nItems==0);
  }

public boolean isFull()     // true if queue is full
  {
  return (nItems==maxSize);
  }

public int size()           // number of items in queue
  {
  return nItems;
  }


public void display()
{ int startFront = front;

  for (int j = front ;j <nItems; j++ )
  {  
      System.out.println(queArray[j]);
      if (j == nItems-1 )
        {       j=0;
                System.out.println(queArray[j]);
        }   


      if (j==startFront-1)
          return;

       }
          }
         }  

 class QueueApp
  {
        public static void main(String[] args)
  {
  Queue theQueue = new Queue(5);  // queue holds 5 items

  theQueue.insert(10);            // insert 4 items
  theQueue.insert(20);
  theQueue.insert(30);
  theQueue.insert(40);

  theQueue.remove();              // remove 3 items
  theQueue.remove();              //    (10, 20, 30)
  theQueue.remove();

  theQueue.insert(50);            // insert 4 more items
  theQueue.insert(60);            //    (wraps around)
  theQueue.insert(70);
  theQueue.insert(80);


  theQueue.display();


  while( !theQueue.isEmpty() )    // remove and display
     {                            //    all items
    long n = theQueue.remove();  // (40, 50, 60, 70, 80)
     System.out.print(n);
     System.out.print(" ");
     }
  System.out.println("");

  }  // end main()
}  // end class QueueApp

Okay, this is the basic, out of the book, queue code. I am attempting to create a display method that will show the queue in order, from front to back. (This is an assignment, i know this is not practical….) If i run the program as is, it will display the queue in order from front to rear(at least that is what i believe i did). The problem i am having is if i change the nItems, it ceases to work. For example if you add the line of code, theQueue.remove(); right above the call to the display, the method ceases to work, i know it is because the front is now = to 4, instead of 3,and it will not enter the for method which needs the front to be < nItems, 4<4 is not true so the for loop does not initiate.

Answer 1

Simply use something like:

public void display() {
    for (int i = 0; i < nItems; i++) {
        System.out.println(queArray[(front + i) % maxSize]);
    }
}