I've written a Queue class for an assignment and a Linked List for a seperate assignment. However, now the teacher wants us to implement the LinkedList class into our Queue class. I don't understand what he means by that? Why would you need linked lists if the queue is an array which already has everything in order?

This is my queue class:

public class Queue{
	private int maxQueueSize;
	private int count;
	private int queueFront;
	private int queueRear;
 
	//array of references to the objects that store queue elements
	private DataElement[] list;
 
	public Queue(){
 
		maxQueueSize = 100;
 
	}
 
	public Queue(int queueSize){
 
		maxQueueSize = queueSize;
 
	}
 
	public void intializeQueue(){
 
		for(int i = queueFront; i <= queueRear; 
                       i = (i + 1) % maxQueueSize)
 
        		list[i] = null;
 
    			queueFront = 0;
    			queueRear = maxQueueSize - 1;
    			count = 0;
	}
 
	public boolean isEmptyQueue(){
 
		return (count == 0);
	}
 
	public boolean isFullQueue(){
 
		return count == maxQueueSize;
	}
 
	public DataElement front(){
 
		if(isEmptyQueue())
			return null;
 
			DataElement temp = list[queueFront].getCopy();
			return temp;
	}
 
	public DataElement back(){
 
		if(isEmptyQueue())
			return null;
 
			DataElement temp = list[queueRear].getCopy();
			return temp;
	}
 
	public boolean addQueue(DataElement queueElement){
 
 
    if(isFullQueue())
        return false;
 
    queueRear = (queueRear + 1) % maxQueueSize;
                              //use the mod 
                              //operator to advance queueRear 
                              //because the array is circular
    count++;
    list[queueRear] = queueElement.getCopy();
    return true;		
}
 
	public boolean deleteQueue(){
 
 
    if(isEmptyQueue())
       return false;
 
    count--;
    list[queueFront] = null;
    queueFront = (queueFront + 1) % maxQueueSize;
                                //use the mod 
                                //operator to advance queueFront 
                                //because the array is circular
 
	  return true; 
}
 
 
 
}

And here is my Linked List class:

public abstract class LinkedList{
 
 
	protected class LinkedListNode{   // inner class node definition
		protected DataElement info;
		protected LinkedListNode link;
	}
 
	protected LinkedListNode first; //variable to store the address of the first node of the list
	protected LinkedListNode last;  // variable to store the address of the last node of the list
 
	protected int count; //variable to store the number of nodes in the list
 
	//default constructor
	//initializes the list to an empty state.
	//postcondition: first = null, last = null, count = 0
 
	public LinkedList(){
		first = null;  //
		last = null;  //empty list
		count = 0;   //
	}
 
	public LinkedList(LinkedList list)
    {
         copy(list);
    }
 
    private void copy(LinkedList list)
    {
        LinkedListNode pointer;
        LinkedListNode newNode;
 
        if(list.first == null)
        {
            first = null;
            last = null;
            count = 0;
        }
        else
        {
            count = list.count;
            pointer = list.first;
            first = new LinkedListNode();
            first.info = pointer.info.getCopy();
            first.link = null;
            last = first;
            pointer = pointer.link;
 
            while(pointer != null)
            {
                newNode = new LinkedListNode();
                newNode.info = pointer.info.getCopy();
                newNode.link = null;
                last.link = newNode;
                last = newNode;
                pointer = pointer.link;
            }
        }
    }
 
 
 
	//method to determine whether the list is empty
	//postcondition: returns true if the list is empty; false otherwise
 
 
	public boolean isEmptyList(){
		return (count == 0);
	}
 
 
 
	//method to initialize the list to an empty state
	//postcondition: first = null, last = null, count = 0
 
	public void initializeList(){
		first = null;
		last = null;
		count = 0;
	}
 
 
	//method to output the data contained in each node
 
	public void print(){
 
	if(count == 0){
		System.out.println("Cannot print an empty list.");
	}
	else{
 
			LinkedListNode pointer = new LinkedListNode();
		pointer = first;
		while(pointer != null){
 
 
		System.out.println(pointer.info.toString());
		pointer=pointer.link;
		}
 
	}
 
	}
 
 
	//method to return the number of nodes in the list.
	//postcondition: the value of count is returned
 
	public int length(){
		return count;
	}
 
	//method to return a reference of the object 
	//containing the data of the first node of the list
	//precondition: the list must exist and must not be empty.
	//postcondition: the reference of the object that contains
	//the info of the first node is returned
 
	public DataElement front(){
 
 
				return first.info.getCopy();
		}
 
 
	//Method to return a reference of object containing
        //the data of the last node of the list.
        //Precondition: The list must exist and must not be empty.
        //Postcondition: The reference of the object that
        //contains the info of the last node
        //is returned.
    public DataElement back(){
    	return last.info.getCopy();
    }
 
    //Method to insert newItem in the list.
        //Postcondition: first points to the new list
        //               and newItem is inserted at the
        //               beginning of the list. Also,
        //               last points to the last node and
        //               count is incremented by 1.
    public void insertFirst(DataElement newItem){
    	LinkedListNode newNode = new LinkedListNode();
    newNode.info = newItem.getCopy();
    newNode.link = first;
    first = newNode;
 
    if(last == null){
 
    last = newNode;
    count++;
    }
 
 
    }
 
    //Method to insert newItem at the end of the list.
        //Postcondition: first points to the new list and
        //newItem is inserted at the end
        //of the list. Also, last points to
        //the last node and
        //count is incremented by 1.
    public void insertLast(DataElement newItem){
 
 LinkedListNode newNode = new LinkedListNode();   
    	newNode.info = newItem.getCopy();
    	newNode.link = null;
    	if(first == null){
    		first = newNode;
    		last = newNode;
    	}
    	else{
 
    	last.link = newNode;
    	last = newNode;
    	count++;
    	}
    }
 
    		public void deleteFront()
    {
        if(first.link != null) 
        {
            first = first.link;
            count--;
        }
        else
        {
            first = null;
            last = null;
            count = 0;
        }
    }
 
    public void deleteLast()
    {
        LinkedListNode pointer = first;
        if(pointer.link == null)
        {
            first = null;
            last = null;
            count = 0;
        }
        else 
        {
            while(pointer.link.link != null) pointer = pointer.link;
            pointer.link = null;
            last = pointer;
            count--;
        }
    }
 
     public void splitMid(LinkedList list)
    {
        int stopPoint = list.count / 2;
        LinkedListNode pointer = null;
        pointer = list.first.link;
        for(int i = 0; i < stopPoint; i++)
        {
            pointer = pointer.link;
        }
        LinkedListNode secondFirst = pointer.link;
 
        pointer.link.link = null;
 
    }
 
 
        public double average(){
 
 
    			double sum=0;
    			LinkedListNode pointer = new LinkedListNode();
 
 
    			pointer = first;
 
    			while(first != null){
 
    		pointer = first;
    			IntElement temp = (IntElement)pointer.info.getCopy();
 
    			sum = sum + temp.getNum();
    			first = pointer.link; 
 
 
    			}
 
    		double average = (sum/59.0);
    		return average;
 
    		}
 
    		public double stdDev(double avg){
    			double stdAvg = avg;
    			double variance = 0;
    			LinkedListNode pointer = new LinkedListNode();
 
    			while(first!= null){
 
    				pointer = first;
    				IntElement temp = (IntElement)pointer.info.getCopy();
    				variance = (variance + ((temp.getNum() - stdAvg)*(temp.getNum() - stdAvg)));
    				first = pointer.link;
 
 
    				}
 
    			return (Math.sqrt(variance/59));
    		}
 
 
    //Method to determine whether searchItem is in the list.
    //Postcondition: Returns true if searchItem is found
    //in the list; false otherwise.
    public abstract boolean search(DataElement searchItem);	
 
		//Method to delete deleteItem from the list.
        //Postcondition: If found, the node containing
        //deleteItem is deleted from the
        //list. Also first points to the first
        //node, last points to the last
        //node of the updated list, and count
        //is decremented by 1.
    public abstract void deleteNode(DataElement deleteItem);
 
 
 
 
  	}

I was checking out a few sites for examples such as

Simple Queue (FIFO) based on LinkedList : QueueCollections Data StructureJava

Create a queue using LinkedList class : QueueCollections Data StructureJava

But I don't really understand what they're doing different besides using polymorphism. Any explanation is appreciated.


If it helps with context, this is what we have to do.

 1. Implement a Linked Queue of type DataElement as described in the lecture. 
    You may use either the custom Queue Class approach the Queue Interface as 
    described in the lecture.
 
 2. Implement a Linked Queue using your existing Linked-List Class. 
 
 
 3.  An Application Challenge:
	Write a program to encode and decode a message using
	a repeating key set.
	Implement your solution using the Queue Class you developed.
 
	Test Case:
		Message: "All programmers are playwrights and all
                       computers are lousy actors.“	
		Key Set:  {5, 12, -3, 8, -9, 4, 10}


--- Update ---

And another problem I'm having is a NullPointerException when using the addQueue method. 

public class TestQueueClass{
 
	public static void main(String[] args){
 
		Queue q = new Queue();
		DataElement data = new IntElement(5);
		q.addQueue(data);
 
 
	}
}

--------------------Configuration: <Default>--------------------
QueueRear: 0 DataElement: 5
Queue Copy: 5
Exception in thread "main" java.lang.NullPointerException
    at Queue.addQueue(Queue.java:83)
    at TestQueueClass.main(TestQueueClass.java:7)
 
Process completed.

It points to the line in the addQueue method which it takes a copy of the DataElement (which is currently 5) and places it in the list position of 0. How can that possibly be null?!


Edit: Nevermind, I fixed it. I overlooked not defining the array properly up top...

Updated:

public class TestQueueClass{
 
	public static void main(String[] args){
 
		Queue q = new Queue();
		q.intializeQueue();
		q.addQueue(new IntElement(5));
		q.addQueue(new IntElement(12));
		q.addQueue(new IntElement(-3));
		q.addQueue(new IntElement(8));
		q.addQueue(new IntElement(-9));
		q.addQueue(new IntElement(4));
		q.addQueue(new IntElement(10));
 
 
 
		q.addQueue(q.front());
		q.deleteQueue();
			System.out.println("Front of queue: " + q.front());
		System.out.println("Back of queue: " + q.back());
 
	}
}