public class DEList implements DoubleEndedListInterface{
	private Node firstNode;
	private Node lastNode;
	private int length;

	public DEList()
	{
		clear();
	} // end default constructor

	public final void clear()      //any function called in a constructor must be declared final
	{
		firstNode = null;
		lastNode = null;
		length = 0;
	} // end clear


	public boolean isEmpty()
	{
		return (firstNode == null && lastNode == null);
	}


	public void display()
	{
	    Node currentNode = firstNode;

	    while(currentNode != null){
			System.out.print(currentNode.data + " ");
			currentNode = currentNode.next;
		}
		System.out.println();
	}

    //Have the class implement the functions shown below:
    //     1.   addFirst(Object newEntry)
    //     2.   addLast(Object newEntry)
    //     3.   getFirst()
    //     4.   removeFirst()
    //     5.   removeLast()
    //     6.   getLast()

    public void addFirst(Object newEntry)
    {
		Node newNode = new Node(newEntry);

		if (isEmpty()){
			firstNode = newNode;
			lastNode = newNode;
		}
		else                   // add to end of nonempty list
		{
            newNode.next = firstNode;
            firstNode = newNode;
		} // end if

		length++;
	}


    public void addLast(Object newEntry)
    {
		Node newNode = new Node(newEntry);

		if (isEmpty()){
			firstNode = newNode;
			lastNode = newNode;
		}
		else                   // add to end of nonempty list
		{
            lastNode.next = newNode;
            lastNode = newNode;
		} // end if

		length++;
	}


    /** Precondition: list is non-empty().
      */
    public Object removeFirst()
    {
		Object result = firstNode.data;

		if(length == 1){
			firstNode = null;
			lastNode = null;
		}
		else{
			firstNode = firstNode.next;
		}

		length--;
		return result;
	}


    public Object removeLast()
    {
		Object result = lastNode.data;

		if(length == 1){
			firstNode = null;
			lastNode = null;
		}
		else{
			Node nextToLast = getNodeAt(length-1);
			nextToLast.next = null;
			lastNode = nextToLast;
		}

		length--;
		return result;
	}


    public Object getFirst()
    {
		Object result = firstNode.data;

		return result;
	}


    public Object getLast()
    {
		Object result = lastNode.data;

		return result;
	}



	/* < Implementations of the public methods add, remove, replace, getEntry, contains,
	     getLength, isEmpty, isFull, and display go here. > */

	// ---------------private!-----------------------------

	/** Task: Returns a reference to the node at a given position.
	 *  Precondition: List is not empty; 1 <= givenPosition <= length. */
	private Node getNodeAt(int givenPosition)
	{
		Node currentNode = firstNode;

		// traverse the list to locate the desired node
		for (int counter = 1; counter < givenPosition; counter++)
			currentNode = currentNode.next;

		return currentNode;
    } // end getNodeAt


	private class Node // private inner class
	{
		private Object data; // data portion
		private Node   next; // link to next node

		private Node(Object dataPortion)
		{
			data = dataPortion;
			next = null;
		} // end constructor

		private Node(Object dataPortion, Node nextNode)
		{
			data = dataPortion;
			next = nextNode;
		} // end constructor
	} // end Node
}