RingQueue ADT nullpointer exception

[J-Ideas]

Hi, i am working on a generic RingQueue ADT, however i came across a nullpointer exception during runtime and i cannot figure out what exactly my problem was.

/** Node of a singly linked list of Integer objects*/
public class Node<E>{
	private E element;  // assume elements are Integeregers
	private Node<E> next;
	/** Creates a Node with the given element and next node */
	public Node(E element, Node n){
		this.element = element;
		next=n;
	}
	/** Returns the element of this node */
	public E getElement(){return element;}
	/** Returns the next node of this node */
	public Node getNext(){return this.next;}
	
	//Modified methods
	/** Sets the element of this node */
	public void setElement(E elem){System.out.println(elem); element = elem;}
	/** Sets the next node of this node */
	public void setNext(Node n){this.next=n;}
}

public class RingQueue<E> implements Queue<E>{
	private Node<E> head,tail;
	private int size, limit;

	public RingQueue(int s){
		head = tail = null;
		limit = s;		//max size of queue
		size = 0;			//num of elements in queue
		setupQ(head,limit);
		tail = head;
	}


        // other methods omitted ...


	/**
	 * Insert an element at the rear of the queue
	 * @param element to be inserted
	 * @exception QueueFullException if queue is full
	 **/
	public void enqueue (E element) throws QueueFullException{
		if(isFull())
			throw new QueueFullException("Queue already full");
		else{
			if(!isEmpty())
				tail = tail.getNext();
			tail.setElement(element);  // this line always give me problem
			size++;
		}
	}

above are my implementation of the RingQ adt, whenever the tail.setElement() method is invoked... or rather the setElement() of Node class is invoked, the program crashes.

Please advise