public class QueueReferenceBased implements QueueInterface {
  private Node front, back;
  
  public QueueReferenceBased() {
    front = back = null;   
  }  // end default constructor
  
  // queue operations:
  public boolean isEmpty() {
    return front == null;
  }  // end isEmpty

  public void dequeueAll() {
    front = back = null;
  }  // end dequeueAll

  public void enqueue(Object newItem) {
    Node newNode = new Node(newItem);

    // insert the new node
    if (isEmpty()) {
      // insertion into empty queue
      front = back = newNode;
    }
    else {
      // insertion into nonempty queue
      back.setNext(newNode);
      back=newNode;
    }  // end if
    
  }  // end enqueue

  public Object dequeue() throws QueueException {
    if (!isEmpty()) {
      // queue is not empty; remove front
      Node tmp = front;
      front=front.getNext();
      return tmp.getItem();
    }
    else {
      throw new QueueException("QueueException on dequeue:" 
                             + "queue empty");
    }  // end if
  }  // end dequeue

  public Object peek() throws QueueException {
    if (!isEmpty()) {  
      // queue is not empty; retrieve front
      return front.getItem();
    }
    else {
      throw new QueueException("QueueException on peek:" 
                             + "queue empty");
    }  // end if
  }  // end peek
   
} // end QueueReferenceBased