For Queue<E>, we are using the LinkedList<E> implementation.
The following operations are permitted:

  public void add(E value); // add an element to the back of 
                            // the queue
  public E remove();        // remove an element from the front 
                            // of the queue
  public boolean isEmpty(); // returns true iff the queue is 
                            // empty
  public int size();        // returns the size of the queue