// Textbook fragment 05.13
public interface Queue<E> {
 /**
  * Returns the number of elements in the queue.
  * @return number of elements in the queue.
  */
  public int size();
 /**
  * Returns whether the queue is empty.
  * @return true if the queue is empty, false otherwise.
  */
  public boolean isEmpty();
 /**
  * Inspects the element at the front of the queue.
  * @return element at the front of the queue.
  * @exception EmptyQueueException if the queue is empty.
  */
  public E front() throws EmptyQueueException;
 /**
  * Inserts an element at the rear of the queue.
  * @param element new element to be inserted.
  */
  public void enqueue (E element);
 /**
  * Removes the element at the front of the queue.
  * @return element removed.
  * @exception EmptyQueueException if the queue is empty.
  */
  public E dequeue() throws EmptyQueueException;
}