/**
  * Interface for a queue: a collection of objects
  * that are inserted and removed according to the 
  * last-in first-out principle.
  */

public interface Queue {  
  /** @return number of elements in the queue. */
  public int size();  

  /** @return true if the queue is empty, false otherwise. */
  public boolean isEmpty(); 

  /**
   * Inspect the element at the front of the queue.
   *
   * @return element at the front of the queue.
   * @throws QueueEmptyException if the queue is empty.
   */
  public Object front() throws QueueEmptyException; 
  
  /** 
   * Insert an element at the rear of the queue.
   *
   * @param element new element to be inserted.
   * @throws QueueFullException if the queue is full.
   */
  public void enqueue (Object element) throws QueueFullException; 
  
  /** 
   * Remove the element at the front of the queue.
   *
   * @return element removed.
   * @throws QueueEmptyException if the queue is empty.
   */
  public Object dequeue() throws QueueEmptyException; 
}