public interface Queue {
  
  /** Add o to the queue. 
   *  Precondition: isFull() returns false. */
  void enqueue(Object o);
  
  /** Return the front element.
   *  Precondition: size() != 0 */
  Object peek(); //or head()
  
  /** Remove and return the front element.
   *  Precondition: size() != 0 */
  Object dequeue();
  
  /** Return the current size of this queue. */
  int size();
  
  /** Return whether the queue is full or not. */
  boolean isFull();
}