public interface DoubleEndedListInterface extends ListInterface{
   //2. This is the behavior imposed on any class that
   //   decides to implement this interface. Tell the students
   //   that an interface can be extended just like a class.

   public void addFirst(Object newEntry);
   public void addLast(Object newEntry);
   public Object removeFirst();
   public Object removeLast();
   public Object getFirst();
   public Object getLast();
}