/**
 * Collections of objects amenable to search with binary search.
 * 
 * @author Samuel A. Rebelsky
 * @version 1.0 of November 1999
 */
public interface BinarySearchable 
{
  /** 
   * Get the ``middle'' key in the collection. 
   * Pre: The collection is nonempty.
   * Post: Returns some designated element in the collection.
   */
  public Object middle();

  /** 
   * Get elements in the collection that are smaller than the
   * middle element.
   * Pre: The collection is nonempty.
   * Pre: The collection has not been modified since the last call
   *   to middle.
   * Post: Returns the smaller elements.
   */
  public BinarySearchable smaller();

  /** 
   * Get elements in the collection that are larger than the
   * middle element.
   * Pre: The collection is nonempty.
   * Pre: The collection has not been modified since the last call
   *   to middle.
   * Post: Returns the larger elements.
   */
  public BinarySearchable larger();

  /**
   * Determine if the collection is empty.
   */
  public boolean isEmpty();
} // BinarySearchable