// file:    EmptyList.java
// author:  Robert Keller
// purpose: EmptyList class of Poly package

package Poly;

import java.io.*;
import java.lang.*;

/**
  *  EmptyList is the sub-class of List containing the empty list.
 **/

public class EmptyList extends List
  {

  /**
    *  EmptyList() constructs an empty list 
   **/

  public EmptyList() {}			// constructor


  /**
    *  isEmpty() tells whether the List is empty.
   **/

  public boolean isEmpty() { return true; }


  /**
    *  nonEmpty() tells whether the List is non-empty
   **/

  public boolean nonEmpty() { return false; }
    

  /**
    *  first() returns the first element of a non-empty List.
    * @exception EmptyListException Can't take first of an empty List.
    *
   **/ 

  public Object first() throws EmptyListException
    {
    throw new EmptyListException("first of empty List");
    }


  /**
    *  rest() returns the rest of a non-empty List.
    * @exception EmptyListException Can't take rest of an empty List.
   **/ 

  public List rest() throws EmptyListException
    {
    throw new EmptyListException("rest of empty List");
    }

  public NonEmptyList getNonEmpty() throws EmptyListException
    {
    throw new EmptyListException("getNonEmpty of empty List");
    }

  public String toString()
    {
    return "()";
    }
  
  }  // class EmptyList