// file:    ListEnum.java
// author:  Robert Keller
// purpose: List enumeration class of Poly package

package Poly;

import java.lang.*;
import java.util.Enumeration;

/**
  *  ListEnum is an enumeration class for the class List.  It 
  *  implements the interface java.util.Enumeration, i.e. the methods:
  *  hasMoreElements() and nextElement().  Note that because ListEnum
  *  implements Enumeration, the return type for nextElement() is Object
  *  and not Poly.
 **/

public class ListEnum implements java.util.Enumeration
  {
  List L;			// current list
 
  /**
    *  ListEnum constructs a ListEnum from a List.
   **/

  public ListEnum(List L)	// constructor
    {
    this.L = L;
    }


  /**
    *  hasMoreElements() indicates whether there are more elements left in 
    *  the enumeration.
   **/

  public boolean hasMoreElements()
    {
    return L.nonEmpty();
    }


  /**
    *  nextElement returns the next element in the enumeration.
   **/

  public Object nextElement() 
    {
    try
      {
      if( L.isEmpty() )
	throw new java.util.NoSuchElementException("No next element in EmptyList");

      NonEmptyList L1 = L.getNonEmpty();
      Object result = L1.first();
      L = L1.rest();
      return result;
      }
    catch( EmptyListException e )
      {
      System.err.println("*** Internal error, empty list should not occur");
      throw new java.util.NoSuchElementException("No next element in EmptyList");
      }
    }
  }