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

package Poly;

import java.io.*;
import java.lang.*;
	
/**
  *  NonEmptyList is the sub-class of List consisting of non-empty 
  *  lists.  Every NonEmptyList has a first and a rest.
 **/

public class NonEmptyList extends List
  {
  private Object First;
  private Object Rest;


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

  public boolean isEmpty() { return false; }


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

  public boolean nonEmpty() { return true; }


  /**
    *  first() returns the first element of a NonEmptyList.
    * @exception EmptyListException is never thrown.
   **/ 

  public Object first()
    {
    return First;
    }


  /**
    *  rest() returns the rest of a NonEmptyList.
    * @exception EmptyListException is never thrown.
   **/ 

  public List rest()
    {
    if( Rest instanceof Seed ) 
      {
      Rest = ((Seed)Rest).grow();
      }
    return (List)Rest;
    }


  /**
    *  NonEmptyList is the constructor for a List, given a First and a Rest.
    *
    *  Use static method cons to avoid using 'new' explicitly.
   **/ 

  public NonEmptyList(Object First, List Rest)
    {
    this.First = First;
    this.Rest = Rest;
    }

  public NonEmptyList(Object First, Seed Rest)
    {
    this.First = First;
    this.Rest = Rest;
    }

  /**
    *  getNonEmpty() returns this list.
    * @exception EmptyListException is never thrown.
   **/ 

  public NonEmptyList getNonEmpty()
    {
    return this;
    }

  /**
    *  toString() converts List to string, e.g. for printing
   **/

  public String toString()
    {
    try
      {
      StringBuffer buff = new StringBuffer();

      buff.append("(");

      buff.append(first().toString());

      List L = rest();

      // print the rest of the items

      for( ; ; )
	{
        if( L instanceof Incremental && !((Incremental)L).grown() )
          {
          buff.append(" ...");
          break;
          }
        if( L.isEmpty() )
          break;
	NonEmptyList L2 = L.getNonEmpty();
	buff.append(" ");
	buff.append(L2.first().toString());
	L = L2.rest();
	}

      buff.append(")");
      return buff.toString();
      }
    catch( EmptyListException e )
      {
      System.err.println("*** Internal error, NonEmptyList expected");
      return "";
      }
    }


  /**
    *  setFirst() sets the first element of a NonEmptyList.
   **/ 

  public void setFirst(Object value)
    {
    First = value;
    }
  }  // class NonEmptyList