/*

  * $Source$

  * $Revision$

  *

  * Copyright (C) 2000 William Chesters

  *

  * Part of Melati (http://melati.org), a framework for the rapid

  * development of clean, maintainable web applications.

  *

  * Melati is free software; Permission is granted to copy, distribute

  * and/or modify this software under the terms either:

  *

  * a) the GNU General Public License as published by the Free Software

  *    Foundation; either version 2 of the License, or (at your option)

  *    any later version,

  *

  *    or

  *

  * b) any version of the Melati Software License, as published

  *    at http://melati.org

  *

  * You should have received a copy of the GNU General Public License and

  * the Melati Software License along with this program;

  * if not, write to the Free Software Foundation, Inc.,

  * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA to obtain the

  * GNU General Public License and visit http://melati.org to obtain the

  * Melati Software License.

  *

  * Feel free to contact the Developers of Melati (http://melati.org),

  * if you would like to work out a different arrangement than the options

  * outlined here.  It is our intention to allow Melati to be used by as

  * wide an audience as possible.

  *

  * This program is distributed in the hope that it will be useful,

  * but WITHOUT ANY WARRANTY; without even the implied warranty of

  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

  * GNU General Public License for more details.

  *

  * Contact details for copyright holder:

  *

  *     William Chesters <williamc At paneris.org>

  *     http://paneris.org/~williamc

  *     Obrechtstraat 114, 2517VX Den Haag, The Netherlands

  */

 package org.melati.poem.util;

 import java.util.Enumeration;

 /**

  * An implementation of the Lisp <code>cons</code> function on 

  * enumerations ie it allows you to add an object to the front 

  * of an <code>Enumeration</code>.

  */

 public class ConsEnumeration<T> implements SkipEnumeration<T> {

   private boolean hadHd = false;

   private T hd;

   private Enumeration<T> tl;

   /**

    * Constructor with head and tail.

    * @param head single element

    * @param tail the rest

    */

   public ConsEnumeration(T head, Enumeration<T> tail) {

     this.hd = head;

     this.tl = tail == null ?(Enumeration<T>) new EmptyEnumeration<T>() : tail;

   }

   @Override

   public synchronized boolean hasMoreElements() {

     return !hadHd || tl.hasMoreElements();

   }

   @Override

   public synchronized T nextElement() {

     if (!hadHd) 

       try {

         return hd;

       }

       finally {

         hadHd = true;

       }

     else

       return tl.nextElement();

   }

   @Override

   public synchronized void skip() {

     if (!hadHd)

       hadHd = true;

     else if (tl instanceof SkipEnumeration)

       ((SkipEnumeration<T>)tl).skip();

     else

       tl.nextElement();

   }

 }