/*

  * $Source$

  * $Revision$

  *

  * 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;

 import java.util.NoSuchElementException;

 /**

  * An <code>Enumeration</code> created by flattening 

  * an <code>Enumeration</code> of <code>Enumeration</code>s 

  * or flattening two <code>Enumeration</code>s .

  */

 public class FlattenedEnumeration<T> implements Enumeration<T> {

   private Enumeration<Enumeration<T>> enumerations;

   private Enumeration<T> enumeration = null;

   /**

    * Constructor.

    * @param enumerations an Enumeration of Enumerations

    */

   public FlattenedEnumeration(Enumeration<Enumeration<T>> enumerations) {

     this.enumerations = enumerations;

   }

   /**

    * Constructor given two Enumerations to flatten into one.

    * @param a head Enumeration

    * @param b tail Enumeration

    */

   public FlattenedEnumeration(Enumeration<T> a, Enumeration<T> b) {

     this.enumerations =

         new ConsEnumeration<Enumeration<T>>(a, new ConsEnumeration<Enumeration<T>>(b, new EmptyEnumeration<Enumeration<T>>()));

   }

   private void probe() {

     while ((enumeration == null || !enumeration.hasMoreElements()) &&

            enumerations.hasMoreElements())

       enumeration = (Enumeration<T>)enumerations.nextElement();

   }

   /**

    * {@inheritDoc}

    * @see java.util.Enumeration#hasMoreElements()

    */

   public synchronized boolean hasMoreElements() {

     probe();

     return enumeration != null && enumeration.hasMoreElements();

   }

   /**

    * {@inheritDoc}

    * @see java.util.Enumeration#nextElement()

    */

   public synchronized T nextElement() {

     probe();

     if (enumeration == null)

       throw new NoSuchElementException();

     return enumeration.nextElement();

   }

 }