FlattenedEnumeration.java

/*
 * $Source$
 * $Revision$
 *
 * 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();
  }
}