ConsEnumeration.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;

/**
 * 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();
  }
}