1 /*
2 * $Source$
3 * $Revision$
4 *
5 * Copyright (C) 2000 William Chesters
6 *
7 * Part of Melati (http://melati.org), a framework for the rapid
8 * development of clean, maintainable web applications.
9 *
10 * Melati is free software; Permission is granted to copy, distribute
11 * and/or modify this software under the terms either:
12 *
13 * a) the GNU General Public License as published by the Free Software
14 * Foundation; either version 2 of the License, or (at your option)
15 * any later version,
16 *
17 * or
18 *
19 * b) any version of the Melati Software License, as published
20 * at http://melati.org
21 *
22 * You should have received a copy of the GNU General Public License and
23 * the Melati Software License along with this program;
24 * if not, write to the Free Software Foundation, Inc.,
25 * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA to obtain the
26 * GNU General Public License and visit http://melati.org to obtain the
27 * Melati Software License.
28 *
29 * Feel free to contact the Developers of Melati (http://melati.org),
30 * if you would like to work out a different arrangement than the options
31 * outlined here. It is our intention to allow Melati to be used by as
32 * wide an audience as possible.
33 *
34 * This program is distributed in the hope that it will be useful,
35 * but WITHOUT ANY WARRANTY; without even the implied warranty of
36 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
37 * GNU General Public License for more details.
38 *
39 * Contact details for copyright holder:
40 *
41 * William Chesters <williamc At paneris.org>
42 * http://paneris.org/~williamc
43 * Obrechtstraat 114, 2517VX Den Haag, The Netherlands
44 */
45
46 package org.melati.poem.util;
47
48 import java.util.Enumeration;
49
50 /**
51 * An implementation of the Lisp <code>cons</code> function on
52 * enumerations ie it allows you to add an object to the front
53 * of an <code>Enumeration</code>.
54 */
55 public class ConsEnumeration<T> implements SkipEnumeration<T> {
56 private boolean hadHd = false;
57 private T hd;
58 private Enumeration<T> tl;
59
60 /**
61 * Constructor with head and tail.
62 * @param head single element
63 * @param tail the rest
64 */
65 public ConsEnumeration(T head, Enumeration<T> tail) {
66 this.hd = head;
67 this.tl = tail == null ?(Enumeration<T>) new EmptyEnumeration<T>() : tail;
68 }
69
70 @Override
71 public synchronized boolean hasMoreElements() {
72 return !hadHd || tl.hasMoreElements();
73 }
74
75 @Override
76 public synchronized T nextElement() {
77 if (!hadHd)
78 try {
79 return hd;
80 }
81 finally {
82 hadHd = true;
83 }
84 else
85 return tl.nextElement();
86 }
87
88 @Override
89 public synchronized void skip() {
90 if (!hadHd)
91 hadHd = true;
92 else if (tl instanceof SkipEnumeration)
93 ((SkipEnumeration<T>)tl).skip();
94 else
95 tl.nextElement();
96 }
97 }