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 import java.util.NoSuchElementException;
50
51 /**
52 * An <code>Enumeration</code> created by filtering another
53 * according to an abstract inclusion function.
54 * <p>
55 * Implementations must provide {@link #isIncluded(Object)}.
56 */
57 public abstract class FilteredEnumeration <T> implements SkipEnumeration<T> {
58 private Enumeration<T> base;
59 private int finished = -1;
60 private T next;
61
62 /**
63 * Constructor.
64 * @param base the Enumeration we are based upon
65 */
66 public FilteredEnumeration(Enumeration<T> base) {
67 this.base = base;
68 }
69
70 protected abstract boolean isIncluded(T o);
71
72 private void probe() {
73 if (finished == -1) {
74 for (;;) {
75 if (!base.hasMoreElements()) {
76 finished = 1;
77 next = null;
78 break;
79 }
80
81 next = base.nextElement();
82 if (isIncluded(next)) {
83 finished = 0;
84 break;
85 }
86 }
87 }
88 }
89
90 /**
91 * {@inheritDoc}
92 * @see java.util.Enumeration#hasMoreElements()
93 */
94 public synchronized boolean hasMoreElements() {
95 probe();
96 return finished == 0;
97 }
98
99 /**
100 * {@inheritDoc}
101 * @see java.util.Enumeration#nextElement()
102 */
103 public synchronized T nextElement() {
104 if (!hasMoreElements())
105 throw new NoSuchElementException();
106
107 finished = -1;
108 return next;
109 }
110
111 /**
112 * {@inheritDoc}
113 * @see org.melati.poem.util.SkipEnumeration#skip()
114 */
115 public synchronized void skip() {
116 nextElement();
117 }
118 }