Source code: gnu/java/util/DoubleEnumeration.java
1 /* gnu.java.util.DoubleEnumeration
2 Copyright (C) 1998, 1999, 2001 Free Software Foundation, Inc.
3
4 This file is part of GNU Classpath.
5
6 GNU Classpath is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 2, or (at your option)
9 any later version.
10
11 GNU Classpath is distributed in the hope that it will be useful, but
12 WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 General Public License for more details.
15
16 You should have received a copy of the GNU General Public License
17 along with GNU Classpath; see the file COPYING. If not, write to the
18 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
19 02110-1301 USA.
20
21 Linking this library statically or dynamically with other modules is
22 making a combined work based on this library. Thus, the terms and
23 conditions of the GNU General Public License cover the whole
24 combination.
25
26 As a special exception, the copyright holders of this library give you
27 permission to link this library with independent modules to produce an
28 executable, regardless of the license terms of these independent
29 modules, and to copy and distribute the resulting executable under
30 terms of your choice, provided that you also meet, for each linked
31 independent module, the terms and conditions of the license of that
32 module. An independent module is a module which is not derived from
33 or based on this library. If you modify this library, you may extend
34 this exception to your version of the library, but you are not
35 obligated to do so. If you do not wish to do so, delete this
36 exception statement from your version. */
37
38 package gnu.java.util;
39
40 import java.util.Enumeration;
41 import java.util.NoSuchElementException;
42
43
44 /**
45 * This is a helper class that combines two Enumerations.
46 * It returns the elements of the first Enumeration until it has
47 * no more elements and then returns the elements of the second
48 * Enumeration.<br>
49 *
50 * In the default case:
51 * <pre>
52 * doubleEnum = new DoubleEnumeration(enum1, enum2);
53 * while (doubleEnum.hasMoreElements()) {
54 * Object o = doubleEnum.nextElement();
55 * do_something(o);
56 * }
57 * </pre>
58 * it calls hasMoreElements of the Enumerations as few times as
59 * possible.
60 * The references to the Enumerations are cleared as soon as they have no
61 * more elements to help garbage collecting.
62 *
63 * @author Jochen Hoenicke
64 * @author Mark Wielaard (mark@klomp.org)
65 */
66 public class DoubleEnumeration implements Enumeration
67 {
68 /**
69 * This is true as long as one of the enumerations has more
70 * elements.
71 * Only valid when hasChecked is true.
72 * Set in <code>hasMoreElements()</code>
73 */
74 private boolean hasMore;
75 /**
76 * This is true, if it is sure that hasMore indicates wether there are
77 * more elements.
78 * Set to true in <code>hasMoreElements()</code>.
79 * Set to false in <code>getNextElement()</code>.
80 */
81 private boolean hasChecked;
82 /**
83 * The first enumeration.
84 */
85 private Enumeration e1;
86 /**
87 * The second enumeration.
88 */
89 private Enumeration e2;
90
91 /**
92 * Creates a new Enumeration combining the given two enumerations.
93 * The enumerations mustn't be accessed by other classes.
94 */
95 public DoubleEnumeration(Enumeration e1, Enumeration e2)
96 {
97 this.e1 = e1;
98 this.e2 = e2;
99 hasChecked = false;
100 }
101
102 /**
103 * Returns true, if at least one of the two enumerations has more
104 * elements.
105 */
106 public boolean hasMoreElements()
107 {
108 if (hasChecked)
109 return hasMore;
110
111 hasMore = (e1 != null && e1.hasMoreElements());
112
113 if (!hasMore) {
114 e1 = e2;
115 e2 = null;
116 hasMore = (e1 != null && e1.hasMoreElements());
117 }
118
119 hasChecked = true;
120 return hasMore;
121 }
122
123 /**
124 * Returns the next element. This returns the next element of the
125 * first enumeration, if it has more elements, otherwise the next
126 * element of the second enumeration. If both enumeration don't have
127 * any elements it throws a <code>NoSuchElementException</code>.
128 */
129 public Object nextElement()
130 {
131 if (!hasMoreElements())
132 throw new NoSuchElementException();
133 else {
134 hasChecked = false;
135 return e1.nextElement();
136 }
137 }
138 }