001 /*
002 * Copyright 2000-2006 Sun Microsystems, Inc. All Rights Reserved.
003 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004 *
005 * This code is free software; you can redistribute it and/or modify it
006 * under the terms of the GNU General Public License version 2 only, as
007 * published by the Free Software Foundation. Sun designates this
008 * particular file as subject to the "Classpath" exception as provided
009 * by Sun in the LICENSE file that accompanied this code.
010 *
011 * This code is distributed in the hope that it will be useful, but WITHOUT
012 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
014 * version 2 for more details (a copy is included in the LICENSE file that
015 * accompanied this code).
016 *
017 * You should have received a copy of the GNU General Public License version
018 * 2 along with this work; if not, write to the Free Software Foundation,
019 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020 *
021 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
022 * CA 95054 USA or visit www.sun.com if you need additional information or
023 * have any questions.
024 */
025
026 package java.util;
027
028 /**
029 * <p>Hash table and linked list implementation of the <tt>Set</tt> interface,
030 * with predictable iteration order. This implementation differs from
031 * <tt>HashSet</tt> in that it maintains a doubly-linked list running through
032 * all of its entries. This linked list defines the iteration ordering,
033 * which is the order in which elements were inserted into the set
034 * (<i>insertion-order</i>). Note that insertion order is <i>not</i> affected
035 * if an element is <i>re-inserted</i> into the set. (An element <tt>e</tt>
036 * is reinserted into a set <tt>s</tt> if <tt>s.add(e)</tt> is invoked when
037 * <tt>s.contains(e)</tt> would return <tt>true</tt> immediately prior to
038 * the invocation.)
039 *
040 * <p>This implementation spares its clients from the unspecified, generally
041 * chaotic ordering provided by {@link HashSet}, without incurring the
042 * increased cost associated with {@link TreeSet}. It can be used to
043 * produce a copy of a set that has the same order as the original, regardless
044 * of the original set's implementation:
045 * <pre>
046 * void foo(Set s) {
047 * Set copy = new LinkedHashSet(s);
048 * ...
049 * }
050 * </pre>
051 * This technique is particularly useful if a module takes a set on input,
052 * copies it, and later returns results whose order is determined by that of
053 * the copy. (Clients generally appreciate having things returned in the same
054 * order they were presented.)
055 *
056 * <p>This class provides all of the optional <tt>Set</tt> operations, and
057 * permits null elements. Like <tt>HashSet</tt>, it provides constant-time
058 * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and
059 * <tt>remove</tt>), assuming the hash function disperses elements
060 * properly among the buckets. Performance is likely to be just slightly
061 * below that of <tt>HashSet</tt>, due to the added expense of maintaining the
062 * linked list, with one exception: Iteration over a <tt>LinkedHashSet</tt>
063 * requires time proportional to the <i>size</i> of the set, regardless of
064 * its capacity. Iteration over a <tt>HashSet</tt> is likely to be more
065 * expensive, requiring time proportional to its <i>capacity</i>.
066 *
067 * <p>A linked hash set has two parameters that affect its performance:
068 * <i>initial capacity</i> and <i>load factor</i>. They are defined precisely
069 * as for <tt>HashSet</tt>. Note, however, that the penalty for choosing an
070 * excessively high value for initial capacity is less severe for this class
071 * than for <tt>HashSet</tt>, as iteration times for this class are unaffected
072 * by capacity.
073 *
074 * <p><strong>Note that this implementation is not synchronized.</strong>
075 * If multiple threads access a linked hash set concurrently, and at least
076 * one of the threads modifies the set, it <em>must</em> be synchronized
077 * externally. This is typically accomplished by synchronizing on some
078 * object that naturally encapsulates the set.
079 *
080 * If no such object exists, the set should be "wrapped" using the
081 * {@link Collections#synchronizedSet Collections.synchronizedSet}
082 * method. This is best done at creation time, to prevent accidental
083 * unsynchronized access to the set: <pre>
084 * Set s = Collections.synchronizedSet(new LinkedHashSet(...));</pre>
085 *
086 * <p>The iterators returned by this class's <tt>iterator</tt> method are
087 * <em>fail-fast</em>: if the set is modified at any time after the iterator
088 * is created, in any way except through the iterator's own <tt>remove</tt>
089 * method, the iterator will throw a {@link ConcurrentModificationException}.
090 * Thus, in the face of concurrent modification, the iterator fails quickly
091 * and cleanly, rather than risking arbitrary, non-deterministic behavior at
092 * an undetermined time in the future.
093 *
094 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
095 * as it is, generally speaking, impossible to make any hard guarantees in the
096 * presence of unsynchronized concurrent modification. Fail-fast iterators
097 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
098 * Therefore, it would be wrong to write a program that depended on this
099 * exception for its correctness: <i>the fail-fast behavior of iterators
100 * should be used only to detect bugs.</i>
101 *
102 * <p>This class is a member of the
103 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
104 * Java Collections Framework</a>.
105 *
106 * @param <E> the type of elements maintained by this set
107 *
108 * @author Josh Bloch
109 * @version 1.24 07/05/05
110 * @see Object#hashCode()
111 * @see Collection
112 * @see Set
113 * @see HashSet
114 * @see TreeSet
115 * @see Hashtable
116 * @since 1.4
117 */
118
119 public class LinkedHashSet<E> extends HashSet<E> implements Set<E>,
120 Cloneable, java.io.Serializable {
121
122 private static final long serialVersionUID = -2851667679971038690L;
123
124 /**
125 * Constructs a new, empty linked hash set with the specified initial
126 * capacity and load factor.
127 *
128 * @param initialCapacity the initial capacity of the linked hash set
129 * @param loadFactor the load factor of the linked hash set
130 * @throws IllegalArgumentException if the initial capacity is less
131 * than zero, or if the load factor is nonpositive
132 */
133 public LinkedHashSet(int initialCapacity, float loadFactor) {
134 super (initialCapacity, loadFactor, true);
135 }
136
137 /**
138 * Constructs a new, empty linked hash set with the specified initial
139 * capacity and the default load factor (0.75).
140 *
141 * @param initialCapacity the initial capacity of the LinkedHashSet
142 * @throws IllegalArgumentException if the initial capacity is less
143 * than zero
144 */
145 public LinkedHashSet(int initialCapacity) {
146 super (initialCapacity, .75f, true);
147 }
148
149 /**
150 * Constructs a new, empty linked hash set with the default initial
151 * capacity (16) and load factor (0.75).
152 */
153 public LinkedHashSet() {
154 super (16, .75f, true);
155 }
156
157 /**
158 * Constructs a new linked hash set with the same elements as the
159 * specified collection. The linked hash set is created with an initial
160 * capacity sufficient to hold the elements in the specified collection
161 * and the default load factor (0.75).
162 *
163 * @param c the collection whose elements are to be placed into
164 * this set
165 * @throws NullPointerException if the specified collection is null
166 */
167 public LinkedHashSet(Collection<? extends E> c) {
168 super (Math.max(2 * c.size(), 11), .75f, true);
169 addAll(c);
170 }
171 }
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