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| 1 | +/* | |
| 2 | + * Copyright (c) 2000, 2006, Oracle and/or its affiliates. All rights reserved. | |
| 3 | + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. | |
| 4 | + * | |
| 5 | + * This code is free software; you can redistribute it and/or modify it | |
| 6 | + * under the terms of the GNU General Public License version 2 only, as | |
| 7 | + * published by the Free Software Foundation. Oracle designates this | |
| 8 | + * particular file as subject to the "Classpath" exception as provided | |
| 9 | + * by Oracle in the LICENSE file that accompanied this code. | |
| 10 | + * | |
| 11 | + * This code is distributed in the hope that it will be useful, but WITHOUT | |
| 12 | + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | |
| 13 | + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | |
| 14 | + * version 2 for more details (a copy is included in the LICENSE file that | |
| 15 | + * accompanied this code). | |
| 16 | + * | |
| 17 | + * You should have received a copy of the GNU General Public License version | |
| 18 | + * 2 along with this work; if not, write to the Free Software Foundation, | |
| 19 | + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. | |
| 20 | + * | |
| 21 | + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA | |
| 22 | + * or visit www.oracle.com if you need additional information or have any | |
| 23 | + * questions. | |
| 24 | + */ | |
| 25 | + | |
| 26 | +package java.util; | |
| 27 | + | |
| 28 | +/** | |
| 29 | + * <p>Hash table and linked list implementation of the <tt>Set</tt> interface, | |
| 30 | + * with predictable iteration order. This implementation differs from | |
| 31 | + * <tt>HashSet</tt> in that it maintains a doubly-linked list running through | |
| 32 | + * all of its entries. This linked list defines the iteration ordering, | |
| 33 | + * which is the order in which elements were inserted into the set | |
| 34 | + * (<i>insertion-order</i>). Note that insertion order is <i>not</i> affected | |
| 35 | + * if an element is <i>re-inserted</i> into the set. (An element <tt>e</tt> | |
| 36 | + * is reinserted into a set <tt>s</tt> if <tt>s.add(e)</tt> is invoked when | |
| 37 | + * <tt>s.contains(e)</tt> would return <tt>true</tt> immediately prior to | |
| 38 | + * the invocation.) | |
| 39 | + * | |
| 40 | + * <p>This implementation spares its clients from the unspecified, generally | |
| 41 | + * chaotic ordering provided by {@link HashSet}, without incurring the | |
| 42 | + * increased cost associated with {@link TreeSet}. It can be used to | |
| 43 | + * produce a copy of a set that has the same order as the original, regardless | |
| 44 | + * of the original set's implementation: | |
| 45 | + * <pre> | |
| 46 | + * void foo(Set s) { | |
| 47 | + * Set copy = new LinkedHashSet(s); | |
| 48 | + * ... | |
| 49 | + * } | |
| 50 | + * </pre> | |
| 51 | + * This technique is particularly useful if a module takes a set on input, | |
| 52 | + * copies it, and later returns results whose order is determined by that of | |
| 53 | + * the copy. (Clients generally appreciate having things returned in the same | |
| 54 | + * order they were presented.) | |
| 55 | + * | |
| 56 | + * <p>This class provides all of the optional <tt>Set</tt> operations, and | |
| 57 | + * permits null elements. Like <tt>HashSet</tt>, it provides constant-time | |
| 58 | + * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and | |
| 59 | + * <tt>remove</tt>), assuming the hash function disperses elements | |
| 60 | + * properly among the buckets. Performance is likely to be just slightly | |
| 61 | + * below that of <tt>HashSet</tt>, due to the added expense of maintaining the | |
| 62 | + * linked list, with one exception: Iteration over a <tt>LinkedHashSet</tt> | |
| 63 | + * requires time proportional to the <i>size</i> of the set, regardless of | |
| 64 | + * its capacity. Iteration over a <tt>HashSet</tt> is likely to be more | |
| 65 | + * expensive, requiring time proportional to its <i>capacity</i>. | |
| 66 | + * | |
| 67 | + * <p>A linked hash set has two parameters that affect its performance: | |
| 68 | + * <i>initial capacity</i> and <i>load factor</i>. They are defined precisely | |
| 69 | + * as for <tt>HashSet</tt>. Note, however, that the penalty for choosing an | |
| 70 | + * excessively high value for initial capacity is less severe for this class | |
| 71 | + * than for <tt>HashSet</tt>, as iteration times for this class are unaffected | |
| 72 | + * by capacity. | |
| 73 | + * | |
| 74 | + * <p><strong>Note that this implementation is not synchronized.</strong> | |
| 75 | + * If multiple threads access a linked hash set concurrently, and at least | |
| 76 | + * one of the threads modifies the set, it <em>must</em> be synchronized | |
| 77 | + * externally. This is typically accomplished by synchronizing on some | |
| 78 | + * object that naturally encapsulates the set. | |
| 79 | + * | |
| 80 | + * If no such object exists, the set should be "wrapped" using the | |
| 81 | + * {@link Collections#synchronizedSet Collections.synchronizedSet} | |
| 82 | + * method. This is best done at creation time, to prevent accidental | |
| 83 | + * unsynchronized access to the set: <pre> | |
| 84 | + * Set s = Collections.synchronizedSet(new LinkedHashSet(...));</pre> | |
| 85 | + * | |
| 86 | + * <p>The iterators returned by this class's <tt>iterator</tt> method are | |
| 87 | + * <em>fail-fast</em>: if the set is modified at any time after the iterator | |
| 88 | + * is created, in any way except through the iterator's own <tt>remove</tt> | |
| 89 | + * method, the iterator will throw a {@link ConcurrentModificationException}. | |
| 90 | + * Thus, in the face of concurrent modification, the iterator fails quickly | |
| 91 | + * and cleanly, rather than risking arbitrary, non-deterministic behavior at | |
| 92 | + * an undetermined time in the future. | |
| 93 | + * | |
| 94 | + * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed | |
| 95 | + * as it is, generally speaking, impossible to make any hard guarantees in the | |
| 96 | + * presence of unsynchronized concurrent modification. Fail-fast iterators | |
| 97 | + * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. | |
| 98 | + * Therefore, it would be wrong to write a program that depended on this | |
| 99 | + * 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 | + * @see Object#hashCode() | |
| 110 | + * @see Collection | |
| 111 | + * @see Set | |
| 112 | + * @see HashSet | |
| 113 | + * @see TreeSet | |
| 114 | + * @see Hashtable | |
| 115 | + * @since 1.4 | |
| 116 | + */ | |
| 117 | + | |
| 118 | +public class LinkedHashSet<E> | |
| 119 | + extends HashSet<E> | |
| 120 | + implements Set<E>, 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 | +} | |