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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 | } | ||