1 /*
2 * Copyright (c) 2000, 2018, 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
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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
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24 */
25
26 package java.util;
27
28 /**
29 * <p>Hash table and linked list implementation of the {@code Set} interface,
30 * with predictable iteration order. This implementation differs from
31 * {@code HashSet} 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 {@code e}
36 * is reinserted into a set {@code s} if {@code s.add(e)} is invoked when
37 * {@code s.contains(e)} would return {@code true} 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 {@code Set} operations, and
57 * permits null elements. Like {@code HashSet}, it provides constant-time
58 * performance for the basic operations ({@code add}, {@code contains} and
59 * {@code remove}), assuming the hash function disperses elements
60 * properly among the buckets. Performance is likely to be just slightly
61 * below that of {@code HashSet}, due to the added expense of maintaining the
62 * linked list, with one exception: Iteration over a {@code LinkedHashSet}
63 * requires time proportional to the <i>size</i> of the set, regardless of
64 * its capacity. Iteration over a {@code HashSet} 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 {@code HashSet}. Note, however, that the penalty for choosing an
70 * excessively high value for initial capacity is less severe for this class
71 * than for {@code HashSet}, 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 {@code iterator} 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 {@code remove}
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 {@code ConcurrentModificationException} 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}/java.base/java/util/package-summary.html#CollectionsFramework">
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
172 /**
173 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
174 * and <em>fail-fast</em> {@code Spliterator} over the elements in this set.
175 *
176 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
177 * {@link Spliterator#DISTINCT}, and {@code ORDERED}. Implementations
178 * should document the reporting of additional characteristic values.
179 *
180 * @implNote
181 * The implementation creates a
182 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
183 * from the set's {@code Iterator}. The spliterator inherits the
184 * <em>fail-fast</em> properties of the set's iterator.
185 * The created {@code Spliterator} additionally reports
186 * {@link Spliterator#SUBSIZED}.
187 *
188 * @return a {@code Spliterator} over the elements in this set
189 * @since 1.8
190 */
191 @Override
192 public Spliterator<E> spliterator() {
193 return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
194 }
195 }
196