| LinkedHashSet.java |
1 /*
2 * %W% %E%
3 *
4 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
5 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6 */
7
8 package java.util;
9
10 /**
11 * <p>Hash table and linked list implementation of the <tt>Set</tt> interface,
12 * with predictable iteration order. This implementation differs from
13 * <tt>HashSet</tt> in that it maintains a doubly-linked list running through
14 * all of its entries. This linked list defines the iteration ordering,
15 * which is the order in which elements were inserted into the set
16 * (<i>insertion-order</i>). Note that insertion order is <i>not</i> affected
17 * if an element is <i>re-inserted</i> into the set. (An element <tt>e</tt>
18 * is reinserted into a set <tt>s</tt> if <tt>s.add(e)</tt> is invoked when
19 * <tt>s.contains(e)</tt> would return <tt>true</tt> immediately prior to
20 * the invocation.)
21 *
22 * <p>This implementation spares its clients from the unspecified, generally
23 * chaotic ordering provided by {@link HashSet}, without incurring the
24 * increased cost associated with {@link TreeSet}. It can be used to
25 * produce a copy of a set that has the same order as the original, regardless
26 * of the original set's implementation:
27 * <pre>
28 * void foo(Set s) {
29 * Set copy = new LinkedHashSet(s);
30 * ...
31 * }
32 * </pre>
33 * This technique is particularly useful if a module takes a set on input,
34 * copies it, and later returns results whose order is determined by that of
35 * the copy. (Clients generally appreciate having things returned in the same
36 * order they were presented.)
37 *
38 * <p>This class provides all of the optional <tt>Set</tt> operations, and
39 * permits null elements. Like <tt>HashSet</tt>, it provides constant-time
40 * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and
41 * <tt>remove</tt>), assuming the hash function disperses elements
42 * properly among the buckets. Performance is likely to be just slightly
43 * below that of <tt>HashSet</tt>, due to the added expense of maintaining the
44 * linked list, with one exception: Iteration over a <tt>LinkedHashSet</tt>
45 * requires time proportional to the <i>size</i> of the set, regardless of
46 * its capacity. Iteration over a <tt>HashSet</tt> is likely to be more
47 * expensive, requiring time proportional to its <i>capacity</i>.
48 *
49 * <p>A linked hash set has two parameters that affect its performance:
50 * <i>initial capacity</i> and <i>load factor</i>. They are defined precisely
51 * as for <tt>HashSet</tt>. Note, however, that the penalty for choosing an
52 * excessively high value for initial capacity is less severe for this class
53 * than for <tt>HashSet</tt>, as iteration times for this class are unaffected
54 * by capacity.
55 *
56 * <p><strong>Note that this implementation is not synchronized.</strong>
57 * If multiple threads access a linked hash set concurrently, and at least
58 * one of the threads modifies the set, it <em>must</em> be synchronized
59 * externally. This is typically accomplished by synchronizing on some
60 * object that naturally encapsulates the set.
61 *
62 * If no such object exists, the set should be "wrapped" using the
63 * {@link Collections#synchronizedSet Collections.synchronizedSet}
64 * method. This is best done at creation time, to prevent accidental
65 * unsynchronized access to the set: <pre>
66 * Set s = Collections.synchronizedSet(new LinkedHashSet(...));</pre>
67 *
68 * <p>The iterators returned by this class's <tt>iterator</tt> method are
69 * <em>fail-fast</em>: if the set is modified at any time after the iterator
70 * is created, in any way except through the iterator's own <tt>remove</tt>
71 * method, the iterator will throw a {@link ConcurrentModificationException}.
72 * Thus, in the face of concurrent modification, the iterator fails quickly
73 * and cleanly, rather than risking arbitrary, non-deterministic behavior at
74 * an undetermined time in the future.
75 *
76 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
77 * as it is, generally speaking, impossible to make any hard guarantees in the
78 * presence of unsynchronized concurrent modification. Fail-fast iterators
79 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
80 * Therefore, it would be wrong to write a program that depended on this
81 * exception for its correctness: <i>the fail-fast behavior of iterators
82 * should be used only to detect bugs.</i>
83 *
84 * <p>This class is a member of the
85 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
86 * Java Collections Framework</a>.
87 *
88 * @param <E> the type of elements maintained by this set
89 *
90 * @author Josh Bloch
91 * @version %I% %E%
92 * @see Object#hashCode()
93 * @see Collection
94 * @see Set
95 * @see HashSet
96 * @see TreeSet
97 * @see Hashtable
98 * @since 1.4
99 */
100
101 public class LinkedHashSet<E>
102 extends HashSet<E>
103 implements Set<E>, Cloneable, java.io.Serializable {
104
105 private static final long serialVersionUID = -2851667679971038690L;
106
107 /**
108 * Constructs a new, empty linked hash set with the specified initial
109 * capacity and load factor.
110 *
111 * @param initialCapacity the initial capacity of the linked hash set
112 * @param loadFactor the load factor of the linked hash set
113 * @throws IllegalArgumentException if the initial capacity is less
114 * than zero, or if the load factor is nonpositive
115 */
116 public LinkedHashSet(int initialCapacity, float loadFactor) {
117 super(initialCapacity, loadFactor, true);
118 }
119
120 /**
121 * Constructs a new, empty linked hash set with the specified initial
122 * capacity and the default load factor (0.75).
123 *
124 * @param initialCapacity the initial capacity of the LinkedHashSet
125 * @throws IllegalArgumentException if the initial capacity is less
126 * than zero
127 */
128 public LinkedHashSet(int initialCapacity) {
129 super(initialCapacity, .75f, true);
130 }
131
132 /**
133 * Constructs a new, empty linked hash set with the default initial
134 * capacity (16) and load factor (0.75).
135 */
136 public LinkedHashSet() {
137 super(16, .75f, true);
138 }
139
140 /**
141 * Constructs a new linked hash set with the same elements as the
142 * specified collection. The linked hash set is created with an initial
143 * capacity sufficient to hold the elements in the specified collection
144 * and the default load factor (0.75).
145 *
146 * @param c the collection whose elements are to be placed into
147 * this set
148 * @throws NullPointerException if the specified collection is null
149 */
150 public LinkedHashSet(Collection<? extends E> c) {
151 super(Math.max(2*c.size(), 11), .75f, true);
152 addAll(c);
153 }
154 }
155