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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
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:
47 * Set copy = new LinkedHashSet(s);
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.)
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>.
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
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.
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>
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.
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>
102 * <p>This class is a member of the
103 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
104 * Java Collections Framework</a>.
106 * @param <E> the type of elements maintained by this set
109 * @see Object#hashCode()
118 public class LinkedHashSet<E>
120 implements Set<E>, Cloneable, java.io.Serializable {
122 private static final long serialVersionUID = -2851667679971038690L;
125 * Constructs a new, empty linked hash set with the specified initial
126 * capacity and load factor.
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
133 public LinkedHashSet(int initialCapacity, float loadFactor) {
134 super(initialCapacity, loadFactor, true);
138 * Constructs a new, empty linked hash set with the specified initial
139 * capacity and the default load factor (0.75).
141 * @param initialCapacity the initial capacity of the LinkedHashSet
142 * @throws IllegalArgumentException if the initial capacity is less
145 public LinkedHashSet(int initialCapacity) {
146 super(initialCapacity, .75f, true);
150 * Constructs a new, empty linked hash set with the default initial
151 * capacity (16) and load factor (0.75).
153 public LinkedHashSet() {
154 super(16, .75f, true);
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).
163 * @param c the collection whose elements are to be placed into
165 * @throws NullPointerException if the specified collection is null
167 public LinkedHashSet(Collection<? extends E> c) {
168 super(Math.max(2*c.size(), 11), .75f, true);