1. /*

  2.  * @(#)LinkedHashSet.java	1.8 03/01/20

  3.  *

  4.  * Copyright 2003 Sun Microsystems, Inc. All rights reserved.

  5.  * SUN 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 m) {

  29.  *         Set copy = new LinkedHashSet(m);

  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 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> If

  57.  * multiple threads access a linked hash set concurrently, and at least one of

  58.  * the threads modifies the set, it <em>must</em> be synchronized externally.

  59.  * This is typically accomplished by synchronizing on some object that

  60.  * naturally encapsulates the set.  If no such object exists, the set should

  61.  * be "wrapped" using the <tt>Collections.synchronizedSet</tt>method.  This is

  62.  * best done at creation time, to prevent accidental unsynchronized access:

  63.  * <pre>

  64.  *     Set s = Collections.synchronizedSet(new LinkedHashSet(...));

  65.  * </pre>

  66.  *

  67.  * <p>The iterators returned by the this class's <tt>iterator</tt> method are

  68.  * <em>fail-fast</em>: if the set is modified at any time after the iterator

  69.  * is created, in any way except through the iterator's own remove method, the

  70.  * iterator will throw a <tt>ConcurrentModificationException</tt>.  Thus, in

  71.  * the face of concurrent modification, the Iterator fails quickly and

  72.  * cleanly, rather than risking arbitrary, non-deterministic behavior at an

  73.  * undetermined time in the future.

  74.  *

  75.  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed

  76.  * as it is, generally speaking, impossible to make any hard guarantees in the

  77.  * presence of unsynchronized concurrent modification.  Fail-fast iterators

  78.  * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. 

  79.  * Therefore, it would be wrong to write a program that depended on this

  80.  * exception for its correctness:   <i>the fail-fast behavior of iterators

  81.  * should be used only to detect bugs.</i>

  82.  *

  83.  * <p>This class is a member of the 

  84.  * <a href="{@docRoot}/../guide/collections/index.html">

  85.  * Java Collections Framework</a>.

  86.  *

  87.  * @author  Josh Bloch

  88.  * @version 1.8 01/20/03

  89.  * @see     Object#hashCode()

  90.  * @see     Collection

  91.  * @see     Set

  92.  * @see     HashSet

  93.  * @see     TreeSet

  94.  * @see     Hashtable

  95.  * @since   1.4

  96.  */

  97. 

  98. public class LinkedHashSet extends HashSet

  99.     implements Set, Cloneable, java.io.Serializable {

  100.     /**

  101.      * Constructs a new, empty linked hash set with the specified initial

  102.      * capacity and load factor. 

  103.      *

  104.      * @param      initialCapacity the initial capacity of the linked hash set

  105.      * @param      loadFactor      the load factor of the linked hash set.

  106.      * @throws     IllegalArgumentException  if the initial capacity is less

  107.      *               than zero, or if the load factor is nonpositive.

  108.      */

  109.     public LinkedHashSet(int initialCapacity, float loadFactor) {

  110.         super(initialCapacity, loadFactor, true);

  111.     }

  112. 

  113.     /**

  114.      * Constructs a new, empty linked hash set with the specified initial

  115.      * capacity and the default load factor (0.75).

  116.      *

  117.      * @param   initialCapacity   the initial capacity of the LinkedHashSet.

  118.      * @throws  IllegalArgumentException if the initial capacity is less

  119.      *              than zero.

  120.      */

  121.     public LinkedHashSet(int initialCapacity) {

  122.         super(initialCapacity, .75f, true);

  123.     }

  124. 

  125.     /**

  126.      * Constructs a new, empty linked hash set with the default initial

  127.      * capacity (16) and load factor (0.75).

  128.      */

  129.     public LinkedHashSet() {

  130.         super(16, .75f, true);

  131.     }

  132. 

  133.     /**

  134.      * Constructs a new linked hash set with the same elements as the

  135.      * specified collection.  The linked hash set is created with an initial

  136.      * capacity sufficient to hold the elements in the specified collection

  137.      * and the default load factor (0.75).

  138.      *

  139.      * @param c  the collection whose elements are to be placed into 

  140.      *           this set.

  141.      * @throws NullPointerException if the specified collection is null.

  142.      */

  143.     public LinkedHashSet(Collection c) {

  144.         super(Math.max(2*c.size(), 11), .75f, true);

  145.         addAll(c);

  146.     }

  147. }