001/* LinkedHashSet.java -- a set backed by a LinkedHashMap, for linked
002   list traversal.
003   Copyright (C) 2001, 2004, 2005, 2007 Free Software Foundation, Inc.
004
005This file is part of GNU Classpath.
006
007GNU Classpath is free software; you can redistribute it and/or modify
008it under the terms of the GNU General Public License as published by
009the Free Software Foundation; either version 2, or (at your option)
010any later version.
011
012GNU Classpath is distributed in the hope that it will be useful, but
013WITHOUT ANY WARRANTY; without even the implied warranty of
014MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
015General Public License for more details.
016
017You should have received a copy of the GNU General Public License
018along with GNU Classpath; see the file COPYING.  If not, write to the
019Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02002110-1301 USA.
021
022Linking this library statically or dynamically with other modules is
023making a combined work based on this library.  Thus, the terms and
024conditions of the GNU General Public License cover the whole
025combination.
026
027As a special exception, the copyright holders of this library give you
028permission to link this library with independent modules to produce an
029executable, regardless of the license terms of these independent
030modules, and to copy and distribute the resulting executable under
031terms of your choice, provided that you also meet, for each linked
032independent module, the terms and conditions of the license of that
033module.  An independent module is a module which is not derived from
034or based on this library.  If you modify this library, you may extend
035this exception to your version of the library, but you are not
036obligated to do so.  If you do not wish to do so, delete this
037exception statement from your version. */
038
039
040package java.util;
041
042import java.io.Serializable;
043
044/**
045 * This class provides a hashtable-backed implementation of the
046 * Set interface, with predictable traversal order.
047 * <p>
048 *
049 * It uses a hash-bucket approach; that is, hash collisions are handled
050 * by linking the new node off of the pre-existing node (or list of
051 * nodes).  In this manner, techniques such as linear probing (which
052 * can cause primary clustering) and rehashing (which does not fit very
053 * well with Java's method of precomputing hash codes) are avoided.  In
054 * addition, this maintains a doubly-linked list which tracks insertion
055 * order.  Note that the insertion order is not modified if an
056 * <code>add</code> simply reinserts an element in the set.
057 * <p>
058 *
059 * One of the nice features of tracking insertion order is that you can
060 * copy a set, and regardless of the implementation of the original,
061 * produce the same results when iterating over the copy.  This is possible
062 * without needing the overhead of <code>TreeSet</code>.
063 * <p>
064 *
065 * Under ideal circumstances (no collisions), LinkedHashSet offers O(1)
066 * performance on most operations.  In the worst case (all elements map
067 * to the same hash code -- very unlikely), most operations are O(n).
068 * <p>
069 *
070 * LinkedHashSet accepts the null entry.  It is not synchronized, so if
071 * you need multi-threaded access, consider using:<br>
072 * <code>Set s = Collections.synchronizedSet(new LinkedHashSet(...));</code>
073 * <p>
074 *
075 * The iterators are <i>fail-fast</i>, meaning that any structural
076 * modification, except for <code>remove()</code> called on the iterator
077 * itself, cause the iterator to throw a
078 * {@link ConcurrentModificationException} rather than exhibit
079 * non-deterministic behavior.
080 *
081 * @author Eric Blake (ebb9@email.byu.edu)
082 * @see Object#hashCode()
083 * @see Collection
084 * @see Set
085 * @see HashSet
086 * @see TreeSet
087 * @see Collections#synchronizedSet(Set)
088 * @since 1.4
089 * @status updated to 1.4
090 */
091public class LinkedHashSet<T> extends HashSet<T>
092  implements Set<T>, Cloneable, Serializable
093{
094  /**
095   * Compatible with JDK 1.4.
096   */
097  private static final long serialVersionUID = -2851667679971038690L;
098
099  /**
100   * Construct a new, empty HashSet whose backing HashMap has the default
101   * capacity (11) and loadFactor (0.75).
102   */
103  public LinkedHashSet()
104  {
105    super();
106  }
107
108  /**
109   * Construct a new, empty HashSet whose backing HashMap has the supplied
110   * capacity and the default load factor (0.75).
111   *
112   * @param initialCapacity the initial capacity of the backing HashMap
113   * @throws IllegalArgumentException if the capacity is negative
114   */
115  public LinkedHashSet(int initialCapacity)
116  {
117    super(initialCapacity);
118  }
119
120  /**
121   * Construct a new, empty HashSet whose backing HashMap has the supplied
122   * capacity and load factor.
123   *
124   * @param initialCapacity the initial capacity of the backing HashMap
125   * @param loadFactor the load factor of the backing HashMap
126   * @throws IllegalArgumentException if either argument is negative, or
127   *         if loadFactor is POSITIVE_INFINITY or NaN
128   */
129  public LinkedHashSet(int initialCapacity, float loadFactor)
130  {
131    super(initialCapacity, loadFactor);
132  }
133
134  /**
135   * Construct a new HashSet with the same elements as are in the supplied
136   * collection (eliminating any duplicates, of course). The backing storage
137   * has twice the size of the collection, or the default size of 11,
138   * whichever is greater; and the default load factor (0.75).
139   *
140   * @param c a collection of initial set elements
141   * @throws NullPointerException if c is null
142   */
143  public LinkedHashSet(Collection<? extends T> c)
144  {
145    super(c);
146  }
147
148  /**
149   * Helper method which initializes the backing Map.
150   *
151   * @param capacity the initial capacity
152   * @param load the initial load factor
153   * @return the backing HashMap
154   */
155  HashMap<T, String> init(int capacity, float load)
156  {
157    return new LinkedHashMap<T, String>(capacity, load);
158  }
159}