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    
005    This file is part of GNU Classpath.
006    
007    GNU Classpath is free software; you can redistribute it and/or modify
008    it under the terms of the GNU General Public License as published by
009    the Free Software Foundation; either version 2, or (at your option)
010    any later version.
011    
012    GNU Classpath is distributed in the hope that it will be useful, but
013    WITHOUT ANY WARRANTY; without even the implied warranty of
014    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
015    General Public License for more details.
016    
017    You should have received a copy of the GNU General Public License
018    along with GNU Classpath; see the file COPYING.  If not, write to the
019    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
020    02110-1301 USA.
021    
022    Linking this library statically or dynamically with other modules is
023    making a combined work based on this library.  Thus, the terms and
024    conditions of the GNU General Public License cover the whole
025    combination.
026    
027    As a special exception, the copyright holders of this library give you
028    permission to link this library with independent modules to produce an
029    executable, regardless of the license terms of these independent
030    modules, and to copy and distribute the resulting executable under
031    terms of your choice, provided that you also meet, for each linked
032    independent module, the terms and conditions of the license of that
033    module.  An independent module is a module which is not derived from
034    or based on this library.  If you modify this library, you may extend
035    this exception to your version of the library, but you are not
036    obligated to do so.  If you do not wish to do so, delete this
037    exception statement from your version. */
038    
039    
040    package java.util;
041    
042    import 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     */
091    public 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    }