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    1   /*
    2    * Copyright (c) 2000, 2006, Oracle and/or its affiliates. All rights reserved.
    3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
    4    *
    5    * This code is free software; you can redistribute it and/or modify it
    6    * under the terms of the GNU General Public License version 2 only, as
    7    * published by the Free Software Foundation.  Oracle designates this
    8    * particular file as subject to the "Classpath" exception as provided
    9    * by Oracle in the LICENSE file that accompanied this code.
   10    *
   11    * This code is distributed in the hope that it will be useful, but WITHOUT
   12    * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   13    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   14    * version 2 for more details (a copy is included in the LICENSE file that
   15    * accompanied this code).
   16    *
   17    * You should have received a copy of the GNU General Public License version
   18    * 2 along with this work; if not, write to the Free Software Foundation,
   19    * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   20    *
   21    * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   22    * or visit www.oracle.com if you need additional information or have any
   23    * questions.
   24    */
   25   
   26   package java.util;
   27   
   28   /**
   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
   38    * the invocation.)
   39    *
   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:
   45    * <pre>
   46    *     void foo(Set s) {
   47    *         Set copy = new LinkedHashSet(s);
   48    *         ...
   49    *     }
   50    * </pre>
   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.)
   55    *
   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>.
   66    *
   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
   72    * by capacity.
   73    *
   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.
   79    *
   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>
   85    *
   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.
   93    *
   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>
  101    *
  102    * <p>This class is a member of the
  103    * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  104    * Java Collections Framework</a>.
  105    *
  106    * @param <E> the type of elements maintained by this set
  107    *
  108    * @author  Josh Bloch
  109    * @see     Object#hashCode()
  110    * @see     Collection
  111    * @see     Set
  112    * @see     HashSet
  113    * @see     TreeSet
  114    * @see     Hashtable
  115    * @since   1.4
  116    */
  117   
  118   public class LinkedHashSet<E>
  119       extends HashSet<E>
  120       implements Set<E>, Cloneable, java.io.Serializable {
  121   
  122       private static final long serialVersionUID = -2851667679971038690L;
  123   
  124       /**
  125        * Constructs a new, empty linked hash set with the specified initial
  126        * capacity and load factor.
  127        *
  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
  132        */
  133       public LinkedHashSet(int initialCapacity, float loadFactor) {
  134           super(initialCapacity, loadFactor, true);
  135       }
  136   
  137       /**
  138        * Constructs a new, empty linked hash set with the specified initial
  139        * capacity and the default load factor (0.75).
  140        *
  141        * @param   initialCapacity   the initial capacity of the LinkedHashSet
  142        * @throws  IllegalArgumentException if the initial capacity is less
  143        *              than zero
  144        */
  145       public LinkedHashSet(int initialCapacity) {
  146           super(initialCapacity, .75f, true);
  147       }
  148   
  149       /**
  150        * Constructs a new, empty linked hash set with the default initial
  151        * capacity (16) and load factor (0.75).
  152        */
  153       public LinkedHashSet() {
  154           super(16, .75f, true);
  155       }
  156   
  157       /**
  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).
  162        *
  163        * @param c  the collection whose elements are to be placed into
  164        *           this set
  165        * @throws NullPointerException if the specified collection is null
  166        */
  167       public LinkedHashSet(Collection<? extends E> c) {
  168           super(Math.max(2*c.size(), 11), .75f, true);
  169           addAll(c);
  170       }
  171   }

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