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    1   /*
    2    * Copyright (c) 1997, 2010, 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    * This class implements the <tt>Set</tt> interface, backed by a hash table
   30    * (actually a <tt>HashMap</tt> instance).  It makes no guarantees as to the
   31    * iteration order of the set; in particular, it does not guarantee that the
   32    * order will remain constant over time.  This class permits the <tt>null</tt>
   33    * element.
   34    *
   35    * <p>This class offers constant time performance for the basic operations
   36    * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),
   37    * assuming the hash function disperses the elements properly among the
   38    * buckets.  Iterating over this set requires time proportional to the sum of
   39    * the <tt>HashSet</tt> instance's size (the number of elements) plus the
   40    * "capacity" of the backing <tt>HashMap</tt> instance (the number of
   41    * buckets).  Thus, it's very important not to set the initial capacity too
   42    * high (or the load factor too low) if iteration performance is important.
   43    *
   44    * <p><strong>Note that this implementation is not synchronized.</strong>
   45    * If multiple threads access a hash set concurrently, and at least one of
   46    * the threads modifies the set, it <i>must</i> be synchronized externally.
   47    * This is typically accomplished by synchronizing on some object that
   48    * naturally encapsulates the set.
   49    *
   50    * If no such object exists, the set should be "wrapped" using the
   51    * {@link Collections#synchronizedSet Collections.synchronizedSet}
   52    * method.  This is best done at creation time, to prevent accidental
   53    * unsynchronized access to the set:<pre>
   54    *   Set s = Collections.synchronizedSet(new HashSet(...));</pre>
   55    *
   56    * <p>The iterators returned by this class's <tt>iterator</tt> method are
   57    * <i>fail-fast</i>: if the set is modified at any time after the iterator is
   58    * created, in any way except through the iterator's own <tt>remove</tt>
   59    * method, the Iterator throws a {@link ConcurrentModificationException}.
   60    * Thus, in the face of concurrent modification, the iterator fails quickly
   61    * and cleanly, rather than risking arbitrary, non-deterministic behavior at
   62    * an undetermined time in the future.
   63    *
   64    * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
   65    * as it is, generally speaking, impossible to make any hard guarantees in the
   66    * presence of unsynchronized concurrent modification.  Fail-fast iterators
   67    * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
   68    * Therefore, it would be wrong to write a program that depended on this
   69    * exception for its correctness: <i>the fail-fast behavior of iterators
   70    * should be used only to detect bugs.</i>
   71    *
   72    * <p>This class is a member of the
   73    * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   74    * Java Collections Framework</a>.
   75    *
   76    * @param <E> the type of elements maintained by this set
   77    *
   78    * @author  Josh Bloch
   79    * @author  Neal Gafter
   80    * @see     Collection
   81    * @see     Set
   82    * @see     TreeSet
   83    * @see     HashMap
   84    * @since   1.2
   85    */
   86   
   87   public class HashSet<E>
   88       extends AbstractSet<E>
   89       implements Set<E>, Cloneable, java.io.Serializable
   90   {
   91       static final long serialVersionUID = -5024744406713321676L;
   92   
   93       private transient HashMap<E,Object> map;
   94   
   95       // Dummy value to associate with an Object in the backing Map
   96       private static final Object PRESENT = new Object();
   97   
   98       /**
   99        * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
  100        * default initial capacity (16) and load factor (0.75).
  101        */
  102       public HashSet() {
  103           map = new HashMap<>();
  104       }
  105   
  106       /**
  107        * Constructs a new set containing the elements in the specified
  108        * collection.  The <tt>HashMap</tt> is created with default load factor
  109        * (0.75) and an initial capacity sufficient to contain the elements in
  110        * the specified collection.
  111        *
  112        * @param c the collection whose elements are to be placed into this set
  113        * @throws NullPointerException if the specified collection is null
  114        */
  115       public HashSet(Collection<? extends E> c) {
  116           map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
  117           addAll(c);
  118       }
  119   
  120       /**
  121        * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
  122        * the specified initial capacity and the specified load factor.
  123        *
  124        * @param      initialCapacity   the initial capacity of the hash map
  125        * @param      loadFactor        the load factor of the hash map
  126        * @throws     IllegalArgumentException if the initial capacity is less
  127        *             than zero, or if the load factor is nonpositive
  128        */
  129       public HashSet(int initialCapacity, float loadFactor) {
  130           map = new HashMap<>(initialCapacity, loadFactor);
  131       }
  132   
  133       /**
  134        * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
  135        * the specified initial capacity and default load factor (0.75).
  136        *
  137        * @param      initialCapacity   the initial capacity of the hash table
  138        * @throws     IllegalArgumentException if the initial capacity is less
  139        *             than zero
  140        */
  141       public HashSet(int initialCapacity) {
  142           map = new HashMap<>(initialCapacity);
  143       }
  144   
  145       /**
  146        * Constructs a new, empty linked hash set.  (This package private
  147        * constructor is only used by LinkedHashSet.) The backing
  148        * HashMap instance is a LinkedHashMap with the specified initial
  149        * capacity and the specified load factor.
  150        *
  151        * @param      initialCapacity   the initial capacity of the hash map
  152        * @param      loadFactor        the load factor of the hash map
  153        * @param      dummy             ignored (distinguishes this
  154        *             constructor from other int, float constructor.)
  155        * @throws     IllegalArgumentException if the initial capacity is less
  156        *             than zero, or if the load factor is nonpositive
  157        */
  158       HashSet(int initialCapacity, float loadFactor, boolean dummy) {
  159           map = new LinkedHashMap<>(initialCapacity, loadFactor);
  160       }
  161   
  162       /**
  163        * Returns an iterator over the elements in this set.  The elements
  164        * are returned in no particular order.
  165        *
  166        * @return an Iterator over the elements in this set
  167        * @see ConcurrentModificationException
  168        */
  169       public Iterator<E> iterator() {
  170           return map.keySet().iterator();
  171       }
  172   
  173       /**
  174        * Returns the number of elements in this set (its cardinality).
  175        *
  176        * @return the number of elements in this set (its cardinality)
  177        */
  178       public int size() {
  179           return map.size();
  180       }
  181   
  182       /**
  183        * Returns <tt>true</tt> if this set contains no elements.
  184        *
  185        * @return <tt>true</tt> if this set contains no elements
  186        */
  187       public boolean isEmpty() {
  188           return map.isEmpty();
  189       }
  190   
  191       /**
  192        * Returns <tt>true</tt> if this set contains the specified element.
  193        * More formally, returns <tt>true</tt> if and only if this set
  194        * contains an element <tt>e</tt> such that
  195        * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
  196        *
  197        * @param o element whose presence in this set is to be tested
  198        * @return <tt>true</tt> if this set contains the specified element
  199        */
  200       public boolean contains(Object o) {
  201           return map.containsKey(o);
  202       }
  203   
  204       /**
  205        * Adds the specified element to this set if it is not already present.
  206        * More formally, adds the specified element <tt>e</tt> to this set if
  207        * this set contains no element <tt>e2</tt> such that
  208        * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
  209        * If this set already contains the element, the call leaves the set
  210        * unchanged and returns <tt>false</tt>.
  211        *
  212        * @param e element to be added to this set
  213        * @return <tt>true</tt> if this set did not already contain the specified
  214        * element
  215        */
  216       public boolean add(E e) {
  217           return map.put(e, PRESENT)==null;
  218       }
  219   
  220       /**
  221        * Removes the specified element from this set if it is present.
  222        * More formally, removes an element <tt>e</tt> such that
  223        * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
  224        * if this set contains such an element.  Returns <tt>true</tt> if
  225        * this set contained the element (or equivalently, if this set
  226        * changed as a result of the call).  (This set will not contain the
  227        * element once the call returns.)
  228        *
  229        * @param o object to be removed from this set, if present
  230        * @return <tt>true</tt> if the set contained the specified element
  231        */
  232       public boolean remove(Object o) {
  233           return map.remove(o)==PRESENT;
  234       }
  235   
  236       /**
  237        * Removes all of the elements from this set.
  238        * The set will be empty after this call returns.
  239        */
  240       public void clear() {
  241           map.clear();
  242       }
  243   
  244       /**
  245        * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
  246        * themselves are not cloned.
  247        *
  248        * @return a shallow copy of this set
  249        */
  250       public Object clone() {
  251           try {
  252               HashSet<E> newSet = (HashSet<E>) super.clone();
  253               newSet.map = (HashMap<E, Object>) map.clone();
  254               return newSet;
  255           } catch (CloneNotSupportedException e) {
  256               throw new InternalError();
  257           }
  258       }
  259   
  260       /**
  261        * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
  262        * serialize it).
  263        *
  264        * @serialData The capacity of the backing <tt>HashMap</tt> instance
  265        *             (int), and its load factor (float) are emitted, followed by
  266        *             the size of the set (the number of elements it contains)
  267        *             (int), followed by all of its elements (each an Object) in
  268        *             no particular order.
  269        */
  270       private void writeObject(java.io.ObjectOutputStream s)
  271           throws java.io.IOException {
  272           // Write out any hidden serialization magic
  273           s.defaultWriteObject();
  274   
  275           // Write out HashMap capacity and load factor
  276           s.writeInt(map.capacity());
  277           s.writeFloat(map.loadFactor());
  278   
  279           // Write out size
  280           s.writeInt(map.size());
  281   
  282           // Write out all elements in the proper order.
  283           for (E e : map.keySet())
  284               s.writeObject(e);
  285       }
  286   
  287       /**
  288        * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
  289        * deserialize it).
  290        */
  291       private void readObject(java.io.ObjectInputStream s)
  292           throws java.io.IOException, ClassNotFoundException {
  293           // Read in any hidden serialization magic
  294           s.defaultReadObject();
  295   
  296           // Read in HashMap capacity and load factor and create backing HashMap
  297           int capacity = s.readInt();
  298           float loadFactor = s.readFloat();
  299           map = (((HashSet)this) instanceof LinkedHashSet ?
  300                  new LinkedHashMap<E,Object>(capacity, loadFactor) :
  301                  new HashMap<E,Object>(capacity, loadFactor));
  302   
  303           // Read in size
  304           int size = s.readInt();
  305   
  306           // Read in all elements in the proper order.
  307           for (int i=0; i<size; i++) {
  308               E e = (E) s.readObject();
  309               map.put(e, PRESENT);
  310           }
  311       }
  312   }

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