1 /*
   2  * Copyright (c) 1997, 2012, 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
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   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     @Override
 251     public HashSet<E> clone() {
 252         try {
 253             @SuppressWarnings("unchecked")
 254             HashSet<E> newSet = (HashSet<E>) super.clone();
 255             newSet.map = (HashMap<E, Object>) map.clone();
 256             return newSet;
 257         } catch (CloneNotSupportedException e) {
 258             throw new InternalError(e);
 259         }
 260     }
 261 
 262     /**
 263      * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
 264      * serialize it).
 265      *
 266      * @serialData The capacity of the backing <tt>HashMap</tt> instance
 267      *             (int), and its load factor (float) are emitted, followed by
 268      *             the size of the set (the number of elements it contains)
 269      *             (int), followed by all of its elements (each an Object) in
 270      *             no particular order.
 271      */
 272     private void writeObject(java.io.ObjectOutputStream s)
 273         throws java.io.IOException {
 274         // Write out any hidden serialization magic
 275         s.defaultWriteObject();
 276 
 277         // Write out HashMap capacity and load factor
 278         s.writeInt(map.capacity());
 279         s.writeFloat(map.loadFactor());
 280 
 281         // Write out size
 282         s.writeInt(map.size());
 283 
 284         // Write out all elements in the proper order.
 285         for (E e : map.keySet())
 286             s.writeObject(e);
 287     }
 288 
 289     /**
 290      * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
 291      * deserialize it).
 292      */
 293     private void readObject(java.io.ObjectInputStream s)
 294         throws java.io.IOException, ClassNotFoundException {
 295         // Read in any hidden serialization magic
 296         s.defaultReadObject();
 297 
 298         // Read in HashMap capacity and load factor and create backing HashMap
 299         int capacity = s.readInt();
 300         float loadFactor = s.readFloat();
 301         map = (((HashSet)this) instanceof LinkedHashSet ?
 302                new LinkedHashMap<E,Object>(capacity, loadFactor) :
 303                new HashMap<E,Object>(capacity, loadFactor));
 304 
 305         // Read in size
 306         int size = s.readInt();
 307 
 308         // Read in all elements in the proper order.
 309         for (int i=0; i<size; i++) {
 310             @SuppressWarnings("unchecked")
 311             E e = (E) s.readObject();
 312             map.put(e, PRESENT);
 313         }
 314     }
 315 }