1 /*

     2  * Copyright 1997-2006 Sun Microsystems, Inc.  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.  Sun designates this

     8  * particular file as subject to the "Classpath" exception as provided

     9  * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

    22  * CA 95054 USA or visit www.sun.com if you need additional information or

    23  * have any 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<E,Object>();

   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<E,Object>(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<E,Object>(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<E,Object>(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<E,Object>(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 (Iterator i=map.keySet().iterator(); i.hasNext(); )

   284             s.writeObject(i.next());

   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 }