001    /* HashSet.java -- a class providing a HashMap-backed Set
002       Copyright (C) 1998, 1999, 2001, 2004, 2005  Free Software Foundation, Inc.
003    
004    This file is part of GNU Classpath.
005    
006    GNU Classpath is free software; you can redistribute it and/or modify
007    it under the terms of the GNU General Public License as published by
008    the Free Software Foundation; either version 2, or (at your option)
009    any later version.
010    
011    GNU Classpath is distributed in the hope that it will be useful, but
012    WITHOUT ANY WARRANTY; without even the implied warranty of
013    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014    General Public License for more details.
015    
016    You should have received a copy of the GNU General Public License
017    along with GNU Classpath; see the file COPYING.  If not, write to the
018    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
019    02110-1301 USA.
020    
021    Linking this library statically or dynamically with other modules is
022    making a combined work based on this library.  Thus, the terms and
023    conditions of the GNU General Public License cover the whole
024    combination.
025    
026    As a special exception, the copyright holders of this library give you
027    permission to link this library with independent modules to produce an
028    executable, regardless of the license terms of these independent
029    modules, and to copy and distribute the resulting executable under
030    terms of your choice, provided that you also meet, for each linked
031    independent module, the terms and conditions of the license of that
032    module.  An independent module is a module which is not derived from
033    or based on this library.  If you modify this library, you may extend
034    this exception to your version of the library, but you are not
035    obligated to do so.  If you do not wish to do so, delete this
036    exception statement from your version. */
037    
038    
039    package java.util;
040    
041    import java.io.IOException;
042    import java.io.ObjectInputStream;
043    import java.io.ObjectOutputStream;
044    import java.io.Serializable;
045    
046    /**
047     * This class provides a HashMap-backed implementation of the Set interface.
048     * <p>
049     *
050     * Most operations are O(1), assuming no hash collisions.  In the worst
051     * case (where all hashes collide), operations are O(n). Setting the
052     * initial capacity too low will force many resizing operations, but
053     * setting the initial capacity too high (or loadfactor too low) leads
054     * to wasted memory and slower iteration.
055     * <p>
056     *
057     * HashSet accepts the null key and null values.  It is not synchronized,
058     * so if you need multi-threaded access, consider using:<br>
059     * <code>Set s = Collections.synchronizedSet(new HashSet(...));</code>
060     * <p>
061     *
062     * The iterators are <i>fail-fast</i>, meaning that any structural
063     * modification, except for <code>remove()</code> called on the iterator
064     * itself, cause the iterator to throw a
065     * {@link ConcurrentModificationException} rather than exhibit
066     * non-deterministic behavior.
067     *
068     * @author Jon Zeppieri
069     * @author Eric Blake (ebb9@email.byu.edu)
070     * @see Collection
071     * @see Set
072     * @see TreeSet
073     * @see Collections#synchronizedSet(Set)
074     * @see HashMap
075     * @see LinkedHashSet
076     * @since 1.2
077     * @status updated to 1.4
078     */
079    public class HashSet<T> extends AbstractSet<T>
080      implements Set<T>, Cloneable, Serializable
081    {
082      /**
083       * Compatible with JDK 1.2.
084       */
085      private static final long serialVersionUID = -5024744406713321676L;
086    
087      /**
088       * The HashMap which backs this Set.
089       */
090      private transient HashMap<T, String> map;
091    
092      /**
093       * Construct a new, empty HashSet whose backing HashMap has the default
094       * capacity (11) and loadFacor (0.75).
095       */
096      public HashSet()
097      {
098        this(HashMap.DEFAULT_CAPACITY, HashMap.DEFAULT_LOAD_FACTOR);
099      }
100    
101      /**
102       * Construct a new, empty HashSet whose backing HashMap has the supplied
103       * capacity and the default load factor (0.75).
104       *
105       * @param initialCapacity the initial capacity of the backing HashMap
106       * @throws IllegalArgumentException if the capacity is negative
107       */
108      public HashSet(int initialCapacity)
109      {
110        this(initialCapacity, HashMap.DEFAULT_LOAD_FACTOR);
111      }
112    
113      /**
114       * Construct a new, empty HashSet whose backing HashMap has the supplied
115       * capacity and load factor.
116       *
117       * @param initialCapacity the initial capacity of the backing HashMap
118       * @param loadFactor the load factor of the backing HashMap
119       * @throws IllegalArgumentException if either argument is negative, or
120       *         if loadFactor is POSITIVE_INFINITY or NaN
121       */
122      public HashSet(int initialCapacity, float loadFactor)
123      {
124        map = init(initialCapacity, loadFactor);
125      }
126    
127      /**
128       * Construct a new HashSet with the same elements as are in the supplied
129       * collection (eliminating any duplicates, of course). The backing storage
130       * has twice the size of the collection, or the default size of 11,
131       * whichever is greater; and the default load factor (0.75).
132       *
133       * @param c a collection of initial set elements
134       * @throws NullPointerException if c is null
135       */
136      public HashSet(Collection<? extends T> c)
137      {
138        this(Math.max(2 * c.size(), HashMap.DEFAULT_CAPACITY));
139        addAll(c);
140      }
141    
142      /**
143       * Adds the given Object to the set if it is not already in the Set.
144       * This set permits a null element.
145       *
146       * @param o the Object to add to this Set
147       * @return true if the set did not already contain o
148       */
149      public boolean add(T o)
150      {
151        return map.put(o, "") == null;
152      }
153    
154      /**
155       * Empties this Set of all elements; this takes constant time.
156       */
157      public void clear()
158      {
159        map.clear();
160      }
161    
162      /**
163       * Returns a shallow copy of this Set. The Set itself is cloned; its
164       * elements are not.
165       *
166       * @return a shallow clone of the set
167       */
168      public Object clone()
169      {
170        HashSet<T> copy = null;
171        try
172          {
173            copy = (HashSet<T>) super.clone();
174          }
175        catch (CloneNotSupportedException x)
176          {
177            // Impossible to get here.
178          }
179        copy.map = (HashMap<T, String>) map.clone();
180        return copy;
181      }
182    
183      /**
184       * Returns true if the supplied element is in this Set.
185       *
186       * @param o the Object to look for
187       * @return true if it is in the set
188       */
189      public boolean contains(Object o)
190      {
191        return map.containsKey(o);
192      }
193    
194      /**
195       * Returns true if this set has no elements in it.
196       *
197       * @return <code>size() == 0</code>.
198       */
199      public boolean isEmpty()
200      {
201        return map.size == 0;
202      }
203    
204      /**
205       * Returns an Iterator over the elements of this Set, which visits the
206       * elements in no particular order.  For this class, the Iterator allows
207       * removal of elements. The iterator is fail-fast, and will throw a
208       * ConcurrentModificationException if the set is modified externally.
209       *
210       * @return a set iterator
211       * @see ConcurrentModificationException
212       */
213      public Iterator<T> iterator()
214      {
215        // Avoid creating intermediate keySet() object by using non-public API.
216        return map.iterator(HashMap.KEYS);
217      }
218    
219      /**
220       * Removes the supplied Object from this Set if it is in the Set.
221       *
222       * @param o the object to remove
223       * @return true if an element was removed
224       */
225      public boolean remove(Object o)
226      {
227        return (map.remove(o) != null);
228      }
229    
230      /**
231       * Returns the number of elements in this Set (its cardinality).
232       *
233       * @return the size of the set
234       */
235      public int size()
236      {
237        return map.size;
238      }
239    
240      /**
241       * Helper method which initializes the backing Map. Overridden by
242       * LinkedHashSet for correct semantics.
243       *
244       * @param capacity the initial capacity
245       * @param load the initial load factor
246       * @return the backing HashMap
247       */
248      HashMap init(int capacity, float load)
249      {
250        return new HashMap(capacity, load);
251      }
252    
253      /**
254       * Serializes this object to the given stream.
255       *
256       * @param s the stream to write to
257       * @throws IOException if the underlying stream fails
258       * @serialData the <i>capacity</i> (int) and <i>loadFactor</i> (float)
259       *             of the backing store, followed by the set size (int),
260       *             then a listing of its elements (Object) in no order
261       */
262      private void writeObject(ObjectOutputStream s) throws IOException
263      {
264        s.defaultWriteObject();
265        // Avoid creating intermediate keySet() object by using non-public API.
266        Iterator<T> it = map.iterator(HashMap.KEYS);
267        s.writeInt(map.buckets.length);
268        s.writeFloat(map.loadFactor);
269        s.writeInt(map.size);
270        while (it.hasNext())
271          s.writeObject(it.next());
272      }
273    
274      /**
275       * Deserializes this object from the given stream.
276       *
277       * @param s the stream to read from
278       * @throws ClassNotFoundException if the underlying stream fails
279       * @throws IOException if the underlying stream fails
280       * @serialData the <i>capacity</i> (int) and <i>loadFactor</i> (float)
281       *             of the backing store, followed by the set size (int),
282       *             then a listing of its elements (Object) in no order
283       */
284      private void readObject(ObjectInputStream s)
285        throws IOException, ClassNotFoundException
286      {
287        s.defaultReadObject();
288    
289        map = init(s.readInt(), s.readFloat());
290        for (int size = s.readInt(); size > 0; size--)
291          map.put((T) s.readObject(), "");
292      }
293    }