001: /*
002: * Copyright 1997-2006 Sun Microsystems, Inc. All Rights Reserved.
003: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004: *
005: * This code is free software; you can redistribute it and/or modify it
006: * under the terms of the GNU General Public License version 2 only, as
007: * published by the Free Software Foundation. Sun designates this
008: * particular file as subject to the "Classpath" exception as provided
009: * by Sun in the LICENSE file that accompanied this code.
010: *
011: * This code is distributed in the hope that it will be useful, but WITHOUT
012: * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013: * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
014: * version 2 for more details (a copy is included in the LICENSE file that
015: * accompanied this code).
016: *
017: * You should have received a copy of the GNU General Public License version
018: * 2 along with this work; if not, write to the Free Software Foundation,
019: * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020: *
021: * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
022: * CA 95054 USA or visit www.sun.com if you need additional information or
023: * have any questions.
024: */
025:
026: package java.util;
027:
028: /**
029: * This class implements the <tt>Set</tt> interface, backed by a hash table
030: * (actually a <tt>HashMap</tt> instance). It makes no guarantees as to the
031: * iteration order of the set; in particular, it does not guarantee that the
032: * order will remain constant over time. This class permits the <tt>null</tt>
033: * element.
034: *
035: * <p>This class offers constant time performance for the basic operations
036: * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),
037: * assuming the hash function disperses the elements properly among the
038: * buckets. Iterating over this set requires time proportional to the sum of
039: * the <tt>HashSet</tt> instance's size (the number of elements) plus the
040: * "capacity" of the backing <tt>HashMap</tt> instance (the number of
041: * buckets). Thus, it's very important not to set the initial capacity too
042: * high (or the load factor too low) if iteration performance is important.
043: *
044: * <p><strong>Note that this implementation is not synchronized.</strong>
045: * If multiple threads access a hash set concurrently, and at least one of
046: * the threads modifies the set, it <i>must</i> be synchronized externally.
047: * This is typically accomplished by synchronizing on some object that
048: * naturally encapsulates the set.
049: *
050: * If no such object exists, the set should be "wrapped" using the
051: * {@link Collections#synchronizedSet Collections.synchronizedSet}
052: * method. This is best done at creation time, to prevent accidental
053: * unsynchronized access to the set:<pre>
054: * Set s = Collections.synchronizedSet(new HashSet(...));</pre>
055: *
056: * <p>The iterators returned by this class's <tt>iterator</tt> method are
057: * <i>fail-fast</i>: if the set is modified at any time after the iterator is
058: * created, in any way except through the iterator's own <tt>remove</tt>
059: * method, the Iterator throws a {@link ConcurrentModificationException}.
060: * Thus, in the face of concurrent modification, the iterator fails quickly
061: * and cleanly, rather than risking arbitrary, non-deterministic behavior at
062: * an undetermined time in the future.
063: *
064: * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
065: * as it is, generally speaking, impossible to make any hard guarantees in the
066: * presence of unsynchronized concurrent modification. Fail-fast iterators
067: * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
068: * Therefore, it would be wrong to write a program that depended on this
069: * exception for its correctness: <i>the fail-fast behavior of iterators
070: * should be used only to detect bugs.</i>
071: *
072: * <p>This class is a member of the
073: * <a href="{@docRoot}/../technotes/guides/collections/index.html">
074: * Java Collections Framework</a>.
075: *
076: * @param <E> the type of elements maintained by this set
077: *
078: * @author Josh Bloch
079: * @author Neal Gafter
080: * @version 1.43, 05/05/07
081: * @see Collection
082: * @see Set
083: * @see TreeSet
084: * @see HashMap
085: * @since 1.2
086: */
087:
088: public class HashSet<E> extends AbstractSet<E> implements Set<E>,
089: Cloneable, java.io.Serializable {
090: static final long serialVersionUID = -5024744406713321676L;
091:
092: private transient HashMap<E, Object> map;
093:
094: // Dummy value to associate with an Object in the backing Map
095: private static final Object PRESENT = new Object();
096:
097: /**
098: * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
099: * default initial capacity (16) and load factor (0.75).
100: */
101: public HashSet() {
102: map = new HashMap<E, Object>();
103: }
104:
105: /**
106: * Constructs a new set containing the elements in the specified
107: * collection. The <tt>HashMap</tt> is created with default load factor
108: * (0.75) and an initial capacity sufficient to contain the elements in
109: * the specified collection.
110: *
111: * @param c the collection whose elements are to be placed into this set
112: * @throws NullPointerException if the specified collection is null
113: */
114: public HashSet(Collection<? extends E> c) {
115: map = new HashMap<E, Object>(Math.max(
116: (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 ? e==null : 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 ? e2==null : 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 ? e==null : 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 ? new LinkedHashMap<E, Object>(
300: 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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