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 ? 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 (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 }