1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent;
37 import java.util.*;
38
39 /**
40 * A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList}
41 * for all of its operations. Thus, it shares the same basic properties:
42 * <ul>
43 * <li>It is best suited for applications in which set sizes generally
44 * stay small, read-only operations
45 * vastly outnumber mutative operations, and you need
46 * to prevent interference among threads during traversal.
47 * <li>It is thread-safe.
48 * <li>Mutative operations (<tt>add</tt>, <tt>set</tt>, <tt>remove</tt>, etc.)
49 * are expensive since they usually entail copying the entire underlying
50 * array.
51 * <li>Iterators do not support the mutative <tt>remove</tt> operation.
52 * <li>Traversal via iterators is fast and cannot encounter
53 * interference from other threads. Iterators rely on
54 * unchanging snapshots of the array at the time the iterators were
55 * constructed.
56 * </ul>
57 *
58 * <p> <b>Sample Usage.</b> The following code sketch uses a
59 * copy-on-write set to maintain a set of Handler objects that
60 * perform some action upon state updates.
61 *
62 * <pre> {@code
63 * class Handler { void handle(); ... }
64 *
65 * class X {
66 * private final CopyOnWriteArraySet<Handler> handlers
67 * = new CopyOnWriteArraySet<Handler>();
68 * public void addHandler(Handler h) { handlers.add(h); }
69 *
70 * private long internalState;
71 * private synchronized void changeState() { internalState = ...; }
72 *
73 * public void update() {
74 * changeState();
75 * for (Handler handler : handlers)
76 * handler.handle();
77 * }
78 * }}</pre>
79 *
80 * <p>This class is a member of the
81 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
82 * Java Collections Framework</a>.
83 *
84 * @see CopyOnWriteArrayList
85 * @since 1.5
86 * @author Doug Lea
87 * @param <E> the type of elements held in this collection
88 */
89 public class CopyOnWriteArraySet<E> extends AbstractSet<E>
90 implements java.io.Serializable {
91 private static final long serialVersionUID = 5457747651344034263L;
92
93 private final CopyOnWriteArrayList<E> al;
94
95 /**
96 * Creates an empty set.
97 */
98 public CopyOnWriteArraySet() {
99 al = new CopyOnWriteArrayList<E>();
100 }
101
102 /**
103 * Creates a set containing all of the elements of the specified
104 * collection.
105 *
106 * @param c the collection of elements to initially contain
107 * @throws NullPointerException if the specified collection is null
108 */
109 public CopyOnWriteArraySet(Collection<? extends E> c) {
110 if (c.getClass() == CopyOnWriteArraySet.class) {
111 CopyOnWriteArraySet<E> s = (CopyOnWriteArraySet<E>) c;
112 al = (CopyOnWriteArrayList<E>) s.al.clone();
113 } else if (c instanceof Set) {
114 al = new CopyOnWriteArrayList<E>(c);
115 } else {
116 al = new CopyOnWriteArrayList<E>();
117 al.addAllAbsent(c);
118 }
119 }
120
121 /**
122 * Returns the number of elements in this set.
123 *
124 * @return the number of elements in this set
125 */
126 public int size() {
127 return al.size();
128 }
129
130 /**
131 * Returns <tt>true</tt> if this set contains no elements.
132 *
133 * @return <tt>true</tt> if this set contains no elements
134 */
135 public boolean isEmpty() {
136 return al.isEmpty();
137 }
138
139 /**
140 * Returns <tt>true</tt> if this set contains the specified element.
141 * More formally, returns <tt>true</tt> if and only if this set
142 * contains an element <tt>e</tt> such that
143 * <tt>(o==null ? e==null : o.equals(e))</tt>.
144 *
145 * @param o element whose presence in this set is to be tested
146 * @return <tt>true</tt> if this set contains the specified element
147 */
148 public boolean contains(Object o) {
149 return al.contains(o);
150 }
151
152 /**
153 * Returns an array containing all of the elements in this set.
154 * If this set makes any guarantees as to what order its elements
155 * are returned by its iterator, this method must return the
156 * elements in the same order.
157 *
158 * <p>The returned array will be "safe" in that no references to it
159 * are maintained by this set. (In other words, this method must
160 * allocate a new array even if this set is backed by an array).
161 * The caller is thus free to modify the returned array.
162 *
163 * <p>This method acts as bridge between array-based and collection-based
164 * APIs.
165 *
166 * @return an array containing all the elements in this set
167 */
168 public Object[] toArray() {
169 return al.toArray();
170 }
171
172 /**
173 * Returns an array containing all of the elements in this set; the
174 * runtime type of the returned array is that of the specified array.
175 * If the set fits in the specified array, it is returned therein.
176 * Otherwise, a new array is allocated with the runtime type of the
177 * specified array and the size of this set.
178 *
179 * <p>If this set fits in the specified array with room to spare
180 * (i.e., the array has more elements than this set), the element in
181 * the array immediately following the end of the set is set to
182 * <tt>null</tt>. (This is useful in determining the length of this
183 * set <i>only</i> if the caller knows that this set does not contain
184 * any null elements.)
185 *
186 * <p>If this set makes any guarantees as to what order its elements
187 * are returned by its iterator, this method must return the elements
188 * in the same order.
189 *
190 * <p>Like the {@link #toArray()} method, this method acts as bridge between
191 * array-based and collection-based APIs. Further, this method allows
192 * precise control over the runtime type of the output array, and may,
193 * under certain circumstances, be used to save allocation costs.
194 *
195 * <p>Suppose <tt>x</tt> is a set known to contain only strings.
196 * The following code can be used to dump the set into a newly allocated
197 * array of <tt>String</tt>:
198 *
199 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
200 *
201 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
202 * <tt>toArray()</tt>.
203 *
204 * @param a the array into which the elements of this set are to be
205 * stored, if it is big enough; otherwise, a new array of the same
206 * runtime type is allocated for this purpose.
207 * @return an array containing all the elements in this set
208 * @throws ArrayStoreException if the runtime type of the specified array
209 * is not a supertype of the runtime type of every element in this
210 * set
211 * @throws NullPointerException if the specified array is null
212 */
213 public <T> T[] toArray(T[] a) {
214 return al.toArray(a);
215 }
216
217 /**
218 * Removes all of the elements from this set.
219 * The set will be empty after this call returns.
220 */
221 public void clear() {
222 al.clear();
223 }
224
225 /**
226 * Removes the specified element from this set if it is present.
227 * More formally, removes an element <tt>e</tt> such that
228 * <tt>(o==null ? e==null : o.equals(e))</tt>,
229 * if this set contains such an element. Returns <tt>true</tt> if
230 * this set contained the element (or equivalently, if this set
231 * changed as a result of the call). (This set will not contain the
232 * element once the call returns.)
233 *
234 * @param o object to be removed from this set, if present
235 * @return <tt>true</tt> if this set contained the specified element
236 */
237 public boolean remove(Object o) {
238 return al.remove(o);
239 }
240
241 /**
242 * Adds the specified element to this set if it is not already present.
243 * More formally, adds the specified element <tt>e</tt> to this set if
244 * the set contains no element <tt>e2</tt> such that
245 * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
246 * If this set already contains the element, the call leaves the set
247 * unchanged and returns <tt>false</tt>.
248 *
249 * @param e element to be added to this set
250 * @return <tt>true</tt> if this set did not already contain the specified
251 * element
252 */
253 public boolean add(E e) {
254 return al.addIfAbsent(e);
255 }
256
257 /**
258 * Returns <tt>true</tt> if this set contains all of the elements of the
259 * specified collection. If the specified collection is also a set, this
260 * method returns <tt>true</tt> if it is a <i>subset</i> of this set.
261 *
262 * @param c collection to be checked for containment in this set
263 * @return <tt>true</tt> if this set contains all of the elements of the
264 * specified collection
265 * @throws NullPointerException if the specified collection is null
266 * @see #contains(Object)
267 */
268 public boolean containsAll(Collection<?> c) {
269 return al.containsAll(c);
270 }
271
272 /**
273 * Adds all of the elements in the specified collection to this set if
274 * they're not already present. If the specified collection is also a
275 * set, the <tt>addAll</tt> operation effectively modifies this set so
276 * that its value is the <i>union</i> of the two sets. The behavior of
277 * this operation is undefined if the specified collection is modified
278 * while the operation is in progress.
279 *
280 * @param c collection containing elements to be added to this set
281 * @return <tt>true</tt> if this set changed as a result of the call
282 * @throws NullPointerException if the specified collection is null
283 * @see #add(Object)
284 */
285 public boolean addAll(Collection<? extends E> c) {
286 return al.addAllAbsent(c) > 0;
287 }
288
289 /**
290 * Removes from this set all of its elements that are contained in the
291 * specified collection. If the specified collection is also a set,
292 * this operation effectively modifies this set so that its value is the
293 * <i>asymmetric set difference</i> of the two sets.
294 *
295 * @param c collection containing elements to be removed from this set
296 * @return <tt>true</tt> if this set changed as a result of the call
297 * @throws ClassCastException if the class of an element of this set
298 * is incompatible with the specified collection (optional)
299 * @throws NullPointerException if this set contains a null element and the
300 * specified collection does not permit null elements (optional),
301 * or if the specified collection is null
302 * @see #remove(Object)
303 */
304 public boolean removeAll(Collection<?> c) {
305 return al.removeAll(c);
306 }
307
308 /**
309 * Retains only the elements in this set that are contained in the
310 * specified collection. In other words, removes from this set all of
311 * its elements that are not contained in the specified collection. If
312 * the specified collection is also a set, this operation effectively
313 * modifies this set so that its value is the <i>intersection</i> of the
314 * two sets.
315 *
316 * @param c collection containing elements to be retained in this set
317 * @return <tt>true</tt> if this set changed as a result of the call
318 * @throws ClassCastException if the class of an element of this set
319 * is incompatible with the specified collection (optional)
320 * @throws NullPointerException if this set contains a null element and the
321 * specified collection does not permit null elements (optional),
322 * or if the specified collection is null
323 * @see #remove(Object)
324 */
325 public boolean retainAll(Collection<?> c) {
326 return al.retainAll(c);
327 }
328
329 /**
330 * Returns an iterator over the elements contained in this set
331 * in the order in which these elements were added.
332 *
333 * <p>The returned iterator provides a snapshot of the state of the set
334 * when the iterator was constructed. No synchronization is needed while
335 * traversing the iterator. The iterator does <em>NOT</em> support the
336 * <tt>remove</tt> method.
337 *
338 * @return an iterator over the elements in this set
339 */
340 public Iterator<E> iterator() {
341 return al.iterator();
342 }
343
344 /**
345 * Compares the specified object with this set for equality.
346 * Returns {@code true} if the specified object is the same object
347 * as this object, or if it is also a {@link Set} and the elements
348 * returned by an {@linkplain List#iterator() iterator} over the
349 * specified set are the same as the elements returned by an
350 * iterator over this set. More formally, the two iterators are
351 * considered to return the same elements if they return the same
352 * number of elements and for every element {@code e1} returned by
353 * the iterator over the specified set, there is an element
354 * {@code e2} returned by the iterator over this set such that
355 * {@code (e1==null ? e2==null : e1.equals(e2))}.
356 *
357 * @param o object to be compared for equality with this set
358 * @return {@code true} if the specified object is equal to this set
359 */
360 public boolean equals(Object o) {
361 if (o == this)
362 return true;
363 if (!(o instanceof Set))
364 return false;
365 Set<?> set = (Set<?>)(o);
366 Iterator<?> it = set.iterator();
367
368 // Uses O(n^2) algorithm that is only appropriate
369 // for small sets, which CopyOnWriteArraySets should be.
370
371 // Use a single snapshot of underlying array
372 Object[] elements = al.getArray();
373 int len = elements.length;
374 // Mark matched elements to avoid re-checking
375 boolean[] matched = new boolean[len];
376 int k = 0;
377 outer: while (it.hasNext()) {
378 if (++k > len)
379 return false;
380 Object x = it.next();
381 for (int i = 0; i < len; ++i) {
382 if (!matched[i] && eq(x, elements[i])) {
383 matched[i] = true;
384 continue outer;
385 }
386 }
387 return false;
388 }
389 return k == len;
390 }
391
392 /**
393 * Tests for equality, coping with nulls.
394 */
395 private static boolean eq(Object o1, Object o2) {
396 return (o1 == null) ? o2 == null : o1.equals(o2);
397 }
398 }