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.Collection;
38 import java.util.Set;
39 import java.util.AbstractSet;
40 import java.util.Iterator;
41 import java.util.Spliterator;
42 import java.util.Spliterators;
43 import java.util.function.Predicate;
44 import java.util.function.Consumer;
45
46 /**
47 * A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList}
48 * for all of its operations. Thus, it shares the same basic properties:
49 * <ul>
50 * <li>It is best suited for applications in which set sizes generally
51 * stay small, read-only operations
52 * vastly outnumber mutative operations, and you need
53 * to prevent interference among threads during traversal.
54 * <li>It is thread-safe.
55 * <li>Mutative operations ({@code add}, {@code set}, {@code remove}, etc.)
56 * are expensive since they usually entail copying the entire underlying
57 * array.
58 * <li>Iterators do not support the mutative {@code remove} operation.
59 * <li>Traversal via iterators is fast and cannot encounter
60 * interference from other threads. Iterators rely on
61 * unchanging snapshots of the array at the time the iterators were
62 * constructed.
63 * </ul>
64 *
65 * <p><b>Sample Usage.</b> The following code sketch uses a
66 * copy-on-write set to maintain a set of Handler objects that
67 * perform some action upon state updates.
68 *
69 * <pre> {@code
70 * class Handler { void handle(); ... }
71 *
72 * class X {
73 * private final CopyOnWriteArraySet<Handler> handlers
74 * = new CopyOnWriteArraySet<Handler>();
75 * public void addHandler(Handler h) { handlers.add(h); }
76 *
77 * private long internalState;
78 * private synchronized void changeState() { internalState = ...; }
79 *
80 * public void update() {
81 * changeState();
82 * for (Handler handler : handlers)
83 * handler.handle();
84 * }
85 * }}</pre>
86 *
87 * <p>This class is a member of the
88 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
89 * Java Collections Framework</a>.
90 *
91 * @see CopyOnWriteArrayList
92 * @since 1.5
93 * @author Doug Lea
94 * @param <E> the type of elements held in this collection
95 */
96 public class CopyOnWriteArraySet<E> extends AbstractSet<E>
97 implements java.io.Serializable {
98 private static final long serialVersionUID = 5457747651344034263L;
99
100 private final CopyOnWriteArrayList<E> al;
101
102 /**
103 * Creates an empty set.
104 */
105 public CopyOnWriteArraySet() {
106 al = new CopyOnWriteArrayList<E>();
107 }
108
109 /**
110 * Creates a set containing all of the elements of the specified
111 * collection.
112 *
113 * @param c the collection of elements to initially contain
114 * @throws NullPointerException if the specified collection is null
115 */
116 public CopyOnWriteArraySet(Collection<? extends E> c) {
117 if (c.getClass() == CopyOnWriteArraySet.class) {
118 @SuppressWarnings("unchecked") CopyOnWriteArraySet<E> cc =
119 (CopyOnWriteArraySet<E>)c;
120 al = new CopyOnWriteArrayList<E>(cc.al);
121 }
122 else {
123 al = new CopyOnWriteArrayList<E>();
124 al.addAllAbsent(c);
125 }
126 }
127
128 /**
129 * Returns the number of elements in this set.
130 *
131 * @return the number of elements in this set
132 */
133 public int size() {
134 return al.size();
135 }
136
137 /**
138 * Returns {@code true} if this set contains no elements.
139 *
140 * @return {@code true} if this set contains no elements
141 */
142 public boolean isEmpty() {
143 return al.isEmpty();
144 }
145
146 /**
147 * Returns {@code true} if this set contains the specified element.
148 * More formally, returns {@code true} if and only if this set
149 * contains an element {@code e} such that
150 * <tt>(o==null ? e==null : o.equals(e))</tt>.
151 *
152 * @param o element whose presence in this set is to be tested
153 * @return {@code true} if this set contains the specified element
154 */
155 public boolean contains(Object o) {
156 return al.contains(o);
157 }
158
159 /**
160 * Returns an array containing all of the elements in this set.
161 * If this set makes any guarantees as to what order its elements
162 * are returned by its iterator, this method must return the
163 * elements in the same order.
164 *
165 * <p>The returned array will be "safe" in that no references to it
166 * are maintained by this set. (In other words, this method must
167 * allocate a new array even if this set is backed by an array).
168 * The caller is thus free to modify the returned array.
169 *
170 * <p>This method acts as bridge between array-based and collection-based
171 * APIs.
172 *
173 * @return an array containing all the elements in this set
174 */
175 public Object[] toArray() {
176 return al.toArray();
177 }
178
179 /**
180 * Returns an array containing all of the elements in this set; the
181 * runtime type of the returned array is that of the specified array.
182 * If the set fits in the specified array, it is returned therein.
183 * Otherwise, a new array is allocated with the runtime type of the
184 * specified array and the size of this set.
185 *
186 * <p>If this set fits in the specified array with room to spare
187 * (i.e., the array has more elements than this set), the element in
188 * the array immediately following the end of the set is set to
189 * {@code null}. (This is useful in determining the length of this
190 * set <i>only</i> if the caller knows that this set does not contain
191 * any null elements.)
192 *
193 * <p>If this set makes any guarantees as to what order its elements
194 * are returned by its iterator, this method must return the elements
195 * in the same order.
196 *
197 * <p>Like the {@link #toArray()} method, this method acts as bridge between
198 * array-based and collection-based APIs. Further, this method allows
199 * precise control over the runtime type of the output array, and may,
200 * under certain circumstances, be used to save allocation costs.
201 *
202 * <p>Suppose {@code x} is a set known to contain only strings.
203 * The following code can be used to dump the set into a newly allocated
204 * array of {@code String}:
205 *
206 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
207 *
208 * Note that {@code toArray(new Object[0])} is identical in function to
209 * {@code toArray()}.
210 *
211 * @param a the array into which the elements of this set are to be
212 * stored, if it is big enough; otherwise, a new array of the same
213 * runtime type is allocated for this purpose.
214 * @return an array containing all the elements in this set
215 * @throws ArrayStoreException if the runtime type of the specified array
216 * is not a supertype of the runtime type of every element in this
217 * set
218 * @throws NullPointerException if the specified array is null
219 */
220 public <T> T[] toArray(T[] a) {
221 return al.toArray(a);
222 }
223
224 /**
225 * Removes all of the elements from this set.
226 * The set will be empty after this call returns.
227 */
228 public void clear() {
229 al.clear();
230 }
231
232 /**
233 * Removes the specified element from this set if it is present.
234 * More formally, removes an element {@code e} such that
235 * <tt>(o==null ? e==null : o.equals(e))</tt>,
236 * if this set contains such an element. Returns {@code true} if
237 * this set contained the element (or equivalently, if this set
238 * changed as a result of the call). (This set will not contain the
239 * element once the call returns.)
240 *
241 * @param o object to be removed from this set, if present
242 * @return {@code true} if this set contained the specified element
243 */
244 public boolean remove(Object o) {
245 return al.remove(o);
246 }
247
248 /**
249 * Adds the specified element to this set if it is not already present.
250 * More formally, adds the specified element {@code e} to this set if
251 * the set contains no element {@code e2} such that
252 * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
253 * If this set already contains the element, the call leaves the set
254 * unchanged and returns {@code false}.
255 *
256 * @param e element to be added to this set
257 * @return {@code true} if this set did not already contain the specified
258 * element
259 */
260 public boolean add(E e) {
261 return al.addIfAbsent(e);
262 }
263
264 /**
265 * Returns {@code true} if this set contains all of the elements of the
266 * specified collection. If the specified collection is also a set, this
267 * method returns {@code true} if it is a <i>subset</i> of this set.
268 *
269 * @param c collection to be checked for containment in this set
270 * @return {@code true} if this set contains all of the elements of the
271 * specified collection
272 * @throws NullPointerException if the specified collection is null
273 * @see #contains(Object)
274 */
275 public boolean containsAll(Collection<?> c) {
276 return al.containsAll(c);
277 }
278
279 /**
280 * Adds all of the elements in the specified collection to this set if
281 * they're not already present. If the specified collection is also a
282 * set, the {@code addAll} operation effectively modifies this set so
283 * that its value is the <i>union</i> of the two sets. The behavior of
284 * this operation is undefined if the specified collection is modified
285 * while the operation is in progress.
286 *
287 * @param c collection containing elements to be added to this set
288 * @return {@code true} if this set changed as a result of the call
289 * @throws NullPointerException if the specified collection is null
290 * @see #add(Object)
291 */
292 public boolean addAll(Collection<? extends E> c) {
293 return al.addAllAbsent(c) > 0;
294 }
295
296 /**
297 * Removes from this set all of its elements that are contained in the
298 * specified collection. If the specified collection is also a set,
299 * this operation effectively modifies this set so that its value is the
300 * <i>asymmetric set difference</i> of the two sets.
301 *
302 * @param c collection containing elements to be removed from this set
303 * @return {@code true} if this set changed as a result of the call
304 * @throws ClassCastException if the class of an element of this set
305 * is incompatible with the specified collection (optional)
306 * @throws NullPointerException if this set contains a null element and the
307 * specified collection does not permit null elements (optional),
308 * or if the specified collection is null
309 * @see #remove(Object)
310 */
311 public boolean removeAll(Collection<?> c) {
312 return al.removeAll(c);
313 }
314
315 /**
316 * Retains only the elements in this set that are contained in the
317 * specified collection. In other words, removes from this set all of
318 * its elements that are not contained in the specified collection. If
319 * the specified collection is also a set, this operation effectively
320 * modifies this set so that its value is the <i>intersection</i> of the
321 * two sets.
322 *
323 * @param c collection containing elements to be retained in this set
324 * @return {@code true} if this set changed as a result of the call
325 * @throws ClassCastException if the class of an element of this set
326 * is incompatible with the specified collection (optional)
327 * @throws NullPointerException if this set contains a null element and the
328 * specified collection does not permit null elements (optional),
329 * or if the specified collection is null
330 * @see #remove(Object)
331 */
332 public boolean retainAll(Collection<?> c) {
333 return al.retainAll(c);
334 }
335
336 /**
337 * Returns an iterator over the elements contained in this set
338 * in the order in which these elements were added.
339 *
340 * <p>The returned iterator provides a snapshot of the state of the set
341 * when the iterator was constructed. No synchronization is needed while
342 * traversing the iterator. The iterator does <em>NOT</em> support the
343 * {@code remove} method.
344 *
345 * @return an iterator over the elements in this set
346 */
347 public Iterator<E> iterator() {
348 return al.iterator();
349 }
350
351 /**
352 * Compares the specified object with this set for equality.
353 * Returns {@code true} if the specified object is the same object
354 * as this object, or if it is also a {@link Set} and the elements
355 * returned by an {@linkplain Set#iterator() iterator} over the
356 * specified set are the same as the elements returned by an
357 * iterator over this set. More formally, the two iterators are
358 * considered to return the same elements if they return the same
359 * number of elements and for every element {@code e1} returned by
360 * the iterator over the specified set, there is an element
361 * {@code e2} returned by the iterator over this set such that
362 * {@code (e1==null ? e2==null : e1.equals(e2))}.
363 *
364 * @param o object to be compared for equality with this set
365 * @return {@code true} if the specified object is equal to this set
366 */
367 public boolean equals(Object o) {
368 if (o == this)
369 return true;
370 if (!(o instanceof Set))
371 return false;
372 Set<?> set = (Set<?>)(o);
373 Iterator<?> it = set.iterator();
374
375 // Uses O(n^2) algorithm that is only appropriate
376 // for small sets, which CopyOnWriteArraySets should be.
377
378 // Use a single snapshot of underlying array
379 Object[] elements = al.getArray();
380 int len = elements.length;
381 // Mark matched elements to avoid re-checking
382 boolean[] matched = new boolean[len];
383 int k = 0;
384 outer: while (it.hasNext()) {
385 if (++k > len)
386 return false;
387 Object x = it.next();
388 for (int i = 0; i < len; ++i) {
389 if (!matched[i] && eq(x, elements[i])) {
390 matched[i] = true;
391 continue outer;
392 }
393 }
394 return false;
395 }
396 return k == len;
397 }
398
399 public boolean removeAll(Predicate<? super E> filter) {
400 return al.removeAll(filter);
401 }
402
403 public void forEach(Consumer<? super E> action) {
404 al.forEach(action);
405 }
406
407 /**
408 * Returns a {@link Spliterator} over the elements in this set in the order
409 * in which these elements were added.
410 *
411 * <p>The {@code Spliterator} reports {@link Spliterator#IMMUTABLE},
412 * {@link Spliterator#DISTINCT}, {@link Spliterator#SIZED}, and
413 * {@link Spliterator#SUBSIZED}.
414 *
415 * <p>The spliterator provides a snapshot of the state of the set
416 * when the spliterator was constructed. No synchronization is needed while
417 * operating on the spliterator.
418 *
419 * @return a {@code Spliterator} over the elements in this set
420 * @since 1.8
421 */
422 public Spliterator<E> spliterator() {
423 return Spliterators.spliterator
424 (al.getArray(), Spliterator.IMMUTABLE | Spliterator.DISTINCT);
425 }
426
427 /**
428 * Tests for equality, coping with nulls.
429 */
430 private static boolean eq(Object o1, Object o2) {
431 return (o1 == null) ? o2 == null : o1.equals(o2);
432 }
433 }