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 38 import java.util.AbstractSet; 39 import java.util.Collection; 40 import java.util.Iterator; 41 import java.util.Objects; 42 import java.util.Set; 43 import java.util.Spliterator; 44 import java.util.Spliterators; 45 import java.util.function.Consumer; 46 import java.util.function.Predicate; 47 48 // BEGIN android-note 49 // removed link to collections framework docs 50 // fixed framework docs link to "Collection#optional" 51 // END android-note 52 53 /** 54 * A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList} 55 * for all of its operations. Thus, it shares the same basic properties: 56 * <ul> 57 * <li>It is best suited for applications in which set sizes generally 58 * stay small, read-only operations 59 * vastly outnumber mutative operations, and you need 60 * to prevent interference among threads during traversal. 61 * <li>It is thread-safe. 62 * <li>Mutative operations ({@code add}, {@code set}, {@code remove}, etc.) 63 * are expensive since they usually entail copying the entire underlying 64 * array. 65 * <li>Iterators do not support the mutative {@code remove} operation. 66 * <li>Traversal via iterators is fast and cannot encounter 67 * interference from other threads. Iterators rely on 68 * unchanging snapshots of the array at the time the iterators were 69 * constructed. 70 * </ul> 71 * 72 * <p><b>Sample Usage.</b> The following code sketch uses a 73 * copy-on-write set to maintain a set of Handler objects that 74 * perform some action upon state updates. 75 * 76 * <pre> {@code 77 * class Handler { void handle(); ... } 78 * 79 * class X { 80 * private final CopyOnWriteArraySet<Handler> handlers 81 * = new CopyOnWriteArraySet<>(); 82 * public void addHandler(Handler h) { handlers.add(h); } 83 * 84 * private long internalState; 85 * private synchronized void changeState() { internalState = ...; } 86 * 87 * public void update() { 88 * changeState(); 89 * for (Handler handler : handlers) 90 * handler.handle(); 91 * } 92 * }}</pre> 93 * 94 * @see CopyOnWriteArrayList 95 * @since 1.5 96 * @author Doug Lea 97 * @param <E> the type of elements held in this set 98 */ 99 public class CopyOnWriteArraySet<E> extends AbstractSet<E> 100 implements java.io.Serializable { 101 private static final long serialVersionUID = 5457747651344034263L; 102 103 private final CopyOnWriteArrayList<E> al; 104 105 /** 106 * Creates an empty set. 107 */ CopyOnWriteArraySet()108 public CopyOnWriteArraySet() { 109 al = new CopyOnWriteArrayList<E>(); 110 } 111 112 /** 113 * Creates a set containing all of the elements of the specified 114 * collection. 115 * 116 * @param c the collection of elements to initially contain 117 * @throws NullPointerException if the specified collection is null 118 */ CopyOnWriteArraySet(Collection<? extends E> c)119 public CopyOnWriteArraySet(Collection<? extends E> c) { 120 if (c.getClass() == CopyOnWriteArraySet.class) { 121 @SuppressWarnings("unchecked") CopyOnWriteArraySet<E> cc = 122 (CopyOnWriteArraySet<E>)c; 123 al = new CopyOnWriteArrayList<E>(cc.al); 124 } 125 else { 126 al = new CopyOnWriteArrayList<E>(); 127 al.addAllAbsent(c); 128 } 129 } 130 131 /** 132 * Returns the number of elements in this set. 133 * 134 * @return the number of elements in this set 135 */ size()136 public int size() { 137 return al.size(); 138 } 139 140 /** 141 * Returns {@code true} if this set contains no elements. 142 * 143 * @return {@code true} if this set contains no elements 144 */ isEmpty()145 public boolean isEmpty() { 146 return al.isEmpty(); 147 } 148 149 /** 150 * Returns {@code true} if this set contains the specified element. 151 * More formally, returns {@code true} if and only if this set 152 * contains an element {@code e} such that {@code Objects.equals(o, e)}. 153 * 154 * @param o element whose presence in this set is to be tested 155 * @return {@code true} if this set contains the specified element 156 */ contains(Object o)157 public boolean contains(Object o) { 158 return al.contains(o); 159 } 160 161 /** 162 * Returns an array containing all of the elements in this set. 163 * If this set makes any guarantees as to what order its elements 164 * are returned by its iterator, this method must return the 165 * elements in the same order. 166 * 167 * <p>The returned array will be "safe" in that no references to it 168 * are maintained by this set. (In other words, this method must 169 * allocate a new array even if this set is backed by an array). 170 * The caller is thus free to modify the returned array. 171 * 172 * <p>This method acts as bridge between array-based and collection-based 173 * APIs. 174 * 175 * @return an array containing all the elements in this set 176 */ toArray()177 public Object[] toArray() { 178 return al.toArray(); 179 } 180 181 /** 182 * Returns an array containing all of the elements in this set; the 183 * runtime type of the returned array is that of the specified array. 184 * If the set fits in the specified array, it is returned therein. 185 * Otherwise, a new array is allocated with the runtime type of the 186 * specified array and the size of this set. 187 * 188 * <p>If this set fits in the specified array with room to spare 189 * (i.e., the array has more elements than this set), the element in 190 * the array immediately following the end of the set is set to 191 * {@code null}. (This is useful in determining the length of this 192 * set <i>only</i> if the caller knows that this set does not contain 193 * any null elements.) 194 * 195 * <p>If this set makes any guarantees as to what order its elements 196 * are returned by its iterator, this method must return the elements 197 * in the same order. 198 * 199 * <p>Like the {@link #toArray()} method, this method acts as bridge between 200 * array-based and collection-based APIs. Further, this method allows 201 * precise control over the runtime type of the output array, and may, 202 * under certain circumstances, be used to save allocation costs. 203 * 204 * <p>Suppose {@code x} is a set known to contain only strings. 205 * The following code can be used to dump the set into a newly allocated 206 * array of {@code String}: 207 * 208 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre> 209 * 210 * Note that {@code toArray(new Object[0])} is identical in function to 211 * {@code toArray()}. 212 * 213 * @param a the array into which the elements of this set are to be 214 * stored, if it is big enough; otherwise, a new array of the same 215 * runtime type is allocated for this purpose. 216 * @return an array containing all the elements in this set 217 * @throws ArrayStoreException if the runtime type of the specified array 218 * is not a supertype of the runtime type of every element in this 219 * set 220 * @throws NullPointerException if the specified array is null 221 */ toArray(T[] a)222 public <T> T[] toArray(T[] a) { 223 return al.toArray(a); 224 } 225 226 /** 227 * Removes all of the elements from this set. 228 * The set will be empty after this call returns. 229 */ clear()230 public void clear() { 231 al.clear(); 232 } 233 234 /** 235 * Removes the specified element from this set if it is present. 236 * More formally, removes an element {@code e} such that 237 * {@code Objects.equals(o, e)}, if this set contains such an element. 238 * Returns {@code true} if this set contained the element (or 239 * equivalently, if this set changed as a result of the call). 240 * (This set will not contain the element once the call returns.) 241 * 242 * @param o object to be removed from this set, if present 243 * @return {@code true} if this set contained the specified element 244 */ remove(Object o)245 public boolean remove(Object o) { 246 return al.remove(o); 247 } 248 249 /** 250 * Adds the specified element to this set if it is not already present. 251 * More formally, adds the specified element {@code e} to this set if 252 * the set contains no element {@code e2} such that 253 * {@code Objects.equals(e, e2)}. 254 * If this set already contains the element, the call leaves the set 255 * unchanged and returns {@code false}. 256 * 257 * @param e element to be added to this set 258 * @return {@code true} if this set did not already contain the specified 259 * element 260 */ add(E e)261 public boolean add(E e) { 262 return al.addIfAbsent(e); 263 } 264 265 /** 266 * Returns {@code true} if this set contains all of the elements of the 267 * specified collection. If the specified collection is also a set, this 268 * method returns {@code true} if it is a <i>subset</i> of this set. 269 * 270 * @param c collection to be checked for containment in this set 271 * @return {@code true} if this set contains all of the elements of the 272 * specified collection 273 * @throws NullPointerException if the specified collection is null 274 * @see #contains(Object) 275 */ containsAll(Collection<?> c)276 public boolean containsAll(Collection<?> c) { 277 return (c instanceof Set) 278 ? compareSets(al.getArray(), (Set<?>) c) >= 0 279 : al.containsAll(c); 280 } 281 282 /** 283 * Tells whether the objects in snapshot (regarded as a set) are a 284 * superset of the given set. 285 * 286 * @return -1 if snapshot is not a superset, 0 if the two sets 287 * contain precisely the same elements, and 1 if snapshot is a 288 * proper superset of the given set 289 */ compareSets(Object[] snapshot, Set<?> set)290 private static int compareSets(Object[] snapshot, Set<?> set) { 291 // Uses O(n^2) algorithm, that is only appropriate for small 292 // sets, which CopyOnWriteArraySets should be. 293 // 294 // Optimize up to O(n) if the two sets share a long common prefix, 295 // as might happen if one set was created as a copy of the other set. 296 297 final int len = snapshot.length; 298 // Mark matched elements to avoid re-checking 299 final boolean[] matched = new boolean[len]; 300 301 // j is the largest int with matched[i] true for { i | 0 <= i < j } 302 int j = 0; 303 outer: for (Object x : set) { 304 for (int i = j; i < len; i++) { 305 if (!matched[i] && Objects.equals(x, snapshot[i])) { 306 matched[i] = true; 307 if (i == j) 308 do { j++; } while (j < len && matched[j]); 309 continue outer; 310 } 311 } 312 return -1; 313 } 314 return (j == len) ? 0 : 1; 315 } 316 317 /** 318 * Adds all of the elements in the specified collection to this set if 319 * they're not already present. If the specified collection is also a 320 * set, the {@code addAll} operation effectively modifies this set so 321 * that its value is the <i>union</i> of the two sets. The behavior of 322 * this operation is undefined if the specified collection is modified 323 * while the operation is in progress. 324 * 325 * @param c collection containing elements to be added to this set 326 * @return {@code true} if this set changed as a result of the call 327 * @throws NullPointerException if the specified collection is null 328 * @see #add(Object) 329 */ addAll(Collection<? extends E> c)330 public boolean addAll(Collection<? extends E> c) { 331 return al.addAllAbsent(c) > 0; 332 } 333 334 /** 335 * Removes from this set all of its elements that are contained in the 336 * specified collection. If the specified collection is also a set, 337 * this operation effectively modifies this set so that its value is the 338 * <i>asymmetric set difference</i> of the two sets. 339 * 340 * @param c collection containing elements to be removed from this set 341 * @return {@code true} if this set changed as a result of the call 342 * @throws ClassCastException if the class of an element of this set 343 * is incompatible with the specified collection 344 * (<a href="../Collection.html#optional-restrictions">optional</a>) 345 * @throws NullPointerException if this set contains a null element and the 346 * specified collection does not permit null elements 347 * (<a href="../Collection.html#optional-restrictions">optional</a>), 348 * or if the specified collection is null 349 * @see #remove(Object) 350 */ removeAll(Collection<?> c)351 public boolean removeAll(Collection<?> c) { 352 return al.removeAll(c); 353 } 354 355 /** 356 * Retains only the elements in this set that are contained in the 357 * specified collection. In other words, removes from this set all of 358 * its elements that are not contained in the specified collection. If 359 * the specified collection is also a set, this operation effectively 360 * modifies this set so that its value is the <i>intersection</i> of the 361 * two sets. 362 * 363 * @param c collection containing elements to be retained in this set 364 * @return {@code true} if this set changed as a result of the call 365 * @throws ClassCastException if the class of an element of this set 366 * is incompatible with the specified collection 367 * (<a href="../Collection.html#optional-restrictions">optional</a>) 368 * @throws NullPointerException if this set contains a null element and the 369 * specified collection does not permit null elements 370 * (<a href="../Collection.html#optional-restrictions">optional</a>), 371 * or if the specified collection is null 372 * @see #remove(Object) 373 */ retainAll(Collection<?> c)374 public boolean retainAll(Collection<?> c) { 375 return al.retainAll(c); 376 } 377 378 /** 379 * Returns an iterator over the elements contained in this set 380 * in the order in which these elements were added. 381 * 382 * <p>The returned iterator provides a snapshot of the state of the set 383 * when the iterator was constructed. No synchronization is needed while 384 * traversing the iterator. The iterator does <em>NOT</em> support the 385 * {@code remove} method. 386 * 387 * @return an iterator over the elements in this set 388 */ iterator()389 public Iterator<E> iterator() { 390 return al.iterator(); 391 } 392 393 /** 394 * Compares the specified object with this set for equality. 395 * Returns {@code true} if the specified object is the same object 396 * as this object, or if it is also a {@link Set} and the elements 397 * returned by an {@linkplain Set#iterator() iterator} over the 398 * specified set are the same as the elements returned by an 399 * iterator over this set. More formally, the two iterators are 400 * considered to return the same elements if they return the same 401 * number of elements and for every element {@code e1} returned by 402 * the iterator over the specified set, there is an element 403 * {@code e2} returned by the iterator over this set such that 404 * {@code Objects.equals(e1, e2)}. 405 * 406 * @param o object to be compared for equality with this set 407 * @return {@code true} if the specified object is equal to this set 408 */ equals(Object o)409 public boolean equals(Object o) { 410 return (o == this) 411 || ((o instanceof Set) 412 && compareSets(al.getArray(), (Set<?>) o) == 0); 413 } 414 removeIf(Predicate<? super E> filter)415 public boolean removeIf(Predicate<? super E> filter) { 416 return al.removeIf(filter); 417 } 418 forEach(Consumer<? super E> action)419 public void forEach(Consumer<? super E> action) { 420 al.forEach(action); 421 } 422 423 /** 424 * Returns a {@link Spliterator} over the elements in this set in the order 425 * in which these elements were added. 426 * 427 * <p>The {@code Spliterator} reports {@link Spliterator#IMMUTABLE}, 428 * {@link Spliterator#DISTINCT}, {@link Spliterator#SIZED}, and 429 * {@link Spliterator#SUBSIZED}. 430 * 431 * <p>The spliterator provides a snapshot of the state of the set 432 * when the spliterator was constructed. No synchronization is needed while 433 * operating on the spliterator. 434 * 435 * @return a {@code Spliterator} over the elements in this set 436 * @since 1.8 437 */ spliterator()438 public Spliterator<E> spliterator() { 439 return Spliterators.spliterator 440 (al.getArray(), Spliterator.IMMUTABLE | Spliterator.DISTINCT); 441 } 442 } 443