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 /** 49 * A {@link Set} that uses an internal {@link CopyOnWriteArrayList} 50 * for all of its operations. Thus, it shares the same basic properties: 51 * <ul> 52 * <li>It is best suited for applications in which set sizes generally 53 * stay small, read-only operations 54 * vastly outnumber mutative operations, and you need 55 * to prevent interference among threads during traversal. 56 * <li>It is thread-safe. 57 * <li>Mutative operations ({@code add}, {@code set}, {@code remove}, etc.) 58 * are expensive since they usually entail copying the entire underlying 59 * array. 60 * <li>Iterators do not support the mutative {@code remove} operation. 61 * <li>Traversal via iterators is fast and cannot encounter 62 * interference from other threads. Iterators rely on 63 * unchanging snapshots of the array at the time the iterators were 64 * constructed. 65 * </ul> 66 * 67 * <p><b>Sample Usage.</b> The following code sketch uses a 68 * copy-on-write set to maintain a set of Handler objects that 69 * perform some action upon state updates. 70 * 71 * <pre> {@code 72 * class Handler { void handle() { ... } } 73 * 74 * class X { 75 * private final CopyOnWriteArraySet<Handler> handlers 76 * = new CopyOnWriteArraySet<>(); 77 * public void addHandler(Handler h) { handlers.add(h); } 78 * 79 * private long internalState; 80 * private synchronized void changeState() { internalState = ...; } 81 * 82 * public void update() { 83 * changeState(); 84 * for (Handler handler : handlers) 85 * handler.handle(); 86 * } 87 * }}</pre> 88 * 89 * <p>This class is a member of the 90 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> 91 * Java Collections Framework</a>. 92 * 93 * @see CopyOnWriteArrayList 94 * @since 1.5 95 * @author Doug Lea 96 * @param <E> the type of elements held in this set 97 */ 98 public class CopyOnWriteArraySet<E> extends AbstractSet<E> 99 implements java.io.Serializable { 100 private static final long serialVersionUID = 5457747651344034263L; 101 102 private final CopyOnWriteArrayList<E> al; 103 104 /** 105 * Creates an empty set. 106 */ CopyOnWriteArraySet()107 public CopyOnWriteArraySet() { 108 al = new CopyOnWriteArrayList<E>(); 109 } 110 111 /** 112 * Creates a set containing all of the elements of the specified 113 * collection. 114 * 115 * @param c the collection of elements to initially contain 116 * @throws NullPointerException if the specified collection is null 117 */ CopyOnWriteArraySet(Collection<? extends E> c)118 public CopyOnWriteArraySet(Collection<? extends E> c) { 119 if (c.getClass() == CopyOnWriteArraySet.class) { 120 @SuppressWarnings("unchecked") CopyOnWriteArraySet<E> cc = 121 (CopyOnWriteArraySet<E>)c; 122 al = new CopyOnWriteArrayList<E>(cc.al); 123 } 124 else { 125 al = new CopyOnWriteArrayList<E>(); 126 al.addAllAbsent(c); 127 } 128 } 129 130 /** 131 * Returns the number of elements in this set. 132 * 133 * @return the number of elements in this set 134 */ size()135 public int size() { 136 return al.size(); 137 } 138 139 /** 140 * Returns {@code true} if this set contains no elements. 141 * 142 * @return {@code true} if this set contains no elements 143 */ isEmpty()144 public boolean isEmpty() { 145 return al.isEmpty(); 146 } 147 148 /** 149 * Returns {@code true} if this set contains the specified element. 150 * More formally, returns {@code true} if and only if this set 151 * contains an element {@code e} such that {@code Objects.equals(o, e)}. 152 * 153 * @param o element whose presence in this set is to be tested 154 * @return {@code true} if this set contains the specified element 155 */ contains(Object o)156 public boolean contains(Object o) { 157 return al.contains(o); 158 } 159 160 /** 161 * Returns an array containing all of the elements in this set. 162 * If this set makes any guarantees as to what order its elements 163 * are returned by its iterator, this method must return the 164 * elements in the same order. 165 * 166 * <p>The returned array will be "safe" in that no references to it 167 * are maintained by this set. (In other words, this method must 168 * allocate a new array even if this set is backed by an array). 169 * The caller is thus free to modify the returned array. 170 * 171 * <p>This method acts as bridge between array-based and collection-based 172 * APIs. 173 * 174 * @return an array containing all the elements in this set 175 */ toArray()176 public Object[] toArray() { 177 return al.toArray(); 178 } 179 180 /** 181 * Returns an array containing all of the elements in this set; the 182 * runtime type of the returned array is that of the specified array. 183 * If the set fits in the specified array, it is returned therein. 184 * Otherwise, a new array is allocated with the runtime type of the 185 * specified array and the size of this set. 186 * 187 * <p>If this set fits in the specified array with room to spare 188 * (i.e., the array has more elements than this set), the element in 189 * the array immediately following the end of the set is set to 190 * {@code null}. (This is useful in determining the length of this 191 * set <i>only</i> if the caller knows that this set does not contain 192 * any null elements.) 193 * 194 * <p>If this set makes any guarantees as to what order its elements 195 * are returned by its iterator, this method must return the elements 196 * in the same order. 197 * 198 * <p>Like the {@link #toArray()} method, this method acts as bridge between 199 * array-based and collection-based APIs. Further, this method allows 200 * precise control over the runtime type of the output array, and may, 201 * under certain circumstances, be used to save allocation costs. 202 * 203 * <p>Suppose {@code x} is a set known to contain only strings. 204 * The following code can be used to dump the set into a newly allocated 205 * array of {@code String}: 206 * 207 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre> 208 * 209 * Note that {@code toArray(new Object[0])} is identical in function to 210 * {@code toArray()}. 211 * 212 * @param a the array into which the elements of this set are to be 213 * stored, if it is big enough; otherwise, a new array of the same 214 * runtime type is allocated for this purpose. 215 * @return an array containing all the elements in this set 216 * @throws ArrayStoreException if the runtime type of the specified array 217 * is not a supertype of the runtime type of every element in this 218 * set 219 * @throws NullPointerException if the specified array is null 220 */ toArray(T[] a)221 public <T> T[] toArray(T[] a) { 222 return al.toArray(a); 223 } 224 225 /** 226 * Removes all of the elements from this set. 227 * The set will be empty after this call returns. 228 */ clear()229 public void clear() { 230 al.clear(); 231 } 232 233 /** 234 * Removes the specified element from this set if it is present. 235 * More formally, removes an element {@code e} such that 236 * {@code Objects.equals(o, e)}, if this set contains such an element. 237 * Returns {@code true} if this set contained the element (or 238 * equivalently, if this set changed as a result of the call). 239 * (This set will not contain the 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 */ remove(Object o)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 * {@code Objects.equals(e, e2)}. 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 */ add(E e)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 */ containsAll(Collection<?> c)275 public boolean containsAll(Collection<?> c) { 276 return (c instanceof Set) 277 ? compareSets(al.getArray(), (Set<?>) c) >= 0 278 : al.containsAll(c); 279 } 280 281 /** 282 * Tells whether the objects in snapshot (regarded as a set) are a 283 * superset of the given set. 284 * 285 * @return -1 if snapshot is not a superset, 0 if the two sets 286 * contain precisely the same elements, and 1 if snapshot is a 287 * proper superset of the given set 288 */ compareSets(Object[] snapshot, Set<?> set)289 private static int compareSets(Object[] snapshot, Set<?> set) { 290 // Uses O(n^2) algorithm, that is only appropriate for small 291 // sets, which CopyOnWriteArraySets should be. 292 // 293 // Optimize up to O(n) if the two sets share a long common prefix, 294 // as might happen if one set was created as a copy of the other set. 295 296 final int len = snapshot.length; 297 // Mark matched elements to avoid re-checking 298 final boolean[] matched = new boolean[len]; 299 300 // j is the largest int with matched[i] true for { i | 0 <= i < j } 301 int j = 0; 302 outer: for (Object x : set) { 303 for (int i = j; i < len; i++) { 304 if (!matched[i] && Objects.equals(x, snapshot[i])) { 305 matched[i] = true; 306 if (i == j) 307 do { j++; } while (j < len && matched[j]); 308 continue outer; 309 } 310 } 311 return -1; 312 } 313 return (j == len) ? 0 : 1; 314 } 315 316 /** 317 * Adds all of the elements in the specified collection to this set if 318 * they're not already present. If the specified collection is also a 319 * set, the {@code addAll} operation effectively modifies this set so 320 * that its value is the <i>union</i> of the two sets. The behavior of 321 * this operation is undefined if the specified collection is modified 322 * while the operation is in progress. 323 * 324 * @param c collection containing elements to be added to this set 325 * @return {@code true} if this set changed as a result of the call 326 * @throws NullPointerException if the specified collection is null 327 * @see #add(Object) 328 */ addAll(Collection<? extends E> c)329 public boolean addAll(Collection<? extends E> c) { 330 return al.addAllAbsent(c) > 0; 331 } 332 333 /** 334 * Removes from this set all of its elements that are contained in the 335 * specified collection. If the specified collection is also a set, 336 * this operation effectively modifies this set so that its value is the 337 * <i>asymmetric set difference</i> of the two sets. 338 * 339 * @param c collection containing elements to be removed from this set 340 * @return {@code true} if this set changed as a result of the call 341 * @throws ClassCastException if the class of an element of this set 342 * is incompatible with the specified collection 343 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) 344 * @throws NullPointerException if this set contains a null element and the 345 * specified collection does not permit null elements 346 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>), 347 * or if the specified collection is null 348 * @see #remove(Object) 349 */ removeAll(Collection<?> c)350 public boolean removeAll(Collection<?> c) { 351 return al.removeAll(c); 352 } 353 354 /** 355 * Retains only the elements in this set that are contained in the 356 * specified collection. In other words, removes from this set all of 357 * its elements that are not contained in the specified collection. If 358 * the specified collection is also a set, this operation effectively 359 * modifies this set so that its value is the <i>intersection</i> of the 360 * two sets. 361 * 362 * @param c collection containing elements to be retained in this set 363 * @return {@code true} if this set changed as a result of the call 364 * @throws ClassCastException if the class of an element of this set 365 * is incompatible with the specified collection 366 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) 367 * @throws NullPointerException if this set contains a null element and the 368 * specified collection does not permit null elements 369 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>), 370 * or if the specified collection is null 371 * @see #remove(Object) 372 */ retainAll(Collection<?> c)373 public boolean retainAll(Collection<?> c) { 374 return al.retainAll(c); 375 } 376 377 /** 378 * Returns an iterator over the elements contained in this set 379 * in the order in which these elements were added. 380 * 381 * <p>The returned iterator provides a snapshot of the state of the set 382 * when the iterator was constructed. No synchronization is needed while 383 * traversing the iterator. The iterator does <em>NOT</em> support the 384 * {@code remove} method. 385 * 386 * @return an iterator over the elements in this set 387 */ iterator()388 public Iterator<E> iterator() { 389 return al.iterator(); 390 } 391 392 /** 393 * Compares the specified object with this set for equality. 394 * Returns {@code true} if the specified object is the same object 395 * as this object, or if it is also a {@link Set} and the elements 396 * returned by an {@linkplain Set#iterator() iterator} over the 397 * specified set are the same as the elements returned by an 398 * iterator over this set. More formally, the two iterators are 399 * considered to return the same elements if they return the same 400 * number of elements and for every element {@code e1} returned by 401 * the iterator over the specified set, there is an element 402 * {@code e2} returned by the iterator over this set such that 403 * {@code Objects.equals(e1, e2)}. 404 * 405 * @param o object to be compared for equality with this set 406 * @return {@code true} if the specified object is equal to this set 407 */ equals(Object o)408 public boolean equals(Object o) { 409 return (o == this) 410 || ((o instanceof Set) 411 && compareSets(al.getArray(), (Set<?>) o) == 0); 412 } 413 414 /** 415 * @throws NullPointerException {@inheritDoc} 416 */ removeIf(Predicate<? super E> filter)417 public boolean removeIf(Predicate<? super E> filter) { 418 return al.removeIf(filter); 419 } 420 421 /** 422 * @throws NullPointerException {@inheritDoc} 423 */ forEach(Consumer<? super E> action)424 public void forEach(Consumer<? super E> action) { 425 al.forEach(action); 426 } 427 428 /** 429 * Returns a {@link Spliterator} over the elements in this set in the order 430 * in which these elements were added. 431 * 432 * <p>The {@code Spliterator} reports {@link Spliterator#IMMUTABLE}, 433 * {@link Spliterator#DISTINCT}, {@link Spliterator#SIZED}, and 434 * {@link Spliterator#SUBSIZED}. 435 * 436 * <p>The spliterator provides a snapshot of the state of the set 437 * when the spliterator was constructed. No synchronization is needed while 438 * operating on the spliterator. 439 * 440 * @return a {@code Spliterator} over the elements in this set 441 * @since 1.8 442 */ spliterator()443 public Spliterator<E> spliterator() { 444 return Spliterators.spliterator 445 (al.getArray(), Spliterator.IMMUTABLE | Spliterator.DISTINCT); 446 } 447 } 448