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