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/licenses/publicdomain 5 */ 6 7package java.util.concurrent; 8import 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> 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 * } 54 * </pre> 55 * 56 * @see CopyOnWriteArrayList 57 * @since 1.5 58 * @author Doug Lea 59 * @param <E> the type of elements held in this collection 60 */ 61public class CopyOnWriteArraySet<E> extends AbstractSet<E> 62 implements java.io.Serializable { 63 private static final long serialVersionUID = 5457747651344034263L; 64 65 private final CopyOnWriteArrayList<E> al; 66 67 /** 68 * Creates an empty set. 69 */ 70 public CopyOnWriteArraySet() { 71 al = new CopyOnWriteArrayList<E>(); 72 } 73 74 /** 75 * Creates a set containing all of the elements of the specified 76 * collection. 77 * 78 * @param c the collection of elements to initially contain 79 * @throws NullPointerException if the specified collection is null 80 */ 81 public CopyOnWriteArraySet(Collection<? extends E> c) { 82 al = new CopyOnWriteArrayList<E>(); 83 al.addAllAbsent(c); 84 } 85 86 /** 87 * Returns the number of elements in this set. 88 * 89 * @return the number of elements in this set 90 */ 91 public int size() { 92 return al.size(); 93 } 94 95 /** 96 * Returns <tt>true</tt> if this set contains no elements. 97 * 98 * @return <tt>true</tt> if this set contains no elements 99 */ 100 public boolean isEmpty() { 101 return al.isEmpty(); 102 } 103 104 /** 105 * Returns <tt>true</tt> if this set contains the specified element. 106 * More formally, returns <tt>true</tt> if and only if this set 107 * contains an element <tt>e</tt> such that 108 * <tt>(o==null ? e==null : o.equals(e))</tt>. 109 * 110 * @param o element whose presence in this set is to be tested 111 * @return <tt>true</tt> if this set contains the specified element 112 */ 113 public boolean contains(Object o) { 114 return al.contains(o); 115 } 116 117 /** 118 * Returns an array containing all of the elements in this set. 119 * If this set makes any guarantees as to what order its elements 120 * are returned by its iterator, this method must return the 121 * elements in the same order. 122 * 123 * <p>The returned array will be "safe" in that no references to it 124 * are maintained by this set. (In other words, this method must 125 * allocate a new array even if this set is backed by an array). 126 * The caller is thus free to modify the returned array. 127 * 128 * <p>This method acts as bridge between array-based and collection-based 129 * APIs. 130 * 131 * @return an array containing all the elements in this set 132 */ 133 public Object[] toArray() { 134 return al.toArray(); 135 } 136 137 /** 138 * Returns an array containing all of the elements in this set; the 139 * runtime type of the returned array is that of the specified array. 140 * If the set fits in the specified array, it is returned therein. 141 * Otherwise, a new array is allocated with the runtime type of the 142 * specified array and the size of this set. 143 * 144 * <p>If this set fits in the specified array with room to spare 145 * (i.e., the array has more elements than this set), the element in 146 * the array immediately following the end of the set is set to 147 * <tt>null</tt>. (This is useful in determining the length of this 148 * set <i>only</i> if the caller knows that this set does not contain 149 * any null elements.) 150 * 151 * <p>If this set makes any guarantees as to what order its elements 152 * are returned by its iterator, this method must return the elements 153 * in the same order. 154 * 155 * <p>Like the {@link #toArray()} method, this method acts as bridge between 156 * array-based and collection-based APIs. Further, this method allows 157 * precise control over the runtime type of the output array, and may, 158 * under certain circumstances, be used to save allocation costs. 159 * 160 * <p>Suppose <tt>x</tt> is a set known to contain only strings. 161 * The following code can be used to dump the set into a newly allocated 162 * array of <tt>String</tt>: 163 * 164 * <pre> 165 * String[] y = x.toArray(new String[0]);</pre> 166 * 167 * Note that <tt>toArray(new Object[0])</tt> is identical in function to 168 * <tt>toArray()</tt>. 169 * 170 * @param a the array into which the elements of this set are to be 171 * stored, if it is big enough; otherwise, a new array of the same 172 * runtime type is allocated for this purpose. 173 * @return an array containing all the elements in this set 174 * @throws ArrayStoreException if the runtime type of the specified array 175 * is not a supertype of the runtime type of every element in this 176 * set 177 * @throws NullPointerException if the specified array is null 178 */ 179 public <T> T[] toArray(T[] a) { 180 return al.toArray(a); 181 } 182 183 /** 184 * Removes all of the elements from this set. 185 * The set will be empty after this call returns. 186 */ 187 public void clear() { 188 al.clear(); 189 } 190 191 /** 192 * Removes the specified element from this set if it is present. 193 * More formally, removes an element <tt>e</tt> such that 194 * <tt>(o==null ? e==null : o.equals(e))</tt>, 195 * if this set contains such an element. Returns <tt>true</tt> if 196 * this set contained the element (or equivalently, if this set 197 * changed as a result of the call). (This set will not contain the 198 * element once the call returns.) 199 * 200 * @param o object to be removed from this set, if present 201 * @return <tt>true</tt> if this set contained the specified element 202 */ 203 public boolean remove(Object o) { 204 return al.remove(o); 205 } 206 207 /** 208 * Adds the specified element to this set if it is not already present. 209 * More formally, adds the specified element <tt>e</tt> to this set if 210 * the set contains no element <tt>e2</tt> such that 211 * <tt>(e==null ? e2==null : e.equals(e2))</tt>. 212 * If this set already contains the element, the call leaves the set 213 * unchanged and returns <tt>false</tt>. 214 * 215 * @param e element to be added to this set 216 * @return <tt>true</tt> if this set did not already contain the specified 217 * element 218 */ 219 public boolean add(E e) { 220 return al.addIfAbsent(e); 221 } 222 223 /** 224 * Returns <tt>true</tt> if this set contains all of the elements of the 225 * specified collection. If the specified collection is also a set, this 226 * method returns <tt>true</tt> if it is a <i>subset</i> of this set. 227 * 228 * @param c collection to be checked for containment in this set 229 * @return <tt>true</tt> if this set contains all of the elements of the 230 * specified collection 231 * @throws NullPointerException if the specified collection is null 232 * @see #contains(Object) 233 */ 234 public boolean containsAll(Collection<?> c) { 235 return al.containsAll(c); 236 } 237 238 /** 239 * Adds all of the elements in the specified collection to this set if 240 * they're not already present. If the specified collection is also a 241 * set, the <tt>addAll</tt> operation effectively modifies this set so 242 * that its value is the <i>union</i> of the two sets. The behavior of 243 * this operation is undefined if the specified collection is modified 244 * while the operation is in progress. 245 * 246 * @param c collection containing elements to be added to this set 247 * @return <tt>true</tt> if this set changed as a result of the call 248 * @throws NullPointerException if the specified collection is null 249 * @see #add(Object) 250 */ 251 public boolean addAll(Collection<? extends E> c) { 252 return al.addAllAbsent(c) > 0; 253 } 254 255 /** 256 * Removes from this set all of its elements that are contained in the 257 * specified collection. If the specified collection is also a set, 258 * this operation effectively modifies this set so that its value is the 259 * <i>asymmetric set difference</i> of the two sets. 260 * 261 * @param c collection containing elements to be removed from this set 262 * @return <tt>true</tt> if this set changed as a result of the call 263 * @throws ClassCastException if the class of an element of this set 264 * is incompatible with the specified collection (optional) 265 * @throws NullPointerException if this set contains a null element and the 266 * specified collection does not permit null elements (optional), 267 * or if the specified collection is null 268 * @see #remove(Object) 269 */ 270 public boolean removeAll(Collection<?> c) { 271 return al.removeAll(c); 272 } 273 274 /** 275 * Retains only the elements in this set that are contained in the 276 * specified collection. In other words, removes from this set all of 277 * its elements that are not contained in the specified collection. If 278 * the specified collection is also a set, this operation effectively 279 * modifies this set so that its value is the <i>intersection</i> of the 280 * two sets. 281 * 282 * @param c collection containing elements to be retained in this set 283 * @return <tt>true</tt> if this set changed as a result of the call 284 * @throws ClassCastException if the class of an element of this set 285 * is incompatible with the specified collection (optional) 286 * @throws NullPointerException if this set contains a null element and the 287 * specified collection does not permit null elements (optional), 288 * or if the specified collection is null 289 * @see #remove(Object) 290 */ 291 public boolean retainAll(Collection<?> c) { 292 return al.retainAll(c); 293 } 294 295 /** 296 * Returns an iterator over the elements contained in this set 297 * in the order in which these elements were added. 298 * 299 * <p>The returned iterator provides a snapshot of the state of the set 300 * when the iterator was constructed. No synchronization is needed while 301 * traversing the iterator. The iterator does <em>NOT</em> support the 302 * <tt>remove</tt> method. 303 * 304 * @return an iterator over the elements in this set 305 */ 306 public Iterator<E> iterator() { 307 return al.iterator(); 308 } 309 310 /** 311 * Compares the specified object with this set for equality. 312 * Returns {@code true} if the specified object is the same object 313 * as this object, or if it is also a {@link Set} and the elements 314 * returned by an {@linkplain List#iterator() iterator} over the 315 * specified set are the same as the elements returned by an 316 * iterator over this set. More formally, the two iterators are 317 * considered to return the same elements if they return the same 318 * number of elements and for every element {@code e1} returned by 319 * the iterator over the specified set, there is an element 320 * {@code e2} returned by the iterator over this set such that 321 * {@code (e1==null ? e2==null : e1.equals(e2))}. 322 * 323 * @param o object to be compared for equality with this set 324 * @return {@code true} if the specified object is equal to this set 325 */ 326 public boolean equals(Object o) { 327 if (o == this) 328 return true; 329 if (!(o instanceof Set)) 330 return false; 331 Set<?> set = (Set<?>)(o); 332 Iterator<?> it = set.iterator(); 333 334 // Uses O(n^2) algorithm that is only appropriate 335 // for small sets, which CopyOnWriteArraySets should be. 336 337 // Use a single snapshot of underlying array 338 Object[] elements = al.getArray(); 339 int len = elements.length; 340 // Mark matched elements to avoid re-checking 341 boolean[] matched = new boolean[len]; 342 int k = 0; 343 outer: while (it.hasNext()) { 344 if (++k > len) 345 return false; 346 Object x = it.next(); 347 for (int i = 0; i < len; ++i) { 348 if (!matched[i] && eq(x, elements[i])) { 349 matched[i] = true; 350 continue outer; 351 } 352 } 353 return false; 354 } 355 return k == len; 356 } 357 358 /** 359 * Test for equality, coping with nulls. 360 */ 361 private static boolean eq(Object o1, Object o2) { 362 return (o1 == null ? o2 == null : o1.equals(o2)); 363 } 364} 365