Java多线程系列--“JUC集合”03之 CopyOnWriteArraySet
概要
本章是JUC系列中的CopyOnWriteArraySet篇。接下来，会先对CopyOnWriteArraySet进行基本介绍，然后再说明它的原理，接着通过代码去分析，最后通过示例更进一步的了解CopyOnWriteArraySet。内容包括：
CopyOnWriteArraySet介绍
CopyOnWriteArraySet原理和数据结构
CopyOnWriteArraySet函数列表
CopyOnWriteArraySet源码(JDK1.7.0_40版本)
CopyOnWriteArraySet示例
转载请注明出处：http://www.cnblogs.com/skywang12345/p/3498497.html
HashSet。有意思的是，CopyOnWriteArraySet和HashSet虽然都继承于共同的父类AbstractSet；但是，HashSet是通过“散列表(HashMap)”实现的，而CopyOnWriteArraySet则是通过“动态数组(CopyOnWriteArrayList)”实现的，并不是散列表。
和CopyOnWriteArrayList类似，CopyOnWriteArraySet具有以下特性：
1. 它最适合于具有以下特征的应用程序：Set 大小通常保持很小，只读操作远多于可变操作，需要在遍历期间防止线程间的冲突。
2. 它是线程安全的。
3. 因为通常需要复制整个基础数组，所以可变操作（add()、set() 和 remove() 等等）的开销很大。
4. 迭代器支持hasNext(), next()等不可变操作，但不支持可变 remove()等 操作。
5. 使用迭代器进行遍历的速度很快，并且不会与其他线程发生冲突。在构造迭代器时，迭代器依赖于不变的数组快照。
建议：在学习CopyOnWriteArraySet之前，先通过"Java 集合系列16之 HashSet详细介绍(源码解析)和使用示例"对HashSet进行了解。
说明：
1. CopyOnWriteArraySet继承于AbstractSet，这就意味着它是一个集合。
2. CopyOnWriteArraySet包含CopyOnWriteArrayList对象，它是通过CopyOnWriteArrayList实现的。而CopyOnWriteArrayList本质是个动态数组队列，
所以CopyOnWriteArraySet相当于通过通过动态数组实现的“集合”！ CopyOnWriteArrayList中允许有重复的元素；但是，CopyOnWriteArraySet是一个集合，所以它不能有重复集合。因此，CopyOnWriteArrayList额外提供了addIfAbsent()和addAllAbsent()这两个添加元素的API，通过这些API来添加元素时，只有当元素不存在时才执行添加操作！
至于CopyOnWriteArraySet的“线程安全”机制，和CopyOnWriteArrayList一样，是通过volatile和互斥锁来实现的。这个在前一章节介绍CopyOnWriteArrayList时数据结构时，已经进行了说明，这里就不再重复叙述了。
docRoot}/../technotes/guides/collections/index.html">
82 * Java Collections Framework.
83 *
84 * @see CopyOnWriteArrayList
85 * @since 1.5
86 * @author Doug Lea
87 * @param the type of elements held in this collection
88 */
89 public class CopyOnWriteArraySet extends AbstractSet
90 implements java.io.Serializable {
91 private static final long serialVersionUID = 5457747651344034263L;
92
93 private final CopyOnWriteArrayList al;
94
95 /**
96 * Creates an empty set.
97 */
98 public CopyOnWriteArraySet() {
99 al = new CopyOnWriteArrayList();
100 }
101
102 /**
103 * Creates a set containing all of the elements of the specified
104 * collection.
105 *
106 * @param c the collection of elements to initially contain
107 * @throws NullPointerException if the specified collection is null
108 */
109 public CopyOnWriteArraySet(Collection<? extends E> c) {
110 al = new CopyOnWriteArrayList();
111 al.addAllAbsent(c);
112 }
113
114 /**
115 * Returns the number of elements in this set.
116 *
117 * @return the number of elements in this set
118 */
119 public int size() {
120 return al.size();
121 }
122
123 /**
124 * Returns true if this set contains no elements.
125 *
126 * @return true if this set contains no elements
127 */
128 public boolean isEmpty() {
129 return al.isEmpty();
130 }
131
132 /**
133 * Returns true if this set contains the specified element.
134 * More formally, returns true if and only if this set
135 * contains an element e such that
136 * (o==null ? e==null : o.equals(e)).
137 *
138 * @param o element whose presence in this set is to be tested
139 * @return true if this set contains the specified element
140 */
141 public boolean contains(Object o) {
142 return al.contains(o);
143 }
144
145 /**
146 * Returns an array containing all of the elements in this set.
147 * If this set makes any guarantees as to what order its elements
148 * are returned by its iterator, this method must return the
149 * elements in the same order.
150 *
151 * The returned array will be "safe" in that no references to it
152 * are maintained by this set. (In other words, this method must
153 * allocate a new array even if this set is backed by an array).
154 * The caller is thus free to modify the returned array.
155 *
156 * This method acts as bridge between array-based and collection-based
157 * APIs.
158 *
159 * @return an array containing all the elements in this set
160 */
161 public Object[] toArray() {
162 return al.toArray();
163 }
164
165 /**
166 * Returns an array containing all of the elements in this set; the
167 * runtime type of the returned array is that of the specified array.
168 * If the set fits in the specified array, it is returned therein.
169 * Otherwise, a new array is allocated with the runtime type of the
170 * specified array and the size of this set.
171 *
172 * If this set fits in the specified array with room to spare
173 * (i.e., the array has more elements than this set), the element in
174 * the array immediately following the end of the set is set to
175 * null. (This is useful in determining the length of this
176 * set only if the caller knows that this set does not contain
177 * any null elements.)
178 *
179 * If this set makes any guarantees as to what order its elements
180 * are returned by its iterator, this method must return the elements
181 * in the same order.
182 *
183 * Like the {
@link #toArray()} method, this method acts as bridge between
184 * array-based and collection-based APIs. Further, this method allows
185 * precise control over the runtime type of the output array, and may,
186 * under certain circumstances, be used to save allocation costs.
187 *
188 * Suppose x is a set known to contain only strings.
189 * The following code can be used to dump the set into a newly allocated
190 * array of String:
191 *
192 *
193 * String[] y = x.toArray(new String[0]);
194 *
195 * Note that toArray(new Object[0]) is identical in function to
196 * toArray().
197 *
198 * @param a the array into which the elements of this set are to be
199 * stored, if it is big enough; otherwise, a new array of the same
200 * runtime type is allocated for this purpose.
201 * @return an array containing all the elements in this set
202 * @throws ArrayStoreException if the runtime type of the specified array
203 * is not a supertype of the runtime type of every element in this
204 * set
205 * @throws NullPointerException if the specified array is null
206 */
207 public T[] toArray(T[] a) {
208 return al.toArray(a);
209 }
210
211 /**
212 * Removes all of the elements from this set.
213 * The set will be empty after this call returns.
214 */
215 public void clear() {
216 al.clear();
217 }
218
219 /**
220 * Removes the specified element from this set if it is present.
221 * More formally, removes an element e such that
222 * (o==null ? e==null : o.equals(e)),
223 * if this set contains such an element. Returns true if
224 * this set contained the element (or equivalently, if this set
225 * changed as a result of the call). (This set will not contain the
226 * element once the call returns.)
227 *
228 * @param o object to be removed from this set, if present
229 * @return true if this set contained the specified element
230 */
231 public boolean remove(Object o) {
232 return al.remove(o);
233 }
234
235 /**
236 * Adds the specified element to this set if it is not already present.
237 * More formally, adds the specified element e to this set if
238 * the set contains no element e2 such that
239 * (e==null ? e2==null : e.equals(e2)).
240 * If this set already contains the element, the call leaves the set
241 * unchanged and returns false.
242 *
243 * @param e element to be added to this set
244 * @return true if this set did not already contain the specified
245 * element
246 */
247 public boolean add(E e) {
248 return al.addIfAbsent(e);
249 }
250
251 /**
252 * Returns true if this set contains all of the elements of the
253 * specified collection. If the specified collection is also a set, this
254 * method returns true if it is a subset of this set.
255 *
256 * @param c collection to be checked for containment in this set
257 * @return true if this set contains all of the elements of the
258 * specified collection
259 * @throws NullPointerException if the specified collection is null
260 * @see #contains(Object)
261 */
262 public boolean containsAll(Collection<?> c) {
263 return al.containsAll(c);
264 }
265
266 /**
267 * Adds all of the elements in the specified collection to this set if
268 * they're not already present. If the specified collection is also a
269 * set, the addAll operation effectively modifies this set so
270 * that its value is the union of the two sets. The behavior of
271 * this operation is undefined if the specified collection is modified
272 * while the operation is in progress.
273 *
274 * @param c collection containing elements to be added to this set
275 * @return true if this set changed as a result of the call
276 * @throws NullPointerException if the specified collection is null
277 * @see #add(Object)
278 */
279 public boolean addAll(Collection<? extends E> c) {
280 return al.addAllAbsent(c) > 0;
281 }
282
283 /**
284 * Removes from this set all of its elements that are contained in the
285 * specified collection. If the specified collection is also a set,
286 * this operation effectively modifies this set so that its value is the
287 * asymmetric set difference of the two sets.
288 *
289 * @param c collection containing elements to be removed from this set
290 * @return true if this set changed as a result of the call
291 * @throws ClassCastException if the class of an element of this set
292 * is incompatible with the specified collection (optional)
293 * @throws NullPointerException if this set contains a null element and the
294 * specified collection does not permit null elements (optional),
295 * or if the specified collection is null
296 * @see #remove(Object)
297 */
298 public boolean removeAll(Collection<?> c) {
299 return al.removeAll(c);
300 }
301
302 /**
303 * Retains only the elements in this set that are contained in the
304 * specified collection. In other words, removes from this set all of
305 * its elements that are not contained in the specified collection. If
306 * the specified collection is also a set, this operation effectively
307 * modifies this set so that its value is the intersection of the
308 * two sets.
309 *
310 * @param c collection containing elements to be retained in this set
311 * @return true if this set changed as a result of the call
312 * @throws ClassCastException if the class of an element of this set
313 * is incompatible with the specified collection (optional)
314 * @throws NullPointerException if this set contains a null element and the
315 * specified collection does not permit null elements (optional),
316 * or if the specified collection is null
317 * @see #remove(Object)
318 */
319 public boolean retainAll(Collection<?> c) {
320 return al.retainAll(c);
321 }
322
323 /**
324 * Returns an iterator over the elements contained in this set
325 * in the order in which these elements were added.
326 *
327 * The returned iterator provides a snapshot of the state of the set
328 * when the iterator was constructed. No synchronization is needed while
329 * traversing the iterator. The iterator does NOT support the
330 * remove method.
331 *
332 * @return an iterator over the elements in this set
333 */
334 public Iterator iterator() {
335 return al.iterator();
336 }
337
338 /**
339 * Compares the specified object with this set for equality.
340 * Returns {@code true} if the specified object is the same object
341 * as this object, or if it is also a {@link Set} and the elements
342 * returned by an {@linkplain List#iterator() iterator} over the
343 * specified set are the same as the elements returned by an
344 * iterator over this set. More formally, the two iterators are
345 * considered to return the same elements if they return the same
346 * number of elements and for every element {@code e1} returned by
347 * the iterator over the specified set, there is an element
348 * {@code e2} returned by the iterator over this set such that
349 * {@code (e1==null ? e2==null : e1.equals(e2))}.
350 *
351 * @param o object to be compared for equality with this set
352 * @return {@code true} if the specified object is equal to this set
353 */
354 public boolean equals(Object o) {
355 if (o == this)
356 return true;
357 if (!(o instanceof Set))
358 return false;
359 Set<?> set = (Set<?>)(o);
360 Iterator<?> it = set.iterator();
361
362 // Uses O(n^2) algorithm that is only appropriate
363 // for small sets, which CopyOnWriteArraySets should be.
364
365 // Use a single snapshot of underlying array
366 Object[] elements = al.getArray();
367 int len = elements.length;
368 // Mark matched elements to avoid re-checking
369 boolean[] matched = new boolean[len];
370 int k = 0;
371 outer: while (it.hasNext()) {
372 if (++k > len)
373 return false;
374 Object x = it.next();
375 for (int i = 0; i < len; ++i) {
376 if (!matched[i] && eq(x, elements[i])) {
377 matched[i] = true;
378 continue outer;
379 }
380 }
381 return false;
382 }
383 return k == len;
384 }
385
386 /**
387 * Test for equality, coping with nulls.
388 */
389 private static boolean eq(Object o1, Object o2) {
390 return (o1 == null ? o2 == null : o1.equals(o2));
391 }
392 }
CopyOnWriteArraySet是通过CopyOnWriteArrayList实现的，它的API基本上都是通过调用CopyOnWriteArrayList的API来实现的。相信对CopyOnWriteArrayList了解的话，对CopyOnWriteArraySet的了解是水到渠成的事；所以，这里就不再对CopyOnWriteArraySet的代码进行详细的解析了。若对CopyOnWriteArrayList不了解，请参考“Java多线程系列--“JUC集合”02之 CopyOnWriteArrayList”。
Java多线程系列--“JUC集合”01之 框架
2. Java多线程系列--“JUC集合”02之 CopyOnWriteArrayLis
3. Java多线程系列目录(共xx篇)
4. Java 集合系列目录(Category)
说明：
1. CopyOnWriteArraySet继承于AbstractSet，这就意味着它是一个集合。
2. CopyOnWriteArraySet包含CopyOnWriteArrayList对象，它是通过CopyOnWriteArrayList实现的。而CopyOnWriteArrayList本质是个动态数组队列，
所以CopyOnWriteArraySet相当于通过通过动态数组实现的“集合”！ CopyOnWriteArrayList中允许有重复的元素；但是，CopyOnWriteArraySet是一个集合，所以它不能有重复集合。因此，CopyOnWriteArrayList额外提供了addIfAbsent()和addAllAbsent()这两个添加元素的API，通过这些API来添加元素时，只有当元素不存在时才执行添加操作！
至于CopyOnWriteArraySet的“线程安全”机制，和CopyOnWriteArrayList一样，是通过volatile和互斥锁来实现的。这个在前一章节介绍CopyOnWriteArrayList时数据结构时，已经进行了说明，这里就不再重复叙述了。
docRoot}/../technotes/guides/collections/index.html">
82 * Java Collections Framework.
83 *
84 * @see CopyOnWriteArrayList
85 * @since 1.5
86 * @author Doug Lea
87 * @param the type of elements held in this collection
88 */
89 public class CopyOnWriteArraySet extends AbstractSet
90 implements java.io.Serializable {
91 private static final long serialVersionUID = 5457747651344034263L;
92
93 private final CopyOnWriteArrayList al;
94
95 /**
96 * Creates an empty set.
97 */
98 public CopyOnWriteArraySet() {
99 al = new CopyOnWriteArrayList();
100 }
101
102 /**
103 * Creates a set containing all of the elements of the specified
104 * collection.
105 *
106 * @param c the collection of elements to initially contain
107 * @throws NullPointerException if the specified collection is null
108 */
109 public CopyOnWriteArraySet(Collection<? extends E> c) {
110 al = new CopyOnWriteArrayList();
111 al.addAllAbsent(c);
112 }
113
114 /**
115 * Returns the number of elements in this set.
116 *
117 * @return the number of elements in this set
118 */
119 public int size() {
120 return al.size();
121 }
122
123 /**
124 * Returns true if this set contains no elements.
125 *
126 * @return true if this set contains no elements
127 */
128 public boolean isEmpty() {
129 return al.isEmpty();
130 }
131
132 /**
133 * Returns true if this set contains the specified element.
134 * More formally, returns true if and only if this set
135 * contains an element e such that
136 * (o==null ? e==null : o.equals(e)).
137 *
138 * @param o element whose presence in this set is to be tested
139 * @return true if this set contains the specified element
140 */
141 public boolean contains(Object o) {
142 return al.contains(o);
143 }
144
145 /**
146 * Returns an array containing all of the elements in this set.
147 * If this set makes any guarantees as to what order its elements
148 * are returned by its iterator, this method must return the
149 * elements in the same order.
150 *
151 * The returned array will be "safe" in that no references to it
152 * are maintained by this set. (In other words, this method must
153 * allocate a new array even if this set is backed by an array).
154 * The caller is thus free to modify the returned array.
155 *
156 * This method acts as bridge between array-based and collection-based
157 * APIs.
158 *
159 * @return an array containing all the elements in this set
160 */
161 public Object[] toArray() {
162 return al.toArray();
163 }
164
165 /**
166 * Returns an array containing all of the elements in this set; the
167 * runtime type of the returned array is that of the specified array.
168 * If the set fits in the specified array, it is returned therein.
169 * Otherwise, a new array is allocated with the runtime type of the
170 * specified array and the size of this set.
171 *
172 * If this set fits in the specified array with room to spare
173 * (i.e., the array has more elements than this set), the element in
174 * the array immediately following the end of the set is set to
175 * null. (This is useful in determining the length of this
176 * set only if the caller knows that this set does not contain
177 * any null elements.)
178 *
179 * If this set makes any guarantees as to what order its elements
180 * are returned by its iterator, this method must return the elements
181 * in the same order.
182 *
183 * Like the {
@link #toArray()} method, this method acts as bridge between
184 * array-based and collection-based APIs. Further, this method allows
185 * precise control over the runtime type of the output array, and may,
186 * under certain circumstances, be used to save allocation costs.
187 *
188 * Suppose x is a set known to contain only strings.
189 * The following code can be used to dump the set into a newly allocated
190 * array of String:
191 *
192 *
193 * String[] y = x.toArray(new String[0]);
194 *
195 * Note that toArray(new Object[0]) is identical in function to
196 * toArray().
197 *
198 * @param a the array into which the elements of this set are to be
199 * stored, if it is big enough; otherwise, a new array of the same
200 * runtime type is allocated for this purpose.
201 * @return an array containing all the elements in this set
202 * @throws ArrayStoreException if the runtime type of the specified array
203 * is not a supertype of the runtime type of every element in this
204 * set
205 * @throws NullPointerException if the specified array is null
206 */
207 public T[] toArray(T[] a) {
208 return al.toArray(a);
209 }
210
211 /**
212 * Removes all of the elements from this set.
213 * The set will be empty after this call returns.
214 */
215 public void clear() {
216 al.clear();
217 }
218
219 /**
220 * Removes the specified element from this set if it is present.
221 * More formally, removes an element e such that
222 * (o==null ? e==null : o.equals(e)),
223 * if this set contains such an element. Returns true if
224 * this set contained the element (or equivalently, if this set
225 * changed as a result of the call). (This set will not contain the
226 * element once the call returns.)
227 *
228 * @param o object to be removed from this set, if present
229 * @return true if this set contained the specified element
230 */
231 public boolean remove(Object o) {
232 return al.remove(o);
233 }
234
235 /**
236 * Adds the specified element to this set if it is not already present.
237 * More formally, adds the specified element e to this set if
238 * the set contains no element e2 such that
239 * (e==null ? e2==null : e.equals(e2)).
240 * If this set already contains the element, the call leaves the set
241 * unchanged and returns false.
242 *
243 * @param e element to be added to this set
244 * @return true if this set did not already contain the specified
245 * element
246 */
247 public boolean add(E e) {
248 return al.addIfAbsent(e);
249 }
250
251 /**
252 * Returns true if this set contains all of the elements of the
253 * specified collection. If the specified collection is also a set, this
254 * method returns true if it is a subset of this set.
255 *
256 * @param c collection to be checked for containment in this set
257 * @return true if this set contains all of the elements of the
258 * specified collection
259 * @throws NullPointerException if the specified collection is null
260 * @see #contains(Object)
261 */
262 public boolean containsAll(Collection<?> c) {
263 return al.containsAll(c);
264 }
265
266 /**
267 * Adds all of the elements in the specified collection to this set if
268 * they're not already present. If the specified collection is also a
269 * set, the addAll operation effectively modifies this set so
270 * that its value is the union of the two sets. The behavior of
271 * this operation is undefined if the specified collection is modified
272 * while the operation is in progress.
273 *
274 * @param c collection containing elements to be added to this set
275 * @return true if this set changed as a result of the call
276 * @throws NullPointerException if the specified collection is null
277 * @see #add(Object)
278 */
279 public boolean addAll(Collection<? extends E> c) {
280 return al.addAllAbsent(c) > 0;
281 }
282
283 /**
284 * Removes from this set all of its elements that are contained in the
285 * specified collection. If the specified collection is also a set,
286 * this operation effectively modifies this set so that its value is the
287 * asymmetric set difference of the two sets.
288 *
289 * @param c collection containing elements to be removed from this set
290 * @return true if this set changed as a result of the call
291 * @throws ClassCastException if the class of an element of this set
292 * is incompatible with the specified collection (optional)
293 * @throws NullPointerException if this set contains a null element and the
294 * specified collection does not permit null elements (optional),
295 * or if the specified collection is null
296 * @see #remove(Object)
297 */
298 public boolean removeAll(Collection<?> c) {
299 return al.removeAll(c);
300 }
301
302 /**
303 * Retains only the elements in this set that are contained in the
304 * specified collection. In other words, removes from this set all of
305 * its elements that are not contained in the specified collection. If
306 * the specified collection is also a set, this operation effectively
307 * modifies this set so that its value is the intersection of the
308 * two sets.
309 *
310 * @param c collection containing elements to be retained in this set
311 * @return true if this set changed as a result of the call
312 * @throws ClassCastException if the class of an element of this set
313 * is incompatible with the specified collection (optional)
314 * @throws NullPointerException if this set contains a null element and the
315 * specified collection does not permit null elements (optional),
316 * or if the specified collection is null
317 * @see #remove(Object)
318 */
319 public boolean retainAll(Collection<?> c) {
320 return al.retainAll(c);
321 }
322
323 /**
324 * Returns an iterator over the elements contained in this set
325 * in the order in which these elements were added.
326 *
327 * The returned iterator provides a snapshot of the state of the set
328 * when the iterator was constructed. No synchronization is needed while
329 * traversing the iterator. The iterator does NOT support the
330 * remove method.
331 *
332 * @return an iterator over the elements in this set
333 */
334 public Iterator iterator() {
335 return al.iterator();
336 }
337
338 /**
339 * Compares the specified object with this set for equality.
340 * Returns {@code true} if the specified object is the same object
341 * as this object, or if it is also a {@link Set} and the elements
342 * returned by an {@linkplain List#iterator() iterator} over the
343 * specified set are the same as the elements returned by an
344 * iterator over this set. More formally, the two iterators are
345 * considered to return the same elements if they return the same
346 * number of elements and for every element {@code e1} returned by
347 * the iterator over the specified set, there is an element
348 * {@code e2} returned by the iterator over this set such that
349 * {@code (e1==null ? e2==null : e1.equals(e2))}.
350 *
351 * @param o object to be compared for equality with this set
352 * @return {@code true} if the specified object is equal to this set
353 */
354 public boolean equals(Object o) {
355 if (o == this)
356 return true;
357 if (!(o instanceof Set))
358 return false;
359 Set<?> set = (Set<?>)(o);
360 Iterator<?> it = set.iterator();
361
362 // Uses O(n^2) algorithm that is only appropriate
363 // for small sets, which CopyOnWriteArraySets should be.
364
365 // Use a single snapshot of underlying array
366 Object[] elements = al.getArray();
367 int len = elements.length;
368 // Mark matched elements to avoid re-checking
369 boolean[] matched = new boolean[len];
370 int k = 0;
371 outer: while (it.hasNext()) {
372 if (++k > len)
373 return false;
374 Object x = it.next();
375 for (int i = 0; i < len; ++i) {
376 if (!matched[i] && eq(x, elements[i])) {
377 matched[i] = true;
378 continue outer;
379 }
380 }
381 return false;
382 }
383 return k == len;
384 }
385
386 /**
387 * Test for equality, coping with nulls.
388 */
389 private static boolean eq(Object o1, Object o2) {
390 return (o1 == null ? o2 == null : o1.equals(o2));
391 }
392 }
CopyOnWriteArraySet是通过CopyOnWriteArrayList实现的，它的API基本上都是通过调用CopyOnWriteArrayList的API来实现的。相信对CopyOnWriteArrayList了解的话，对CopyOnWriteArraySet的了解是水到渠成的事；所以，这里就不再对CopyOnWriteArraySet的代码进行详细的解析了。若对CopyOnWriteArrayList不了解，请参考“Java多线程系列--“JUC集合”02之 CopyOnWriteArrayList”。
Java多线程系列--“JUC集合”01之 框架
2. Java多线程系列--“JUC集合”02之 CopyOnWriteArrayLis
3. Java多线程系列目录(共xx篇)
4. Java 集合系列目录(Category)
The returned array will be "safe" in that no references to it
152 * are maintained by this set. (In other words, this method must 153 * allocate a new array even if this set is backed by an array). 154 * The caller is thus free to modify the returned array. 155 * 156 *This method acts as bridge between array-based and collection-based
157 * APIs. 158 * 159 * @return an array containing all the elements in this set 160 */ 161 public Object[] toArray() { 162 return al.toArray(); 163 } 164 165 /** 166 * Returns an array containing all of the elements in this set; the 167 * runtime type of the returned array is that of the specified array. 168 * If the set fits in the specified array, it is returned therein. 169 * Otherwise, a new array is allocated with the runtime type of the 170 * specified array and the size of this set. 171 * 172 *If this set fits in the specified array with room to spare
173 * (i.e., the array has more elements than this set), the element in 174 * the array immediately following the end of the set is set to 175 * null. (This is useful in determining the length of this 176 * set only if the caller knows that this set does not contain 177 * any null elements.) 178 * 179 *If this set makes any guarantees as to what order its elements
180 * are returned by its iterator, this method must return the elements 181 * in the same order. 182 * 183 *Like the {
@link #toArray()} method, this method acts as bridge between 184 * array-based and collection-based APIs. Further, this method allows 185 * precise control over the runtime type of the output array, and may, 186 * under certain circumstances, be used to save allocation costs. 187 * 188 *Suppose x is a set known to contain only strings.
189 * The following code can be used to dump the set into a newly allocated 190 * array of String: 191 * 192 *193 * String[] y = x.toArray(new String[0]); 194 * 195 * Note that toArray(new Object[0]) is identical in function to 196 * toArray(). 197 * 198 * @param a the array into which the elements of this set are to be 199 * stored, if it is big enough; otherwise, a new array of the same 200 * runtime type is allocated for this purpose. 201 * @return an array containing all the elements in this set 202 * @throws ArrayStoreException if the runtime type of the specified array 203 * is not a supertype of the runtime type of every element in this 204 * set 205 * @throws NullPointerException if the specified array is null 206 */ 207 public
The returned iterator provides a snapshot of the state of the set
328 * when the iterator was constructed. No synchronization is needed while 329 * traversing the iterator. The iterator does NOT support the 330 * remove method. 331 * 332 * @return an iterator over the elements in this set 333 */ 334 public IteratorCopyOnWriteArraySet是通过CopyOnWriteArrayList实现的，它的API基本上都是通过调用CopyOnWriteArrayList的API来实现的。相信对CopyOnWriteArrayList了解的话，对CopyOnWriteArraySet的了解是水到渠成的事；所以，这里就不再对CopyOnWriteArraySet的代码进行详细的解析了。若对CopyOnWriteArrayList不了解，请参考“Java多线程系列--“JUC集合”02之 CopyOnWriteArrayList”。