ViewVC Help
View File | Revision Log | Show Annotations | Download File | Root Listing
root/jsr166/jsr166/src/jdk7/java/util/concurrent/CopyOnWriteArraySet.java
Revision: 1.3
Committed: Sun Jan 18 20:17:32 2015 UTC (9 years, 4 months ago) by jsr166
Branch: MAIN
CVS Tags: HEAD
Changes since 1.2: +1 -0 lines
Log Message: 
exactly one blank line before and after package statements

File Contents

# User Rev Content
1 dl 1.1 /*
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 jsr166 1.3
9 dl 1.1 import java.util.*;
10    
11     /**
12     * A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList}
13     * for all of its operations. Thus, it shares the same basic properties:
14     * <ul>
15     * <li>It is best suited for applications in which set sizes generally
16     * stay small, read-only operations
17     * vastly outnumber mutative operations, and you need
18     * to prevent interference among threads during traversal.
19     * <li>It is thread-safe.
20 jsr166 1.2 * <li>Mutative operations ({@code add}, {@code set}, {@code remove}, etc.)
21 dl 1.1 * are expensive since they usually entail copying the entire underlying
22     * array.
23 jsr166 1.2 * <li>Iterators do not support the mutative {@code remove} operation.
24 dl 1.1 * <li>Traversal via iterators is fast and cannot encounter
25     * interference from other threads. Iterators rely on
26     * unchanging snapshots of the array at the time the iterators were
27     * constructed.
28     * </ul>
29     *
30     * <p><b>Sample Usage.</b> The following code sketch uses a
31     * copy-on-write set to maintain a set of Handler objects that
32     * perform some action upon state updates.
33     *
34     * <pre> {@code
35     * class Handler { void handle(); ... }
36     *
37     * class X {
38     * private final CopyOnWriteArraySet<Handler> handlers
39     * = new CopyOnWriteArraySet<Handler>();
40     * public void addHandler(Handler h) { handlers.add(h); }
41     *
42     * private long internalState;
43     * private synchronized void changeState() { internalState = ...; }
44     *
45     * public void update() {
46     * changeState();
47     * for (Handler handler : handlers)
48     * handler.handle();
49     * }
50     * }}</pre>
51     *
52     * <p>This class is a member of the
53     * <a href="{@docRoot}/../technotes/guides/collections/index.html">
54     * Java Collections Framework</a>.
55     *
56     * @see CopyOnWriteArrayList
57     * @since 1.5
58     * @author Doug Lea
59     * @param <E> the type of elements held in this collection
60     */
61     public 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 jsr166 1.2 * Returns {@code true} if this set contains no elements.
97 dl 1.1 *
98 jsr166 1.2 * @return {@code true} if this set contains no elements
99 dl 1.1 */
100     public boolean isEmpty() {
101     return al.isEmpty();
102     }
103    
104     /**
105 jsr166 1.2 * Returns {@code true} if this set contains the specified element.
106     * More formally, returns {@code true} if and only if this set
107     * contains an element {@code e} such that
108 dl 1.1 * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
109     *
110     * @param o element whose presence in this set is to be tested
111 jsr166 1.2 * @return {@code true} if this set contains the specified element
112 dl 1.1 */
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 jsr166 1.2 * {@code null}. (This is useful in determining the length of this
148 dl 1.1 * 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 jsr166 1.2 * <p>Suppose {@code x} is a set known to contain only strings.
161 dl 1.1 * The following code can be used to dump the set into a newly allocated
162 jsr166 1.2 * array of {@code String}:
163 dl 1.1 *
164     * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
165     *
166 jsr166 1.2 * Note that {@code toArray(new Object[0])} is identical in function to
167     * {@code toArray()}.
168 dl 1.1 *
169     * @param a the array into which the elements of this set are to be
170     * stored, if it is big enough; otherwise, a new array of the same
171     * runtime type is allocated for this purpose.
172     * @return an array containing all the elements in this set
173     * @throws ArrayStoreException if the runtime type of the specified array
174     * is not a supertype of the runtime type of every element in this
175     * set
176     * @throws NullPointerException if the specified array is null
177     */
178     public <T> T[] toArray(T[] a) {
179     return al.toArray(a);
180     }
181    
182     /**
183     * Removes all of the elements from this set.
184     * The set will be empty after this call returns.
185     */
186     public void clear() {
187     al.clear();
188     }
189    
190     /**
191     * Removes the specified element from this set if it is present.
192 jsr166 1.2 * More formally, removes an element {@code e} such that
193 dl 1.1 * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
194 jsr166 1.2 * if this set contains such an element. Returns {@code true} if
195 dl 1.1 * this set contained the element (or equivalently, if this set
196     * changed as a result of the call). (This set will not contain the
197     * element once the call returns.)
198     *
199     * @param o object to be removed from this set, if present
200 jsr166 1.2 * @return {@code true} if this set contained the specified element
201 dl 1.1 */
202     public boolean remove(Object o) {
203     return al.remove(o);
204     }
205    
206     /**
207     * Adds the specified element to this set if it is not already present.
208 jsr166 1.2 * More formally, adds the specified element {@code e} to this set if
209     * the set contains no element {@code e2} such that
210 dl 1.1 * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
211     * If this set already contains the element, the call leaves the set
212 jsr166 1.2 * unchanged and returns {@code false}.
213 dl 1.1 *
214     * @param e element to be added to this set
215 jsr166 1.2 * @return {@code true} if this set did not already contain the specified
216 dl 1.1 * element
217     */
218     public boolean add(E e) {
219     return al.addIfAbsent(e);
220     }
221    
222     /**
223 jsr166 1.2 * Returns {@code true} if this set contains all of the elements of the
224 dl 1.1 * specified collection. If the specified collection is also a set, this
225 jsr166 1.2 * method returns {@code true} if it is a <i>subset</i> of this set.
226 dl 1.1 *
227     * @param c collection to be checked for containment in this set
228 jsr166 1.2 * @return {@code true} if this set contains all of the elements of the
229 dl 1.1 * specified collection
230     * @throws NullPointerException if the specified collection is null
231     * @see #contains(Object)
232     */
233     public boolean containsAll(Collection<?> c) {
234     return al.containsAll(c);
235     }
236    
237     /**
238     * Adds all of the elements in the specified collection to this set if
239     * they're not already present. If the specified collection is also a
240 jsr166 1.2 * set, the {@code addAll} operation effectively modifies this set so
241 dl 1.1 * that its value is the <i>union</i> of the two sets. The behavior of
242     * this operation is undefined if the specified collection is modified
243     * while the operation is in progress.
244     *
245     * @param c collection containing elements to be added to this set
246 jsr166 1.2 * @return {@code true} if this set changed as a result of the call
247 dl 1.1 * @throws NullPointerException if the specified collection is null
248     * @see #add(Object)
249     */
250     public boolean addAll(Collection<? extends E> c) {
251     return al.addAllAbsent(c) > 0;
252     }
253    
254     /**
255     * Removes from this set all of its elements that are contained in the
256     * specified collection. If the specified collection is also a set,
257     * this operation effectively modifies this set so that its value is the
258     * <i>asymmetric set difference</i> of the two sets.
259     *
260     * @param c collection containing elements to be removed from this set
261 jsr166 1.2 * @return {@code true} if this set changed as a result of the call
262 dl 1.1 * @throws ClassCastException if the class of an element of this set
263     * is incompatible with the specified collection (optional)
264     * @throws NullPointerException if this set contains a null element and the
265     * specified collection does not permit null elements (optional),
266     * or if the specified collection is null
267     * @see #remove(Object)
268     */
269     public boolean removeAll(Collection<?> c) {
270     return al.removeAll(c);
271     }
272    
273     /**
274     * Retains only the elements in this set that are contained in the
275     * specified collection. In other words, removes from this set all of
276     * its elements that are not contained in the specified collection. If
277     * the specified collection is also a set, this operation effectively
278     * modifies this set so that its value is the <i>intersection</i> of the
279     * two sets.
280     *
281     * @param c collection containing elements to be retained in this set
282 jsr166 1.2 * @return {@code true} if this set changed as a result of the call
283 dl 1.1 * @throws ClassCastException if the class of an element of this set
284     * is incompatible with the specified collection (optional)
285     * @throws NullPointerException if this set contains a null element and the
286     * specified collection does not permit null elements (optional),
287     * or if the specified collection is null
288     * @see #remove(Object)
289     */
290     public boolean retainAll(Collection<?> c) {
291     return al.retainAll(c);
292     }
293    
294     /**
295     * Returns an iterator over the elements contained in this set
296     * in the order in which these elements were added.
297     *
298     * <p>The returned iterator provides a snapshot of the state of the set
299     * when the iterator was constructed. No synchronization is needed while
300     * traversing the iterator. The iterator does <em>NOT</em> support the
301 jsr166 1.2 * {@code remove} method.
302 dl 1.1 *
303     * @return an iterator over the elements in this set
304     */
305     public Iterator<E> iterator() {
306     return al.iterator();
307     }
308    
309     /**
310     * Compares the specified object with this set for equality.
311     * Returns {@code true} if the specified object is the same object
312     * as this object, or if it is also a {@link Set} and the elements
313     * returned by an {@linkplain List#iterator() iterator} over the
314     * specified set are the same as the elements returned by an
315     * iterator over this set. More formally, the two iterators are
316     * considered to return the same elements if they return the same
317     * number of elements and for every element {@code e1} returned by
318     * the iterator over the specified set, there is an element
319     * {@code e2} returned by the iterator over this set such that
320     * {@code (e1==null ? e2==null : e1.equals(e2))}.
321     *
322     * @param o object to be compared for equality with this set
323     * @return {@code true} if the specified object is equal to this set
324     */
325     public boolean equals(Object o) {
326     if (o == this)
327     return true;
328     if (!(o instanceof Set))
329     return false;
330     Set<?> set = (Set<?>)(o);
331     Iterator<?> it = set.iterator();
332    
333     // Uses O(n^2) algorithm that is only appropriate
334     // for small sets, which CopyOnWriteArraySets should be.
335    
336     // Use a single snapshot of underlying array
337     Object[] elements = al.getArray();
338     int len = elements.length;
339     // Mark matched elements to avoid re-checking
340     boolean[] matched = new boolean[len];
341     int k = 0;
342     outer: while (it.hasNext()) {
343     if (++k > len)
344     return false;
345     Object x = it.next();
346     for (int i = 0; i < len; ++i) {
347     if (!matched[i] && eq(x, elements[i])) {
348     matched[i] = true;
349     continue outer;
350     }
351     }
352     return false;
353     }
354     return k == len;
355     }
356    
357     /**
358     * Tests for equality, coping with nulls.
359     */
360     private static boolean eq(Object o1, Object o2) {
361     return (o1 == null) ? o2 == null : o1.equals(o2);
362     }
363     }