util/concurrent/ConcurrentSkipListSet.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.*;

/**
 * A scalable concurrent {@link NavigableSet} implementation based on
 * a {@link ConcurrentSkipListMap}.  The elements of the set are kept
 * sorted according to their {@linkplain Comparable natural ordering},
 * or by a {@link Comparator} provided at set creation time, depending
 * on which constructor is used.
 *
 * <p>This implementation provides expected average <i>log(n)</i> time
 * cost for the {@code contains}, {@code add}, and {@code remove}
 * operations and their variants.  Insertion, removal, and access
 * operations safely execute concurrently by multiple threads.
 * Iterators are <i>weakly consistent</i>, returning elements
 * reflecting the state of the set at some point at or since the
 * creation of the iterator.  They do <em>not</em> throw {@link
 * ConcurrentModificationException}, and may proceed concurrently with
 * other operations.  Ascending ordered views and their iterators are
 * faster than descending ones.
 *
 * <p>Beware that, unlike in most collections, the {@code size}
 * method is <em>not</em> a constant-time operation. Because of the
 * asynchronous nature of these sets, determining the current number
 * of elements requires a traversal of the elements, and so may report
 * inaccurate results if this collection is modified during traversal.
 * Additionally, the bulk operations {@code addAll},
 * {@code removeAll}, {@code retainAll}, {@code containsAll},
 * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
 * to be performed atomically. For example, an iterator operating
 * concurrently with an {@code addAll} operation might view only some
 * of the added elements.
 *
 * <p>This class and its iterators implement all of the
 * <em>optional</em> methods of the {@link Set} and {@link Iterator}
 * interfaces. Like most other concurrent collection implementations,
 * this class does not permit the use of {@code null} elements,
 * because {@code null} arguments and return values cannot be reliably
 * distinguished from the absence of elements.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author Doug Lea
 * @param <E> the type of elements maintained by this set
 * @since 1.6
 */
public class ConcurrentSkipListSet<E>
    extends AbstractSet<E>
    implements NavigableSet<E>, Cloneable, java.io.Serializable {

    private static final long serialVersionUID = -2479143111061671589L;

    /**
     * The underlying map. Uses Boolean.TRUE as value for each
     * element.  This field is declared final for the sake of thread
     * safety, which entails some ugliness in clone()
     */
    private final ConcurrentNavigableMap<E,Object> m;

    /**
     * Constructs a new, empty set that orders its elements according to
     * their {@linkplain Comparable natural ordering}.
     */
    public ConcurrentSkipListSet() {
        m = new ConcurrentSkipListMap<E,Object>();
    }

    /**
     * Constructs a new, empty set that orders its elements according to
     * the specified comparator.
     *
     * @param comparator the comparator that will be used to order this set.
     *        If {@code null}, the {@linkplain Comparable natural
     *        ordering} of the elements will be used.
     */
    public ConcurrentSkipListSet(Comparator<? super E> comparator) {
        m = new ConcurrentSkipListMap<E,Object>(comparator);
    }

    /**
     * Constructs a new set containing the elements in the specified
     * collection, that orders its elements according to their
     * {@linkplain Comparable natural ordering}.
     *
     * @param c The elements that will comprise the new set
     * @throws ClassCastException if the elements in {@code c} are
     *         not {@link Comparable}, or are not mutually comparable
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public ConcurrentSkipListSet(Collection<? extends E> c) {
        m = new ConcurrentSkipListMap<E,Object>();
        addAll(c);
    }

    /**
     * Constructs a new set containing the same elements and using the
     * same ordering as the specified sorted set.
     *
     * @param s sorted set whose elements will comprise the new set
     * @throws NullPointerException if the specified sorted set or any
     *         of its elements are null
     */
    public ConcurrentSkipListSet(SortedSet<E> s) {
        m = new ConcurrentSkipListMap<E,Object>(s.comparator());
        addAll(s);
    }

    /**
     * For use by submaps
     */
    ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
        this.m = m;
    }

    /**
     * Returns a shallow copy of this {@code ConcurrentSkipListSet}
     * instance. (The elements themselves are not cloned.)
     *
     * @return a shallow copy of this set
     */
    public ConcurrentSkipListSet<E> clone() {
        try {
            @SuppressWarnings("unchecked")
            ConcurrentSkipListSet<E> clone =
                (ConcurrentSkipListSet<E>) super.clone();
            clone.setMap(new ConcurrentSkipListMap<E,Object>(m));
            return clone;
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
        }
    }

    /* ---------------- Set operations -------------- */

    /**
     * Returns the number of elements in this set.  If this set
     * contains more than {@code Integer.MAX_VALUE} elements, it
     * returns {@code Integer.MAX_VALUE}.
     *
     * <p>Beware that, unlike in most collections, this method is
     * <em>NOT</em> a constant-time operation. Because of the
     * asynchronous nature of these sets, determining the current
     * number of elements requires traversing them all to count them.
     * Additionally, it is possible for the size to change during
     * execution of this method, in which case the returned result
     * will be inaccurate. Thus, this method is typically not very
     * useful in concurrent applications.
     *
     * @return the number of elements in this set
     */
    public int size() {
        return m.size();
    }

    /**
     * Returns {@code true} if this set contains no elements.
     * @return {@code true} if this set contains no elements
     */
    public boolean isEmpty() {
        return m.isEmpty();
    }

    /**
     * Returns {@code true} if this set contains the specified element.
     * More formally, returns {@code true} if and only if this set
     * contains an element {@code e} such that {@code o.equals(e)}.
     *
     * @param o object to be checked for containment in this set
     * @return {@code true} if this set contains the specified element
     * @throws ClassCastException if the specified element cannot be
     *         compared with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     */
    public boolean contains(Object o) {
        return m.containsKey(o);
    }

    /**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element {@code e} to this set if
     * the set contains no element {@code e2} such that {@code e.equals(e2)}.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns {@code false}.
     *
     * @param e element to be added to this set
     * @return {@code true} if this set did not already contain the
     *         specified element
     * @throws ClassCastException if {@code e} cannot be compared
     *         with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return m.putIfAbsent(e, Boolean.TRUE) == null;
    }

    /**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element {@code e} such that
     * {@code o.equals(e)}, if this set contains such an element.
     * Returns {@code true} if this set contained the element (or
     * equivalently, if this set changed as a result of the call).
     * (This set will not contain the element once the call returns.)
     *
     * @param o object to be removed from this set, if present
     * @return {@code true} if this set contained the specified element
     * @throws ClassCastException if {@code o} cannot be compared
     *         with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     */
    public boolean remove(Object o) {
        return m.remove(o, Boolean.TRUE);
    }

    /**
     * Removes all of the elements from this set.
     */
    public void clear() {
        m.clear();
    }

    /**
     * Returns an iterator over the elements in this set in ascending order.
     *
     * @return an iterator over the elements in this set in ascending order
     */
    public Iterator<E> iterator() {
        return m.navigableKeySet().iterator();
    }

    /**
     * Returns an iterator over the elements in this set in descending order.
     *
     * @return an iterator over the elements in this set in descending order
     */
    public Iterator<E> descendingIterator() {
        return m.descendingKeySet().iterator();
    }


    /* ---------------- AbstractSet Overrides -------------- */

    /**
     * Compares the specified object with this set for equality.  Returns
     * {@code true} if the specified object is also a set, the two sets
     * have the same size, and every member of the specified set is
     * contained in this set (or equivalently, every member of this set is
     * contained in the specified set).  This definition ensures that the
     * equals method works properly across different implementations of the
     * set interface.
     *
     * @param o the object to be compared for equality with this set
     * @return {@code true} if the specified object is equal to this set
     */
    public boolean equals(Object o) {
        // Override AbstractSet version to avoid calling size()
        if (o == this)
            return true;
        if (!(o instanceof Set))
            return false;
        Collection<?> c = (Collection<?>) o;
        try {
            return containsAll(c) && c.containsAll(this);
        } catch (ClassCastException unused) {
            return false;
        } catch (NullPointerException unused) {
            return false;
        }
    }

    /**
     * Removes from this set all of its elements that are contained in
     * the specified collection.  If the specified collection is also
     * a set, this operation effectively modifies this set so that its
     * value is the <i>asymmetric set difference</i> of the two sets.
     *
     * @param  c collection containing elements to be removed from this set
     * @return {@code true} if this set changed as a result of the call
     * @throws ClassCastException if the types of one or more elements in this
     *         set are incompatible with the specified collection
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public boolean removeAll(Collection<?> c) {
        // Override AbstractSet version to avoid unnecessary call to size()
        boolean modified = false;
        for (Object e : c)
            if (remove(e))
                modified = true;
        return modified;
    }

    /* ---------------- Relational operations -------------- */

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    public E lower(E e) {
        return m.lowerKey(e);
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    public E floor(E e) {
        return m.floorKey(e);
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    public E ceiling(E e) {
        return m.ceilingKey(e);
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    public E higher(E e) {
        return m.higherKey(e);
    }

    public E pollFirst() {
        Map.Entry<E,Object> e = m.pollFirstEntry();
        return (e == null) ? null : e.getKey();
    }

    public E pollLast() {
        Map.Entry<E,Object> e = m.pollLastEntry();
        return (e == null) ? null : e.getKey();
    }


    /* ---------------- SortedSet operations -------------- */


    public Comparator<? super E> comparator() {
        return m.comparator();
    }

    /**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E first() {
        return m.firstKey();
    }

    /**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E last() {
        return m.lastKey();
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code fromElement} or
     *         {@code toElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public NavigableSet<E> subSet(E fromElement,
                                  boolean fromInclusive,
                                  E toElement,
                                  boolean toInclusive) {
        return new ConcurrentSkipListSet<E>
            (m.subMap(fromElement, fromInclusive,
                      toElement,   toInclusive));
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code toElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
        return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code fromElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
        return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code fromElement} or
     *         {@code toElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public NavigableSet<E> subSet(E fromElement, E toElement) {
        return subSet(fromElement, true, toElement, false);
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code toElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public NavigableSet<E> headSet(E toElement) {
        return headSet(toElement, false);
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if {@code fromElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public NavigableSet<E> tailSet(E fromElement) {
        return tailSet(fromElement, true);
    }

    /**
     * Returns a reverse order view of the elements contained in this set.
     * The descending set is backed by this set, so changes to the set are
     * reflected in the descending set, and vice-versa.
     *
     * <p>The returned set has an ordering equivalent to
     * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
     * The expression {@code s.descendingSet().descendingSet()} returns a
     * view of {@code s} essentially equivalent to {@code s}.
     *
     * @return a reverse order view of this set
     */
    public NavigableSet<E> descendingSet() {
        return new ConcurrentSkipListSet<E>(m.descendingMap());
    }

    // Support for resetting map in clone
    private void setMap(ConcurrentNavigableMap<E,Object> map) {
        UNSAFE.putObjectVolatile(this, mapOffset, map);
    }

    private static final sun.misc.Unsafe UNSAFE;
    private static final long mapOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = ConcurrentSkipListSet.class;
            mapOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("m"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}
Revision:	1.2
Committed:	Wed Jan 16 01:39:37 2013 UTC (11 years, 4 months ago) by jsr166
Branch:	MAIN
Changes since 1.1:	+32 -32 lines
Log Message:	<tt> -> {@code
#	Content
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	/**
11	* A scalable concurrent {@link NavigableSet} implementation based on
12	* a {@link ConcurrentSkipListMap}. The elements of the set are kept
13	* sorted according to their {@linkplain Comparable natural ordering},
14	* or by a {@link Comparator} provided at set creation time, depending
15	* on which constructor is used.
16	*
17	* <p>This implementation provides expected average <i>log(n)</i> time
18	* cost for the {@code contains}, {@code add}, and {@code remove}
19	* operations and their variants. Insertion, removal, and access
20	* operations safely execute concurrently by multiple threads.
21	* Iterators are <i>weakly consistent</i>, returning elements
22	* reflecting the state of the set at some point at or since the
23	* creation of the iterator. They do <em>not</em> throw {@link
24	* ConcurrentModificationException}, and may proceed concurrently with
25	* other operations. Ascending ordered views and their iterators are
26	* faster than descending ones.
27	*
28	* <p>Beware that, unlike in most collections, the {@code size}
29	* method is <em>not</em> a constant-time operation. Because of the
30	* asynchronous nature of these sets, determining the current number
31	* of elements requires a traversal of the elements, and so may report
32	* inaccurate results if this collection is modified during traversal.
33	* Additionally, the bulk operations {@code addAll},
34	* {@code removeAll}, {@code retainAll}, {@code containsAll},
35	* {@code equals}, and {@code toArray} are <em>not</em> guaranteed
36	* to be performed atomically. For example, an iterator operating
37	* concurrently with an {@code addAll} operation might view only some
38	* of the added elements.
39	*
40	* <p>This class and its iterators implement all of the
41	* <em>optional</em> methods of the {@link Set} and {@link Iterator}
42	* interfaces. Like most other concurrent collection implementations,
43	* this class does not permit the use of {@code null} elements,
44	* because {@code null} arguments and return values cannot be reliably
45	* distinguished from the absence of elements.
46	*
47	* <p>This class is a member of the
48	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
49	* Java Collections Framework</a>.
50	*
51	* @author Doug Lea
52	* @param <E> the type of elements maintained by this set
53	* @since 1.6
54	*/
55	public class ConcurrentSkipListSet<E>
56	extends AbstractSet<E>
57	implements NavigableSet<E>, Cloneable, java.io.Serializable {
58
59	private static final long serialVersionUID = -2479143111061671589L;
60
61	/**
62	* The underlying map. Uses Boolean.TRUE as value for each
63	* element. This field is declared final for the sake of thread
64	* safety, which entails some ugliness in clone()
65	*/
66	private final ConcurrentNavigableMap<E,Object> m;
67
68	/**
69	* Constructs a new, empty set that orders its elements according to
70	* their {@linkplain Comparable natural ordering}.
71	*/
72	public ConcurrentSkipListSet() {
73	m = new ConcurrentSkipListMap<E,Object>();
74	}
75
76	/**
77	* Constructs a new, empty set that orders its elements according to
78	* the specified comparator.
79	*
80	* @param comparator the comparator that will be used to order this set.
81	* If {@code null}, the {@linkplain Comparable natural
82	* ordering} of the elements will be used.
83	*/
84	public ConcurrentSkipListSet(Comparator<? super E> comparator) {
85	m = new ConcurrentSkipListMap<E,Object>(comparator);
86	}
87
88	/**
89	* Constructs a new set containing the elements in the specified
90	* collection, that orders its elements according to their
91	* {@linkplain Comparable natural ordering}.
92	*
93	* @param c The elements that will comprise the new set
94	* @throws ClassCastException if the elements in {@code c} are
95	* not {@link Comparable}, or are not mutually comparable
96	* @throws NullPointerException if the specified collection or any
97	* of its elements are null
98	*/
99	public ConcurrentSkipListSet(Collection<? extends E> c) {
100	m = new ConcurrentSkipListMap<E,Object>();
101	addAll(c);
102	}
103
104	/**
105	* Constructs a new set containing the same elements and using the
106	* same ordering as the specified sorted set.
107	*
108	* @param s sorted set whose elements will comprise the new set
109	* @throws NullPointerException if the specified sorted set or any
110	* of its elements are null
111	*/
112	public ConcurrentSkipListSet(SortedSet<E> s) {
113	m = new ConcurrentSkipListMap<E,Object>(s.comparator());
114	addAll(s);
115	}
116
117	/**
118	* For use by submaps
119	*/
120	ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
121	this.m = m;
122	}
123
124	/**
125	* Returns a shallow copy of this {@code ConcurrentSkipListSet}
126	* instance. (The elements themselves are not cloned.)
127	*
128	* @return a shallow copy of this set
129	*/
130	public ConcurrentSkipListSet<E> clone() {
131	try {
132	@SuppressWarnings("unchecked")
133	ConcurrentSkipListSet<E> clone =
134	(ConcurrentSkipListSet<E>) super.clone();
135	clone.setMap(new ConcurrentSkipListMap<E,Object>(m));
136	return clone;
137	} catch (CloneNotSupportedException e) {
138	throw new InternalError();
139	}
140	}
141
142	/* ---------------- Set operations -------------- */
143
144	/**
145	* Returns the number of elements in this set. If this set
146	* contains more than {@code Integer.MAX_VALUE} elements, it
147	* returns {@code Integer.MAX_VALUE}.
148	*
149	* <p>Beware that, unlike in most collections, this method is
150	* <em>NOT</em> a constant-time operation. Because of the
151	* asynchronous nature of these sets, determining the current
152	* number of elements requires traversing them all to count them.
153	* Additionally, it is possible for the size to change during
154	* execution of this method, in which case the returned result
155	* will be inaccurate. Thus, this method is typically not very
156	* useful in concurrent applications.
157	*
158	* @return the number of elements in this set
159	*/
160	public int size() {
161	return m.size();
162	}
163
164	/**
165	* Returns {@code true} if this set contains no elements.
166	* @return {@code true} if this set contains no elements
167	*/
168	public boolean isEmpty() {
169	return m.isEmpty();
170	}
171
172	/**
173	* Returns {@code true} if this set contains the specified element.
174	* More formally, returns {@code true} if and only if this set
175	* contains an element {@code e} such that {@code o.equals(e)}.
176	*
177	* @param o object to be checked for containment in this set
178	* @return {@code true} if this set contains the specified element
179	* @throws ClassCastException if the specified element cannot be
180	* compared with the elements currently in this set
181	* @throws NullPointerException if the specified element is null
182	*/
183	public boolean contains(Object o) {
184	return m.containsKey(o);
185	}
186
187	/**
188	* Adds the specified element to this set if it is not already present.
189	* More formally, adds the specified element {@code e} to this set if
190	* the set contains no element {@code e2} such that {@code e.equals(e2)}.
191	* If this set already contains the element, the call leaves the set
192	* unchanged and returns {@code false}.
193	*
194	* @param e element to be added to this set
195	* @return {@code true} if this set did not already contain the
196	* specified element
197	* @throws ClassCastException if {@code e} cannot be compared
198	* with the elements currently in this set
199	* @throws NullPointerException if the specified element is null
200	*/
201	public boolean add(E e) {
202	return m.putIfAbsent(e, Boolean.TRUE) == null;
203	}
204
205	/**
206	* Removes the specified element from this set if it is present.
207	* More formally, removes an element {@code e} such that
208	* {@code o.equals(e)}, if this set contains such an element.
209	* Returns {@code true} if this set contained the element (or
210	* equivalently, if this set changed as a result of the call).
211	* (This set will not contain the element once the call returns.)
212	*
213	* @param o object to be removed from this set, if present
214	* @return {@code true} if this set contained the specified element
215	* @throws ClassCastException if {@code o} cannot be compared
216	* with the elements currently in this set
217	* @throws NullPointerException if the specified element is null
218	*/
219	public boolean remove(Object o) {
220	return m.remove(o, Boolean.TRUE);
221	}
222
223	/**
224	* Removes all of the elements from this set.
225	*/
226	public void clear() {
227	m.clear();
228	}
229
230	/**
231	* Returns an iterator over the elements in this set in ascending order.
232	*
233	* @return an iterator over the elements in this set in ascending order
234	*/
235	public Iterator<E> iterator() {
236	return m.navigableKeySet().iterator();
237	}
238
239	/**
240	* Returns an iterator over the elements in this set in descending order.
241	*
242	* @return an iterator over the elements in this set in descending order
243	*/
244	public Iterator<E> descendingIterator() {
245	return m.descendingKeySet().iterator();
246	}
247
248
249	/* ---------------- AbstractSet Overrides -------------- */
250
251	/**
252	* Compares the specified object with this set for equality. Returns
253	* {@code true} if the specified object is also a set, the two sets
254	* have the same size, and every member of the specified set is
255	* contained in this set (or equivalently, every member of this set is
256	* contained in the specified set). This definition ensures that the
257	* equals method works properly across different implementations of the
258	* set interface.
259	*
260	* @param o the object to be compared for equality with this set
261	* @return {@code true} if the specified object is equal to this set
262	*/
263	public boolean equals(Object o) {
264	// Override AbstractSet version to avoid calling size()
265	if (o == this)
266	return true;
267	if (!(o instanceof Set))
268	return false;
269	Collection<?> c = (Collection<?>) o;
270	try {
271	return containsAll(c) && c.containsAll(this);
272	} catch (ClassCastException unused) {
273	return false;
274	} catch (NullPointerException unused) {
275	return false;
276	}
277	}
278
279	/**
280	* Removes from this set all of its elements that are contained in
281	* the specified collection. If the specified collection is also
282	* a set, this operation effectively modifies this set so that its
283	* value is the <i>asymmetric set difference</i> of the two sets.
284	*
285	* @param c collection containing elements to be removed from this set
286	* @return {@code true} if this set changed as a result of the call
287	* @throws ClassCastException if the types of one or more elements in this
288	* set are incompatible with the specified collection
289	* @throws NullPointerException if the specified collection or any
290	* of its elements are null
291	*/
292	public boolean removeAll(Collection<?> c) {
293	// Override AbstractSet version to avoid unnecessary call to size()
294	boolean modified = false;
295	for (Object e : c)
296	if (remove(e))
297	modified = true;
298	return modified;
299	}
300
301	/* ---------------- Relational operations -------------- */
302
303	/**
304	* @throws ClassCastException {@inheritDoc}
305	* @throws NullPointerException if the specified element is null
306	*/
307	public E lower(E e) {
308	return m.lowerKey(e);
309	}
310
311	/**
312	* @throws ClassCastException {@inheritDoc}
313	* @throws NullPointerException if the specified element is null
314	*/
315	public E floor(E e) {
316	return m.floorKey(e);
317	}
318
319	/**
320	* @throws ClassCastException {@inheritDoc}
321	* @throws NullPointerException if the specified element is null
322	*/
323	public E ceiling(E e) {
324	return m.ceilingKey(e);
325	}
326
327	/**
328	* @throws ClassCastException {@inheritDoc}
329	* @throws NullPointerException if the specified element is null
330	*/
331	public E higher(E e) {
332	return m.higherKey(e);
333	}
334
335	public E pollFirst() {
336	Map.Entry<E,Object> e = m.pollFirstEntry();
337	return (e == null) ? null : e.getKey();
338	}
339
340	public E pollLast() {
341	Map.Entry<E,Object> e = m.pollLastEntry();
342	return (e == null) ? null : e.getKey();
343	}
344
345
346	/* ---------------- SortedSet operations -------------- */
347
348
349	public Comparator<? super E> comparator() {
350	return m.comparator();
351	}
352
353	/**
354	* @throws NoSuchElementException {@inheritDoc}
355	*/
356	public E first() {
357	return m.firstKey();
358	}
359
360	/**
361	* @throws NoSuchElementException {@inheritDoc}
362	*/
363	public E last() {
364	return m.lastKey();
365	}
366
367	/**
368	* @throws ClassCastException {@inheritDoc}
369	* @throws NullPointerException if {@code fromElement} or
370	* {@code toElement} is null
371	* @throws IllegalArgumentException {@inheritDoc}
372	*/
373	public NavigableSet<E> subSet(E fromElement,
374	boolean fromInclusive,
375	E toElement,
376	boolean toInclusive) {
377	return new ConcurrentSkipListSet<E>
378	(m.subMap(fromElement, fromInclusive,
379	toElement, toInclusive));
380	}
381
382	/**
383	* @throws ClassCastException {@inheritDoc}
384	* @throws NullPointerException if {@code toElement} is null
385	* @throws IllegalArgumentException {@inheritDoc}
386	*/
387	public NavigableSet<E> headSet(E toElement, boolean inclusive) {
388	return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
389	}
390
391	/**
392	* @throws ClassCastException {@inheritDoc}
393	* @throws NullPointerException if {@code fromElement} is null
394	* @throws IllegalArgumentException {@inheritDoc}
395	*/
396	public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
397	return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
398	}
399
400	/**
401	* @throws ClassCastException {@inheritDoc}
402	* @throws NullPointerException if {@code fromElement} or
403	* {@code toElement} is null
404	* @throws IllegalArgumentException {@inheritDoc}
405	*/
406	public NavigableSet<E> subSet(E fromElement, E toElement) {
407	return subSet(fromElement, true, toElement, false);
408	}
409
410	/**
411	* @throws ClassCastException {@inheritDoc}
412	* @throws NullPointerException if {@code toElement} is null
413	* @throws IllegalArgumentException {@inheritDoc}
414	*/
415	public NavigableSet<E> headSet(E toElement) {
416	return headSet(toElement, false);
417	}
418
419	/**
420	* @throws ClassCastException {@inheritDoc}
421	* @throws NullPointerException if {@code fromElement} is null
422	* @throws IllegalArgumentException {@inheritDoc}
423	*/
424	public NavigableSet<E> tailSet(E fromElement) {
425	return tailSet(fromElement, true);
426	}
427
428	/**
429	* Returns a reverse order view of the elements contained in this set.
430	* The descending set is backed by this set, so changes to the set are
431	* reflected in the descending set, and vice-versa.
432	*
433	* <p>The returned set has an ordering equivalent to
434	* <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
435	* The expression {@code s.descendingSet().descendingSet()} returns a
436	* view of {@code s} essentially equivalent to {@code s}.
437	*
438	* @return a reverse order view of this set
439	*/
440	public NavigableSet<E> descendingSet() {
441	return new ConcurrentSkipListSet<E>(m.descendingMap());
442	}
443
444	// Support for resetting map in clone
445	private void setMap(ConcurrentNavigableMap<E,Object> map) {
446	UNSAFE.putObjectVolatile(this, mapOffset, map);
447	}
448
449	private static final sun.misc.Unsafe UNSAFE;
450	private static final long mapOffset;
451	static {
452	try {
453	UNSAFE = sun.misc.Unsafe.getUnsafe();
454	Class<?> k = ConcurrentSkipListSet.class;
455	mapOffset = UNSAFE.objectFieldOffset
456	(k.getDeclaredField("m"));
457	} catch (Exception e) {
458	throw new Error(e);
459	}
460	}
461	}