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
     * {@link Collections#reverseOrder(Comparator) Collections.reverseOrder}{@code (comparator())}.
     * 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:	1.5
Committed:	Sun Jan 18 20:17:32 2015 UTC (9 years, 3 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.4:	+1 -0 lines
Log Message:	exactly one blank line before and after package statements
#	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
9	import java.util.*;
10
11	/**
12	* A scalable concurrent {@link NavigableSet} implementation based on
13	* a {@link ConcurrentSkipListMap}. The elements of the set are kept
14	* sorted according to their {@linkplain Comparable natural ordering},
15	* or by a {@link Comparator} provided at set creation time, depending
16	* on which constructor is used.
17	*
18	* <p>This implementation provides expected average <i>log(n)</i> time
19	* cost for the {@code contains}, {@code add}, and {@code remove}
20	* operations and their variants. Insertion, removal, and access
21	* operations safely execute concurrently by multiple threads.
22	* Iterators are <i>weakly consistent</i>, returning elements
23	* reflecting the state of the set at some point at or since the
24	* creation of the iterator. They do <em>not</em> throw {@link
25	* ConcurrentModificationException}, and may proceed concurrently with
26	* other operations. Ascending ordered views and their iterators are
27	* faster than descending ones.
28	*
29	* <p>Beware that, unlike in most collections, the {@code size}
30	* method is <em>not</em> a constant-time operation. Because of the
31	* asynchronous nature of these sets, determining the current number
32	* of elements requires a traversal of the elements, and so may report
33	* inaccurate results if this collection is modified during traversal.
34	* Additionally, the bulk operations {@code addAll},
35	* {@code removeAll}, {@code retainAll}, {@code containsAll},
36	* {@code equals}, and {@code toArray} are <em>not</em> guaranteed
37	* to be performed atomically. For example, an iterator operating
38	* concurrently with an {@code addAll} operation might view only some
39	* of the added elements.
40	*
41	* <p>This class and its iterators implement all of the
42	* <em>optional</em> methods of the {@link Set} and {@link Iterator}
43	* interfaces. Like most other concurrent collection implementations,
44	* this class does not permit the use of {@code null} elements,
45	* because {@code null} arguments and return values cannot be reliably
46	* distinguished from the absence of elements.
47	*
48	* <p>This class is a member of the
49	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
50	* Java Collections Framework</a>.
51	*
52	* @author Doug Lea
53	* @param <E> the type of elements maintained by this set
54	* @since 1.6
55	*/
56	public class ConcurrentSkipListSet<E>
57	extends AbstractSet<E>
58	implements NavigableSet<E>, Cloneable, java.io.Serializable {
59
60	private static final long serialVersionUID = -2479143111061671589L;
61
62	/**
63	* The underlying map. Uses Boolean.TRUE as value for each
64	* element. This field is declared final for the sake of thread
65	* safety, which entails some ugliness in clone().
66	*/
67	private final ConcurrentNavigableMap<E,Object> m;
68
69	/**
70	* Constructs a new, empty set that orders its elements according to
71	* their {@linkplain Comparable natural ordering}.
72	*/
73	public ConcurrentSkipListSet() {
74	m = new ConcurrentSkipListMap<E,Object>();
75	}
76
77	/**
78	* Constructs a new, empty set that orders its elements according to
79	* the specified comparator.
80	*
81	* @param comparator the comparator that will be used to order this set.
82	* If {@code null}, the {@linkplain Comparable natural
83	* ordering} of the elements will be used.
84	*/
85	public ConcurrentSkipListSet(Comparator<? super E> comparator) {
86	m = new ConcurrentSkipListMap<E,Object>(comparator);
87	}
88
89	/**
90	* Constructs a new set containing the elements in the specified
91	* collection, that orders its elements according to their
92	* {@linkplain Comparable natural ordering}.
93	*
94	* @param c The elements that will comprise the new set
95	* @throws ClassCastException if the elements in {@code c} are
96	* not {@link Comparable}, or are not mutually comparable
97	* @throws NullPointerException if the specified collection or any
98	* of its elements are null
99	*/
100	public ConcurrentSkipListSet(Collection<? extends E> c) {
101	m = new ConcurrentSkipListMap<E,Object>();
102	addAll(c);
103	}
104
105	/**
106	* Constructs a new set containing the same elements and using the
107	* same ordering as the specified sorted set.
108	*
109	* @param s sorted set whose elements will comprise the new set
110	* @throws NullPointerException if the specified sorted set or any
111	* of its elements are null
112	*/
113	public ConcurrentSkipListSet(SortedSet<E> s) {
114	m = new ConcurrentSkipListMap<E,Object>(s.comparator());
115	addAll(s);
116	}
117
118	/**
119	* For use by submaps
120	*/
121	ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
122	this.m = m;
123	}
124
125	/**
126	* Returns a shallow copy of this {@code ConcurrentSkipListSet}
127	* instance. (The elements themselves are not cloned.)
128	*
129	* @return a shallow copy of this set
130	*/
131	public ConcurrentSkipListSet<E> clone() {
132	try {
133	@SuppressWarnings("unchecked")
134	ConcurrentSkipListSet<E> clone =
135	(ConcurrentSkipListSet<E>) super.clone();
136	clone.setMap(new ConcurrentSkipListMap<E,Object>(m));
137	return clone;
138	} catch (CloneNotSupportedException e) {
139	throw new InternalError();
140	}
141	}
142
143	/* ---------------- Set operations -------------- */
144
145	/**
146	* Returns the number of elements in this set. If this set
147	* contains more than {@code Integer.MAX_VALUE} elements, it
148	* returns {@code Integer.MAX_VALUE}.
149	*
150	* <p>Beware that, unlike in most collections, this method is
151	* <em>NOT</em> a constant-time operation. Because of the
152	* asynchronous nature of these sets, determining the current
153	* number of elements requires traversing them all to count them.
154	* Additionally, it is possible for the size to change during
155	* execution of this method, in which case the returned result
156	* will be inaccurate. Thus, this method is typically not very
157	* useful in concurrent applications.
158	*
159	* @return the number of elements in this set
160	*/
161	public int size() {
162	return m.size();
163	}
164
165	/**
166	* Returns {@code true} if this set contains no elements.
167	* @return {@code true} if this set contains no elements
168	*/
169	public boolean isEmpty() {
170	return m.isEmpty();
171	}
172
173	/**
174	* Returns {@code true} if this set contains the specified element.
175	* More formally, returns {@code true} if and only if this set
176	* contains an element {@code e} such that {@code o.equals(e)}.
177	*
178	* @param o object to be checked for containment in this set
179	* @return {@code true} if this set contains the specified element
180	* @throws ClassCastException if the specified element cannot be
181	* compared with the elements currently in this set
182	* @throws NullPointerException if the specified element is null
183	*/
184	public boolean contains(Object o) {
185	return m.containsKey(o);
186	}
187
188	/**
189	* Adds the specified element to this set if it is not already present.
190	* More formally, adds the specified element {@code e} to this set if
191	* the set contains no element {@code e2} such that {@code e.equals(e2)}.
192	* If this set already contains the element, the call leaves the set
193	* unchanged and returns {@code false}.
194	*
195	* @param e element to be added to this set
196	* @return {@code true} if this set did not already contain the
197	* specified element
198	* @throws ClassCastException if {@code e} cannot be compared
199	* with the elements currently in this set
200	* @throws NullPointerException if the specified element is null
201	*/
202	public boolean add(E e) {
203	return m.putIfAbsent(e, Boolean.TRUE) == null;
204	}
205
206	/**
207	* Removes the specified element from this set if it is present.
208	* More formally, removes an element {@code e} such that
209	* {@code o.equals(e)}, if this set contains such an element.
210	* Returns {@code true} if this set contained the element (or
211	* equivalently, if this set changed as a result of the call).
212	* (This set will not contain the element once the call returns.)
213	*
214	* @param o object to be removed from this set, if present
215	* @return {@code true} if this set contained the specified element
216	* @throws ClassCastException if {@code o} cannot be compared
217	* with the elements currently in this set
218	* @throws NullPointerException if the specified element is null
219	*/
220	public boolean remove(Object o) {
221	return m.remove(o, Boolean.TRUE);
222	}
223
224	/**
225	* Removes all of the elements from this set.
226	*/
227	public void clear() {
228	m.clear();
229	}
230
231	/**
232	* Returns an iterator over the elements in this set in ascending order.
233	*
234	* @return an iterator over the elements in this set in ascending order
235	*/
236	public Iterator<E> iterator() {
237	return m.navigableKeySet().iterator();
238	}
239
240	/**
241	* Returns an iterator over the elements in this set in descending order.
242	*
243	* @return an iterator over the elements in this set in descending order
244	*/
245	public Iterator<E> descendingIterator() {
246	return m.descendingKeySet().iterator();
247	}
248
249
250	/* ---------------- AbstractSet Overrides -------------- */
251
252	/**
253	* Compares the specified object with this set for equality. Returns
254	* {@code true} if the specified object is also a set, the two sets
255	* have the same size, and every member of the specified set is
256	* contained in this set (or equivalently, every member of this set is
257	* contained in the specified set). This definition ensures that the
258	* equals method works properly across different implementations of the
259	* set interface.
260	*
261	* @param o the object to be compared for equality with this set
262	* @return {@code true} if the specified object is equal to this set
263	*/
264	public boolean equals(Object o) {
265	// Override AbstractSet version to avoid calling size()
266	if (o == this)
267	return true;
268	if (!(o instanceof Set))
269	return false;
270	Collection<?> c = (Collection<?>) o;
271	try {
272	return containsAll(c) && c.containsAll(this);
273	} catch (ClassCastException unused) {
274	return false;
275	} catch (NullPointerException unused) {
276	return false;
277	}
278	}
279
280	/**
281	* Removes from this set all of its elements that are contained in
282	* the specified collection. If the specified collection is also
283	* a set, this operation effectively modifies this set so that its
284	* value is the <i>asymmetric set difference</i> of the two sets.
285	*
286	* @param c collection containing elements to be removed from this set
287	* @return {@code true} if this set changed as a result of the call
288	* @throws ClassCastException if the types of one or more elements in this
289	* set are incompatible with the specified collection
290	* @throws NullPointerException if the specified collection or any
291	* of its elements are null
292	*/
293	public boolean removeAll(Collection<?> c) {
294	// Override AbstractSet version to avoid unnecessary call to size()
295	boolean modified = false;
296	for (Object e : c)
297	if (remove(e))
298	modified = true;
299	return modified;
300	}
301
302	/* ---------------- Relational operations -------------- */
303
304	/**
305	* @throws ClassCastException {@inheritDoc}
306	* @throws NullPointerException if the specified element is null
307	*/
308	public E lower(E e) {
309	return m.lowerKey(e);
310	}
311
312	/**
313	* @throws ClassCastException {@inheritDoc}
314	* @throws NullPointerException if the specified element is null
315	*/
316	public E floor(E e) {
317	return m.floorKey(e);
318	}
319
320	/**
321	* @throws ClassCastException {@inheritDoc}
322	* @throws NullPointerException if the specified element is null
323	*/
324	public E ceiling(E e) {
325	return m.ceilingKey(e);
326	}
327
328	/**
329	* @throws ClassCastException {@inheritDoc}
330	* @throws NullPointerException if the specified element is null
331	*/
332	public E higher(E e) {
333	return m.higherKey(e);
334	}
335
336	public E pollFirst() {
337	Map.Entry<E,Object> e = m.pollFirstEntry();
338	return (e == null) ? null : e.getKey();
339	}
340
341	public E pollLast() {
342	Map.Entry<E,Object> e = m.pollLastEntry();
343	return (e == null) ? null : e.getKey();
344	}
345
346
347	/* ---------------- SortedSet operations -------------- */
348
349
350	public Comparator<? super E> comparator() {
351	return m.comparator();
352	}
353
354	/**
355	* @throws NoSuchElementException {@inheritDoc}
356	*/
357	public E first() {
358	return m.firstKey();
359	}
360
361	/**
362	* @throws NoSuchElementException {@inheritDoc}
363	*/
364	public E last() {
365	return m.lastKey();
366	}
367
368	/**
369	* @throws ClassCastException {@inheritDoc}
370	* @throws NullPointerException if {@code fromElement} or
371	* {@code toElement} is null
372	* @throws IllegalArgumentException {@inheritDoc}
373	*/
374	public NavigableSet<E> subSet(E fromElement,
375	boolean fromInclusive,
376	E toElement,
377	boolean toInclusive) {
378	return new ConcurrentSkipListSet<E>
379	(m.subMap(fromElement, fromInclusive,
380	toElement, toInclusive));
381	}
382
383	/**
384	* @throws ClassCastException {@inheritDoc}
385	* @throws NullPointerException if {@code toElement} is null
386	* @throws IllegalArgumentException {@inheritDoc}
387	*/
388	public NavigableSet<E> headSet(E toElement, boolean inclusive) {
389	return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
390	}
391
392	/**
393	* @throws ClassCastException {@inheritDoc}
394	* @throws NullPointerException if {@code fromElement} is null
395	* @throws IllegalArgumentException {@inheritDoc}
396	*/
397	public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
398	return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
399	}
400
401	/**
402	* @throws ClassCastException {@inheritDoc}
403	* @throws NullPointerException if {@code fromElement} or
404	* {@code toElement} is null
405	* @throws IllegalArgumentException {@inheritDoc}
406	*/
407	public NavigableSet<E> subSet(E fromElement, E toElement) {
408	return subSet(fromElement, true, toElement, false);
409	}
410
411	/**
412	* @throws ClassCastException {@inheritDoc}
413	* @throws NullPointerException if {@code toElement} is null
414	* @throws IllegalArgumentException {@inheritDoc}
415	*/
416	public NavigableSet<E> headSet(E toElement) {
417	return headSet(toElement, false);
418	}
419
420	/**
421	* @throws ClassCastException {@inheritDoc}
422	* @throws NullPointerException if {@code fromElement} is null
423	* @throws IllegalArgumentException {@inheritDoc}
424	*/
425	public NavigableSet<E> tailSet(E fromElement) {
426	return tailSet(fromElement, true);
427	}
428
429	/**
430	* Returns a reverse order view of the elements contained in this set.
431	* The descending set is backed by this set, so changes to the set are
432	* reflected in the descending set, and vice-versa.
433	*
434	* <p>The returned set has an ordering equivalent to
435	* {@link Collections#reverseOrder(Comparator) Collections.reverseOrder}{@code (comparator())}.
436	* The expression {@code s.descendingSet().descendingSet()} returns a
437	* view of {@code s} essentially equivalent to {@code s}.
438	*
439	* @return a reverse order view of this set
440	*/
441	public NavigableSet<E> descendingSet() {
442	return new ConcurrentSkipListSet<E>(m.descendingMap());
443	}
444
445	// Support for resetting map in clone
446	private void setMap(ConcurrentNavigableMap<E,Object> map) {
447	UNSAFE.putObjectVolatile(this, mapOffset, map);
448	}
449
450	private static final sun.misc.Unsafe UNSAFE;
451	private static final long mapOffset;
452	static {
453	try {
454	UNSAFE = sun.misc.Unsafe.getUnsafe();
455	Class<?> k = ConcurrentSkipListSet.class;
456	mapOffset = UNSAFE.objectFieldOffset
457	(k.getDeclaredField("m"));
458	} catch (Exception e) {
459	throw new Error(e);
460	}
461	}
462	}