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, 4 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
#	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	1.5
9	dl	1.1	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	jsr166	1.2	* cost for the {@code contains}, {@code add}, and {@code remove}
20	dl	1.1	* 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	jsr166	1.2	* <p>Beware that, unlike in most collections, the {@code size}
30	dl	1.1	* 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	jsr166	1.2	* 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	dl	1.1	* to be performed atomically. For example, an iterator operating
38	jsr166	1.2	* concurrently with an {@code addAll} operation might view only some
39	dl	1.1	* 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	jsr166	1.2	* this class does not permit the use of {@code null} elements,
45			* because {@code null} arguments and return values cannot be reliably
46	dl	1.1	* 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	jsr166	1.3	* safety, which entails some ugliness in clone().
66	dl	1.1	*/
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	jsr166	1.2	* If {@code null}, the {@linkplain Comparable natural
83	dl	1.1	* 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	jsr166	1.2	* @throws ClassCastException if the elements in {@code c} are
96	dl	1.1	* 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	jsr166	1.2	* Returns a shallow copy of this {@code ConcurrentSkipListSet}
127	dl	1.1	* 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	jsr166	1.2	* contains more than {@code Integer.MAX_VALUE} elements, it
148			* returns {@code Integer.MAX_VALUE}.
149	dl	1.1	*
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	jsr166	1.2	* Returns {@code true} if this set contains no elements.
167			* @return {@code true} if this set contains no elements
168	dl	1.1	*/
169			public boolean isEmpty() {
170			return m.isEmpty();
171			}
172
173			/**
174	jsr166	1.2	* 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	dl	1.1	*
178			* @param o object to be checked for containment in this set
179	jsr166	1.2	* @return {@code true} if this set contains the specified element
180	dl	1.1	* @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	jsr166	1.2	* 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	dl	1.1	* If this set already contains the element, the call leaves the set
193	jsr166	1.2	* unchanged and returns {@code false}.
194	dl	1.1	*
195			* @param e element to be added to this set
196	jsr166	1.2	* @return {@code true} if this set did not already contain the
197	dl	1.1	* specified element
198	jsr166	1.2	* @throws ClassCastException if {@code e} cannot be compared
199	dl	1.1	* 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	jsr166	1.2	* 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	dl	1.1	* 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	jsr166	1.2	* @return {@code true} if this set contained the specified element
216			* @throws ClassCastException if {@code o} cannot be compared
217	dl	1.1	* 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	jsr166	1.2	* {@code true} if the specified object is also a set, the two sets
255	dl	1.1	* 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	jsr166	1.2	* @return {@code true} if the specified object is equal to this set
263	dl	1.1	*/
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	jsr166	1.2	* @return {@code true} if this set changed as a result of the call
288	dl	1.1	* @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	jsr166	1.4	* {@link Collections#reverseOrder(Comparator) Collections.reverseOrder}{@code (comparator())}.
436	dl	1.1	* 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			}