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
#	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			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	jsr166	1.2	* cost for the {@code contains}, {@code add}, and {@code remove}
19	dl	1.1	* 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	jsr166	1.2	* <p>Beware that, unlike in most collections, the {@code size}
29	dl	1.1	* 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	jsr166	1.2	* 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	dl	1.1	* to be performed atomically. For example, an iterator operating
37	jsr166	1.2	* concurrently with an {@code addAll} operation might view only some
38	dl	1.1	* 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	jsr166	1.2	* this class does not permit the use of {@code null} elements,
44			* because {@code null} arguments and return values cannot be reliably
45	dl	1.1	* 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	jsr166	1.2	* If {@code null}, the {@linkplain Comparable natural
82	dl	1.1	* 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	jsr166	1.2	* @throws ClassCastException if the elements in {@code c} are
95	dl	1.1	* 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	jsr166	1.2	* Returns a shallow copy of this {@code ConcurrentSkipListSet}
126	dl	1.1	* 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	jsr166	1.2	* contains more than {@code Integer.MAX_VALUE} elements, it
147			* returns {@code Integer.MAX_VALUE}.
148	dl	1.1	*
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	jsr166	1.2	* Returns {@code true} if this set contains no elements.
166			* @return {@code true} if this set contains no elements
167	dl	1.1	*/
168			public boolean isEmpty() {
169			return m.isEmpty();
170			}
171
172			/**
173	jsr166	1.2	* 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	dl	1.1	*
177			* @param o object to be checked for containment in this set
178	jsr166	1.2	* @return {@code true} if this set contains the specified element
179	dl	1.1	* @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	jsr166	1.2	* 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	dl	1.1	* If this set already contains the element, the call leaves the set
192	jsr166	1.2	* unchanged and returns {@code false}.
193	dl	1.1	*
194			* @param e element to be added to this set
195	jsr166	1.2	* @return {@code true} if this set did not already contain the
196	dl	1.1	* specified element
197	jsr166	1.2	* @throws ClassCastException if {@code e} cannot be compared
198	dl	1.1	* 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	jsr166	1.2	* 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	dl	1.1	* 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	jsr166	1.2	* @return {@code true} if this set contained the specified element
215			* @throws ClassCastException if {@code o} cannot be compared
216	dl	1.1	* 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	jsr166	1.2	* {@code true} if the specified object is also a set, the two sets
254	dl	1.1	* 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	jsr166	1.2	* @return {@code true} if the specified object is equal to this set
262	dl	1.1	*/
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	jsr166	1.2	* @return {@code true} if this set changed as a result of the call
287	dl	1.1	* @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			}