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.AbstractSet;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.Map;
import java.util.NavigableMap;
import java.util.NavigableSet;
import java.util.Set;
import java.util.SortedSet;
import java.util.Spliterator;

/**
 * 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.
 *
 * <p>Iterators and spliterators are
 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
 *
 * <p>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 class of an element of this set
     *         is incompatible with the specified collection
     * (<a href="{@docRoot}/../api/java/util/Collection.html#optional-restrictions">optional</a>)
     * @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 java.util.NoSuchElementException {@inheritDoc}
     */
    public E first() {
        return m.firstKey();
    }

    /**
     * @throws java.util.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());
    }

    /**
     * Returns a {@link Spliterator} over the elements in this set.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#NONNULL}, {@link Spliterator#DISTINCT},
     * {@link Spliterator#SORTED} and {@link Spliterator#ORDERED}, with an
     * encounter order that is ascending order.  Overriding implementations
     * should document the reporting of additional characteristic values.
     *
     * <p>The spliterator's comparator (see
     * {@link java.util.Spliterator#getComparator()}) is {@code null} if
     * the set's comparator (see {@link #comparator()}) is {@code null}.
     * Otherwise, the spliterator's comparator is the same as or imposes the
     * same total ordering as the set's comparator.
     *
     * @return a {@code Spliterator} over the elements in this set
     * @since 1.8
     */
    public Spliterator<E> spliterator() {
        return (m instanceof ConcurrentSkipListMap)
            ? ((ConcurrentSkipListMap<E,?>)m).keySpliterator()
            : ((ConcurrentSkipListMap.SubMap<E,?>)m).new SubMapKeyIterator();
    }

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

    private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
    private static final long MAP;
    static {
        try {
            MAP = U.objectFieldOffset
                (ConcurrentSkipListSet.class.getDeclaredField("m"));
        } catch (ReflectiveOperationException e) {
            throw new Error(e);
        }
    }
}
Revision:	1.1
Committed:	Sat Mar 26 06:22:50 2016 UTC (8 years, 2 months ago) by jsr166
Branch:	MAIN
Log Message:	fork jdk8 maintenance branch for source and jtreg tests
#	User	Rev	Content
1	jsr166	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
9			import java.util.AbstractSet;
10			import java.util.Collection;
11			import java.util.Collections;
12			import java.util.Comparator;
13			import java.util.Iterator;
14			import java.util.Map;
15			import java.util.NavigableMap;
16			import java.util.NavigableSet;
17			import java.util.Set;
18			import java.util.SortedSet;
19			import java.util.Spliterator;
20
21			/**
22			* A scalable concurrent {@link NavigableSet} implementation based on
23			* a {@link ConcurrentSkipListMap}. The elements of the set are kept
24			* sorted according to their {@linkplain Comparable natural ordering},
25			* or by a {@link Comparator} provided at set creation time, depending
26			* on which constructor is used.
27			*
28			* <p>This implementation provides expected average <i>log(n)</i> time
29			* cost for the {@code contains}, {@code add}, and {@code remove}
30			* operations and their variants. Insertion, removal, and access
31			* operations safely execute concurrently by multiple threads.
32			*
33			* <p>Iterators and spliterators are
34			* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
35			*
36			* <p>Ascending ordered views and their iterators are faster than
37			* descending ones.
38			*
39			* <p>Beware that, unlike in most collections, the {@code size}
40			* method is <em>not</em> a constant-time operation. Because of the
41			* asynchronous nature of these sets, determining the current number
42			* of elements requires a traversal of the elements, and so may report
43			* inaccurate results if this collection is modified during traversal.
44			* Additionally, the bulk operations {@code addAll},
45			* {@code removeAll}, {@code retainAll}, {@code containsAll},
46			* {@code equals}, and {@code toArray} are <em>not</em> guaranteed
47			* to be performed atomically. For example, an iterator operating
48			* concurrently with an {@code addAll} operation might view only some
49			* of the added elements.
50			*
51			* <p>This class and its iterators implement all of the
52			* <em>optional</em> methods of the {@link Set} and {@link Iterator}
53			* interfaces. Like most other concurrent collection implementations,
54			* this class does not permit the use of {@code null} elements,
55			* because {@code null} arguments and return values cannot be reliably
56			* distinguished from the absence of elements.
57			*
58			* <p>This class is a member of the
59			* <a href="{@docRoot}/../technotes/guides/collections/index.html">
60			* Java Collections Framework</a>.
61			*
62			* @author Doug Lea
63			* @param <E> the type of elements maintained by this set
64			* @since 1.6
65			*/
66			public class ConcurrentSkipListSet<E>
67			extends AbstractSet<E>
68			implements NavigableSet<E>, Cloneable, java.io.Serializable {
69
70			private static final long serialVersionUID = -2479143111061671589L;
71
72			/**
73			* The underlying map. Uses Boolean.TRUE as value for each
74			* element. This field is declared final for the sake of thread
75			* safety, which entails some ugliness in clone().
76			*/
77			private final ConcurrentNavigableMap<E,Object> m;
78
79			/**
80			* Constructs a new, empty set that orders its elements according to
81			* their {@linkplain Comparable natural ordering}.
82			*/
83			public ConcurrentSkipListSet() {
84			m = new ConcurrentSkipListMap<E,Object>();
85			}
86
87			/**
88			* Constructs a new, empty set that orders its elements according to
89			* the specified comparator.
90			*
91			* @param comparator the comparator that will be used to order this set.
92			* If {@code null}, the {@linkplain Comparable natural
93			* ordering} of the elements will be used.
94			*/
95			public ConcurrentSkipListSet(Comparator<? super E> comparator) {
96			m = new ConcurrentSkipListMap<E,Object>(comparator);
97			}
98
99			/**
100			* Constructs a new set containing the elements in the specified
101			* collection, that orders its elements according to their
102			* {@linkplain Comparable natural ordering}.
103			*
104			* @param c The elements that will comprise the new set
105			* @throws ClassCastException if the elements in {@code c} are
106			* not {@link Comparable}, or are not mutually comparable
107			* @throws NullPointerException if the specified collection or any
108			* of its elements are null
109			*/
110			public ConcurrentSkipListSet(Collection<? extends E> c) {
111			m = new ConcurrentSkipListMap<E,Object>();
112			addAll(c);
113			}
114
115			/**
116			* Constructs a new set containing the same elements and using the
117			* same ordering as the specified sorted set.
118			*
119			* @param s sorted set whose elements will comprise the new set
120			* @throws NullPointerException if the specified sorted set or any
121			* of its elements are null
122			*/
123			public ConcurrentSkipListSet(SortedSet<E> s) {
124			m = new ConcurrentSkipListMap<E,Object>(s.comparator());
125			addAll(s);
126			}
127
128			/**
129			* For use by submaps
130			*/
131			ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
132			this.m = m;
133			}
134
135			/**
136			* Returns a shallow copy of this {@code ConcurrentSkipListSet}
137			* instance. (The elements themselves are not cloned.)
138			*
139			* @return a shallow copy of this set
140			*/
141			public ConcurrentSkipListSet<E> clone() {
142			try {
143			@SuppressWarnings("unchecked")
144			ConcurrentSkipListSet<E> clone =
145			(ConcurrentSkipListSet<E>) super.clone();
146			clone.setMap(new ConcurrentSkipListMap<E,Object>(m));
147			return clone;
148			} catch (CloneNotSupportedException e) {
149			throw new InternalError();
150			}
151			}
152
153			/* ---------------- Set operations -------------- */
154
155			/**
156			* Returns the number of elements in this set. If this set
157			* contains more than {@code Integer.MAX_VALUE} elements, it
158			* returns {@code Integer.MAX_VALUE}.
159			*
160			* <p>Beware that, unlike in most collections, this method is
161			* <em>NOT</em> a constant-time operation. Because of the
162			* asynchronous nature of these sets, determining the current
163			* number of elements requires traversing them all to count them.
164			* Additionally, it is possible for the size to change during
165			* execution of this method, in which case the returned result
166			* will be inaccurate. Thus, this method is typically not very
167			* useful in concurrent applications.
168			*
169			* @return the number of elements in this set
170			*/
171			public int size() {
172			return m.size();
173			}
174
175			/**
176			* Returns {@code true} if this set contains no elements.
177			* @return {@code true} if this set contains no elements
178			*/
179			public boolean isEmpty() {
180			return m.isEmpty();
181			}
182
183			/**
184			* Returns {@code true} if this set contains the specified element.
185			* More formally, returns {@code true} if and only if this set
186			* contains an element {@code e} such that {@code o.equals(e)}.
187			*
188			* @param o object to be checked for containment in this set
189			* @return {@code true} if this set contains the specified element
190			* @throws ClassCastException if the specified element cannot be
191			* compared with the elements currently in this set
192			* @throws NullPointerException if the specified element is null
193			*/
194			public boolean contains(Object o) {
195			return m.containsKey(o);
196			}
197
198			/**
199			* Adds the specified element to this set if it is not already present.
200			* More formally, adds the specified element {@code e} to this set if
201			* the set contains no element {@code e2} such that {@code e.equals(e2)}.
202			* If this set already contains the element, the call leaves the set
203			* unchanged and returns {@code false}.
204			*
205			* @param e element to be added to this set
206			* @return {@code true} if this set did not already contain the
207			* specified element
208			* @throws ClassCastException if {@code e} cannot be compared
209			* with the elements currently in this set
210			* @throws NullPointerException if the specified element is null
211			*/
212			public boolean add(E e) {
213			return m.putIfAbsent(e, Boolean.TRUE) == null;
214			}
215
216			/**
217			* Removes the specified element from this set if it is present.
218			* More formally, removes an element {@code e} such that
219			* {@code o.equals(e)}, if this set contains such an element.
220			* Returns {@code true} if this set contained the element (or
221			* equivalently, if this set changed as a result of the call).
222			* (This set will not contain the element once the call returns.)
223			*
224			* @param o object to be removed from this set, if present
225			* @return {@code true} if this set contained the specified element
226			* @throws ClassCastException if {@code o} cannot be compared
227			* with the elements currently in this set
228			* @throws NullPointerException if the specified element is null
229			*/
230			public boolean remove(Object o) {
231			return m.remove(o, Boolean.TRUE);
232			}
233
234			/**
235			* Removes all of the elements from this set.
236			*/
237			public void clear() {
238			m.clear();
239			}
240
241			/**
242			* Returns an iterator over the elements in this set in ascending order.
243			*
244			* @return an iterator over the elements in this set in ascending order
245			*/
246			public Iterator<E> iterator() {
247			return m.navigableKeySet().iterator();
248			}
249
250			/**
251			* Returns an iterator over the elements in this set in descending order.
252			*
253			* @return an iterator over the elements in this set in descending order
254			*/
255			public Iterator<E> descendingIterator() {
256			return m.descendingKeySet().iterator();
257			}
258
259
260			/* ---------------- AbstractSet Overrides -------------- */
261
262			/**
263			* Compares the specified object with this set for equality. Returns
264			* {@code true} if the specified object is also a set, the two sets
265			* have the same size, and every member of the specified set is
266			* contained in this set (or equivalently, every member of this set is
267			* contained in the specified set). This definition ensures that the
268			* equals method works properly across different implementations of the
269			* set interface.
270			*
271			* @param o the object to be compared for equality with this set
272			* @return {@code true} if the specified object is equal to this set
273			*/
274			public boolean equals(Object o) {
275			// Override AbstractSet version to avoid calling size()
276			if (o == this)
277			return true;
278			if (!(o instanceof Set))
279			return false;
280			Collection<?> c = (Collection<?>) o;
281			try {
282			return containsAll(c) && c.containsAll(this);
283			} catch (ClassCastException unused) {
284			return false;
285			} catch (NullPointerException unused) {
286			return false;
287			}
288			}
289
290			/**
291			* Removes from this set all of its elements that are contained in
292			* the specified collection. If the specified collection is also
293			* a set, this operation effectively modifies this set so that its
294			* value is the <i>asymmetric set difference</i> of the two sets.
295			*
296			* @param c collection containing elements to be removed from this set
297			* @return {@code true} if this set changed as a result of the call
298			* @throws ClassCastException if the class of an element of this set
299			* is incompatible with the specified collection
300			* (<a href="{@docRoot}/../api/java/util/Collection.html#optional-restrictions">optional</a>)
301			* @throws NullPointerException if the specified collection or any
302			* of its elements are null
303			*/
304			public boolean removeAll(Collection<?> c) {
305			// Override AbstractSet version to avoid unnecessary call to size()
306			boolean modified = false;
307			for (Object e : c)
308			if (remove(e))
309			modified = true;
310			return modified;
311			}
312
313			/* ---------------- Relational operations -------------- */
314
315			/**
316			* @throws ClassCastException {@inheritDoc}
317			* @throws NullPointerException if the specified element is null
318			*/
319			public E lower(E e) {
320			return m.lowerKey(e);
321			}
322
323			/**
324			* @throws ClassCastException {@inheritDoc}
325			* @throws NullPointerException if the specified element is null
326			*/
327			public E floor(E e) {
328			return m.floorKey(e);
329			}
330
331			/**
332			* @throws ClassCastException {@inheritDoc}
333			* @throws NullPointerException if the specified element is null
334			*/
335			public E ceiling(E e) {
336			return m.ceilingKey(e);
337			}
338
339			/**
340			* @throws ClassCastException {@inheritDoc}
341			* @throws NullPointerException if the specified element is null
342			*/
343			public E higher(E e) {
344			return m.higherKey(e);
345			}
346
347			public E pollFirst() {
348			Map.Entry<E,Object> e = m.pollFirstEntry();
349			return (e == null) ? null : e.getKey();
350			}
351
352			public E pollLast() {
353			Map.Entry<E,Object> e = m.pollLastEntry();
354			return (e == null) ? null : e.getKey();
355			}
356
357
358			/* ---------------- SortedSet operations -------------- */
359
360			public Comparator<? super E> comparator() {
361			return m.comparator();
362			}
363
364			/**
365			* @throws java.util.NoSuchElementException {@inheritDoc}
366			*/
367			public E first() {
368			return m.firstKey();
369			}
370
371			/**
372			* @throws java.util.NoSuchElementException {@inheritDoc}
373			*/
374			public E last() {
375			return m.lastKey();
376			}
377
378			/**
379			* @throws ClassCastException {@inheritDoc}
380			* @throws NullPointerException if {@code fromElement} or
381			* {@code toElement} is null
382			* @throws IllegalArgumentException {@inheritDoc}
383			*/
384			public NavigableSet<E> subSet(E fromElement,
385			boolean fromInclusive,
386			E toElement,
387			boolean toInclusive) {
388			return new ConcurrentSkipListSet<E>
389			(m.subMap(fromElement, fromInclusive,
390			toElement, toInclusive));
391			}
392
393			/**
394			* @throws ClassCastException {@inheritDoc}
395			* @throws NullPointerException if {@code toElement} is null
396			* @throws IllegalArgumentException {@inheritDoc}
397			*/
398			public NavigableSet<E> headSet(E toElement, boolean inclusive) {
399			return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
400			}
401
402			/**
403			* @throws ClassCastException {@inheritDoc}
404			* @throws NullPointerException if {@code fromElement} is null
405			* @throws IllegalArgumentException {@inheritDoc}
406			*/
407			public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
408			return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
409			}
410
411			/**
412			* @throws ClassCastException {@inheritDoc}
413			* @throws NullPointerException if {@code fromElement} or
414			* {@code toElement} is null
415			* @throws IllegalArgumentException {@inheritDoc}
416			*/
417			public NavigableSet<E> subSet(E fromElement, E toElement) {
418			return subSet(fromElement, true, toElement, false);
419			}
420
421			/**
422			* @throws ClassCastException {@inheritDoc}
423			* @throws NullPointerException if {@code toElement} is null
424			* @throws IllegalArgumentException {@inheritDoc}
425			*/
426			public NavigableSet<E> headSet(E toElement) {
427			return headSet(toElement, false);
428			}
429
430			/**
431			* @throws ClassCastException {@inheritDoc}
432			* @throws NullPointerException if {@code fromElement} is null
433			* @throws IllegalArgumentException {@inheritDoc}
434			*/
435			public NavigableSet<E> tailSet(E fromElement) {
436			return tailSet(fromElement, true);
437			}
438
439			/**
440			* Returns a reverse order view of the elements contained in this set.
441			* The descending set is backed by this set, so changes to the set are
442			* reflected in the descending set, and vice-versa.
443			*
444			* <p>The returned set has an ordering equivalent to
445			* {@link Collections#reverseOrder(Comparator) Collections.reverseOrder}{@code (comparator())}.
446			* The expression {@code s.descendingSet().descendingSet()} returns a
447			* view of {@code s} essentially equivalent to {@code s}.
448			*
449			* @return a reverse order view of this set
450			*/
451			public NavigableSet<E> descendingSet() {
452			return new ConcurrentSkipListSet<E>(m.descendingMap());
453			}
454
455			/**
456			* Returns a {@link Spliterator} over the elements in this set.
457			*
458			* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
459			* {@link Spliterator#NONNULL}, {@link Spliterator#DISTINCT},
460			* {@link Spliterator#SORTED} and {@link Spliterator#ORDERED}, with an
461			* encounter order that is ascending order. Overriding implementations
462			* should document the reporting of additional characteristic values.
463			*
464			* <p>The spliterator's comparator (see
465			* {@link java.util.Spliterator#getComparator()}) is {@code null} if
466			* the set's comparator (see {@link #comparator()}) is {@code null}.
467			* Otherwise, the spliterator's comparator is the same as or imposes the
468			* same total ordering as the set's comparator.
469			*
470			* @return a {@code Spliterator} over the elements in this set
471			* @since 1.8
472			*/
473			public Spliterator<E> spliterator() {
474			return (m instanceof ConcurrentSkipListMap)
475			? ((ConcurrentSkipListMap<E,?>)m).keySpliterator()
476			: ((ConcurrentSkipListMap.SubMap<E,?>)m).new SubMapKeyIterator();
477			}
478
479			// Support for resetting map in clone
480			private void setMap(ConcurrentNavigableMap<E,Object> map) {
481			U.putObjectVolatile(this, MAP, map);
482			}
483
484			private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
485			private static final long MAP;
486			static {
487			try {
488			MAP = U.objectFieldOffset
489			(ConcurrentSkipListSet.class.getDeclaredField("m"));
490			} catch (ReflectiveOperationException e) {
491			throw new Error(e);
492			}
493			}
494			}