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.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:	1.2
Committed:	Sat Mar 11 17:36:10 2017 UTC (7 years, 2 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.1:	+0 -1 lines
Log Message:	fix unused imports reported by errorprone [RemoveUnusedImports]
#	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.NavigableSet;
16			import java.util.Set;
17			import java.util.SortedSet;
18			import java.util.Spliterator;
19
20			/**
21			* A scalable concurrent {@link NavigableSet} implementation based on
22			* a {@link ConcurrentSkipListMap}. The elements of the set are kept
23			* sorted according to their {@linkplain Comparable natural ordering},
24			* or by a {@link Comparator} provided at set creation time, depending
25			* on which constructor is used.
26			*
27			* <p>This implementation provides expected average <i>log(n)</i> time
28			* cost for the {@code contains}, {@code add}, and {@code remove}
29			* operations and their variants. Insertion, removal, and access
30			* operations safely execute concurrently by multiple threads.
31			*
32			* <p>Iterators and spliterators are
33			* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
34			*
35			* <p>Ascending ordered views and their iterators are faster than
36			* descending ones.
37			*
38			* <p>Beware that, unlike in most collections, the {@code size}
39			* method is <em>not</em> a constant-time operation. Because of the
40			* asynchronous nature of these sets, determining the current number
41			* of elements requires a traversal of the elements, and so may report
42			* inaccurate results if this collection is modified during traversal.
43			* Additionally, the bulk operations {@code addAll},
44			* {@code removeAll}, {@code retainAll}, {@code containsAll},
45			* {@code equals}, and {@code toArray} are <em>not</em> guaranteed
46			* to be performed atomically. For example, an iterator operating
47			* concurrently with an {@code addAll} operation might view only some
48			* of the added elements.
49			*
50			* <p>This class and its iterators implement all of the
51			* <em>optional</em> methods of the {@link Set} and {@link Iterator}
52			* interfaces. Like most other concurrent collection implementations,
53			* this class does not permit the use of {@code null} elements,
54			* because {@code null} arguments and return values cannot be reliably
55			* distinguished from the absence of elements.
56			*
57			* <p>This class is a member of the
58			* <a href="{@docRoot}/../technotes/guides/collections/index.html">
59			* Java Collections Framework</a>.
60			*
61			* @author Doug Lea
62			* @param <E> the type of elements maintained by this set
63			* @since 1.6
64			*/
65			public class ConcurrentSkipListSet<E>
66			extends AbstractSet<E>
67			implements NavigableSet<E>, Cloneable, java.io.Serializable {
68
69			private static final long serialVersionUID = -2479143111061671589L;
70
71			/**
72			* The underlying map. Uses Boolean.TRUE as value for each
73			* element. This field is declared final for the sake of thread
74			* safety, which entails some ugliness in clone().
75			*/
76			private final ConcurrentNavigableMap<E,Object> m;
77
78			/**
79			* Constructs a new, empty set that orders its elements according to
80			* their {@linkplain Comparable natural ordering}.
81			*/
82			public ConcurrentSkipListSet() {
83			m = new ConcurrentSkipListMap<E,Object>();
84			}
85
86			/**
87			* Constructs a new, empty set that orders its elements according to
88			* the specified comparator.
89			*
90			* @param comparator the comparator that will be used to order this set.
91			* If {@code null}, the {@linkplain Comparable natural
92			* ordering} of the elements will be used.
93			*/
94			public ConcurrentSkipListSet(Comparator<? super E> comparator) {
95			m = new ConcurrentSkipListMap<E,Object>(comparator);
96			}
97
98			/**
99			* Constructs a new set containing the elements in the specified
100			* collection, that orders its elements according to their
101			* {@linkplain Comparable natural ordering}.
102			*
103			* @param c The elements that will comprise the new set
104			* @throws ClassCastException if the elements in {@code c} are
105			* not {@link Comparable}, or are not mutually comparable
106			* @throws NullPointerException if the specified collection or any
107			* of its elements are null
108			*/
109			public ConcurrentSkipListSet(Collection<? extends E> c) {
110			m = new ConcurrentSkipListMap<E,Object>();
111			addAll(c);
112			}
113
114			/**
115			* Constructs a new set containing the same elements and using the
116			* same ordering as the specified sorted set.
117			*
118			* @param s sorted set whose elements will comprise the new set
119			* @throws NullPointerException if the specified sorted set or any
120			* of its elements are null
121			*/
122			public ConcurrentSkipListSet(SortedSet<E> s) {
123			m = new ConcurrentSkipListMap<E,Object>(s.comparator());
124			addAll(s);
125			}
126
127			/**
128			* For use by submaps
129			*/
130			ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
131			this.m = m;
132			}
133
134			/**
135			* Returns a shallow copy of this {@code ConcurrentSkipListSet}
136			* instance. (The elements themselves are not cloned.)
137			*
138			* @return a shallow copy of this set
139			*/
140			public ConcurrentSkipListSet<E> clone() {
141			try {
142			@SuppressWarnings("unchecked")
143			ConcurrentSkipListSet<E> clone =
144			(ConcurrentSkipListSet<E>) super.clone();
145			clone.setMap(new ConcurrentSkipListMap<E,Object>(m));
146			return clone;
147			} catch (CloneNotSupportedException e) {
148			throw new InternalError();
149			}
150			}
151
152			/* ---------------- Set operations -------------- */
153
154			/**
155			* Returns the number of elements in this set. If this set
156			* contains more than {@code Integer.MAX_VALUE} elements, it
157			* returns {@code Integer.MAX_VALUE}.
158			*
159			* <p>Beware that, unlike in most collections, this method is
160			* <em>NOT</em> a constant-time operation. Because of the
161			* asynchronous nature of these sets, determining the current
162			* number of elements requires traversing them all to count them.
163			* Additionally, it is possible for the size to change during
164			* execution of this method, in which case the returned result
165			* will be inaccurate. Thus, this method is typically not very
166			* useful in concurrent applications.
167			*
168			* @return the number of elements in this set
169			*/
170			public int size() {
171			return m.size();
172			}
173
174			/**
175			* Returns {@code true} if this set contains no elements.
176			* @return {@code true} if this set contains no elements
177			*/
178			public boolean isEmpty() {
179			return m.isEmpty();
180			}
181
182			/**
183			* Returns {@code true} if this set contains the specified element.
184			* More formally, returns {@code true} if and only if this set
185			* contains an element {@code e} such that {@code o.equals(e)}.
186			*
187			* @param o object to be checked for containment in this set
188			* @return {@code true} if this set contains the specified element
189			* @throws ClassCastException if the specified element cannot be
190			* compared with the elements currently in this set
191			* @throws NullPointerException if the specified element is null
192			*/
193			public boolean contains(Object o) {
194			return m.containsKey(o);
195			}
196
197			/**
198			* Adds the specified element to this set if it is not already present.
199			* More formally, adds the specified element {@code e} to this set if
200			* the set contains no element {@code e2} such that {@code e.equals(e2)}.
201			* If this set already contains the element, the call leaves the set
202			* unchanged and returns {@code false}.
203			*
204			* @param e element to be added to this set
205			* @return {@code true} if this set did not already contain the
206			* specified element
207			* @throws ClassCastException if {@code e} cannot be compared
208			* with the elements currently in this set
209			* @throws NullPointerException if the specified element is null
210			*/
211			public boolean add(E e) {
212			return m.putIfAbsent(e, Boolean.TRUE) == null;
213			}
214
215			/**
216			* Removes the specified element from this set if it is present.
217			* More formally, removes an element {@code e} such that
218			* {@code o.equals(e)}, if this set contains such an element.
219			* Returns {@code true} if this set contained the element (or
220			* equivalently, if this set changed as a result of the call).
221			* (This set will not contain the element once the call returns.)
222			*
223			* @param o object to be removed from this set, if present
224			* @return {@code true} if this set contained the specified element
225			* @throws ClassCastException if {@code o} cannot be compared
226			* with the elements currently in this set
227			* @throws NullPointerException if the specified element is null
228			*/
229			public boolean remove(Object o) {
230			return m.remove(o, Boolean.TRUE);
231			}
232
233			/**
234			* Removes all of the elements from this set.
235			*/
236			public void clear() {
237			m.clear();
238			}
239
240			/**
241			* Returns an iterator over the elements in this set in ascending order.
242			*
243			* @return an iterator over the elements in this set in ascending order
244			*/
245			public Iterator<E> iterator() {
246			return m.navigableKeySet().iterator();
247			}
248
249			/**
250			* Returns an iterator over the elements in this set in descending order.
251			*
252			* @return an iterator over the elements in this set in descending order
253			*/
254			public Iterator<E> descendingIterator() {
255			return m.descendingKeySet().iterator();
256			}
257
258
259			/* ---------------- AbstractSet Overrides -------------- */
260
261			/**
262			* Compares the specified object with this set for equality. Returns
263			* {@code true} if the specified object is also a set, the two sets
264			* have the same size, and every member of the specified set is
265			* contained in this set (or equivalently, every member of this set is
266			* contained in the specified set). This definition ensures that the
267			* equals method works properly across different implementations of the
268			* set interface.
269			*
270			* @param o the object to be compared for equality with this set
271			* @return {@code true} if the specified object is equal to this set
272			*/
273			public boolean equals(Object o) {
274			// Override AbstractSet version to avoid calling size()
275			if (o == this)
276			return true;
277			if (!(o instanceof Set))
278			return false;
279			Collection<?> c = (Collection<?>) o;
280			try {
281			return containsAll(c) && c.containsAll(this);
282			} catch (ClassCastException unused) {
283			return false;
284			} catch (NullPointerException unused) {
285			return false;
286			}
287			}
288
289			/**
290			* Removes from this set all of its elements that are contained in
291			* the specified collection. If the specified collection is also
292			* a set, this operation effectively modifies this set so that its
293			* value is the <i>asymmetric set difference</i> of the two sets.
294			*
295			* @param c collection containing elements to be removed from this set
296			* @return {@code true} if this set changed as a result of the call
297			* @throws ClassCastException if the class of an element of this set
298			* is incompatible with the specified collection
299			* (<a href="{@docRoot}/../api/java/util/Collection.html#optional-restrictions">optional</a>)
300			* @throws NullPointerException if the specified collection or any
301			* of its elements are null
302			*/
303			public boolean removeAll(Collection<?> c) {
304			// Override AbstractSet version to avoid unnecessary call to size()
305			boolean modified = false;
306			for (Object e : c)
307			if (remove(e))
308			modified = true;
309			return modified;
310			}
311
312			/* ---------------- Relational operations -------------- */
313
314			/**
315			* @throws ClassCastException {@inheritDoc}
316			* @throws NullPointerException if the specified element is null
317			*/
318			public E lower(E e) {
319			return m.lowerKey(e);
320			}
321
322			/**
323			* @throws ClassCastException {@inheritDoc}
324			* @throws NullPointerException if the specified element is null
325			*/
326			public E floor(E e) {
327			return m.floorKey(e);
328			}
329
330			/**
331			* @throws ClassCastException {@inheritDoc}
332			* @throws NullPointerException if the specified element is null
333			*/
334			public E ceiling(E e) {
335			return m.ceilingKey(e);
336			}
337
338			/**
339			* @throws ClassCastException {@inheritDoc}
340			* @throws NullPointerException if the specified element is null
341			*/
342			public E higher(E e) {
343			return m.higherKey(e);
344			}
345
346			public E pollFirst() {
347			Map.Entry<E,Object> e = m.pollFirstEntry();
348			return (e == null) ? null : e.getKey();
349			}
350
351			public E pollLast() {
352			Map.Entry<E,Object> e = m.pollLastEntry();
353			return (e == null) ? null : e.getKey();
354			}
355
356
357			/* ---------------- SortedSet operations -------------- */
358
359			public Comparator<? super E> comparator() {
360			return m.comparator();
361			}
362
363			/**
364			* @throws java.util.NoSuchElementException {@inheritDoc}
365			*/
366			public E first() {
367			return m.firstKey();
368			}
369
370			/**
371			* @throws java.util.NoSuchElementException {@inheritDoc}
372			*/
373			public E last() {
374			return m.lastKey();
375			}
376
377			/**
378			* @throws ClassCastException {@inheritDoc}
379			* @throws NullPointerException if {@code fromElement} or
380			* {@code toElement} is null
381			* @throws IllegalArgumentException {@inheritDoc}
382			*/
383			public NavigableSet<E> subSet(E fromElement,
384			boolean fromInclusive,
385			E toElement,
386			boolean toInclusive) {
387			return new ConcurrentSkipListSet<E>
388			(m.subMap(fromElement, fromInclusive,
389			toElement, toInclusive));
390			}
391
392			/**
393			* @throws ClassCastException {@inheritDoc}
394			* @throws NullPointerException if {@code toElement} is null
395			* @throws IllegalArgumentException {@inheritDoc}
396			*/
397			public NavigableSet<E> headSet(E toElement, boolean inclusive) {
398			return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
399			}
400
401			/**
402			* @throws ClassCastException {@inheritDoc}
403			* @throws NullPointerException if {@code fromElement} is null
404			* @throws IllegalArgumentException {@inheritDoc}
405			*/
406			public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
407			return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
408			}
409
410			/**
411			* @throws ClassCastException {@inheritDoc}
412			* @throws NullPointerException if {@code fromElement} or
413			* {@code toElement} is null
414			* @throws IllegalArgumentException {@inheritDoc}
415			*/
416			public NavigableSet<E> subSet(E fromElement, E toElement) {
417			return subSet(fromElement, true, toElement, false);
418			}
419
420			/**
421			* @throws ClassCastException {@inheritDoc}
422			* @throws NullPointerException if {@code toElement} is null
423			* @throws IllegalArgumentException {@inheritDoc}
424			*/
425			public NavigableSet<E> headSet(E toElement) {
426			return headSet(toElement, false);
427			}
428
429			/**
430			* @throws ClassCastException {@inheritDoc}
431			* @throws NullPointerException if {@code fromElement} is null
432			* @throws IllegalArgumentException {@inheritDoc}
433			*/
434			public NavigableSet<E> tailSet(E fromElement) {
435			return tailSet(fromElement, true);
436			}
437
438			/**
439			* Returns a reverse order view of the elements contained in this set.
440			* The descending set is backed by this set, so changes to the set are
441			* reflected in the descending set, and vice-versa.
442			*
443			* <p>The returned set has an ordering equivalent to
444			* {@link Collections#reverseOrder(Comparator) Collections.reverseOrder}{@code (comparator())}.
445			* The expression {@code s.descendingSet().descendingSet()} returns a
446			* view of {@code s} essentially equivalent to {@code s}.
447			*
448			* @return a reverse order view of this set
449			*/
450			public NavigableSet<E> descendingSet() {
451			return new ConcurrentSkipListSet<E>(m.descendingMap());
452			}
453
454			/**
455			* Returns a {@link Spliterator} over the elements in this set.
456			*
457			* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
458			* {@link Spliterator#NONNULL}, {@link Spliterator#DISTINCT},
459			* {@link Spliterator#SORTED} and {@link Spliterator#ORDERED}, with an
460			* encounter order that is ascending order. Overriding implementations
461			* should document the reporting of additional characteristic values.
462			*
463			* <p>The spliterator's comparator (see
464			* {@link java.util.Spliterator#getComparator()}) is {@code null} if
465			* the set's comparator (see {@link #comparator()}) is {@code null}.
466			* Otherwise, the spliterator's comparator is the same as or imposes the
467			* same total ordering as the set's comparator.
468			*
469			* @return a {@code Spliterator} over the elements in this set
470			* @since 1.8
471			*/
472			public Spliterator<E> spliterator() {
473			return (m instanceof ConcurrentSkipListMap)
474			? ((ConcurrentSkipListMap<E,?>)m).keySpliterator()
475			: ((ConcurrentSkipListMap.SubMap<E,?>)m).new SubMapKeyIterator();
476			}
477
478			// Support for resetting map in clone
479			private void setMap(ConcurrentNavigableMap<E,Object> map) {
480			U.putObjectVolatile(this, MAP, map);
481			}
482
483			private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
484			private static final long MAP;
485			static {
486			try {
487			MAP = U.objectFieldOffset
488			(ConcurrentSkipListSet.class.getDeclaredField("m"));
489			} catch (ReflectiveOperationException e) {
490			throw new Error(e);
491			}
492			}
493			}