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]
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8
9	import java.util.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	}