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;
import java.util.stream.Stream;

/**
 * A scalable concurrent {@link NavigableSet} implementation based on
 * a {@link ConcurrentSkipListMap}.  The elements of the set are kept
 * sorted according to their {@linkplain Comparable natural ordering},
 * or by a {@link Comparator} provided at set creation time, depending
 * on which constructor is used.
 *
 * <p>This implementation provides expected average <i>log(n)</i> time
 * cost for the {@code contains}, {@code add}, and {@code remove}
 * operations and their variants.  Insertion, removal, and access
 * operations safely execute concurrently by multiple threads.
 * Iterators are <i>weakly consistent</i>, returning elements
 * reflecting the state of the set at some point at or since the
 * creation of the iterator.  They do <em>not</em> throw {@link
 * java.util.ConcurrentModificationException}, and may proceed
 * concurrently with other operations.  Ascending ordered views and
 * their iterators are faster than descending ones.
 *
 * <p>Beware that, unlike in most collections, the {@code size}
 * method is <em>not</em> a constant-time operation. Because of the
 * asynchronous nature of these sets, determining the current number
 * of elements requires a traversal of the elements, and so may report
 * inaccurate results if this collection is modified during traversal.
 * Additionally, the bulk operations {@code addAll},
 * {@code removeAll}, {@code retainAll}, {@code containsAll},
 * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
 * to be performed atomically. For example, an iterator operating
 * concurrently with an {@code addAll} operation might view only some
 * of the added elements.
 *
 * <p>This class and its iterators implement all of the
 * <em>optional</em> methods of the {@link Set} and {@link Iterator}
 * interfaces. Like most other concurrent collection implementations,
 * this class does not permit the use of {@code null} elements,
 * because {@code null} arguments and return values cannot be reliably
 * distinguished from the absence of elements.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author Doug Lea
 * @param <E> the type of elements maintained by this set
 * @since 1.6
 */
public class ConcurrentSkipListSet<E>
    extends AbstractSet<E>
    implements NavigableSet<E>, Cloneable, java.io.Serializable {

    private static final long serialVersionUID = -2479143111061671589L;

    /**
     * The underlying map. Uses Boolean.TRUE as value for each
     * element.  This field is declared final for the sake of thread
     * safety, which entails some ugliness in clone().
     */
    private final ConcurrentNavigableMap<E,Object> m;

    /**
     * Constructs a new, empty set that orders its elements according to
     * their {@linkplain Comparable natural ordering}.
     */
    public ConcurrentSkipListSet() {
        m = new ConcurrentSkipListMap<E,Object>();
    }

    /**
     * Constructs a new, empty set that orders its elements according to
     * the specified comparator.
     *
     * @param comparator the comparator that will be used to order this set.
     *        If {@code null}, the {@linkplain Comparable natural
     *        ordering} of the elements will be used.
     */
    public ConcurrentSkipListSet(Comparator<? super E> comparator) {
        m = new ConcurrentSkipListMap<E,Object>(comparator);
    }

    /**
     * Constructs a new set containing the elements in the specified
     * collection, that orders its elements according to their
     * {@linkplain Comparable natural ordering}.
     *
     * @param c The elements that will comprise the new set
     * @throws ClassCastException if the elements in {@code c} are
     *         not {@link Comparable}, or are not mutually comparable
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public ConcurrentSkipListSet(Collection<? extends E> c) {
        m = new ConcurrentSkipListMap<E,Object>();
        addAll(c);
    }

    /**
     * Constructs a new set containing the same elements and using the
     * same ordering as the specified sorted set.
     *
     * @param s sorted set whose elements will comprise the new set
     * @throws NullPointerException if the specified sorted set or any
     *         of its elements are null
     */
    public ConcurrentSkipListSet(SortedSet<E> s) {
        m = new ConcurrentSkipListMap<E,Object>(s.comparator());
        addAll(s);
    }

    /**
     * For use by submaps
     */
    ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
        this.m = m;
    }

    /**
     * Returns a shallow copy of this {@code ConcurrentSkipListSet}
     * instance. (The elements themselves are not cloned.)
     *
     * @return a shallow copy of this set
     */
    public ConcurrentSkipListSet<E> clone() {
        try {
            @SuppressWarnings("unchecked")
            ConcurrentSkipListSet<E> clone =
                (ConcurrentSkipListSet<E>) super.clone();
            clone.setMap(new ConcurrentSkipListMap<E,Object>(m));
            return clone;
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
        }
    }

    /* ---------------- Set operations -------------- */

    /**
     * Returns the number of elements in this set.  If this set
     * contains more than {@code Integer.MAX_VALUE} elements, it
     * returns {@code Integer.MAX_VALUE}.
     *
     * <p>Beware that, unlike in most collections, this method is
     * <em>NOT</em> a constant-time operation. Because of the
     * asynchronous nature of these sets, determining the current
     * number of elements requires traversing them all to count them.
     * Additionally, it is possible for the size to change during
     * execution of this method, in which case the returned result
     * will be inaccurate. Thus, this method is typically not very
     * useful in concurrent applications.
     *
     * @return the number of elements in this set
     */
    public int size() {
        return m.size();
    }

    /**
     * Returns {@code true} if this set contains no elements.
     * @return {@code true} if this set contains no elements
     */
    public boolean isEmpty() {
        return m.isEmpty();
    }

    /**
     * Returns {@code true} if this set contains the specified element.
     * More formally, returns {@code true} if and only if this set
     * contains an element {@code e} such that {@code o.equals(e)}.
     *
     * @param o object to be checked for containment in this set
     * @return {@code true} if this set contains the specified element
     * @throws ClassCastException if the specified element cannot be
     *         compared with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     */
    public boolean contains(Object o) {
        return m.containsKey(o);
    }

    /**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element {@code e} to this set if
     * the set contains no element {@code e2} such that {@code e.equals(e2)}.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns {@code false}.
     *
     * @param e element to be added to this set
     * @return {@code true} if this set did not already contain the
     *         specified element
     * @throws ClassCastException if {@code e} cannot be compared
     *         with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return m.putIfAbsent(e, Boolean.TRUE) == null;
    }

    /**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element {@code e} such that
     * {@code o.equals(e)}, if this set contains such an element.
     * Returns {@code true} if this set contained the element (or
     * equivalently, if this set changed as a result of the call).
     * (This set will not contain the element once the call returns.)
     *
     * @param o object to be removed from this set, if present
     * @return {@code true} if this set contained the specified element
     * @throws ClassCastException if {@code o} cannot be compared
     *         with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     */
    public boolean remove(Object o) {
        return m.remove(o, Boolean.TRUE);
    }

    /**
     * Removes all of the elements from this set.
     */
    public void clear() {
        m.clear();
    }

    /**
     * Returns an iterator over the elements in this set in ascending order.
     *
     * @return an iterator over the elements in this set in ascending order
     */
    public Iterator<E> iterator() {
        return m.navigableKeySet().iterator();
    }

    /**
     * Returns an iterator over the elements in this set in descending order.
     *
     * @return an iterator over the elements in this set in descending order
     */
    public Iterator<E> descendingIterator() {
        return m.descendingKeySet().iterator();
    }


    /* ---------------- AbstractSet Overrides -------------- */

    /**
     * Compares the specified object with this set for equality.  Returns
     * {@code true} if the specified object is also a set, the two sets
     * have the same size, and every member of the specified set is
     * contained in this set (or equivalently, every member of this set is
     * contained in the specified set).  This definition ensures that the
     * equals method works properly across different implementations of the
     * set interface.
     *
     * @param o the object to be compared for equality with this set
     * @return {@code true} if the specified object is equal to this set
     */
    public boolean equals(Object o) {
        // Override AbstractSet version to avoid calling size()
        if (o == this)
            return true;
        if (!(o instanceof Set))
            return false;
        Collection<?> c = (Collection<?>) o;
        try {
            return containsAll(c) && c.containsAll(this);
        } catch (ClassCastException unused) {
            return false;
        } catch (NullPointerException unused) {
            return false;
        }
    }

    /**
     * Removes from this set all of its elements that are contained in
     * the specified collection.  If the specified collection is also
     * a set, this operation effectively modifies this set so that its
     * value is the <i>asymmetric set difference</i> of the two sets.
     *
     * @param  c collection containing elements to be removed from this set
     * @return {@code true} if this set changed as a result of the call
     * @throws ClassCastException if the types of one or more elements in this
     *         set are incompatible with the specified collection
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public boolean removeAll(Collection<?> c) {
        // Override AbstractSet version to avoid unnecessary call to size()
        boolean modified = false;
        for (Object e : c)
            if (remove(e))
                modified = true;
        return modified;
    }

    /* ---------------- Relational operations -------------- */

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    public E lower(E e) {
        return m.lowerKey(e);
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    public E floor(E e) {
        return m.floorKey(e);
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    public E ceiling(E e) {
        return m.ceilingKey(e);
    }

    /**
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    public E higher(E e) {
        return m.higherKey(e);
    }

    public E pollFirst() {
        Map.Entry<E,Object> e = m.pollFirstEntry();
        return (e == null) ? null : e.getKey();
    }

    public E pollLast() {
        Map.Entry<E,Object> e = m.pollLastEntry();
        return (e == null) ? null : e.getKey();
    }


    /* ---------------- SortedSet operations -------------- */


    public Comparator<? super E> comparator() {
        return m.comparator();
    }

    /**
     * @throws 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
     */
    @SuppressWarnings("unchecked")
    public Spliterator<E> spliterator() {
        if (m instanceof ConcurrentSkipListMap)
            return ((ConcurrentSkipListMap<E,?>)m).keySpliterator();
        else
            return (Spliterator<E>)((ConcurrentSkipListMap.SubMap<E,?>)m).keyIterator();
    }

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

    private static final sun.misc.Unsafe UNSAFE;
    private static final long mapOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = ConcurrentSkipListSet.class;
            mapOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("m"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}
Revision:	1.40
Committed:	Thu Aug 8 15:13:34 2013 UTC (10 years, 9 months ago) by jsr166
Branch:	MAIN
Changes since 1.39:	+18 -0 lines
Log Message:	add javadoc for spliterator()
#	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	import java.util.AbstractSet;
9	import java.util.Collection;
10	import java.util.Collections;
11	import java.util.Comparator;
12	import java.util.Iterator;
13	import java.util.Map;
14	import java.util.NavigableMap;
15	import java.util.NavigableSet;
16	import java.util.Set;
17	import java.util.SortedSet;
18	import java.util.Spliterator;
19	import java.util.stream.Stream;
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	* Iterators are <i>weakly consistent</i>, returning elements
33	* reflecting the state of the set at some point at or since the
34	* creation of the iterator. They do <em>not</em> throw {@link
35	* java.util.ConcurrentModificationException}, and may proceed
36	* concurrently with other operations. Ascending ordered views and
37	* their iterators are faster than 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 types of one or more elements in this
299	* set are incompatible with the specified collection
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
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	@SuppressWarnings("unchecked")
474	public Spliterator<E> spliterator() {
475	if (m instanceof ConcurrentSkipListMap)
476	return ((ConcurrentSkipListMap<E,?>)m).keySpliterator();
477	else
478	return (Spliterator<E>)((ConcurrentSkipListMap.SubMap<E,?>)m).keyIterator();
479	}
480
481	// Support for resetting map in clone
482	private void setMap(ConcurrentNavigableMap<E,Object> map) {
483	UNSAFE.putObjectVolatile(this, mapOffset, map);
484	}
485
486	private static final sun.misc.Unsafe UNSAFE;
487	private static final long mapOffset;
488	static {
489	try {
490	UNSAFE = sun.misc.Unsafe.getUnsafe();
491	Class<?> k = ConcurrentSkipListSet.class;
492	mapOffset = UNSAFE.objectFieldOffset
493	(k.getDeclaredField("m"));
494	} catch (Exception e) {
495	throw new Error(e);
496	}
497	}
498	}