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.lang.reflect.Field;
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.
 *
 * <p>Bulk operations that add, remove, or examine multiple elements,
 * such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
 * are <em>not</em> guaranteed to be performed atomically.
 * For example, a {@code forEach} traversal concurrent with an {@code
 * addAll} operation might observe 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}/java.base/java/util/package-summary.html#CollectionsFramework">
 * 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 | 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}/java.base/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 {@linkplain Spliterator#getComparator() spliterator's comparator}
     * is {@code null} if the {@linkplain #comparator() set's 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();
    }

    /** Initializes map field; for use in clone. */
    private void setMap(ConcurrentNavigableMap<E,Object> map) {
        Field mapField = java.security.AccessController.doPrivileged(
            (java.security.PrivilegedAction<Field>) () -> {
                try {
                    Field f = ConcurrentSkipListSet.class
                        .getDeclaredField("m");
                    f.setAccessible(true);
                    return f;
                } catch (ReflectiveOperationException e) {
                    throw new Error(e);
                }});
        try {
            mapField.set(this, map);
        } catch (IllegalAccessException e) {
            throw new Error(e);
        }
    }
}
Revision:	1.61
Committed:	Mon Oct 1 00:10:53 2018 UTC (5 years, 8 months ago) by jsr166
Branch:	MAIN
Changes since 1.60:	+2 -2 lines
Log Message:	update to using jdk11 by default, except link to jdk10 javadocs; sync @docRoot references in javadoc with upstream
#	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.lang.reflect.Field;
10	import java.util.AbstractSet;
11	import java.util.Collection;
12	import java.util.Collections;
13	import java.util.Comparator;
14	import java.util.Iterator;
15	import java.util.Map;
16	import java.util.NavigableSet;
17	import java.util.Set;
18	import java.util.SortedSet;
19	import java.util.Spliterator;
20
21	/**
22	* A scalable concurrent {@link NavigableSet} implementation based on
23	* a {@link ConcurrentSkipListMap}. The elements of the set are kept
24	* sorted according to their {@linkplain Comparable natural ordering},
25	* or by a {@link Comparator} provided at set creation time, depending
26	* on which constructor is used.
27	*
28	* <p>This implementation provides expected average <i>log(n)</i> time
29	* cost for the {@code contains}, {@code add}, and {@code remove}
30	* operations and their variants. Insertion, removal, and access
31	* operations safely execute concurrently by multiple threads.
32	*
33	* <p>Iterators and spliterators are
34	* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
35	*
36	* <p>Ascending ordered views and their iterators are faster than
37	* descending ones.
38	*
39	* <p>Beware that, unlike in most collections, the {@code size}
40	* method is <em>not</em> a constant-time operation. Because of the
41	* asynchronous nature of these sets, determining the current number
42	* of elements requires a traversal of the elements, and so may report
43	* inaccurate results if this collection is modified during traversal.
44	*
45	* <p>Bulk operations that add, remove, or examine multiple elements,
46	* such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
47	* are <em>not</em> guaranteed to be performed atomically.
48	* For example, a {@code forEach} traversal concurrent with an {@code
49	* addAll} operation might observe only some 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}/java.base/java/util/package-summary.html#CollectionsFramework">
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 \| NullPointerException unused) {
284	return false;
285	}
286	}
287
288	/**
289	* Removes from this set all of its elements that are contained in
290	* the specified collection. If the specified collection is also
291	* a set, this operation effectively modifies this set so that its
292	* value is the <i>asymmetric set difference</i> of the two sets.
293	*
294	* @param c collection containing elements to be removed from this set
295	* @return {@code true} if this set changed as a result of the call
296	* @throws ClassCastException if the class of an element of this set
297	* is incompatible with the specified collection
298	* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
299	* @throws NullPointerException if the specified collection or any
300	* of its elements are null
301	*/
302	public boolean removeAll(Collection<?> c) {
303	// Override AbstractSet version to avoid unnecessary call to size()
304	boolean modified = false;
305	for (Object e : c)
306	if (remove(e))
307	modified = true;
308	return modified;
309	}
310
311	/* ---------------- Relational operations -------------- */
312
313	/**
314	* @throws ClassCastException {@inheritDoc}
315	* @throws NullPointerException if the specified element is null
316	*/
317	public E lower(E e) {
318	return m.lowerKey(e);
319	}
320
321	/**
322	* @throws ClassCastException {@inheritDoc}
323	* @throws NullPointerException if the specified element is null
324	*/
325	public E floor(E e) {
326	return m.floorKey(e);
327	}
328
329	/**
330	* @throws ClassCastException {@inheritDoc}
331	* @throws NullPointerException if the specified element is null
332	*/
333	public E ceiling(E e) {
334	return m.ceilingKey(e);
335	}
336
337	/**
338	* @throws ClassCastException {@inheritDoc}
339	* @throws NullPointerException if the specified element is null
340	*/
341	public E higher(E e) {
342	return m.higherKey(e);
343	}
344
345	public E pollFirst() {
346	Map.Entry<E,Object> e = m.pollFirstEntry();
347	return (e == null) ? null : e.getKey();
348	}
349
350	public E pollLast() {
351	Map.Entry<E,Object> e = m.pollLastEntry();
352	return (e == null) ? null : e.getKey();
353	}
354
355
356	/* ---------------- SortedSet operations -------------- */
357
358	public Comparator<? super E> comparator() {
359	return m.comparator();
360	}
361
362	/**
363	* @throws java.util.NoSuchElementException {@inheritDoc}
364	*/
365	public E first() {
366	return m.firstKey();
367	}
368
369	/**
370	* @throws java.util.NoSuchElementException {@inheritDoc}
371	*/
372	public E last() {
373	return m.lastKey();
374	}
375
376	/**
377	* @throws ClassCastException {@inheritDoc}
378	* @throws NullPointerException if {@code fromElement} or
379	* {@code toElement} is null
380	* @throws IllegalArgumentException {@inheritDoc}
381	*/
382	public NavigableSet<E> subSet(E fromElement,
383	boolean fromInclusive,
384	E toElement,
385	boolean toInclusive) {
386	return new ConcurrentSkipListSet<E>
387	(m.subMap(fromElement, fromInclusive,
388	toElement, toInclusive));
389	}
390
391	/**
392	* @throws ClassCastException {@inheritDoc}
393	* @throws NullPointerException if {@code toElement} is null
394	* @throws IllegalArgumentException {@inheritDoc}
395	*/
396	public NavigableSet<E> headSet(E toElement, boolean inclusive) {
397	return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
398	}
399
400	/**
401	* @throws ClassCastException {@inheritDoc}
402	* @throws NullPointerException if {@code fromElement} is null
403	* @throws IllegalArgumentException {@inheritDoc}
404	*/
405	public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
406	return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
407	}
408
409	/**
410	* @throws ClassCastException {@inheritDoc}
411	* @throws NullPointerException if {@code fromElement} or
412	* {@code toElement} is null
413	* @throws IllegalArgumentException {@inheritDoc}
414	*/
415	public NavigableSet<E> subSet(E fromElement, E toElement) {
416	return subSet(fromElement, true, toElement, false);
417	}
418
419	/**
420	* @throws ClassCastException {@inheritDoc}
421	* @throws NullPointerException if {@code toElement} is null
422	* @throws IllegalArgumentException {@inheritDoc}
423	*/
424	public NavigableSet<E> headSet(E toElement) {
425	return headSet(toElement, false);
426	}
427
428	/**
429	* @throws ClassCastException {@inheritDoc}
430	* @throws NullPointerException if {@code fromElement} is null
431	* @throws IllegalArgumentException {@inheritDoc}
432	*/
433	public NavigableSet<E> tailSet(E fromElement) {
434	return tailSet(fromElement, true);
435	}
436
437	/**
438	* Returns a reverse order view of the elements contained in this set.
439	* The descending set is backed by this set, so changes to the set are
440	* reflected in the descending set, and vice-versa.
441	*
442	* <p>The returned set has an ordering equivalent to
443	* {@link Collections#reverseOrder(Comparator) Collections.reverseOrder}{@code (comparator())}.
444	* The expression {@code s.descendingSet().descendingSet()} returns a
445	* view of {@code s} essentially equivalent to {@code s}.
446	*
447	* @return a reverse order view of this set
448	*/
449	public NavigableSet<E> descendingSet() {
450	return new ConcurrentSkipListSet<E>(m.descendingMap());
451	}
452
453	/**
454	* Returns a {@link Spliterator} over the elements in this set.
455	*
456	* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
457	* {@link Spliterator#NONNULL}, {@link Spliterator#DISTINCT},
458	* {@link Spliterator#SORTED} and {@link Spliterator#ORDERED}, with an
459	* encounter order that is ascending order. Overriding implementations
460	* should document the reporting of additional characteristic values.
461	*
462	* <p>The {@linkplain Spliterator#getComparator() spliterator's comparator}
463	* is {@code null} if the {@linkplain #comparator() set's comparator}
464	* is {@code null}.
465	* Otherwise, the spliterator's comparator is the same as or imposes the
466	* same total ordering as the set's comparator.
467	*
468	* @return a {@code Spliterator} over the elements in this set
469	* @since 1.8
470	*/
471	public Spliterator<E> spliterator() {
472	return (m instanceof ConcurrentSkipListMap)
473	? ((ConcurrentSkipListMap<E,?>)m).keySpliterator()
474	: ((ConcurrentSkipListMap.SubMap<E,?>)m).new SubMapKeyIterator();
475	}
476
477	/** Initializes map field; for use in clone. */
478	private void setMap(ConcurrentNavigableMap<E,Object> map) {
479	Field mapField = java.security.AccessController.doPrivileged(
480	(java.security.PrivilegedAction<Field>) () -> {
481	try {
482	Field f = ConcurrentSkipListSet.class
483	.getDeclaredField("m");
484	f.setAccessible(true);
485	return f;
486	} catch (ReflectiveOperationException e) {
487	throw new Error(e);
488	}});
489	try {
490	mapField.set(this, map);
491	} catch (IllegalAccessException e) {
492	throw new Error(e);
493	}
494	}
495	}