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().
     */
    @SuppressWarnings("serial") // Conditionally serializable
    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) {
        @SuppressWarnings({"deprecation", "removal"})
        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:	1.63
Committed:	Fri Mar 18 16:01:41 2022 UTC (2 years, 2 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.62:	+1 -0 lines
Log Message:	jdk17+ suppressWarnings, FJ updates
#	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	@SuppressWarnings("serial") // Conditionally serializable
78	private final ConcurrentNavigableMap<E,Object> m;
79
80	/**
81	* Constructs a new, empty set that orders its elements according to
82	* their {@linkplain Comparable natural ordering}.
83	*/
84	public ConcurrentSkipListSet() {
85	m = new ConcurrentSkipListMap<E,Object>();
86	}
87
88	/**
89	* Constructs a new, empty set that orders its elements according to
90	* the specified comparator.
91	*
92	* @param comparator the comparator that will be used to order this set.
93	* If {@code null}, the {@linkplain Comparable natural
94	* ordering} of the elements will be used.
95	*/
96	public ConcurrentSkipListSet(Comparator<? super E> comparator) {
97	m = new ConcurrentSkipListMap<E,Object>(comparator);
98	}
99
100	/**
101	* Constructs a new set containing the elements in the specified
102	* collection, that orders its elements according to their
103	* {@linkplain Comparable natural ordering}.
104	*
105	* @param c The elements that will comprise the new set
106	* @throws ClassCastException if the elements in {@code c} are
107	* not {@link Comparable}, or are not mutually comparable
108	* @throws NullPointerException if the specified collection or any
109	* of its elements are null
110	*/
111	public ConcurrentSkipListSet(Collection<? extends E> c) {
112	m = new ConcurrentSkipListMap<E,Object>();
113	addAll(c);
114	}
115
116	/**
117	* Constructs a new set containing the same elements and using the
118	* same ordering as the specified sorted set.
119	*
120	* @param s sorted set whose elements will comprise the new set
121	* @throws NullPointerException if the specified sorted set or any
122	* of its elements are null
123	*/
124	public ConcurrentSkipListSet(SortedSet<E> s) {
125	m = new ConcurrentSkipListMap<E,Object>(s.comparator());
126	addAll(s);
127	}
128
129	/**
130	* For use by submaps
131	*/
132	ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
133	this.m = m;
134	}
135
136	/**
137	* Returns a shallow copy of this {@code ConcurrentSkipListSet}
138	* instance. (The elements themselves are not cloned.)
139	*
140	* @return a shallow copy of this set
141	*/
142	public ConcurrentSkipListSet<E> clone() {
143	try {
144	@SuppressWarnings("unchecked")
145	ConcurrentSkipListSet<E> clone =
146	(ConcurrentSkipListSet<E>) super.clone();
147	clone.setMap(new ConcurrentSkipListMap<E,Object>(m));
148	return clone;
149	} catch (CloneNotSupportedException e) {
150	throw new InternalError();
151	}
152	}
153
154	/* ---------------- Set operations -------------- */
155
156	/**
157	* Returns the number of elements in this set. If this set
158	* contains more than {@code Integer.MAX_VALUE} elements, it
159	* returns {@code Integer.MAX_VALUE}.
160	*
161	* <p>Beware that, unlike in most collections, this method is
162	* <em>NOT</em> a constant-time operation. Because of the
163	* asynchronous nature of these sets, determining the current
164	* number of elements requires traversing them all to count them.
165	* Additionally, it is possible for the size to change during
166	* execution of this method, in which case the returned result
167	* will be inaccurate. Thus, this method is typically not very
168	* useful in concurrent applications.
169	*
170	* @return the number of elements in this set
171	*/
172	public int size() {
173	return m.size();
174	}
175
176	/**
177	* Returns {@code true} if this set contains no elements.
178	* @return {@code true} if this set contains no elements
179	*/
180	public boolean isEmpty() {
181	return m.isEmpty();
182	}
183
184	/**
185	* Returns {@code true} if this set contains the specified element.
186	* More formally, returns {@code true} if and only if this set
187	* contains an element {@code e} such that {@code o.equals(e)}.
188	*
189	* @param o object to be checked for containment in this set
190	* @return {@code true} if this set contains the specified element
191	* @throws ClassCastException if the specified element cannot be
192	* compared with the elements currently in this set
193	* @throws NullPointerException if the specified element is null
194	*/
195	public boolean contains(Object o) {
196	return m.containsKey(o);
197	}
198
199	/**
200	* Adds the specified element to this set if it is not already present.
201	* More formally, adds the specified element {@code e} to this set if
202	* the set contains no element {@code e2} such that {@code e.equals(e2)}.
203	* If this set already contains the element, the call leaves the set
204	* unchanged and returns {@code false}.
205	*
206	* @param e element to be added to this set
207	* @return {@code true} if this set did not already contain the
208	* specified element
209	* @throws ClassCastException if {@code e} cannot be compared
210	* with the elements currently in this set
211	* @throws NullPointerException if the specified element is null
212	*/
213	public boolean add(E e) {
214	return m.putIfAbsent(e, Boolean.TRUE) == null;
215	}
216
217	/**
218	* Removes the specified element from this set if it is present.
219	* More formally, removes an element {@code e} such that
220	* {@code o.equals(e)}, if this set contains such an element.
221	* Returns {@code true} if this set contained the element (or
222	* equivalently, if this set changed as a result of the call).
223	* (This set will not contain the element once the call returns.)
224	*
225	* @param o object to be removed from this set, if present
226	* @return {@code true} if this set contained the specified element
227	* @throws ClassCastException if {@code o} cannot be compared
228	* with the elements currently in this set
229	* @throws NullPointerException if the specified element is null
230	*/
231	public boolean remove(Object o) {
232	return m.remove(o, Boolean.TRUE);
233	}
234
235	/**
236	* Removes all of the elements from this set.
237	*/
238	public void clear() {
239	m.clear();
240	}
241
242	/**
243	* Returns an iterator over the elements in this set in ascending order.
244	*
245	* @return an iterator over the elements in this set in ascending order
246	*/
247	public Iterator<E> iterator() {
248	return m.navigableKeySet().iterator();
249	}
250
251	/**
252	* Returns an iterator over the elements in this set in descending order.
253	*
254	* @return an iterator over the elements in this set in descending order
255	*/
256	public Iterator<E> descendingIterator() {
257	return m.descendingKeySet().iterator();
258	}
259
260
261	/* ---------------- AbstractSet Overrides -------------- */
262
263	/**
264	* Compares the specified object with this set for equality. Returns
265	* {@code true} if the specified object is also a set, the two sets
266	* have the same size, and every member of the specified set is
267	* contained in this set (or equivalently, every member of this set is
268	* contained in the specified set). This definition ensures that the
269	* equals method works properly across different implementations of the
270	* set interface.
271	*
272	* @param o the object to be compared for equality with this set
273	* @return {@code true} if the specified object is equal to this set
274	*/
275	public boolean equals(Object o) {
276	// Override AbstractSet version to avoid calling size()
277	if (o == this)
278	return true;
279	if (!(o instanceof Set))
280	return false;
281	Collection<?> c = (Collection<?>) o;
282	try {
283	return containsAll(c) && c.containsAll(this);
284	} catch (ClassCastException \| 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}/java.base/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 {@linkplain Spliterator#getComparator() spliterator's comparator}
464	* is {@code null} if the {@linkplain #comparator() set's comparator}
465	* 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	/** Initializes map field; for use in clone. */
479	private void setMap(ConcurrentNavigableMap<E,Object> map) {
480	@SuppressWarnings({"deprecation", "removal"})
481	Field mapField = java.security.AccessController.doPrivileged(
482	(java.security.PrivilegedAction<Field>) () -> {
483	try {
484	Field f = ConcurrentSkipListSet.class
485	.getDeclaredField("m");
486	f.setAccessible(true);
487	return f;
488	} catch (ReflectiveOperationException e) {
489	throw new Error(e);
490	}});
491	try {
492	mapField.set(this, map);
493	} catch (IllegalAccessException e) {
494	throw new Error(e);
495	}
496	}
497	}