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/*
 * 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.*;

// BEGIN android-note
// removed link to collections framework docs
// END android-note

/**
 * 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.
 *
 * 

This implementation provides expected average log(n) 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 weakly consistent, returning elements * reflecting the state of the set at some point at or since the * creation of the iterator. They do not throw {@link * ConcurrentModificationException}, and may proceed concurrently with * other operations. Ascending ordered views and their iterators are * faster than descending ones. * *

Beware that, unlike in most collections, the {@code size} * method is not 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 not guaranteed * to be performed atomically. For example, an iterator operating * concurrently with an {@code addAll} operation might view only some * of the added elements. * *

This class and its iterators implement all of the * optional 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. * * @author Doug Lea * @param the type of elements maintained by this set * @since 1.6 */ public class ConcurrentSkipListSet extends AbstractSet implements NavigableSet, 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 m; /** * Constructs a new, empty set that orders its elements according to * their {@linkplain Comparable natural ordering}. */ public ConcurrentSkipListSet() { m = new ConcurrentSkipListMap(); } /** * 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 comparator) { m = new ConcurrentSkipListMap(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 c) { m = new ConcurrentSkipListMap(); 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 s) { m = new ConcurrentSkipListMap(s.comparator()); addAll(s); } /** * For use by submaps */ ConcurrentSkipListSet(ConcurrentNavigableMap 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 clone() { try { @SuppressWarnings("unchecked") ConcurrentSkipListSet clone = (ConcurrentSkipListSet) super.clone(); clone.setMap(new ConcurrentSkipListMap(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}. * *

Beware that, unlike in most collections, this method is * NOT 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 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 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 asymmetric set difference 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 = m.pollFirstEntry(); return (e == null) ? null : e.getKey(); } public E pollLast() { Map.Entry e = m.pollLastEntry(); return (e == null) ? null : e.getKey(); } /* ---------------- SortedSet operations -------------- */ public Comparator comparator() { return m.comparator(); } /** * @throws NoSuchElementException {@inheritDoc} */ public E first() { return m.firstKey(); } /** * @throws 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 subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) { return new ConcurrentSkipListSet (m.subMap(fromElement, fromInclusive, toElement, toInclusive)); } /** * @throws ClassCastException {@inheritDoc} * @throws NullPointerException if {@code toElement} is null * @throws IllegalArgumentException {@inheritDoc} */ public NavigableSet headSet(E toElement, boolean inclusive) { return new ConcurrentSkipListSet(m.headMap(toElement, inclusive)); } /** * @throws ClassCastException {@inheritDoc} * @throws NullPointerException if {@code fromElement} is null * @throws IllegalArgumentException {@inheritDoc} */ public NavigableSet tailSet(E fromElement, boolean inclusive) { return new ConcurrentSkipListSet(m.tailMap(fromElement, inclusive)); } /** * @throws ClassCastException {@inheritDoc} * @throws NullPointerException if {@code fromElement} or * {@code toElement} is null * @throws IllegalArgumentException {@inheritDoc} */ public NavigableSet 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 headSet(E toElement) { return headSet(toElement, false); } /** * @throws ClassCastException {@inheritDoc} * @throws NullPointerException if {@code fromElement} is null * @throws IllegalArgumentException {@inheritDoc} */ public NavigableSet 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. * *

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 descendingSet() { return new ConcurrentSkipListSet(m.descendingMap()); } // Support for resetting map in clone private void setMap(ConcurrentNavigableMap 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); } } }





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