/*
* @(#)ConcurrentSkipListSet.java 1.4 06/05/10
*
* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util.concurrent;
import java.util.*;
import sun.misc.Unsafe;
/**
* 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 <tt>contains</tt>, <tt>add</tt>, and <tt>remove</tt>
* 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
* 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 <tt>size</tt>
* 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. Additionally, the
* bulk operations <tt>addAll</tt>, <tt>removeAll</tt>,
* <tt>retainAll</tt>, and <tt>containsAll</tt> are <em>not</em>
* guaranteed to be performed atomically. For example, an iterator
* operating concurrently with an <tt>addAll</tt> 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 <tt>null</tt> elements,
* because <tt>null</tt> 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 <tt>null</tt>, 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 <tt>c</tt> 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 <tt>ConcurrentSkipListSet</tt>
* instance. (The elements themselves are not cloned.)
*
* @return a shallow copy of this set
*/
public ConcurrentSkipListSet<E> clone() {
ConcurrentSkipListSet<E> clone = null;
try {
clone = (ConcurrentSkipListSet<E>) super.clone();
clone.setMap(new ConcurrentSkipListMap(m));
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
return clone;
}
/* ---------------- Set operations -------------- */
/**
* Returns the number of elements in this set. If this set
* contains more than <tt>Integer.MAX_VALUE</tt> elements, it
* returns <tt>Integer.MAX_VALUE</tt>.
*
* <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 <tt>true</tt> if this set contains no elements.
* @return <tt>true</tt> if this set contains no elements
*/
public boolean isEmpty() {
return m.isEmpty();
}
/**
* Returns <tt>true</tt> if this set contains the specified element.
* More formally, returns <tt>true</tt> if and only if this set
* contains an element <tt>e</tt> such that <tt>o.equals(e)</tt>.
*
* @param o object to be checked for containment in this set
* @return <tt>true</tt> 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 <tt>e</tt> to this set if
* the set contains no element <tt>e2</tt> such that <tt>e.equals(e2)</tt>.
* If this set already contains the element, the call leaves the set
* unchanged and returns <tt>false</tt>.
*
* @param e element to be added to this set
* @return <tt>true</tt> if this set did not already contain the
* specified element
* @throws ClassCastException if <tt>e</tt> 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 <tt>e</tt> such that
* <tt>o.equals(e)</tt>, if this set contains such an element.
* Returns <tt>true</tt> 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 <tt>true</tt> if this set contained the specified element
* @throws ClassCastException if <tt>o</tt> 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
* <tt>true</tt> 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 <tt>true</tt> 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 <tt>true</tt> 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 (Iterator<?> i = c.iterator(); i.hasNext(); )
if (remove(i.next()))
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 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<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
* <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
* 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(m.descendingMap());
}
// Support for resetting map in clone
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long mapOffset;
static {
try {
mapOffset = unsafe.objectFieldOffset
(ConcurrentSkipListSet.class.getDeclaredField("m"));
} catch (Exception ex) { throw new Error(ex); }
}
private void setMap(ConcurrentNavigableMap<E,Object> map) {
unsafe.putObjectVolatile(this, mapOffset, map);
}
}
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