java.util.concurrent

Class ConcurrentSkipListSet<E>

Implemented Interfaces:
Cloneable, Collection<E>, Iterable<E>, NavigableSet<E>, Serializable, Set<E>, SortedSet<E>

public class ConcurrentSkipListSet<E>
extends AbstractSet<E>
implements NavigableSet<E>, Cloneable, Serializable

A scalable concurrent NavigableSet implementation based on a ConcurrentSkipListMap. The elements of the set are kept sorted according to their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.

This implementation provides expected average log(n) time cost for the contains, add, and 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 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 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. Additionally, the bulk operations addAll, removeAll, retainAll, and containsAll are not guaranteed to be performed atomically. For example, an iterator operating concurrently with an addAll operation might view only some of the added elements.

This class and its iterators implement all of the optional methods of the Set and Iterator interfaces. Like most other concurrent collection implementations, this class does not permit the use of null elements, because null arguments and return values cannot be reliably distinguished from the absence of elements.

This class is a member of the ../../../../technotes/guides/collections/index.html"> Java Collections Framework.

Since:
1.6
See Also:
Serialized Form

Constructor Summary

ConcurrentSkipListSet()
Constructs a new, empty set that orders its elements according to their natural ordering.
ConcurrentSkipListSet(E> comparator)
Constructs a new, empty set that orders its elements according to the specified comparator.
ConcurrentSkipListSet(E> c)
Constructs a new set containing the elements in the specified collection, that orders its elements according to their natural ordering.
ConcurrentSkipListSet(SortedSet s)
Constructs a new set containing the same elements and using the same ordering as the specified sorted set.

Method Summary

boolean
add(E e)
Adds the specified element to this set if it is not already present.
E
ceiling(E e)
void
clear()
Removes all of the elements from this set.
ConcurrentSkipListSet
clone()
Returns a shallow copy of this ConcurrentSkipListSet instance.
boolean
contains(Object o)
Returns true if this set contains the specified element.
Iterator
descendingIterator()
Returns an iterator over the elements in this set in descending order.
NavigableSet
descendingSet()
Returns a reverse order view of the elements contained in this set.
boolean
equals(Object o)
Compares the specified object with this set for equality.
E
first()
E
floor(E e)
NavigableSet
headSet(E toElement)
NavigableSet
headSet(E toElement, boolean inclusive)
E
higher(E e)
boolean
isEmpty()
Returns true if this set contains no elements.
Iterator
iterator()
Returns an iterator over the elements in this set in ascending order.
E
last()
E
lower(E e)
E
pollFirst()
E
pollLast()
boolean
remove(Object o)
Removes the specified element from this set if it is present.
boolean
removeAll(Collection c)
Removes from this set all of its elements that are contained in the specified collection.
int
size()
Returns the number of elements in this set.
NavigableSet
subSet(E fromElement, E toElement)
NavigableSet
subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
Comparator
super E> comparator()
NavigableSet
tailSet(E fromElement)
NavigableSet
tailSet(E fromElement, boolean inclusive)

Methods inherited from class java.util.AbstractSet<E>

equals, hashCode, removeAll

Methods inherited from class java.util.AbstractCollection<E>

T[] toArray, add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

ConcurrentSkipListSet

public ConcurrentSkipListSet()
Constructs a new, empty set that orders its elements according to their natural ordering.

ConcurrentSkipListSet

public ConcurrentSkipListSet(E> comparator)
Constructs a new, empty set that orders its elements according to the specified comparator.
Parameters:
comparator - the comparator that will be used to order this set. If null, the natural ordering of the elements will be used.

ConcurrentSkipListSet

public ConcurrentSkipListSet(E> c)
Constructs a new set containing the elements in the specified collection, that orders its elements according to their natural ordering.
Parameters:
c - The elements that will comprise the new set
Throws:
ClassCastException - if the elements in c are not Comparable, or are not mutually comparable
NullPointerException - if the specified collection or any of its elements are null

ConcurrentSkipListSet

public ConcurrentSkipListSet(SortedSet s)
Constructs a new set containing the same elements and using the same ordering as the specified sorted set.
Parameters:
s - sorted set whose elements will comprise the new set
Throws:
NullPointerException - if the specified sorted set or any of its elements are null

Method Details

add

public boolean add(E e)
Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if the set contains no element e2 such that e.equals(e2). If this set already contains the element, the call leaves the set unchanged and returns false.
Specified by:
add in interface Set<E>
add in interface Collection<E>
Overrides:
add in interface AbstractCollection<E>
Parameters:
e - element to be added to this set
Returns:
true if this set did not already contain the specified element
Throws:
ClassCastException - if e cannot be compared with the elements currently in this set
NullPointerException - if the specified element is null

ceiling

public E ceiling(E e)
Specified by:
ceiling in interface NavigableSet<E>
Throws:
ClassCastException -
NullPointerException - if the specified element is null

clear

public void clear()
Removes all of the elements from this set.
Specified by:
clear in interface Set<E>
clear in interface Collection<E>
Overrides:
clear in interface AbstractCollection<E>

clone

public ConcurrentSkipListSet clone()
Returns a shallow copy of this ConcurrentSkipListSet instance. (The elements themselves are not cloned.)
Overrides:
clone in interface Object
Returns:
a shallow copy of this set

contains

public boolean contains(Object o)
Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that o.equals(e).
Specified by:
contains in interface Set<E>
contains in interface Collection<E>
Overrides:
contains in interface AbstractCollection<E>
Parameters:
o - object to be checked for containment in this set
Returns:
true if this set contains the specified element
Throws:
ClassCastException - if the specified element cannot be compared with the elements currently in this set
NullPointerException - if the specified element is null

descendingIterator

public Iterator descendingIterator()
Returns an iterator over the elements in this set in descending order.
Specified by:
descendingIterator in interface NavigableSet<E>
Returns:
an iterator over the elements in this set in descending order

descendingSet

public NavigableSet descendingSet()
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 Collections.reverseOrder(comparator()). The expression s.descendingSet().descendingSet() returns a view of s essentially equivalent to s.

Specified by:
descendingSet in interface NavigableSet<E>
Returns:
a reverse order view of this set

equals

public boolean equals(Object o)
Compares the specified object with this set for equality. Returns 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.
Specified by:
equals in interface Set<E>
equals in interface Collection<E>
Overrides:
equals in interface AbstractSet<E>
Parameters:
o - the object to be compared for equality with this set
Returns:
true if the specified object is equal to this set

first

public E first()
Specified by:
first in interface SortedSet<E>

floor

public E floor(E e)
Specified by:
floor in interface NavigableSet<E>
Throws:
ClassCastException -
NullPointerException - if the specified element is null

headSet

public NavigableSet headSet(E toElement)
Specified by:
headSet in interface NavigableSet<E>
headSet in interface SortedSet<E>

headSet

public NavigableSet headSet(E toElement,
                               boolean inclusive)
Specified by:
headSet in interface NavigableSet<E>

higher

public E higher(E e)
Specified by:
higher in interface NavigableSet<E>
Throws:
ClassCastException -
NullPointerException - if the specified element is null

isEmpty

public boolean isEmpty()
Returns true if this set contains no elements.
Specified by:
isEmpty in interface Set<E>
isEmpty in interface Collection<E>
Overrides:
isEmpty in interface AbstractCollection<E>
Returns:
true if this set contains no elements

iterator

public Iterator iterator()
Returns an iterator over the elements in this set in ascending order.
Specified by:
iterator in interface NavigableSet<E>
iterator in interface Set<E>
iterator in interface Collection<E>
iterator in interface Iterable<E>
Overrides:
iterator in interface AbstractCollection<E>
Returns:
an iterator over the elements in this set in ascending order

last

public E last()
Specified by:
last in interface SortedSet<E>

lower

public E lower(E e)
Specified by:
lower in interface NavigableSet<E>
Throws:
ClassCastException -
NullPointerException - if the specified element is null

pollFirst

public E pollFirst()
Specified by:
pollFirst in interface NavigableSet<E>

pollLast

public E pollLast()
Specified by:
pollLast in interface NavigableSet<E>

remove

public boolean remove(Object o)
Removes the specified element from this set if it is present. More formally, removes an element e such that o.equals(e), if this set contains such an element. Returns 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.)
Specified by:
remove in interface Set<E>
remove in interface Collection<E>
Overrides:
remove in interface AbstractCollection<E>
Parameters:
o - object to be removed from this set, if present
Returns:
true if this set contained the specified element
Throws:
ClassCastException - if o cannot be compared with the elements currently in this set
NullPointerException - if the specified element is null

removeAll

public boolean removeAll(Collection c)
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.
Specified by:
removeAll in interface Set<E>
removeAll in interface Collection<E>
Overrides:
removeAll in interface AbstractSet<E>
Parameters:
c - collection containing elements to be removed from this set
Returns:
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
NullPointerException - if the specified collection or any of its elements are null

size

public int size()
Returns the number of elements in this set. If this set contains more than Integer.MAX_VALUE elements, it returns 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.

Specified by:
size in interface Set<E>
size in interface Collection<E>
Overrides:
size in interface AbstractCollection<E>
Returns:
the number of elements in this set

subSet

public NavigableSet subSet(E fromElement,
                              E toElement)
Specified by:
subSet in interface NavigableSet<E>
subSet in interface SortedSet<E>
Throws:
ClassCastException -
NullPointerException - if fromElement or toElement is null
IllegalArgumentException -

subSet

public NavigableSet subSet(E fromElement,
                              boolean fromInclusive,
                              E toElement,
                              boolean toInclusive)
Specified by:
subSet in interface NavigableSet<E>
Throws:
ClassCastException -
NullPointerException - if fromElement or toElement is null
IllegalArgumentException -

super E> comparator

public Comparatorsuper E> comparator()
Specified by:
super E> comparator in interface SortedSet<E>

tailSet

public NavigableSet tailSet(E fromElement)
Specified by:
tailSet in interface NavigableSet<E>
tailSet in interface SortedSet<E>

tailSet

public NavigableSet tailSet(E fromElement,
                               boolean inclusive)
Specified by:
tailSet in interface NavigableSet<E>