edu.emory.mathcs.backport.java.util.concurrent

Class ConcurrentSkipListSet

Implemented Interfaces:
Cloneable, NavigableSet, java.io.Serializable, SortedSet

public class ConcurrentSkipListSet
extends AbstractSet
implements NavigableSet, Cloneable, java.io.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 java.util.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.

Author:
Doug Lea
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(Collection c)
Constructs a new set containing the elements in the specified collection, that orders its elements according to their natural ordering.
ConcurrentSkipListSet(Comparator comparator)
Constructs a new, empty set that orders its elements according to the specified comparator.
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(Object e)
Adds the specified element to this set if it is not already present.
Object
ceiling(Object e)
void
clear()
Removes all of the elements from this set.
Object
clone()
Returns a shallow copy of this ConcurrentSkipListSet instance.
Comparator
comparator()
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.
Object
first()
Object
floor(Object e)
SortedSet
headSet(Object toElement)
NavigableSet
headSet(Object toElement, boolean inclusive)
Object
higher(Object 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.
Object
last()
Object
lower(Object e)
Object
pollFirst()
Object
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.
SortedSet
subSet(Object fromElement, Object toElement)
NavigableSet
subSet(Object fromElement, boolean fromInclusive, Object toElement, boolean toInclusive)
SortedSet
tailSet(Object fromElement)
NavigableSet
tailSet(Object fromElement, boolean inclusive)

Methods inherited from class edu.emory.mathcs.backport.java.util.AbstractSet

toArray, toArray

Constructor Details

ConcurrentSkipListSet

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

ConcurrentSkipListSet

public ConcurrentSkipListSet(Collection 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

ConcurrentSkipListSet

public ConcurrentSkipListSet(Comparator 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(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

Method Details

add

public boolean add(Object 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.
Parameters:
e - element to be added to this set
Returns:
true if this set did not already contain the specified element

ceiling

public Object ceiling(Object e)
Specified by:
ceiling in interface NavigableSet

clear

public void clear()
Removes all of the elements from this set.

clone

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

comparator

public Comparator comparator()

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).
Parameters:
o - object to be checked for containment in this set
Returns:
true if this set contains the specified element

descendingIterator

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

floor

public Object floor(Object e)
Specified by:
floor in interface NavigableSet

headSet

public SortedSet headSet(Object toElement)
Specified by:
headSet in interface NavigableSet

headSet

public NavigableSet headSet(Object toElement,
                            boolean inclusive)
Specified by:
headSet in interface NavigableSet

higher

public Object higher(Object e)
Specified by:
higher in interface NavigableSet

isEmpty

public boolean isEmpty()
Returns true if this set contains no elements.
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
Returns:
an iterator over the elements in this set in ascending order

last

public Object last()

lower

public Object lower(Object e)
Specified by:
lower in interface NavigableSet

pollFirst

public Object pollFirst()
Specified by:
pollFirst in interface NavigableSet

pollLast

public Object pollLast()
Specified by:
pollLast in interface NavigableSet

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.)
Parameters:
o - object to be removed from this set, if present
Returns:
true if this set contained the specified element

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.
Parameters:
c - collection containing elements to be removed from this set
Returns:
true if this set changed as a result of the call

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.

Returns:
the number of elements in this set

subSet

public SortedSet subSet(Object fromElement,
                        Object toElement)
Specified by:
subSet in interface NavigableSet

subSet

public NavigableSet subSet(Object fromElement,
                           boolean fromInclusive,
                           Object toElement,
                           boolean toInclusive)
Specified by:
subSet in interface NavigableSet

tailSet

public SortedSet tailSet(Object fromElement)
Specified by:
tailSet in interface NavigableSet

tailSet

public NavigableSet tailSet(Object fromElement,
                            boolean inclusive)
Specified by:
tailSet in interface NavigableSet