java.util.concurrent
Class ConcurrentSkipListSet<E>

java.lang.Object
  java.util.AbstractCollection<E>
      java.util.AbstractSet<E>
          java.util.concurrent.ConcurrentSkipListSet<E>

Type Parameters:

E - the type of elements maintained by this set

All Implemented Interfaces:

java.io.Serializable, java.lang.Cloneable, java.lang.Iterable<E>, java.util.Collection<E>, NavigableSet<E>, java.util.Set<E>, java.util.SortedSet<E>

public class ConcurrentSkipListSet<E>
extends java.util.AbstractSet<E>
implements NavigableSet<E>, java.lang.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 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, and so may report inaccurate results if this collection is modified during traversal. Additionally, the bulk operations addAll, removeAll, retainAll, containsAll, equals, and toArray 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 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(java.util.Collection<? extends E> c)
Constructs a new set containing the elements in the specified collection, that orders its elements according to their natural ordering.

ConcurrentSkipListSet(java.util.Comparator<? super E> comparator)
Constructs a new, empty set that orders its elements according to the specified comparator.

ConcurrentSkipListSet(java.util.SortedSet<E> 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) Returns the least element in this set greater than or equal to the given element, or null if there is no such element.
void	clear() Removes all of the elements from this set.
ConcurrentSkipListSet<E>	clone() Returns a shallow copy of this ConcurrentSkipListSet instance.
java.util.Comparator<? super E>	comparator()
boolean	contains(java.lang.Object o) Returns true if this set contains the specified element.
java.util.Iterator<E>	descendingIterator() Returns an iterator over the elements in this set in descending order.
NavigableSet<E>	descendingSet() Returns a reverse order view of the elements contained in this set.
boolean	equals(java.lang.Object o) Compares the specified object with this set for equality.
E	first()
E	floor(E e) Returns the greatest element in this set less than or equal to the given element, or null if there is no such element.
NavigableSet<E>	headSet(E toElement)
NavigableSet<E>	headSet(E toElement, boolean inclusive) Returns a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement.
E	higher(E e) Returns the least element in this set strictly greater than the given element, or null if there is no such element.
boolean	isEmpty() Returns true if this set contains no elements.
java.util.Iterator<E>	iterator() Returns an iterator over the elements in this set in ascending order.
E	last()
E	lower(E e) Returns the greatest element in this set strictly less than the given element, or null if there is no such element.
E	pollFirst() Retrieves and removes the first (lowest) element, or returns null if this set is empty.
E	pollLast() Retrieves and removes the last (highest) element, or returns null if this set is empty.
boolean	remove(java.lang.Object o) Removes the specified element from this set if it is present.
boolean	removeAll(java.util.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<E>	subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) Returns a view of the portion of this set whose elements range from fromElement to toElement.
NavigableSet<E>	subSet(E fromElement, E toElement)
NavigableSet<E>	tailSet(E fromElement)
NavigableSet<E>	tailSet(E fromElement, boolean inclusive) Returns a view of the portion of this set whose elements are greater than (or equal to, if inclusive is true) fromElement.

Methods inherited from class java.util.AbstractSet

hashCode

Methods inherited from class java.util.AbstractCollection

addAll, containsAll, retainAll, toArray, toArray, toString

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Set

addAll, containsAll, hashCode, retainAll, toArray, toArray

Constructor Detail

ConcurrentSkipListSet

public ConcurrentSkipListSet()

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

ConcurrentSkipListSet

public ConcurrentSkipListSet(java.util.Comparator<? super 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(java.util.Collection<? extends 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:

java.lang.ClassCastException - if the elements in c are not Comparable, or are not mutually comparable

java.lang.NullPointerException - if the specified collection or any of its elements are null

ConcurrentSkipListSet

public ConcurrentSkipListSet(java.util.SortedSet<E> 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:

java.lang.NullPointerException - if the specified sorted set or any of its elements are null

Method Detail

clone

public ConcurrentSkipListSet<E> clone()

Returns a shallow copy of this ConcurrentSkipListSet instance. (The elements themselves are not cloned.)

Overrides:

clone in class java.lang.Object

Returns:

a shallow copy of this set

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 java.util.Collection<E>

Specified by:

size in interface java.util.Set<E>

Specified by:

size in class java.util.AbstractCollection<E>

Returns:

the number of elements in this set

isEmpty

public boolean isEmpty()

Returns true if this set contains no elements.

Specified by:

isEmpty in interface java.util.Collection<E>

Specified by:

isEmpty in interface java.util.Set<E>

Overrides:

isEmpty in class java.util.AbstractCollection<E>

Returns:

true if this set contains no elements

contains

public boolean contains(java.lang.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 java.util.Collection<E>

Specified by:

contains in interface java.util.Set<E>

Overrides:

contains in class java.util.AbstractCollection<E>

Parameters:

o - object to be checked for containment in this set

Returns:

true if this set contains the specified element

Throws:

java.lang.ClassCastException - if the specified element cannot be compared with the elements currently in this set

java.lang.NullPointerException - if the specified element is null

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 java.util.Collection<E>

Specified by:

add in interface java.util.Set<E>

Overrides:

add in class java.util.AbstractCollection<E>

Parameters:

e - element to be added to this set

Returns:

true if this set did not already contain the specified element

Throws:

java.lang.ClassCastException - if e cannot be compared with the elements currently in this set

java.lang.NullPointerException - if the specified element is null

remove

public boolean remove(java.lang.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 java.util.Collection<E>

Specified by:

remove in interface java.util.Set<E>

Overrides:

remove in class java.util.AbstractCollection<E>

Parameters:

o - object to be removed from this set, if present

Returns:

true if this set contained the specified element

Throws:

java.lang.ClassCastException - if o cannot be compared with the elements currently in this set

java.lang.NullPointerException - if the specified element is null

clear

public void clear()

Removes all of the elements from this set.

Specified by:

clear in interface java.util.Collection<E>

Specified by:

clear in interface java.util.Set<E>

Overrides:

clear in class java.util.AbstractCollection<E>

iterator

public java.util.Iterator<E> iterator()

Returns an iterator over the elements in this set in ascending order.

Specified by:

iterator in interface java.lang.Iterable<E>

Specified by:

iterator in interface java.util.Collection<E>

Specified by:

iterator in interface NavigableSet<E>

Specified by:

iterator in interface java.util.Set<E>

Specified by:

iterator in class java.util.AbstractCollection<E>

Returns:

an iterator over the elements in this set in ascending order

descendingIterator

public java.util.Iterator<E> 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

equals

public boolean equals(java.lang.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 java.util.Collection<E>

Specified by:

equals in interface java.util.Set<E>

Overrides:

equals in class java.util.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

removeAll

public boolean removeAll(java.util.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 java.util.Collection<E>

Specified by:

removeAll in interface java.util.Set<E>

Overrides:

removeAll in class java.util.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:

java.lang.ClassCastException - if the types of one or more elements in this set are incompatible with the specified collection

java.lang.NullPointerException - if the specified collection or any of its elements are null

lower

public E lower(E e)

Description copied from interface: NavigableSet

Returns the greatest element in this set strictly less than the given element, or null if there is no such element.

Specified by:

lower in interface NavigableSet<E>

Parameters:

e - the value to match

Returns:

the greatest element less than e, or null if there is no such element

Throws:

java.lang.ClassCastException - if the specified element cannot be compared with the elements currently in the set

java.lang.NullPointerException - if the specified element is null

floor

public E floor(E e)

Description copied from interface: NavigableSet

Returns the greatest element in this set less than or equal to the given element, or null if there is no such element.

Specified by:

floor in interface NavigableSet<E>

Parameters:

e - the value to match

Returns:

the greatest element less than or equal to e, or null if there is no such element

Throws:

java.lang.ClassCastException - if the specified element cannot be compared with the elements currently in the set

java.lang.NullPointerException - if the specified element is null

ceiling

public E ceiling(E e)

Description copied from interface: NavigableSet

Returns the least element in this set greater than or equal to the given element, or null if there is no such element.

Specified by:

ceiling in interface NavigableSet<E>

Parameters:

e - the value to match

Returns:

the least element greater than or equal to e, or null if there is no such element

Throws:

java.lang.ClassCastException - if the specified element cannot be compared with the elements currently in the set

java.lang.NullPointerException - if the specified element is null

higher

public E higher(E e)

Description copied from interface: NavigableSet

Returns the least element in this set strictly greater than the given element, or null if there is no such element.

Specified by:

higher in interface NavigableSet<E>

Parameters:

e - the value to match

Returns:

the least element greater than e, or null if there is no such element

Throws:

java.lang.ClassCastException - if the specified element cannot be compared with the elements currently in the set

java.lang.NullPointerException - if the specified element is null

pollFirst

public E pollFirst()

Description copied from interface: NavigableSet

Retrieves and removes the first (lowest) element, or returns null if this set is empty.

Specified by:

pollFirst in interface NavigableSet<E>

Returns:

the first element, or null if this set is empty

pollLast

public E pollLast()

Description copied from interface: NavigableSet

Retrieves and removes the last (highest) element, or returns null if this set is empty.

Specified by:

pollLast in interface NavigableSet<E>

Returns:

the last element, or null if this set is empty

comparator

public java.util.Comparator<? super E> comparator()

Specified by:

comparator in interface java.util.SortedSet<E>

first

public E first()

Specified by:

first in interface java.util.SortedSet<E>

Throws:

java.util.NoSuchElementException

last

public E last()

Specified by:

last in interface java.util.SortedSet<E>

Throws:

java.util.NoSuchElementException

subSet

public NavigableSet<E> subSet(E fromElement,
                              boolean fromInclusive,
                              E toElement,
                              boolean toInclusive)

Description copied from interface: NavigableSet

Returns a view of the portion of this set whose elements range from fromElement to toElement. If fromElement and toElement are equal, the returned set is empty unless fromExclusive and toExclusive are both true. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

The returned set will throw an IllegalArgumentException on an attempt to insert an element outside its range.

Specified by:

subSet in interface NavigableSet<E>

Parameters:

fromElement - low endpoint of the returned set

fromInclusive - true if the low endpoint is to be included in the returned view

toElement - high endpoint of the returned set

toInclusive - true if the high endpoint is to be included in the returned view

Returns:

a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive

Throws:

java.lang.ClassCastException - if fromElement and toElement cannot be compared to one another using this set's comparator (or, if the set has no comparator, using natural ordering). Implementations may, but are not required to, throw this exception if fromElement or toElement cannot be compared to elements currently in the set.

java.lang.NullPointerException - if fromElement or toElement is null

java.lang.IllegalArgumentException - if fromElement is greater than toElement; or if this set itself has a restricted range, and fromElement or toElement lies outside the bounds of the range.

headSet

public NavigableSet<E> headSet(E toElement,
                               boolean inclusive)

Description copied from interface: NavigableSet

Returns a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

The returned set will throw an IllegalArgumentException on an attempt to insert an element outside its range.

Specified by:

headSet in interface NavigableSet<E>

Parameters:

toElement - high endpoint of the returned set

inclusive - true if the high endpoint is to be included in the returned view

Returns:

a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement

Throws:

java.lang.ClassCastException - if toElement is not compatible with this set's comparator (or, if the set has no comparator, if toElement does not implement Comparable). Implementations may, but are not required to, throw this exception if toElement cannot be compared to elements currently in the set.

java.lang.NullPointerException - if toElement is null

java.lang.IllegalArgumentException - if this set itself has a restricted range, and toElement lies outside the bounds of the range

tailSet

public NavigableSet<E> tailSet(E fromElement,
                               boolean inclusive)

Description copied from interface: NavigableSet

Returns a view of the portion of this set whose elements are greater than (or equal to, if inclusive is true) fromElement. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

The returned set will throw an IllegalArgumentException on an attempt to insert an element outside its range.

Specified by:

tailSet in interface NavigableSet<E>

Parameters:

fromElement - low endpoint of the returned set

inclusive - true if the low endpoint is to be included in the returned view

Returns:

a view of the portion of this set whose elements are greater than or equal to fromElement

Throws:

java.lang.ClassCastException - if fromElement is not compatible with this set's comparator (or, if the set has no comparator, if fromElement does not implement Comparable). Implementations may, but are not required to, throw this exception if fromElement cannot be compared to elements currently in the set.

java.lang.NullPointerException - if fromElement is null

java.lang.IllegalArgumentException - if this set itself has a restricted range, and fromElement lies outside the bounds of the range

subSet

public NavigableSet<E> subSet(E fromElement,
                              E toElement)

Description copied from interface: NavigableSet

Equivalent to subSet(fromElement, true, toElement, false).

Specified by:

subSet in interface NavigableSet<E>

Specified by:

subSet in interface java.util.SortedSet<E>

Throws:

java.lang.ClassCastException

java.lang.NullPointerException - if fromElement or toElement is null

java.lang.IllegalArgumentException

headSet

public NavigableSet<E> headSet(E toElement)

Description copied from interface: NavigableSet

Equivalent to headSet(toElement, false).

Specified by:

headSet in interface NavigableSet<E>

Specified by:

headSet in interface java.util.SortedSet<E>

Throws:

java.lang.ClassCastException

java.lang.NullPointerException - if toElement is null

java.lang.IllegalArgumentException - na

tailSet

public NavigableSet<E> tailSet(E fromElement)

Description copied from interface: NavigableSet

Equivalent to tailSet(fromElement, true).

Specified by:

tailSet in interface NavigableSet<E>

Specified by:

tailSet in interface java.util.SortedSet<E>

Throws:

java.lang.ClassCastException

java.lang.NullPointerException - if fromElement is null

java.lang.IllegalArgumentException

descendingSet

public NavigableSet<E> 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