ConcurrentSkipListSet

public class ConcurrentSkipListSet 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 {@linkplain Comparable 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 and spliterators are weakly consistent.

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.

Public Constructor Summary

ConcurrentSkipListSet()
Constructs a new, empty set that orders its elements according to their {@linkplain Comparable natural ordering}.
ConcurrentSkipListSet(Comparator<? super E> comparator)
Constructs a new, empty set that orders its elements according to the specified comparator.
ConcurrentSkipListSet(Collection<? extends E> c)
Constructs a new set containing the elements in the specified collection, that orders its elements according to their {@linkplain Comparable natural ordering}.
ConcurrentSkipListSet(SortedSet<E> s)
Constructs a new set containing the same elements and using the same ordering as the specified sorted set.

Public 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.
Comparator<? super E>
comparator()
Returns the comparator used to order the elements in this set, or null if this set uses the {@linkplain Comparable natural ordering} of its elements.
boolean
contains(Object o)
Returns true if this set contains the specified element.
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(Object o)
Compares the specified object with this set for equality.
E
first()
Returns the first (lowest) element currently in this set.
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, boolean inclusive)
Returns a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement.
NavigableSet<E>
headSet(E toElement)
Returns a view of the portion of this set whose elements are strictly less than toElement.

Equivalent to headSet(toElement, false).

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.
Iterator<E>
iterator()
Returns an iterator over the elements in this set in ascending order.
E
last()
Returns the last (highest) element currently in this set.
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(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.
Spliterator<E>
spliterator()
Returns a Spliterator over the 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)
Returns a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive.

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

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.
NavigableSet<E>
tailSet(E fromElement)
Returns a view of the portion of this set whose elements are greater than or equal to fromElement.

Equivalent to tailSet(fromElement, true).

Inherited Method Summary

Public Constructors

public ConcurrentSkipListSet ()

Constructs a new, empty set that orders its elements according to their {@linkplain Comparable natural ordering}.

public ConcurrentSkipListSet (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 {@linkplain Comparable natural ordering} of the elements will be used.

public ConcurrentSkipListSet (Collection<? extends E> c)

Constructs a new set containing the elements in the specified collection, that orders its elements according to their {@linkplain Comparable 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

public ConcurrentSkipListSet (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
NullPointerException if the specified sorted set or any of its elements are null

Public Methods

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.

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

public 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.

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
ClassCastException
NullPointerException if the specified element is null

public void clear ()

Removes all of the elements from this set.

public ConcurrentSkipListSet<E> clone ()

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

Returns
  • a shallow copy of this set

public Comparator<? super E> comparator ()

Returns the comparator used to order the elements in this set, or null if this set uses the {@linkplain Comparable natural ordering} of its elements.

Returns
  • the comparator used to order the elements in this set, or null if this set uses the natural ordering of its elements

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
Throws
ClassCastException if the specified element cannot be compared with the elements currently in this set
NullPointerException if the specified element is null

public Iterator<E> descendingIterator ()

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

Returns
  • an iterator over the elements in this set in descending order

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.

Returns
  • a reverse order view of this set

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

public E first ()

Returns the first (lowest) element currently in this set.

Returns
  • the first (lowest) element currently in this set

public 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.

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
ClassCastException
NullPointerException if the specified element is null

public 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. 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.

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

public NavigableSet<E> headSet (E toElement)

Returns a view of the portion of this set whose elements are strictly less than 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.

Equivalent to headSet(toElement, false).

Parameters
toElement high endpoint (exclusive) of the returned set
Returns
  • a view of the portion of this set whose elements are strictly less than toElement

public 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.

Parameters
e the value to match
Returns
  • the least element greater than e, or null if there is no such element
Throws
ClassCastException
NullPointerException if the specified element is null

public boolean isEmpty ()

Returns true if this set contains no elements.

Returns
  • true if this set contains no elements

public Iterator<E> iterator ()

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

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

public E last ()

Returns the last (highest) element currently in this set.

Returns
  • the last (highest) element currently in this set

public 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.

Parameters
e the value to match
Returns
  • the greatest element less than e, or null if there is no such element
Throws
ClassCastException
NullPointerException if the specified element is null

public E pollFirst ()

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

Returns
  • the first element, or null if this set is empty

public E pollLast ()

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

Returns
  • the last element, or null if this set is empty

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
Throws
ClassCastException if o cannot be compared with the elements currently in this set
NullPointerException if the specified element is null

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
Throws
ClassCastException if the class of an element of this set is incompatible with the specified collection (optional)
NullPointerException if the specified collection or any of its elements are null

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

public Spliterator<E> spliterator ()

Returns a Spliterator over the elements in this set.

The Spliterator reports Spliterator.CONCURRENT, Spliterator.NONNULL, Spliterator.DISTINCT, Spliterator.SORTED and Spliterator.ORDERED, with an encounter order that is ascending order. Overriding implementations should document the reporting of additional characteristic values.

The spliterator's comparator (see Spliterator.getComparator()) is null if the set's comparator (see comparator()) is null. Otherwise, the spliterator's comparator is the same as or imposes the same total ordering as the set's comparator.

Returns
  • a Spliterator over the elements in this set

public 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. If fromElement and toElement are equal, the returned set is empty unless fromInclusive and toInclusive 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.

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
ClassCastException
NullPointerException if fromElement or toElement is null
IllegalArgumentException

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

Returns a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive. (If fromElement and toElement are equal, the returned set is empty.) 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.

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

Parameters
fromElement low endpoint (inclusive) of the returned set
toElement high endpoint (exclusive) of the returned set
Returns
  • a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive
Throws
ClassCastException
NullPointerException if fromElement or toElement is null
IllegalArgumentException

public 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. 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.

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

public NavigableSet<E> tailSet (E fromElement)

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

Equivalent to tailSet(fromElement, true).

Parameters
fromElement low endpoint (inclusive) of the returned set
Returns
  • a view of the portion of this set whose elements are greater than or equal to fromElement