 Type Parameters:
E
 the type of elements maintained by this set
 All Implemented Interfaces:
Serializable
,Cloneable
,Iterable<E>
,Collection<E>
,NavigableSet<E>
,SequencedCollection<E>
,SequencedSet<E>
,Set<E>
,SortedSet<E>
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 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 constanttime 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.
Bulk operations that add, remove, or examine multiple elements,
such as AbstractCollection.addAll(java.util.Collection<? extends E>)
, Collection.removeIf(java.util.function.Predicate<? super E>)
or Iterable.forEach(java.util.function.Consumer<? super T>)
,
are not guaranteed to be performed atomically.
For example, a forEach
traversal concurrent with an
addAll
operation might observe 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:

Constructor Summary
ConstructorDescriptionConstructs a new, empty set that orders its elements according to their natural ordering.ConcurrentSkipListSet
(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
(Comparator<? super E> comparator) Constructs a new, empty set that orders its elements according to the specified comparator.Constructs a new set containing the same elements and using the same ordering as the specified sorted set. 
Method Summary
Modifier and TypeMethodDescriptionboolean
Adds the specified element to this set if it is not already present.void
ThrowsUnsupportedOperationException
.void
ThrowsUnsupportedOperationException
.Returns the least element in this set greater than or equal to the given element, ornull
if there is no such element.void
clear()
Removes all of the elements from this set.clone()
Returns a shallow copy of thisConcurrentSkipListSet
instance.Comparator
<? super E> Returns the comparator used to order the elements in this set, ornull
if this set uses the natural ordering of its elements.boolean
Returnstrue
if this set contains the specified element.Returns an iterator over the elements in this set in descending order.Returns a reverse order view of the elements contained in this set.boolean
Compares the specified object with this set for equality.first()
Returns the first (lowest) element currently in this set.Returns the greatest element in this set less than or equal to the given element, ornull
if there is no such element.Returns a view of the portion of this set whose elements are strictly less thantoElement
.Returns a view of the portion of this set whose elements are less than (or equal to, ifinclusive
is true)toElement
.Returns the least element in this set strictly greater than the given element, ornull
if there is no such element.boolean
isEmpty()
Returnstrue
if this set contains no elements.iterator()
Returns an iterator over the elements in this set in ascending order.last()
Returns the last (highest) element currently in this set.Returns the greatest element in this set strictly less than the given element, ornull
if there is no such element.Retrieves and removes the first (lowest) element, or returnsnull
if this set is empty (optional operation).pollLast()
Retrieves and removes the last (highest) element, or returnsnull
if this set is empty (optional operation).boolean
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.Returns aSpliterator
over the elements in this set.Returns a view of the portion of this set whose elements range fromfromElement
totoElement
.Returns a view of the portion of this set whose elements range fromfromElement
, inclusive, totoElement
, exclusive.Returns a view of the portion of this set whose elements are greater than or equal tofromElement
.Returns a view of the portion of this set whose elements are greater than (or equal to, ifinclusive
is true)fromElement
.Methods declared in class java.util.AbstractSet
hashCode
Methods declared in class java.util.AbstractCollection
addAll, containsAll, retainAll, toArray, toArray, toString
Methods declared in interface java.util.Collection
parallelStream, removeIf, stream, toArray
Methods declared in interface java.util.NavigableSet
removeFirst, removeLast, reversed

Constructor Details

ConcurrentSkipListSet
public ConcurrentSkipListSet()Constructs a new, empty set that orders its elements according to their natural ordering. 
ConcurrentSkipListSet
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. Ifnull
, the natural ordering of the elements will be used.

ConcurrentSkipListSet
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 inc
are notComparable
, or are not mutually comparableNullPointerException
 if the specified collection or any of its elements are null

ConcurrentSkipListSet
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

clone
Returns a shallow copy of thisConcurrentSkipListSet
instance. (The elements themselves are not cloned.) 
size
public int size()Returns the number of elements in this set. If this set contains more thanInteger.MAX_VALUE
elements, it returnsInteger.MAX_VALUE
.Beware that, unlike in most collections, this method is NOT a constanttime 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.

isEmpty
public boolean isEmpty()Returnstrue
if this set contains no elements. Specified by:
isEmpty
in interfaceCollection<E>
 Specified by:
isEmpty
in interfaceSet<E>
 Overrides:
isEmpty
in classAbstractCollection<E>
 Returns:
true
if this set contains no elements

contains
Returnstrue
if this set contains the specified element. More formally, returnstrue
if and only if this set contains an elemente
such thato.equals(e)
. Specified by:
contains
in interfaceCollection<E>
 Specified by:
contains
in interfaceSet<E>
 Overrides:
contains
in classAbstractCollection<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 setNullPointerException
 if the specified element is null

add
Adds the specified element to this set if it is not already present. More formally, adds the specified elemente
to this set if the set contains no elemente2
such thate.equals(e2)
. If this set already contains the element, the call leaves the set unchanged and returnsfalse
. Specified by:
add
in interfaceCollection<E>
 Specified by:
add
in interfaceSet<E>
 Overrides:
add
in classAbstractCollection<E>
 Parameters:
e
 element to be added to this set Returns:
true
if this set did not already contain the specified element Throws:
ClassCastException
 ife
cannot be compared with the elements currently in this setNullPointerException
 if the specified element is null

remove
Removes the specified element from this set if it is present. More formally, removes an elemente
such thato.equals(e)
, if this set contains such an element. Returnstrue
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 interfaceCollection<E>
 Specified by:
remove
in interfaceSet<E>
 Overrides:
remove
in classAbstractCollection<E>
 Parameters:
o
 object to be removed from this set, if present Returns:
true
if this set contained the specified element Throws:
ClassCastException
 ifo
cannot be compared with the elements currently in this setNullPointerException
 if the specified element is null

clear
public void clear()Removes all of the elements from this set. Specified by:
clear
in interfaceCollection<E>
 Specified by:
clear
in interfaceSet<E>
 Overrides:
clear
in classAbstractCollection<E>

iterator
Returns an iterator over the elements in this set in ascending order. Specified by:
iterator
in interfaceCollection<E>
 Specified by:
iterator
in interfaceIterable<E>
 Specified by:
iterator
in interfaceNavigableSet<E>
 Specified by:
iterator
in interfaceSet<E>
 Specified by:
iterator
in classAbstractCollection<E>
 Returns:
 an iterator over the elements in this set in ascending order

descendingIterator
Returns an iterator over the elements in this set in descending order. Specified by:
descendingIterator
in interfaceNavigableSet<E>
 Returns:
 an iterator over the elements in this set in descending order

equals
Compares the specified object with this set for equality. Returnstrue
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 interfaceCollection<E>
 Specified by:
equals
in interfaceSet<E>
 Overrides:
equals
in classAbstractSet<E>
 Parameters:
o
 the object to be compared for equality with this set Returns:
true
if the specified object is equal to this set See Also:

removeAll
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 interfaceCollection<E>
 Specified by:
removeAll
in interfaceSet<E>
 Overrides:
removeAll
in classAbstractSet<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 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 See Also:

lower
Description copied from interface:NavigableSet
Returns the greatest element in this set strictly less than the given element, ornull
if there is no such element. Specified by:
lower
in interfaceNavigableSet<E>
 Parameters:
e
 the value to match Returns:
 the greatest element less than
e
, ornull
if there is no such element  Throws:
ClassCastException
 if the specified element cannot be compared with the elements currently in the setNullPointerException
 if the specified element is null

floor
Description copied from interface:NavigableSet
Returns the greatest element in this set less than or equal to the given element, ornull
if there is no such element. Specified by:
floor
in interfaceNavigableSet<E>
 Parameters:
e
 the value to match Returns:
 the greatest element less than or equal to
e
, ornull
if there is no such element  Throws:
ClassCastException
 if the specified element cannot be compared with the elements currently in the setNullPointerException
 if the specified element is null

ceiling
Description copied from interface:NavigableSet
Returns the least element in this set greater than or equal to the given element, ornull
if there is no such element. Specified by:
ceiling
in interfaceNavigableSet<E>
 Parameters:
e
 the value to match Returns:
 the least element greater than or equal to
e
, ornull
if there is no such element  Throws:
ClassCastException
 if the specified element cannot be compared with the elements currently in the setNullPointerException
 if the specified element is null

higher
Description copied from interface:NavigableSet
Returns the least element in this set strictly greater than the given element, ornull
if there is no such element. Specified by:
higher
in interfaceNavigableSet<E>
 Parameters:
e
 the value to match Returns:
 the least element greater than
e
, ornull
if there is no such element  Throws:
ClassCastException
 if the specified element cannot be compared with the elements currently in the setNullPointerException
 if the specified element is null

pollFirst
Description copied from interface:NavigableSet
Retrieves and removes the first (lowest) element, or returnsnull
if this set is empty (optional operation). Specified by:
pollFirst
in interfaceNavigableSet<E>
 Returns:
 the first element, or
null
if this set is empty

pollLast
Description copied from interface:NavigableSet
Retrieves and removes the last (highest) element, or returnsnull
if this set is empty (optional operation). Specified by:
pollLast
in interfaceNavigableSet<E>
 Returns:
 the last element, or
null
if this set is empty

comparator
Description copied from interface:SortedSet
Returns the comparator used to order the elements in this set, ornull
if this set uses the natural ordering of its elements. Specified by:
comparator
in interfaceSortedSet<E>
 Returns:
 the comparator used to order the elements in this set,
or
null
if this set uses the natural ordering of its elements

first

last

addFirst
ThrowsUnsupportedOperationException
. The encounter order induced by this set's comparison method determines the position of elements, so explicit positioning is not supported. Specified by:
addFirst
in interfaceSequencedCollection<E>
 Specified by:
addFirst
in interfaceSortedSet<E>
 Parameters:
e
 the element to be added Throws:
UnsupportedOperationException
 always Since:
 21

addLast
ThrowsUnsupportedOperationException
. The encounter order induced by this set's comparison method determines the position of elements, so explicit positioning is not supported. Specified by:
addLast
in interfaceSequencedCollection<E>
 Specified by:
addLast
in interfaceSortedSet<E>
 Parameters:
e
 the element to be added. Throws:
UnsupportedOperationException
 always Since:
 21

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 fromfromElement
totoElement
. IffromElement
andtoElement
are equal, the returned set is empty unlessfromInclusive
andtoInclusive
are both true. The returned set is backed by this set, so changes in the returned set are reflected in this set, and viceversa. 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 interfaceNavigableSet<E>
 Parameters:
fromElement
 low endpoint of the returned setfromInclusive
true
if the low endpoint is to be included in the returned viewtoElement
 high endpoint of the returned settoInclusive
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, totoElement
, exclusive  Throws:
ClassCastException
 iffromElement
andtoElement
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 iffromElement
ortoElement
cannot be compared to elements currently in the set.NullPointerException
 iffromElement
ortoElement
is nullIllegalArgumentException
 iffromElement
is greater thantoElement
; or if this set itself has a restricted range, andfromElement
ortoElement
lies outside the bounds of the range.

headSet
Description copied from interface:NavigableSet
Returns a view of the portion of this set whose elements are less than (or equal to, ifinclusive
is true)toElement
. The returned set is backed by this set, so changes in the returned set are reflected in this set, and viceversa. 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 interfaceNavigableSet<E>
 Parameters:
toElement
 high endpoint of the returned setinclusive
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:
ClassCastException
 iftoElement
is not compatible with this set's comparator (or, if the set has no comparator, iftoElement
does not implementComparable
). Implementations may, but are not required to, throw this exception iftoElement
cannot be compared to elements currently in the set.NullPointerException
 iftoElement
is nullIllegalArgumentException
 if this set itself has a restricted range, andtoElement
lies outside the bounds of the range

tailSet
Description copied from interface:NavigableSet
Returns a view of the portion of this set whose elements are greater than (or equal to, ifinclusive
is true)fromElement
. The returned set is backed by this set, so changes in the returned set are reflected in this set, and viceversa. 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 interfaceNavigableSet<E>
 Parameters:
fromElement
 low endpoint of the returned setinclusive
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:
ClassCastException
 iffromElement
is not compatible with this set's comparator (or, if the set has no comparator, iffromElement
does not implementComparable
). Implementations may, but are not required to, throw this exception iffromElement
cannot be compared to elements currently in the set.NullPointerException
 iffromElement
is nullIllegalArgumentException
 if this set itself has a restricted range, andfromElement
lies outside the bounds of the range

subSet
Description copied from interface:NavigableSet
Returns a view of the portion of this set whose elements range fromfromElement
, inclusive, totoElement
, exclusive. (IffromElement
andtoElement
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 viceversa. 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)
. Specified by:
subSet
in interfaceNavigableSet<E>
 Specified by:
subSet
in interfaceSortedSet<E>
 Parameters:
fromElement
 low endpoint (inclusive) of the returned settoElement
 high endpoint (exclusive) of the returned set Returns:
 a view of the portion of this set whose elements range from
fromElement
, inclusive, totoElement
, exclusive  Throws:
ClassCastException
 iffromElement
andtoElement
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 iffromElement
ortoElement
cannot be compared to elements currently in the set.NullPointerException
 iffromElement
ortoElement
is nullIllegalArgumentException
 iffromElement
is greater thantoElement
; or if this set itself has a restricted range, andfromElement
ortoElement
lies outside the bounds of the range

headSet
Description copied from interface:NavigableSet
Returns a view of the portion of this set whose elements are strictly less thantoElement
. The returned set is backed by this set, so changes in the returned set are reflected in this set, and viceversa. 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)
. Specified by:
headSet
in interfaceNavigableSet<E>
 Specified by:
headSet
in interfaceSortedSet<E>
 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
 Throws:
ClassCastException
 iftoElement
is not compatible with this set's comparator (or, if the set has no comparator, iftoElement
does not implementComparable
). Implementations may, but are not required to, throw this exception iftoElement
cannot be compared to elements currently in the set.NullPointerException
 iftoElement
is nullIllegalArgumentException
 if this set itself has a restricted range, andtoElement
lies outside the bounds of the range

tailSet
Description copied from interface:NavigableSet
Returns a view of the portion of this set whose elements are greater than or equal tofromElement
. The returned set is backed by this set, so changes in the returned set are reflected in this set, and viceversa. 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)
. Specified by:
tailSet
in interfaceNavigableSet<E>
 Specified by:
tailSet
in interfaceSortedSet<E>
 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
 Throws:
ClassCastException
 iffromElement
is not compatible with this set's comparator (or, if the set has no comparator, iffromElement
does not implementComparable
). Implementations may, but are not required to, throw this exception iffromElement
cannot be compared to elements currently in the set.NullPointerException
 iffromElement
is nullIllegalArgumentException
 if this set itself has a restricted range, andfromElement
lies outside the bounds of the range

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 viceversa.The returned set has an ordering equivalent to
Collections.reverseOrder
(comparator())
. The expressions.descendingSet().descendingSet()
returns a view ofs
essentially equivalent tos
. Specified by:
descendingSet
in interfaceNavigableSet<E>
 Returns:
 a reverse order view of this set

spliterator
Returns aSpliterator
over the elements in this set.The
Spliterator
reportsSpliterator.CONCURRENT
,Spliterator.NONNULL
,Spliterator.DISTINCT
,Spliterator.SORTED
andSpliterator.ORDERED
, with an encounter order that is ascending order. Overriding implementations should document the reporting of additional characteristic values.The spliterator's comparator is
null
if the set's comparator isnull
. Otherwise, the spliterator's comparator is the same as or imposes the same total ordering as the set's comparator. Specified by:
spliterator
in interfaceCollection<E>
 Specified by:
spliterator
in interfaceIterable<E>
 Specified by:
spliterator
in interfaceSet<E>
 Specified by:
spliterator
in interfaceSortedSet<E>
 Returns:
 a
Spliterator
over the elements in this set  Since:
 1.8
