E
 the type of elements maintained by this setpublic class ConcurrentSkipListSet<E> extends AbstractSet<E> implements NavigableSet<E>, Cloneable, Serializable
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.
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.
Constructor and Description 

ConcurrentSkipListSet()
Constructs 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.

ConcurrentSkipListSet(SortedSet<E> s)
Constructs a new set containing the same elements and using the
same ordering as the specified sorted set.

Modifier and Type  Method and Description 

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

NavigableSet<E> 
tailSet(E fromElement)
Returns a view of the portion of this set whose elements are
greater than or equal to 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 . 
hashCode
addAll, containsAll, retainAll, toArray, toArray, toString
finalize, getClass, notify, notifyAll, wait, wait, wait
addAll, containsAll, hashCode, retainAll, toArray, toArray
parallelStream, removeIf, stream
public ConcurrentSkipListSet()
public ConcurrentSkipListSet(Comparator<? super E> comparator)
comparator
 the comparator that will be used to order this set.
If null
, the natural
ordering of the elements will be used.public ConcurrentSkipListSet(Collection<? extends E> c)
c
 The elements that will comprise the new setClassCastException
 if the elements in c
are
not Comparable
, or are not mutually comparableNullPointerException
 if the specified collection or any
of its elements are nullpublic ConcurrentSkipListSet(SortedSet<E> s)
s
 sorted set whose elements will comprise the new setNullPointerException
 if the specified sorted set or any
of its elements are nullpublic ConcurrentSkipListSet<E> clone()
ConcurrentSkipListSet
instance. (The elements themselves are not cloned.)public int size()
Integer.MAX_VALUE
elements, it
returns Integer.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.
size
in interface Collection<E>
size
in interface Set<E>
size
in class AbstractCollection<E>
public boolean isEmpty()
true
if this set contains no elements.isEmpty
in interface Collection<E>
isEmpty
in interface Set<E>
isEmpty
in class AbstractCollection<E>
true
if this set contains no elementspublic boolean contains(Object o)
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)
.contains
in interface Collection<E>
contains
in interface Set<E>
contains
in class AbstractCollection<E>
o
 object to be checked for containment in this settrue
if this set contains the specified elementClassCastException
 if the specified element cannot be
compared with the elements currently in this setNullPointerException
 if the specified element is nullpublic boolean add(E e)
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
.add
in interface Collection<E>
add
in interface Set<E>
add
in class AbstractCollection<E>
e
 element to be added to this settrue
if this set did not already contain the
specified elementClassCastException
 if e
cannot be compared
with the elements currently in this setNullPointerException
 if the specified element is nullpublic boolean remove(Object o)
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.)remove
in interface Collection<E>
remove
in interface Set<E>
remove
in class AbstractCollection<E>
o
 object to be removed from this set, if presenttrue
if this set contained the specified elementClassCastException
 if o
cannot be compared
with the elements currently in this setNullPointerException
 if the specified element is nullpublic void clear()
clear
in interface Collection<E>
clear
in interface Set<E>
clear
in class AbstractCollection<E>
public Iterator<E> iterator()
iterator
in interface Iterable<E>
iterator
in interface Collection<E>
iterator
in interface NavigableSet<E>
iterator
in interface Set<E>
iterator
in class AbstractCollection<E>
public Iterator<E> descendingIterator()
descendingIterator
in interface NavigableSet<E>
public boolean equals(Object o)
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.equals
in interface Collection<E>
equals
in interface Set<E>
equals
in class AbstractSet<E>
o
 the object to be compared for equality with this settrue
if the specified object is equal to this setObject.hashCode()
,
HashMap
public boolean removeAll(Collection<?> c)
removeAll
in interface Collection<E>
removeAll
in interface Set<E>
removeAll
in class AbstractSet<E>
c
 collection containing elements to be removed from this settrue
if this set changed as a result of the callClassCastException
 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 nullAbstractCollection.remove(Object)
,
AbstractCollection.contains(Object)
public E lower(E e)
NavigableSet
null
if there is no such element.lower
in interface NavigableSet<E>
e
 the value to matche
,
or null
if there is no such elementClassCastException
 if the specified element cannot be
compared with the elements currently in the setNullPointerException
 if the specified element is nullpublic E floor(E e)
NavigableSet
null
if there is no such element.floor
in interface NavigableSet<E>
e
 the value to matche
,
or null
if there is no such elementClassCastException
 if the specified element cannot be
compared with the elements currently in the setNullPointerException
 if the specified element is nullpublic E ceiling(E e)
NavigableSet
null
if there is no such element.ceiling
in interface NavigableSet<E>
e
 the value to matche
,
or null
if there is no such elementClassCastException
 if the specified element cannot be
compared with the elements currently in the setNullPointerException
 if the specified element is nullpublic E higher(E e)
NavigableSet
null
if there is no such element.higher
in interface NavigableSet<E>
e
 the value to matche
,
or null
if there is no such elementClassCastException
 if the specified element cannot be
compared with the elements currently in the setNullPointerException
 if the specified element is nullpublic E pollFirst()
NavigableSet
null
if this set is empty.pollFirst
in interface NavigableSet<E>
null
if this set is emptypublic E pollLast()
NavigableSet
null
if this set is empty.pollLast
in interface NavigableSet<E>
null
if this set is emptypublic Comparator<? super E> comparator()
java.util.SortedSet
comparator
in interface SortedSet<E>
public E first()
java.util.SortedSet
first
in interface SortedSet<E>
NoSuchElementException
 if this set is emptypublic E last()
java.util.SortedSet
last
in interface SortedSet<E>
NoSuchElementException
 if this set is emptypublic NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
NavigableSet
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 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.
subSet
in interface NavigableSet<E>
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 viewfromElement
, inclusive, to toElement
, exclusiveClassCastException
 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.NullPointerException
 if fromElement
or
toElement
is nullIllegalArgumentException
 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.public NavigableSet<E> headSet(E toElement, boolean inclusive)
NavigableSet
inclusive
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.
headSet
in interface NavigableSet<E>
toElement
 high endpoint of the returned setinclusive
 true
if the high endpoint
is to be included in the returned viewinclusive
is true) toElement
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.NullPointerException
 if toElement
is nullIllegalArgumentException
 if this set itself has a
restricted range, and toElement
lies outside the
bounds of the rangepublic NavigableSet<E> tailSet(E fromElement, boolean inclusive)
NavigableSet
inclusive
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.
tailSet
in interface NavigableSet<E>
fromElement
 low endpoint of the returned setinclusive
 true
if the low endpoint
is to be included in the returned viewfromElement
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.NullPointerException
 if fromElement
is nullIllegalArgumentException
 if this set itself has a
restricted range, and fromElement
lies outside the
bounds of the rangepublic NavigableSet<E> subSet(E fromElement, E toElement)
NavigableSet
The returned set will throw an IllegalArgumentException on an attempt to insert an element outside its range.
Equivalent to subSet(fromElement, true, toElement, false)
.
subSet
in interface NavigableSet<E>
subSet
in interface SortedSet<E>
fromElement
 low endpoint (inclusive) of the returned settoElement
 high endpoint (exclusive) of the returned setClassCastException
 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.NullPointerException
 if fromElement
or
toElement
is nullIllegalArgumentException
 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 rangepublic NavigableSet<E> headSet(E toElement)
NavigableSet
The returned set will throw an IllegalArgumentException on an attempt to insert an element outside its range.
Equivalent to headSet(toElement, false)
.
headSet
in interface NavigableSet<E>
headSet
in interface SortedSet<E>
toElement
 high endpoint (exclusive) of the returned setClassCastException
 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.NullPointerException
 if toElement
is nullIllegalArgumentException
 if this set itself has a
restricted range, and toElement lies outside the
bounds of the rangepublic NavigableSet<E> tailSet(E fromElement)
NavigableSet
The returned set will throw an IllegalArgumentException on an attempt to insert an element outside its range.
Equivalent to tailSet(fromElement, true)
.
tailSet
in interface NavigableSet<E>
tailSet
in interface SortedSet<E>
fromElement
 low endpoint (inclusive) of the returned setClassCastException
 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.NullPointerException
 if fromElement
is nullIllegalArgumentException
 if this set itself has a
restricted range, and fromElement lies outside the
bounds of the rangepublic NavigableSet<E> descendingSet()
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
.
descendingSet
in interface NavigableSet<E>
public Spliterator<E> spliterator()
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.
spliterator
in interface Iterable<E>
spliterator
in interface Collection<E>
spliterator
in interface Set<E>
spliterator
in interface SortedSet<E>
Spliterator
over the elements in this set