 Type Parameters:
E
 the type of elements maintained by this set
 All Superinterfaces:
Collection<E>
,Iterable<E>
,Set<E>
,SortedSet<E>
 All Known Implementing Classes:
ConcurrentSkipListSet
,TreeSet
SortedSet
extended with navigation methods reporting
closest matches for given search targets. Methods lower(E)
,
floor(E)
, ceiling(E)
, and higher(E)
return elements
respectively less than, less than or equal, greater than or equal,
and greater than a given element, returning null
if there
is no such element.
A NavigableSet
may be accessed and traversed in either
ascending or descending order. The descendingSet()
method
returns a view of the set with the senses of all relational and
directional methods inverted. The performance of ascending
operations and views is likely to be faster than that of descending
ones. This interface additionally defines methods pollFirst()
and pollLast()
that return and remove the lowest
and highest element, if one exists, else returning null
.
Methods
subSet(E, boolean, E, boolean)
,
headSet(E, boolean)
, and
tailSet(E, boolean)
differ from the likenamed SortedSet
methods in accepting
additional arguments describing whether lower and upper bounds are
inclusive versus exclusive. Subsets of any NavigableSet
must implement the NavigableSet
interface.
The return values of navigation methods may be ambiguous in
implementations that permit null
elements. However, even
in this case the result can be disambiguated by checking
contains(null)
. To avoid such issues, implementations of
this interface are encouraged to not permit insertion of
null
elements. (Note that sorted sets of Comparable
elements intrinsically do not permit null
.)
Methods
subSet(E, E)
,
headSet(E)
, and
tailSet(E)
are specified to return SortedSet
to allow existing
implementations of SortedSet
to be compatibly retrofitted to
implement NavigableSet
, but extensions and implementations
of this interface are encouraged to override these methods to return
NavigableSet
.
This interface is a member of the Java Collections Framework.
 Since:
 1.6

Method Summary
Modifier and TypeMethodDescriptionReturns the least element in this set greater than or equal to the given element, ornull
if there is no such 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.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.iterator()
Returns an iterator over the elements in this set, in ascending order.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.pollLast()
Retrieves and removes the last (highest) element, or returnsnull
if this set is empty.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 inherited from interface java.util.Collection
parallelStream, removeIf, stream, toArray
Methods inherited from interface java.util.Set
add, addAll, clear, contains, containsAll, equals, hashCode, isEmpty, remove, removeAll, retainAll, size, toArray, toArray
Methods inherited from interface java.util.SortedSet
comparator, first, last, spliterator

Method Details

lower
Returns the greatest element in this set strictly less than the given element, ornull
if there is no such element. 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 and this set does not permit null elements

floor
Returns the greatest element in this set less than or equal to the given element, ornull
if there is no such element. 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 and this set does not permit null elements

ceiling
Returns the least element in this set greater than or equal to the given element, ornull
if there is no such element. 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 and this set does not permit null elements

higher
Returns the least element in this set strictly greater than the given element, ornull
if there is no such element. 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 and this set does not permit null elements

pollFirst
E pollFirst()Retrieves and removes the first (lowest) element, or returnsnull
if this set is empty. Returns:
 the first element, or
null
if this set is empty

pollLast
E pollLast()Retrieves and removes the last (highest) element, or returnsnull
if this set is empty. Returns:
 the last element, or
null
if this set is empty

iterator
Returns an iterator over the elements in this set, in ascending order. 
descendingSet
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 viceversa. If either set is modified while an iteration over either set is in progress (except through the iterator's ownremove
operation), the results of the iteration are undefined.The returned set has an ordering equivalent to
Collections.reverseOrder
(comparator())
. The expressions.descendingSet().descendingSet()
returns a view ofs
essentially equivalent tos
. Returns:
 a reverse order view of this set

descendingIterator
Returns an iterator over the elements in this set, in descending order. Equivalent in effect todescendingSet().iterator()
. Returns:
 an iterator over the elements in this set, in descending order

subSet
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. 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 null and this set does not permit null elementsIllegalArgumentException
 iffromElement
is greater thantoElement
; or if this set itself has a restricted range, andfromElement
ortoElement
lies outside the bounds of the range.

headSet
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. 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 null and this set does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, andtoElement
lies outside the bounds of the range

tailSet
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. 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 null and this set does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, andfromElement
lies outside the bounds of the range

subSet
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 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 null and this set does not permit null elementsIllegalArgumentException
 iffromElement
is greater thantoElement
; or if this set itself has a restricted range, andfromElement
ortoElement
lies outside the bounds of the range

headSet
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 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 null and this set does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, andtoElement
lies outside the bounds of the range

tailSet
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 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 null and this set does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, andfromElement
lies outside the bounds of the range
