E
 the type of elements maintained by this setpublic interface NavigableSet<E> extends SortedSet<E>
SortedSet
extended with navigation methods reporting closest matches for given search targets. Methods
lower
, floor
, ceiling
, and higher
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
, headSet
, and tailSet
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.
Modifier and Type  Method and Description 

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. 
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.

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. 
SortedSet<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. 
Iterator<E> 
iterator()
Returns an iterator over the elements in this set, in ascending order.

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. 
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 . 
SortedSet<E> 
subSet(E fromElement,
E toElement)
Returns a view of the portion of this set whose elements range from fromElement, inclusive, to
toElement, exclusive.

SortedSet<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 . 
comparator, first, last
@Nullable E ceiling(E e)
null
if there is no
such element.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 null and this set does not permit null elementsIterator<E> descendingIterator()
descendingSet().iterator()
.NavigableSet<E> descendingSet()
remove
operation), the
results of the iteration are undefined.
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
.
@Nullable E floor(E e)
null
if there is no
such element.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 null and this set does not permit null elementsSortedSet<E> headSet(E toElement)
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 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 null and this set does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, and toElement lies outside the bounds of the
rangeNavigableSet<E> headSet(E toElement, boolean inclusive)
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.
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 null and this set does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, and toElement
lies outside the bounds of the range@Nullable E higher(E e)
null
if there is no
such element.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 null and this set does not permit null elementsIterator<E> iterator()
@Nullable E lower(E e)
null
if there is no
such element.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 null and this set does not permit null elements@Nullable E pollFirst()
null
if this set is empty.null
if this set is empty@Nullable E pollLast()
null
if this set is empty.null
if this set is emptyNavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
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.
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 null and this set does not permit null elementsIllegalArgumentException
 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.SortedSet<E> subSet(E fromElement, E toElement)
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 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 null and this set does not permit null elementsIllegalArgumentException
 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 rangeSortedSet<E> tailSet(E fromElement)
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 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 null and this set does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, and fromElement lies outside the bounds of the
rangeNavigableSet<E> tailSet(E fromElement, boolean inclusive)
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.
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 null and this set does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, and fromElement
lies outside the bounds of the
range