 Type Parameters:
E
 the type of elements maintained by this set
 All Implemented Interfaces:
Serializable
,Cloneable
,Iterable<E>
,Collection<E>
,NavigableSet<E>
,Set<E>
,SortedSet<E>
public class TreeSet<E> extends AbstractSet<E> implements NavigableSet<E>, Cloneable, Serializable
NavigableSet
implementation based on a TreeMap
.
The elements are ordered using their natural
ordering, or by a Comparator
provided at set creation
time, depending on which constructor is used.
This implementation provides guaranteed log(n) time cost for the basic
operations (add
, remove
and contains
).
Note that the ordering maintained by a set (whether or not an explicit
comparator is provided) must be consistent with equals if it is to
correctly implement the Set
interface. (See Comparable
or Comparator
for a precise definition of consistent with
equals.) This is so because the Set
interface is defined in
terms of the equals
operation, but a TreeSet
instance
performs all element comparisons using its compareTo
(or
compare
) method, so two elements that are deemed equal by this method
are, from the standpoint of the set, equal. The behavior of a set
is welldefined even if its ordering is inconsistent with equals; it
just fails to obey the general contract of the Set
interface.
Note that this implementation is not synchronized.
If multiple threads access a tree set concurrently, and at least one
of the threads modifies the set, it must be synchronized
externally. This is typically accomplished by synchronizing on some
object that naturally encapsulates the set.
If no such object exists, the set should be "wrapped" using the
Collections.synchronizedSortedSet
method. This is best done at creation time, to prevent accidental
unsynchronized access to the set:
SortedSet s = Collections.synchronizedSortedSet(new TreeSet(...));
The iterators returned by this class's iterator
method are
failfast: if the set is modified at any time after the iterator is
created, in any way except through the iterator's own remove
method, the iterator will throw a ConcurrentModificationException
.
Thus, in the face of concurrent modification, the iterator fails quickly
and cleanly, rather than risking arbitrary, nondeterministic behavior at
an undetermined time in the future.
Note that the failfast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Failfast iterators
throw ConcurrentModificationException
on a besteffort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: the failfast behavior of iterators
should be used only to detect bugs.
This class is a member of the Java Collections Framework.
 Since:
 1.2
 See Also:
Collection
,Set
,HashSet
,Comparable
,Comparator
,TreeMap
, Serialized Form

Constructor Summary
Constructors Constructor Description TreeSet()
Constructs a new, empty tree set, sorted according to the natural ordering of its elements.TreeSet(Collection<? extends E> c)
Constructs a new tree set containing the elements in the specified collection, sorted according to the natural ordering of its elements.TreeSet(Comparator<? super E> comparator)
Constructs a new, empty tree set, sorted according to the specified comparator.TreeSet(SortedSet<E> s)
Constructs a new tree set containing the same elements and using the same ordering as the specified sorted set. 
Method Summary
Modifier and Type Method Description boolean
add(E e)
Adds the specified element to this set if it is not already present.boolean
addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this set.E
ceiling(E e)
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.Object
clone()
Returns a shallow copy of thisTreeSet
instance.boolean
contains(Object o)
Returnstrue
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.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, ornull
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 thantoElement
.NavigableSet<E>
headSet(E toElement, boolean inclusive)
Returns a view of the portion of this set whose elements are less than (or equal to, ifinclusive
is true)toElement
.E
higher(E e)
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<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, ornull
if there is no such element.E
pollFirst()
Retrieves and removes the first (lowest) element, or returnsnull
if this set is empty.E
pollLast()
Retrieves and removes the last (highest) element, or returnsnull
if this set is empty.boolean
remove(Object o)
Removes the specified element from this set if it is present.int
size()
Returns the number of elements in this set (its cardinality).Spliterator<E>
spliterator()
Creates a latebinding and failfastSpliterator
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 fromfromElement
totoElement
.SortedSet<E>
subSet(E fromElement, E toElement)
Returns a view of the portion of this set whose elements range fromfromElement
, inclusive, totoElement
, exclusive.SortedSet<E>
tailSet(E fromElement)
Returns a view of the portion of this set whose elements are greater than or equal tofromElement
.NavigableSet<E>
tailSet(E fromElement, boolean inclusive)
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
equals, hashCode, removeAll
Methods declared in class java.util.AbstractCollection
containsAll, retainAll, toArray, toArray, toString
Methods declared in interface java.util.Collection
parallelStream, removeIf, stream, toArray
Methods declared in interface java.util.Set
containsAll, equals, hashCode, removeAll, retainAll, toArray, toArray
Methods declared in interface java.util.SortedSet
comparator

Constructor Details

TreeSet
public TreeSet()Constructs a new, empty tree set, sorted according to the natural ordering of its elements. All elements inserted into the set must implement theComparable
interface. Furthermore, all such elements must be mutually comparable:e1.compareTo(e2)
must not throw aClassCastException
for any elementse1
ande2
in the set. If the user attempts to add an element to the set that violates this constraint (for example, the user attempts to add a string element to a set whose elements are integers), theadd
call will throw aClassCastException
. 
TreeSet
Constructs a new, empty tree set, sorted according to the specified comparator. All elements inserted into the set must be mutually comparable by the specified comparator:comparator.compare(e1, e2)
must not throw aClassCastException
for any elementse1
ande2
in the set. If the user attempts to add an element to the set that violates this constraint, theadd
call will throw aClassCastException
. Parameters:
comparator
 the comparator that will be used to order this set. Ifnull
, the natural ordering of the elements will be used.

TreeSet
Constructs a new tree set containing the elements in the specified collection, sorted according to the natural ordering of its elements. All elements inserted into the set must implement theComparable
interface. Furthermore, all such elements must be mutually comparable:e1.compareTo(e2)
must not throw aClassCastException
for any elementse1
ande2
in the set. Parameters:
c
 collection whose elements will comprise the new set Throws:
ClassCastException
 if the elements inc
are notComparable
, or are not mutually comparableNullPointerException
 if the specified collection is null

TreeSet
Constructs a new tree 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 is null


Method Details

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
 Since:
 1.6

descendingSet
Description copied from interface:NavigableSet
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
. Specified by:
descendingSet
in interfaceNavigableSet<E>
 Returns:
 a reverse order view of this set
 Since:
 1.6

size
public int size()Returns the number of elements in this set (its cardinality). 
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 thatObjects.equals(o, 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 object cannot be compared with the elements currently in the setNullPointerException
 if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

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 thatObjects.equals(e, 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
 if the specified object cannot be compared with the elements currently in this setNullPointerException
 if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

remove
Removes the specified element from this set if it is present. More formally, removes an elemente
such thatObjects.equals(o, 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
 if the specified object cannot be compared with the elements currently in this setNullPointerException
 if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

clear
public void clear()Removes all of the elements from this set. The set will be empty after this call returns. Specified by:
clear
in interfaceCollection<E>
 Specified by:
clear
in interfaceSet<E>
 Overrides:
clear
in classAbstractCollection<E>

addAll
Adds all of the elements in the specified collection to this set. Specified by:
addAll
in interfaceCollection<E>
 Specified by:
addAll
in interfaceSet<E>
 Overrides:
addAll
in classAbstractCollection<E>
 Parameters:
c
 collection containing elements to be added to this set Returns:
true
if this set changed as a result of the call Throws:
ClassCastException
 if the elements provided cannot be compared with the elements currently in the setNullPointerException
 if the specified collection is null or if any element is null and this set uses natural ordering, or its comparator does not permit null elements See Also:
AbstractCollection.add(Object)

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 null and this set uses natural ordering, or its comparator 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. Since:
 1.6

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 null and this set uses natural ordering, or its comparator does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, andtoElement
lies outside the bounds of the range Since:
 1.6

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 null and this set uses natural ordering, or its comparator does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, andfromElement
lies outside the bounds of the range Since:
 1.6

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 null and this set uses natural ordering, or its comparator 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
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 null and this set uses natural ordering, or its comparator does not permit null elementsIllegalArgumentException
 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 null and this set uses natural ordering, or its comparator does not permit null elementsIllegalArgumentException
 if this set itself has a restricted range, andfromElement
lies outside the bounds of the range

first
Description copied from interface:SortedSet
Returns the first (lowest) element currently in this set. Specified by:
first
in interfaceSortedSet<E>
 Returns:
 the first (lowest) element currently in this set
 Throws:
NoSuchElementException
 if this set is empty

last
Description copied from interface:SortedSet
Returns the last (highest) element currently in this set. Specified by:
last
in interfaceSortedSet<E>
 Returns:
 the last (highest) element currently in this set
 Throws:
NoSuchElementException
 if this set is empty

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 and this set uses natural ordering, or its comparator does not permit null elements Since:
 1.6

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 and this set uses natural ordering, or its comparator does not permit null elements Since:
 1.6

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 and this set uses natural ordering, or its comparator does not permit null elements Since:
 1.6

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 and this set uses natural ordering, or its comparator does not permit null elements Since:
 1.6

pollFirst
Description copied from interface:NavigableSet
Retrieves and removes the first (lowest) element, or returnsnull
if this set is empty. Specified by:
pollFirst
in interfaceNavigableSet<E>
 Returns:
 the first element, or
null
if this set is empty  Since:
 1.6

pollLast
Description copied from interface:NavigableSet
Retrieves and removes the last (highest) element, or returnsnull
if this set is empty. Specified by:
pollLast
in interfaceNavigableSet<E>
 Returns:
 the last element, or
null
if this set is empty  Since:
 1.6

clone
Returns a shallow copy of thisTreeSet
instance. (The elements themselves are not cloned.) 
spliterator
Creates a latebinding and failfastSpliterator
over the elements in this set.The
Spliterator
reportsSpliterator.SIZED
,Spliterator.DISTINCT
,Spliterator.SORTED
, andSpliterator.ORDERED
. Overriding implementations should document the reporting of additional characteristic values.The spliterator's comparator (see
Spliterator.getComparator()
) isnull
if the tree set's comparator (seeSortedSet.comparator()
) isnull
. Otherwise, the spliterator's comparator is the same as or imposes the same total ordering as the tree 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
