Added in API level 1

TreeSet

public class TreeSet
extends AbstractSet<E> implements NavigableSet<E>, Cloneable, Serializable

java.lang.Object
   ↳ java.util.AbstractCollection<E>
     ↳ java.util.AbstractSet<E>
       ↳ java.util.TreeSet<E>


A NavigableSet implementation based on a TreeMap. The elements are ordered using their 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 well-defined 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 fail-fast: 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, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast 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. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

The addFirst and addLast methods of this class throw UnsupportedOperationException. The encounter order of elements is determined by the comparison method; therefore, explicit positioning is not supported.

This class is a member of the Java Collections Framework.

Summary

Public constructors

TreeSet()

Constructs a new, empty tree set, 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(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(SortedSet<E> s)

Constructs a new tree set containing the same elements and using the same ordering as the specified sorted set.

Public methods

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.

void addFirst(E e)

Throws UnsupportedOperationException.

void addLast(E e)

Throws UnsupportedOperationException.

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.

Object clone()

Returns a shallow copy of this TreeSet 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.

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, boolean inclusive)

Returns a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement.

SortedSet<E> headSet(E toElement)

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

Equivalent to headSet(toElement, false).

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.

int size()

Returns the number of elements in this set (its cardinality).

Spliterator<E> spliterator()

Creates a late-binding and fail-fast 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.

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.

Equivalent to subSet(fromElement, true, toElement, false).

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.

SortedSet<E> tailSet(E fromElement)

Returns a view of the portion of this set whose elements are greater than or equal to fromElement.

Equivalent to tailSet(fromElement, true).

Inherited methods

Public constructors

TreeSet

Added in API level 1
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 the Comparable interface. Furthermore, all such elements must be mutually comparable: e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 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), the add call will throw a ClassCastException.

TreeSet

Added in API level 1
public TreeSet (Comparator<? super E> comparator)

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 a ClassCastException for any elements e1 and e2 in the set. If the user attempts to add an element to the set that violates this constraint, the add call will throw a ClassCastException.

Parameters
comparator Comparator: the comparator that will be used to order this set. If null, the ordering of the elements will be used.

TreeSet

Added in API level 1
public 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. All elements inserted into the set must implement the Comparable interface. Furthermore, all such elements must be mutually comparable: e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the set.

Parameters
c Collection: collection whose elements will comprise the new set

Throws
ClassCastException if the elements in c are not Comparable, or are not mutually comparable
NullPointerException if the specified collection is null

TreeSet

Added in API level 1
public TreeSet (SortedSet<E> s)

Constructs a new tree set containing the same elements and using the same ordering as the specified sorted set.

Parameters
s SortedSet: sorted set whose elements will comprise the new set

Throws
NullPointerException if the specified sorted set is null

Public methods

add

Added in API level 1
public boolean add (E e)

Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if the set contains no element e2 such that Objects.equals(e, e2). If this set already contains the element, the call leaves the set unchanged and returns false.

Parameters
e E: element to be added to this set

Returns
boolean 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 set
NullPointerException if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

addAll

Added in API level 1
public boolean addAll (Collection<? extends E> c)

Adds all of the elements in the specified collection to this set.

Parameters
c Collection: collection containing elements to be added to this set

Returns
boolean 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 set
NullPointerException 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

addFirst

Added in API level 35
public void addFirst (E e)

Throws UnsupportedOperationException. The encounter order induced by this set's comparison method determines the position of elements, so explicit positioning is not supported.

Parameters
e E: the element to be added

Throws
UnsupportedOperationException always

addLast

Added in API level 35
public void addLast (E e)

Throws UnsupportedOperationException. The encounter order induced by this set's comparison method determines the position of elements, so explicit positioning is not supported.

Parameters
e E: the element to be added.

Throws
UnsupportedOperationException always

ceiling

Added in API level 9
public 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.

Parameters
e E: the value to match

Returns
E the least element greater than or equal to e, or null if there is no such element

Throws
ClassCastException
NullPointerException if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

clear

Added in API level 1
public void clear ()

Removes all of the elements from this set. The set will be empty after this call returns.

clone

Added in API level 1
public Object clone ()

Returns a shallow copy of this TreeSet instance. (The elements themselves are not cloned.)

Returns
Object a shallow copy of this set

comparator

Added in API level 1
public 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.

Returns
Comparator<? super E> the comparator used to order the elements in this set, or null if this set uses the natural ordering of its elements

contains

Added in API level 1
public boolean contains (Object o)

Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that Objects.equals(o, e).

Parameters
o Object: object to be checked for containment in this set

Returns
boolean true if this set contains the specified element

Throws
ClassCastException if the specified object cannot be compared with the elements currently in the set
NullPointerException if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

descendingIterator

Added in API level 9
public Iterator<E> descendingIterator ()

Returns an iterator over the elements in this set in descending order.

Returns
Iterator<E> an iterator over the elements in this set in descending order

descendingSet

Added in API level 9
public 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 vice-versa. If either set is modified while an iteration over either set is in progress (except through the iterator's own 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.

Returns
NavigableSet<E> a reverse order view of this set

first

Added in API level 1
public E first ()

Returns the first (lowest) element currently in this set.

Returns
E the first (lowest) element currently in this set

Throws
NoSuchElementException

floor

Added in API level 9
public 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.

Parameters
e E: the value to match

Returns
E the greatest element less than or equal to e, or null if there is no such element

Throws
ClassCastException
NullPointerException if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

headSet

Added in API level 9
public 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. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. 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 E: high endpoint of the returned set

inclusive boolean: true if the high endpoint is to be included in the returned view

Returns
NavigableSet<E> a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement

Throws
ClassCastException
NullPointerException if toElement is null and this set uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException

headSet

Added in API level 1
public SortedSet<E> headSet (E toElement)

Returns a view of the portion of this set whose elements are strictly less than toElement. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. 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).

Parameters
toElement E: high endpoint (exclusive) of the returned set

Returns
SortedSet<E> a view of the portion of this set whose elements are strictly less than toElement

Throws
ClassCastException
NullPointerException if toElement is null and this set uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException

higher

Added in API level 9
public 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.

Parameters
e E: the value to match

Returns
E the least element greater than e, or null if there is no such element

Throws
ClassCastException
NullPointerException if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

isEmpty

Added in API level 1
public boolean isEmpty ()

Returns true if this set contains no elements.

Returns
boolean true if this set contains no elements

iterator

Added in API level 1
public Iterator<E> iterator ()

Returns an iterator over the elements in this set in ascending order.

Returns
Iterator<E> an iterator over the elements in this set in ascending order

last

Added in API level 1
public E last ()

Returns the last (highest) element currently in this set.

Returns
E the last (highest) element currently in this set

Throws
NoSuchElementException

lower

Added in API level 9
public 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.

Parameters
e E: the value to match

Returns
E the greatest element less than e, or null if there is no such element

Throws
ClassCastException
NullPointerException if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

pollFirst

Added in API level 9
public E pollFirst ()

Retrieves and removes the first (lowest) element, or returns null if this set is empty.

Returns
E the first element, or null if this set is empty

pollLast

Added in API level 9
public E pollLast ()

Retrieves and removes the last (highest) element, or returns null if this set is empty.

Returns
E the last element, or null if this set is empty

remove

Added in API level 1
public boolean remove (Object o)

Removes the specified element from this set if it is present. More formally, removes an element e such that Objects.equals(o, 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.)

Parameters
o Object: object to be removed from this set, if present

Returns
boolean true if this set contained the specified element

Throws
ClassCastException if the specified object cannot be compared with the elements currently in this set
NullPointerException if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

size

Added in API level 1
public int size ()

Returns the number of elements in this set (its cardinality).

Returns
int the number of elements in this set (its cardinality)

spliterator

Added in API level 24
public Spliterator<E> spliterator ()

Creates a late-binding and fail-fast Spliterator over the elements in this set.

The Spliterator reports Spliterator#SIZED, Spliterator#DISTINCT, Spliterator#SORTED, and Spliterator#ORDERED. Overriding implementations should document the reporting of additional characteristic values.

The spliterator's comparator (see Spliterator.getComparator()) is null if the tree set's comparator (see comparator()) is null. Otherwise, the spliterator's comparator is the same as or imposes the same total ordering as the tree set's comparator.

Returns
Spliterator<E> a Spliterator over the elements in this set

subSet

Added in API level 9
public 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. 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 vice-versa. 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 E: low endpoint of the returned set

fromInclusive boolean: true if the low endpoint is to be included in the returned view

toElement E: high endpoint of the returned set

toInclusive boolean: true if the high endpoint is to be included in the returned view

Returns
NavigableSet<E> a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive

Throws
ClassCastException
NullPointerException if fromElement or toElement is null and this set uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException

subSet

Added in API level 1
public 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. (If fromElement and toElement 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 vice-versa. 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).

Parameters
fromElement E: low endpoint (inclusive) of the returned set

toElement E: high endpoint (exclusive) of the returned set

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

Throws
ClassCastException
NullPointerException if fromElement or toElement is null and this set uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException

tailSet

Added in API level 9
public 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. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. 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 E: low endpoint of the returned set

inclusive boolean: true if the low endpoint is to be included in the returned view

Returns
NavigableSet<E> a view of the portion of this set whose elements are greater than or equal to fromElement

Throws
ClassCastException
NullPointerException if fromElement is null and this set uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException

tailSet

Added in API level 1
public SortedSet<E> tailSet (E fromElement)

Returns a view of the portion of this set whose elements are greater than or equal to fromElement. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. 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).

Parameters
fromElement E: low endpoint (inclusive) of the returned set

Returns
SortedSet<E> a view of the portion of this set whose elements are greater than or equal to fromElement

Throws
ClassCastException
NullPointerException if fromElement is null and this set uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException