Java™ Platform
Standard Ed. 6

java.util
Class TreeSet<E>

java.lang.Object
  extended by java.util.AbstractCollection<E>
      extended by java.util.AbstractSet<E>
          extended by java.util.TreeSet<E>
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

A 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 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.

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
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
 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, 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.
 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.
 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).
 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.
 
Methods inherited from class java.util.AbstractSet
equals, hashCode, removeAll
 
Methods inherited from class java.util.AbstractCollection
containsAll, retainAll, toArray, toArray, toString
 
Methods inherited from class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
 
Methods inherited from interface java.util.Set
containsAll, equals, hashCode, removeAll, retainAll, toArray, toArray
 

Constructor Detail

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 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

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 - the comparator that will be used to order this set. If null, the natural ordering of the elements will be used.

TreeSet

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 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

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 - sorted set whose elements will comprise the new set
Throws:
NullPointerException - if the specified sorted set is null
Method Detail

iterator

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

Specified by:
iterator in interface Iterable<E>
Specified by:
iterator in interface Collection<E>
Specified by:
iterator in interface NavigableSet<E>
Specified by:
iterator in interface Set<E>
Specified by:
iterator in class AbstractCollection<E>
Returns:
an iterator over the elements in this set in ascending order

descendingIterator

public Iterator<E> descendingIterator()
Returns an iterator over the elements in this set in descending order.

Specified by:
descendingIterator in interface NavigableSet<E>
Returns:
an iterator over the elements in this set in descending order
Since:
1.6

descendingSet

public NavigableSet<E> 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 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.

Specified by:
descendingSet in interface NavigableSet<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).

Specified by:
size in interface Collection<E>
Specified by:
size in interface Set<E>
Specified by:
size in class AbstractCollection<E>
Returns:
the number of elements in this set (its cardinality)

isEmpty

public boolean isEmpty()
Returns true if this set contains no elements.

Specified by:
isEmpty in interface Collection<E>
Specified by:
isEmpty in interface Set<E>
Overrides:
isEmpty in class AbstractCollection<E>
Returns:
true if this set contains no elements

contains

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 (o==null ? e==null : o.equals(e)).

Specified by:
contains in interface Collection<E>
Specified by:
contains in interface Set<E>
Overrides:
contains in class AbstractCollection<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 set
NullPointerException - if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

add

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 (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.

Specified by:
add in interface Collection<E>
Specified by:
add in interface Set<E>
Overrides:
add in class AbstractCollection<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 set
NullPointerException - if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

remove

public boolean remove(Object o)
Removes the specified element from this set if it is present. More formally, removes an element e such that (o==null ? e==null : 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.)

Specified by:
remove in interface Collection<E>
Specified by:
remove in interface Set<E>
Overrides:
remove in class AbstractCollection<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 set
NullPointerException - 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 interface Collection<E>
Specified by:
clear in interface Set<E>
Overrides:
clear in class AbstractCollection<E>

addAll

public boolean addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this set.

Specified by:
addAll in interface Collection<E>
Specified by:
addAll in interface Set<E>
Overrides:
addAll in class AbstractCollection<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 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
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 from fromElement to toElement. If fromElement and toElement are equal, the returned set is empty unless fromExclusive and toExclusive 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.

Specified by:
subSet in interface NavigableSet<E>
Parameters:
fromElement - low endpoint of the returned set
fromInclusive - true if the low endpoint is to be included in the returned view
toElement - high endpoint of the returned set
toInclusive - 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, to toElement, exclusive
Throws:
ClassCastException - 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 uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException - 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.
Since:
1.6

headSet

public NavigableSet<E> headSet(E toElement,
                               boolean inclusive)
Description copied from interface: NavigableSet
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.

Specified by:
headSet in interface NavigableSet<E>
Parameters:
toElement - high endpoint of the returned set
inclusive - 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 - 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 uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException - if this set itself has a restricted range, and toElement lies outside the bounds of the range
Since:
1.6

tailSet

public NavigableSet<E> tailSet(E fromElement,
                               boolean inclusive)
Description copied from interface: NavigableSet
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.

Specified by:
tailSet in interface NavigableSet<E>
Parameters:
fromElement - low endpoint of the returned set
inclusive - 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 - 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 uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException - if this set itself has a restricted range, and fromElement lies outside the bounds of the range
Since:
1.6

subSet

public SortedSet<E> subSet(E fromElement,
                           E toElement)
Description copied from interface: NavigableSet
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).

Specified by:
subSet in interface NavigableSet<E>
Specified by:
subSet in interface SortedSet<E>
Parameters:
fromElement - low endpoint (inclusive) of the returned set
toElement - high endpoint (exclusive) of the returned set
Returns:
a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive
Throws:
ClassCastException - 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 uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException - 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

headSet

public SortedSet<E> headSet(E toElement)
Description copied from interface: NavigableSet
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).

Specified by:
headSet in interface NavigableSet<E>
Specified by:
headSet in interface SortedSet<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 - 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 uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException - if this set itself has a restricted range, and toElement lies outside the bounds of the range na

tailSet

public SortedSet<E> tailSet(E fromElement)
Description copied from interface: NavigableSet
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).

Specified by:
tailSet in interface NavigableSet<E>
Specified by:
tailSet in interface SortedSet<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 - 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 uses natural ordering, or its comparator does not permit null elements
IllegalArgumentException - if this set itself has a restricted range, and fromElement lies outside the bounds of the range

comparator

public Comparator<? super E> comparator()
Description copied from interface: SortedSet
Returns the comparator used to order the elements in this set, or null if this set uses the natural ordering of its elements.

Specified by:
comparator in interface SortedSet<E>
Returns:
the comparator used to order the elements in this set, or null if this set uses the natural ordering of its elements

first

public E first()
Description copied from interface: SortedSet
Returns the first (lowest) element currently in this set.

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

last

public E last()
Description copied from interface: SortedSet
Returns the last (highest) element currently in this set.

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

lower

public E lower(E e)
Description copied from interface: NavigableSet
Returns the greatest element in this set strictly less than the given element, or null if there is no such element.

Specified by:
lower in interface NavigableSet<E>
Parameters:
e - the value to match
Returns:
the greatest element less than e, or null if there is no such element
Throws:
ClassCastException - if the specified element 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
Since:
1.6

floor

public E floor(E e)
Description copied from interface: NavigableSet
Returns the greatest element in this set less than or equal to the given element, or null if there is no such element.

Specified by:
floor in interface NavigableSet<E>
Parameters:
e - the value to match
Returns:
the greatest element less than or equal to e, or null if there is no such element
Throws:
ClassCastException - if the specified element 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
Since:
1.6

ceiling

public E ceiling(E e)
Description copied from interface: NavigableSet
Returns the least element in this set greater than or equal to the given element, or null if there is no such element.

Specified by:
ceiling in interface NavigableSet<E>
Parameters:
e - the value to match
Returns:
the least element greater than or equal to e, or null if there is no such element
Throws:
ClassCastException - if the specified element 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
Since:
1.6

higher

public E higher(E e)
Description copied from interface: NavigableSet
Returns the least element in this set strictly greater than the given element, or null if there is no such element.

Specified by:
higher in interface NavigableSet<E>
Parameters:
e - the value to match
Returns:
the least element greater than e, or null if there is no such element
Throws:
ClassCastException - if the specified element 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
Since:
1.6

pollFirst

public E pollFirst()
Description copied from interface: NavigableSet
Retrieves and removes the first (lowest) element, or returns null if this set is empty.

Specified by:
pollFirst in interface NavigableSet<E>
Returns:
the first element, or null if this set is empty
Since:
1.6

pollLast

public E pollLast()
Description copied from interface: NavigableSet
Retrieves and removes the last (highest) element, or returns null if this set is empty.

Specified by:
pollLast in interface NavigableSet<E>
Returns:
the last element, or null if this set is empty
Since:
1.6

clone

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

Overrides:
clone in class Object
Returns:
a shallow copy of this set
See Also:
Cloneable

Java™ Platform
Standard Ed. 6

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