java.util
Class TreeSet

java.lang.Object
  java.util.AbstractCollection
      java.util.AbstractSet
          java.util.TreeSet

All Implemented Interfaces:

Cloneable, Collection, Serializable, Set, SortedSet

public class TreeSet

extends AbstractSet

implements SortedSet, Cloneable, Serializable

This class implements the Set interface, backed by a TreeMap instance. This class guarantees that the sorted set will be in ascending element order, sorted according to the natural order of the elements (see Comparable), or by the 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 key comparisons using its compareTo (or compare) method, so two keys 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 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.synchronizedSet 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

Author:

Josh Bloch

See Also:

Collection, Set, HashSet, Comparable, Comparator, Collections.synchronizedSortedSet(SortedSet), TreeMap, Serialized Form

Field Summary

private Set	keySet
private SortedMap	m
private static Object	PRESENT
private static long	serialVersionUID

Constructor Summary

	TreeSet() Constructs a new, empty set, sorted according to the elements' natural order.
	TreeSet(Collection c) Constructs a new set containing the elements in the specified collection, sorted according to the elements' natural order.
	TreeSet(Comparator c) Constructs a new, empty set, sorted according to the specified comparator.
private	TreeSet(SortedMap m) Constructs a set backed by the specified sorted map.
	TreeSet(SortedSet s) Constructs a new set containing the same elements as the specified sorted set, sorted according to the same ordering.

Method Summary

boolean	add(Object o) Adds the specified element to this set if it is not already present.
boolean	addAll(Collection c) Adds all of the elements in the specified collection to this set.
void	clear() Removes all of the elements from this set.
Object	clone() Returns a shallow copy of this TreeSet instance.
Comparator	comparator() Returns the comparator used to order this sorted set, or null if this tree set uses its elements natural ordering.
boolean	contains(Object o) Returns true if this set contains the specified element.
Object	first() Returns the first (lowest) element currently in this sorted set.
SortedSet	headSet(Object toElement) Returns a view of the portion of this set whose elements are strictly less than toElement.
boolean	isEmpty() Returns true if this set contains no elements.
Iterator	iterator() Returns an iterator over the elements in this set.
Object	last() Returns the last (highest) element currently in this sorted set.
private void	readObject(ObjectInputStream s) Reconstitute the TreeSet instance from a stream (that is, deserialize it).
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).
SortedSet	subSet(Object fromElement, Object toElement) Returns a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive.
SortedSet	tailSet(Object fromElement) Returns a view of the portion of this set whose elements are greater than or equal to fromElement.
private void	writeObject(ObjectOutputStream s) Save the state of the TreeSet instance to a stream (that is, serialize it).

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

Field Detail

m

private transient SortedMap m

keySet

private transient Set keySet

PRESENT

private static final Object PRESENT

serialVersionUID

private static final long serialVersionUID

See Also:

Constant Field Values

Constructor Detail

TreeSet

private TreeSet(SortedMap m)

Constructs a set backed by the specified sorted map.

TreeSet

public TreeSet()

Constructs a new, empty set, sorted according to the elements' natural order. 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(Object) call will throw a ClassCastException.

See Also:

Comparable

TreeSet

public TreeSet(Comparator c)

Constructs a new, empty 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(Object) call will throw a ClassCastException.

Parameters:

c - the comparator that will be used to sort this set. A null value indicates that the elements' natural ordering should be used.

TreeSet

public TreeSet(Collection c)

Constructs a new set containing the elements in the specified collection, sorted according to the elements' natural order. All keys inserted into the set must implement the Comparable interface. Furthermore, all such keys must be mutually comparable: k1.compareTo(k2) must not throw a ClassCastException for any elements k1 and k2 in the set.

Parameters:

c - The elements that will comprise the new set.

Throws:

ClassCastException - if the keys in the specified collection are not comparable, or are not mutually comparable.

NullPointerException - if the specified collection is null.

TreeSet

public TreeSet(SortedSet s)

Constructs a new set containing the same elements as the specified sorted set, sorted according to the same ordering.

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 iterator()

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

Specified by:

iterator in interface Set

Specified by:

iterator in class AbstractCollection

Returns:

an iterator over the elements in this set.

size

public int size()

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

Specified by:

size in interface Set

Specified by:

size in class AbstractCollection

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 Set

Overrides:

isEmpty in class AbstractCollection

Returns:

true if this set contains no elements.

contains

public boolean contains(Object o)

Returns true if this set contains the specified element.

Specified by:

contains in interface Set

Overrides:

contains in class AbstractCollection

Parameters:

o - the 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.

add

public boolean add(Object o)

Adds the specified element to this set if it is not already present.

Specified by:

add in interface Set

Overrides:

add in class AbstractCollection

Parameters:

o - element to be added to this set.

Returns:

true if the set did not already contain the specified element.

Throws:

ClassCastException - if the specified object cannot be compared with the elements currently in the set.

remove

public boolean remove(Object o)

Removes the specified element from this set if it is present.

Specified by:

remove in interface Set

Overrides:

remove in class AbstractCollection

Parameters:

o - object to be removed from this set, if present.

Returns:

true if the set contained the specified element.

Throws:

ClassCastException - if the specified object cannot be compared with the elements currently in the set.

clear

public void clear()

Removes all of the elements from this set.

Specified by:

clear in interface Set

Overrides:

clear in class AbstractCollection

addAll

public boolean addAll(Collection c)

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

Specified by:

addAll in interface Set

Overrides:

addAll in class AbstractCollection

Parameters:

c - elements to be added

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 - of the specified collection is null.

See Also:

AbstractCollection.add(Object)

subSet

public SortedSet subSet(Object fromElement,
                        Object 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 sorted set is empty.) The returned sorted set is backed by this set, so changes in the returned sorted set are reflected in this set, and vice-versa. The returned sorted set supports all optional Set operations.

The sorted set returned by this method will throw an IllegalArgumentException if the user attempts to insert an element outside the specified range.

Note: this method always returns a half-open range (which includes its low endpoint but not its high endpoint). If you need a closed range (which includes both endpoints), and the element type allows for calculation of the successor of a specified value, merely request the subrange from lowEndpoint to successor(highEndpoint). For example, suppose that s is a sorted set of strings. The following idiom obtains a view containing all of the strings in s from low to high, inclusive:

     SortedSet sub = s.subSet(low, high+"\0");
 

A similar technique can be used to generate an open range (which contains neither endpoint). The following idiom obtains a view containing all of the strings in s from low to high, exclusive:

     SortedSet sub = s.subSet(low+"\0", high);
 

Specified by:

subSet in interface SortedSet

Parameters:

fromElement - low endpoint (inclusive) of the subSet.

toElement - high endpoint (exclusive) of the subSet.

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).

IllegalArgumentException - if fromElement is greater than toElement.

NullPointerException - if fromElement or toElement is null and this set uses natural order, or its comparator does not tolerate null elements.

headSet

public SortedSet headSet(Object toElement)

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

The sorted set returned by this method will throw an IllegalArgumentException if the user attempts to insert an element greater than or equal to toElement.

Note: this method always returns a view that does not contain its (high) endpoint. If you need a view that does contain this endpoint, and the element type allows for calculation of the successor of a specified value, merely request a headSet bounded by successor(highEndpoint). For example, suppose that s is a sorted set of strings. The following idiom obtains a view containing all of the strings in s that are less than or equal to high:

 SortedSet head = s.headSet(high+"\0");

Specified by:

headSet in interface SortedSet

Parameters:

toElement - high endpoint (exclusive) of the headSet.

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).

IllegalArgumentException - if this set is itself a subSet, headSet, or tailSet, and toElement is not within the specified range of the subSet, headSet, or tailSet.

NullPointerException - if toElement is null and this set uses natural ordering, or its comparator does not tolerate null elements.

tailSet

public SortedSet tailSet(Object fromElement)

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

The sorted set returned by this method will throw an IllegalArgumentException if the user attempts to insert an element less than fromElement. Note: this method always returns a view that contains its (low) endpoint. If you need a view that does not contain this endpoint, and the element type allows for calculation of the successor of a specified value, merely request a tailSet bounded by successor(lowEndpoint). For example, suppose that s is a sorted set of strings. The following idiom obtains a view containing all of the strings in s that are strictly greater than low:

     SortedSet tail = s.tailSet(low+"\0");
 

Specified by:

tailSet in interface SortedSet

Parameters:

fromElement - low endpoint (inclusive) of the tailSet.

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).

IllegalArgumentException - if this set is itself a subSet, headSet, or tailSet, and fromElement is not within the specified range of the subSet, headSet, or tailSet.

NullPointerException - if fromElement is null and this set uses natural ordering, or its comparator does not tolerate null elements.

comparator

public Comparator comparator()

Returns the comparator used to order this sorted set, or null if this tree set uses its elements natural ordering.

Specified by:

comparator in interface SortedSet

Returns:

the comparator used to order this sorted set, or null if this tree set uses its elements natural ordering.

first

public Object first()

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

Specified by:

first in interface SortedSet

Returns:

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

Throws:

NoSuchElementException - sorted set is empty.

last

public Object last()

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

Specified by:

last in interface SortedSet

Returns:

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

Throws:

NoSuchElementException - sorted set is empty.

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

writeObject

private void writeObject(ObjectOutputStream s)
                  throws IOException

Save the state of the TreeSet instance to a stream (that is, serialize it).

Throws:

IOException

readObject

private void readObject(ObjectInputStream s)
                 throws IOException,
                        ClassNotFoundException

Reconstitute the TreeSet instance from a stream (that is, deserialize it).

Throws:

IOException

ClassNotFoundException