java.util

Class HashSet<T>

public class HashSet<T> extends AbstractSet<T> implements Set<T>, Cloneable, Serializable

This class provides a HashMap-backed implementation of the Set interface.

Most operations are O(1), assuming no hash collisions. In the worst case (where all hashes collide), operations are O(n). Setting the initial capacity too low will force many resizing operations, but setting the initial capacity too high (or loadfactor too low) leads to wasted memory and slower iteration.

HashSet accepts the null key and null values. It is not synchronized, so if you need multi-threaded access, consider using:
Set s = Collections.synchronizedSet(new HashSet(...));

The iterators are fail-fast, meaning that any structural modification, except for remove() called on the iterator itself, cause the iterator to throw a {@link ConcurrentModificationException} rather than exhibit non-deterministic behavior.

Since: 1.2

UNKNOWN: updated to 1.4

Constructor Summary
	HashSet() Construct a new, empty HashSet whose backing HashMap has the default capacity (11) and loadFacor (0.75).
	HashSet(int initialCapacity) Construct a new, empty HashSet whose backing HashMap has the supplied capacity and the default load factor (0.75).
	HashSet(int initialCapacity, float loadFactor) Construct a new, empty HashSet whose backing HashMap has the supplied capacity and load factor.
	HashSet(Collection<? extends T> c) Construct a new HashSet with the same elements as are in the supplied collection (eliminating any duplicates, of course).

Method Summary
boolean	add(T o) Adds the given Object to the set if it is not already in the Set.
void	clear() Empties this Set of all elements; this takes constant time.
Object	clone() Returns a shallow copy of this Set.
boolean	contains(Object o) Returns true if the supplied element is in this Set.
boolean	isEmpty() Returns true if this set has no elements in it.
Iterator<T>	iterator() Returns an Iterator over the elements of this Set, which visits the elements in no particular order.
boolean	remove(Object o) Removes the supplied Object from this Set if it is in the Set.
int	size() Returns the number of elements in this Set (its cardinality).

Constructor Detail

HashSet

public HashSet()

Construct a new, empty HashSet whose backing HashMap has the default capacity (11) and loadFacor (0.75).

HashSet

public HashSet(int initialCapacity)

Construct a new, empty HashSet whose backing HashMap has the supplied capacity and the default load factor (0.75).

Parameters: initialCapacity the initial capacity of the backing HashMap

Throws: IllegalArgumentException if the capacity is negative

HashSet

public HashSet(int initialCapacity, float loadFactor)

Construct a new, empty HashSet whose backing HashMap has the supplied capacity and load factor.

Parameters: initialCapacity the initial capacity of the backing HashMap loadFactor the load factor of the backing HashMap

Throws: IllegalArgumentException if either argument is negative, or if loadFactor is POSITIVE_INFINITY or NaN

HashSet

public HashSet(Collection<? extends T> c)

Construct a new HashSet with the same elements as are in the supplied collection (eliminating any duplicates, of course). The backing storage has twice the size of the collection, or the default size of 11, whichever is greater; and the default load factor (0.75).

Parameters: c a collection of initial set elements

Throws: NullPointerException if c is null

Method Detail

add

public boolean add(T o)

Adds the given Object to the set if it is not already in the Set. This set permits a null element.

Parameters: o the Object to add to this Set

Returns: true if the set did not already contain o

clear

public void clear()

Empties this Set of all elements; this takes constant time.

clone

public Object clone()

Returns a shallow copy of this Set. The Set itself is cloned; its elements are not.

Returns: a shallow clone of the set

contains

public boolean contains(Object o)

Returns true if the supplied element is in this Set.

Parameters: o the Object to look for

Returns: true if it is in the set

isEmpty

public boolean isEmpty()

Returns true if this set has no elements in it.

Returns: size() == 0.

iterator

public Iterator<T> iterator()

Returns an Iterator over the elements of this Set, which visits the elements in no particular order. For this class, the Iterator allows removal of elements. The iterator is fail-fast, and will throw a ConcurrentModificationException if the set is modified externally.

Returns: a set iterator

remove

public boolean remove(Object o)

Removes the supplied Object from this Set if it is in the Set.

Parameters: o the object to remove

Returns: true if an element was removed

size

public int size()

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

Returns: the size of the set