java.util
Class HashSet

java.lang.Object
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  +--java.util.AbstractCollection
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        +--java.util.AbstractSet
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              +--java.util.HashSet

public class HashSet

extends AbstractSet

implements Set, Cloneable, Serializable

This class implements the Set interface, backed by a hash table (actually a HashMap). It makes no guarantees as to the iteration order of the Set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.

This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the the hash function disperses the elements properly among the buckets. Iterating over the Set requires time proportional to the sum of the HashSet's size (the number of elements) plus the "capacity" of the backing HashMap (the number of buckets). Thus, it's very important not to set the intial capacity too high (or the load factor too low) if iteration performance is important.

Note that this implementation is not synchronized. If multiple threads access a HashSet concurrently, and at least one of the threads modifies the HashSet, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the HashSet. If no such object exists, the HashSet should be "wrapped" using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the HashSet:

	Set s = Collections.synchronizedSet(new HashSet(...));
 

The Iterators returned by HashSet's iterator method are fail-fast: if the HashSet 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.

Since:

JDK1.2

See Also:

Collection, Set, TreeSet, Collections.synchronizedSet(Set), HashMap, Serialized Form

Constructor Summary

HashSet()
Constructs a new, empty HashSet; the backing HashMap has default capacity and load factor, which is 0.75.

HashSet(Collection c)
Constructs a new HashSet containing the elements in the specified Collection.

HashSet(int initialCapacity)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and default load factor, which is 0.75.

HashSet(int initialCapacity, float loadFactor)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and the specified load factor.

Method Summary

boolean	add(Object o) Adds the specified element to this HashSet if it is not already present.
void	clear() Removes all of the elements from this HashSet.
Object	clone() Returns a shallow copy of this HashSet.
boolean	contains(Object o) Returns true if this HashSet contains the specified element.
boolean	isEmpty() Returns true if this HashSet contains no elements.
Iterator	iterator() Returns an Iterator over the elements in this HashSet.
boolean	remove(Object o) Removes the given element from this HashSet if it is present.
int	size() Returns the number of elements in this HashSet (its cardinality).

Methods inherited from class java.util.AbstractSet

equals, hashCode

Methods inherited from class java.util.AbstractCollection

addAll, containsAll, removeAll, retainAll, toArray, toArray, toString

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

HashSet

public HashSet()

Constructs a new, empty HashSet; the backing HashMap has default capacity and load factor, which is 0.75.

HashSet

public HashSet(Collection c)

Constructs a new HashSet containing the elements in the specified Collection. The capacity of the backing HashMap is twice the size of the specified Collection or eleven (whichever is greater), and the default load factor (which is 0.75) is used.

Throws:

NullPointerException - if the specified collection or one of its elements is null.

HashSet

public HashSet(int initialCapacity,
               float loadFactor)

Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and the specified load factor.

Parameters:

initialCapacity - the initial capacity of the HashMap.

loadFactor - the load factor of the HashMap.

Throws:

IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is less than or equal to zero or greater than one.

HashSet

public HashSet(int initialCapacity)

Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and default load factor, which is 0.75.

Parameters:

initialCapacity - the initial capacity of the hashtable.

Throws:

IllegalArgumentException - if the initial capacity is less than zero.

Method Detail

iterator

public Iterator iterator()

Returns an Iterator over the elements in this HashSet. The elements are returned in no particular order.

Specified by:

iterator in interface Set

Returns:

an Iterator over the elements in this HashSet.

Overrides:

iterator in class AbstractCollection

See Also:

ConcurrentModificationException

size

public int size()

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

Specified by:

size in interface Set

Returns:

the number of elements in this HashSet (its cardinality).

Overrides:

size in class AbstractCollection

isEmpty

public boolean isEmpty()

Returns true if this HashSet contains no elements.

Specified by:

isEmpty in interface Set

Returns:

true if this HashSet contains no elements.

Overrides:

isEmpty in class AbstractCollection

contains

public boolean contains(Object o)

Returns true if this HashSet contains the specified element.

Specified by:

contains in interface Set

Returns:

true if this HashSet contains the specified element.

Throws:

NullPointerException - if the specified element is null.

Overrides:

contains in class AbstractCollection

add

public boolean add(Object o)

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

Specified by:

add in interface Set

Parameters:

o - element to be added to this HashSet.

Returns:

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

Throws:

NullPointerException - if the specified element is null.

Overrides:

add in class AbstractCollection

remove

public boolean remove(Object o)

Removes the given element from this HashSet if it is present.

Specified by:

remove in interface Set

Parameters:

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

Returns:

true if the HashSet contained the specified element.

Throws:

NullPointerException - if the specified element is null.

Overrides:

remove in class AbstractCollection

clear

public void clear()

Removes all of the elements from this HashSet.

Specified by:

clear in interface Set

Overrides:

clear in class AbstractCollection

clone

public Object clone()

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

Returns:

a shallow copy of this HashSet.

Overrides:

clone in class Object