java.util
Class HashSet

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

All Implemented Interfaces:

Cloneable, Collection, Serializable, Set

Direct Known Subclasses:

LinkedHashSet

public class HashSet

extends AbstractSet

implements Set, Cloneable, Serializable

This class implements the Set interface, backed by a hash table (actually a HashMap instance). 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 hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance's size (the number of elements) plus the "capacity" of the backing HashMap instance (the number of buckets). Thus, it's very important not to set the initial 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 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 HashSet instance:

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

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

Field Summary

private HashMap	map
private static Object	PRESENT
(package private) static long	serialVersionUID

Constructor Summary

	HashSet() Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).
	HashSet(Collection c) Constructs a new set containing the elements in the specified collection.
	HashSet(int initialCapacity) Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor, which is 0.75.
	HashSet(int initialCapacity, float loadFactor) Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.
(package private)	HashSet(int initialCapacity, float loadFactor, boolean dummy) Constructs a new, empty linked hash set.

Method Summary

boolean	add(Object o) Adds the specified element to this set if it is not already present.
void	clear() Removes all of the elements from this set.
Object	clone() Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.
boolean	contains(Object o) Returns true if this set contains the specified element.
boolean	isEmpty() Returns true if this set contains no elements.
Iterator	iterator() Returns an iterator over the elements in this set.
private void	readObject(ObjectInputStream s) Reconstitute the HashSet 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).
private void	writeObject(ObjectOutputStream s) Save the state of this HashSet instance to a stream (that is, serialize this set).

Methods inherited from class java.util.AbstractSet

equals, hashCode, removeAll

Methods inherited from class java.util.AbstractCollection

addAll, 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

addAll, containsAll, equals, hashCode, removeAll, retainAll, toArray, toArray

Field Detail

serialVersionUID

static final long serialVersionUID

See Also:

Constant Field Values

map

private transient HashMap map

PRESENT

private static final Object PRESENT

Constructor Detail

HashSet

public HashSet()

Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).

HashSet

public HashSet(Collection c)

Constructs a new set containing the elements in the specified collection. The HashMap is created with default load factor (0.75) and an initial capacity sufficient to contain the elements in the specified collection.

Parameters:

c - the collection whose elements are to be placed into this set.

Throws:

NullPointerException - if the specified collection is null.

HashSet

public HashSet(int initialCapacity,
               float loadFactor)

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

Parameters:

initialCapacity - the initial capacity of the hash map.

loadFactor - the load factor of the hash map.

Throws:

IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive.

HashSet

public HashSet(int initialCapacity)

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

Parameters:

initialCapacity - the initial capacity of the hash table.

Throws:

IllegalArgumentException - if the initial capacity is less than zero.

HashSet

HashSet(int initialCapacity,
        float loadFactor,
        boolean dummy)

Constructs a new, empty linked hash set. (This package private constructor is only used by LinkedHashSet.) The backing HashMap instance is a LinkedHashMap with the specified initial capacity and the specified load factor.

Parameters:

initialCapacity - the initial capacity of the hash map.

loadFactor - the load factor of the hash map.

dummy - ignored (distinguishes this constructor from other int, float constructor.)

Throws:

IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive.

Method Detail

iterator

public Iterator iterator()

Returns an iterator over the elements in this set. The elements are returned in no particular order.

Specified by:

iterator in interface Set

Specified by:

iterator in class AbstractCollection

Returns:

an Iterator over the elements in this set.

See Also:

ConcurrentModificationException

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 - element whose presence in this set is to be tested.

Returns:

true if this set contains the specified element.

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.

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.

clear

public void clear()

Removes all of the elements from this set.

Specified by:

clear in interface Set

Overrides:

clear in class AbstractCollection

clone

public Object clone()

Returns a shallow copy of this HashSet 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 this HashSet instance to a stream (that is, serialize this set).

Throws:

IOException

readObject

private void readObject(ObjectInputStream s)
                 throws IOException,
                        ClassNotFoundException

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

Throws:

IOException

ClassNotFoundException