java.util
Class HashSet

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

All Implemented Interfaces:

Collection, Set

public class HashSet
extends AbstractSet
implements Set

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 ConcurrentModificationException rather than exhibit non-deterministic behavior.

Since:

1.2

See Also:

Collection, Set, TreeSet, Collections.synchronizedSet(Set), HashMap, LinkedHashSet

Constructor Summary

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

HashSet(Collection c)
Construct a new HashSet with the same elements as are in the supplied collection (eliminating any duplicates, of course).

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.

Method Summary

boolean	add(Object 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.
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	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).

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

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

Methods inherited from interface java.util.Set

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

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 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(Object o)

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

Specified by:

add in interface Collection

Specified by:

add in interface Set

Overrides:

add in class AbstractCollection

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.

Specified by:

clear in interface Collection

Specified by:

clear in interface Set

Overrides:

clear in class AbstractCollection

See Also:

Iterator.remove()

contains

public boolean contains(Object o)

Returns true if the supplied element is in this Set.

Specified by:

contains in interface Collection

Specified by:

contains in interface Set

Overrides:

contains in class AbstractCollection

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.

Specified by:

isEmpty in interface Collection

Specified by:

isEmpty in interface Set

Overrides:

isEmpty in class AbstractCollection

Returns:

size() == 0.

See Also:

AbstractCollection.size()

iterator

public Iterator 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.

Specified by:

iterator in interface Collection

Specified by:

iterator in interface Set

Specified by:

iterator in class AbstractCollection

Returns:

a set iterator

See Also:

ConcurrentModificationException

remove

public boolean remove(Object o)

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

Specified by:

remove in interface Collection

Specified by:

remove in interface Set

Overrides:

remove in class AbstractCollection

Parameters:

o - the object to remove

Returns:

true if an element was removed

See Also:

Iterator.remove()

size

public int size()

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

Specified by:

size in interface Collection

Specified by:

size in interface Set

Specified by:

size in class AbstractCollection

Returns:

the size of the set