All Packages  Class Hierarchy  This Package  Previous  Next  Index

Class java.util.HashSet 

java.lang.Object
    |
    +----java.util.AbstractCollection
            |
            +----java.util.AbstractSet
                    |
                    +----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 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. This Set does not permit the null element.

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, ArraySet, Collections.synchronizedSet, HashMap

Constructor Index

o HashSet()
Constructs a new, empty HashSet; the backing HashMap has default capacity and load factor.
o HashSet(Collection)
Constructs a new HashSet containing the elements in the specified Collection.
o HashSet(int)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and default load factor.
o HashSet(int, float)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and the specified load factor.

Method Index

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

Constructors

o HashSet 
public HashSet()
Constructs a new, empty HashSet; the backing HashMap has default capacity and load factor.

o 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, and the default load factor is used.

Throws: NullPointerException
if the specified collection or one of its elements is null.
o 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 hashtable.
loadFactor - a number between 0.0 and 1.0.
Throws: IllegalArgumentException
if the initial capacity is less than or equal to zero, or if the load factor is less than or equal to zero.
o HashSet 
public HashSet(int initialCapacity)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and default load factor.

Parameters:
initialCapacity - the initial capacity of the hashtable.
Throws: IllegalArgumentException
if the initial capacity is less than or equal to zero, or if the load factor is less than or equal to zero.

Methods

o iterator 
public Iterator iterator()
Returns an Iterator over the elements in this HashSet. The elements are returned in no particular order.

Overrides:
iterator in class AbstractCollection
See Also:
ConcurrentModificationException
o size 
public int size()
Returns the number of elements in this HashSet (its cardinality).

Overrides:
size in class AbstractCollection
o isEmpty 
public boolean isEmpty()
Returns true if this HashSet contains no elements.

Overrides:
isEmpty in class AbstractCollection
o contains 
public boolean contains(Object o)
Returns true if this HashSet contains the specified element.

Throws: NullPointerException
if the specified element is null.
Overrides:
contains in class AbstractCollection
o add 
public boolean add(Object o)
Adds the specified element to this HashSet if it is not already present.

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
o remove 
public boolean remove(Object o)
Removes the given element from this HashSet if it is present.

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
o clear 
public void clear()
Removes all of the elements from this HashSet.

Overrides:
clear in class AbstractCollection
o clone 
public Object clone()
Returns a shallow copy of this HashSet. (The elements themselves are not cloned.)

Overrides:
clone in class Object
All Packages  Class Hierarchy  This Package  Previous  Next  Index
Submit a bug or feature