Contents | Package | Class | Tree | Deprecated | Index | Help Java 1.2 Beta 3
PREV | NEXT SHOW LISTS | HIDE LISTS

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 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, HashMap

Constructor Summary
 HashSet()
Constructs a new, empty HashSet; the backing HashMap has default capacity and load factor.
 HashSet(Collection c)
Constructs a new HashSet containing the elements in the specified Collection.
 HashSet(int initialCapacity, float loadFactor)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and the specified load factor.
 HashSet(int initialCapacity)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and default 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
 add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray, toString
 
Methods inherited from class java.lang.Object
 clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

HashSet

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

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.

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.

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.
Method Detail

iterator

public Iterator iterator()
Returns an Iterator over the elements in this HashSet. The elements are returned in no particular order.
Implements:
iterator in interface Set
Overrides:
iterator in class AbstractCollection
See Also:
ConcurrentModificationException

size

public int size()
Returns the number of elements in this HashSet (its cardinality).
Implements:
size in interface Set
Overrides:
size in class AbstractCollection

isEmpty

public boolean isEmpty()
Returns true if this HashSet contains no elements.
Implements:
isEmpty in interface Set
Overrides:
isEmpty in class AbstractCollection

contains

public boolean contains(Object o)
Returns true if this HashSet contains the specified element.
Implements:
contains in interface Set
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.
Implements:
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.
Implements:
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.
Implements:
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.)
Overrides:
clone in class Object

Contents | Package | Class | Tree | Deprecated | Index | Help Java 1.2 Beta 3
PREV | NEXT SHOW LISTS | HIDE LISTS

Submit a bug or feature
Submit comments/suggestions about new javadoc look.
Java is a trademark or registered trademark of Sun Microsystems, Inc. in the US and other countries.
Copyright 1993-1998 Sun Microsystems, Inc. 901 San Antonio Road, Palo Alto, California, 94303, U.S.A. All Rights Reserved.