java.util

Class HashSet<T>

Implemented Interfaces:
Cloneable, Collection<E>, Iterable<E>, Serializable, Set<E>
Known Direct Subclasses:
JobStateReasons, LinkedHashSet<T>

public class HashSet<T>
extends AbstractSet<E>
implements Set<E>, Cloneable, Serializable

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, Serialized Form

Constructor Summary

HashSet()
Construct a new, empty HashSet whose backing HashMap has the default capacity (11) and loadFacor (0.75).
HashSet(T> 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(T 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.
Object
clone()
Returns a shallow copy of this Set.
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<E>

equals, hashCode, removeAll

Methods inherited from class java.util.AbstractCollection<E>

T[] toArray, add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

HashSet

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

HashSet

public HashSet(T> 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

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

Method Details

add

public boolean add(T o)
Adds the given Object to the set if it is not already in the Set. This set permits a null element.
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 Set<E>
clear in interface Collection<E>
Overrides:
clear in interface AbstractCollection<E>

clone

public Object clone()
Returns a shallow copy of this Set. The Set itself is cloned; its elements are not.
Overrides:
clone in interface Object
Returns:
a shallow clone of the set

contains

public boolean contains(Object o)
Returns true if the supplied element is in this Set.
Specified by:
contains in interface Set<E>
contains in interface Collection<E>
Overrides:
contains in interface AbstractCollection<E>
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 Set<E>
isEmpty in interface Collection<E>
Overrides:
isEmpty in interface AbstractCollection<E>
Returns:
size() == 0.

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 Set<E>
iterator in interface Collection<E>
iterator in interface Iterable<E>
Overrides:
iterator in interface AbstractCollection<E>
Returns:
a set iterator

remove

public boolean remove(Object o)
Removes the supplied Object from this Set if it is in the Set.
Specified by:
remove in interface Set<E>
remove in interface Collection<E>
Overrides:
remove in interface AbstractCollection<E>
Parameters:
o - the object to remove
Returns:
true if an element was removed

size

public int size()
Returns the number of elements in this Set (its cardinality).
Specified by:
size in interface Set<E>
size in interface Collection<E>
Overrides:
size in interface AbstractCollection<E>
Returns:
the size of the set

HashSet.java -- a class providing a HashMap-backed Set Copyright (C) 1998, 1999, 2001, 2004, 2005 Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.