| JavaTM 2 Platform Std. Ed. v1.5.0
java.util
Class HashSet<E>
java.lang.Object
 java.util.AbstractCollection<E>
java.util.AbstractSet<E>
java.util.HashSet<E>
- All Implemented Interfaces:
- Serializable, Cloneable, Iterable<E>, Collection<E>, Set<E>
- Direct Known Subclasses:
- JobStateReasons, LinkedHashSet
public class HashSet<E> - extends AbstractSet<E>
- implements Set<E>, 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
- See Also:
Collection,
Set,
TreeSet,
Collections.synchronizedSet(Set),
HashMap,
Serialized Form
|
Constructor Summary |
HashSet()
Constructs a new, empty set; the backing HashMap instance has
default initial capacity (16) and load factor (0.75). |
HashSet(Collection<? extends E> 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. |
|
Method Summary |
boolean |
add(E 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<E> |
iterator()
Returns an iterator over the elements in this set. |
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). |
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<? extends E> 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.
iterator
public Iterator<E> iterator()
- Returns an iterator over the elements in this set. The elements
are returned in no particular order.
- Specified by:
iterator in interface Iterable<E>- Specified by:
iterator in interface Collection<E>- Specified by:
iterator in interface Set<E>- Specified by:
iterator in class AbstractCollection<E>
- 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 Collection<E>- Specified by:
size in interface Set<E>- Specified by:
size in class AbstractCollection<E>
- 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 Collection<E>- Specified by:
isEmpty in interface Set<E>- Overrides:
isEmpty in class AbstractCollection<E>
- 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 Collection<E>- Specified by:
contains in interface Set<E>- Overrides:
contains in class AbstractCollection<E>
- 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(E o)
- Adds the specified element to this set if it is not already
present.
- Specified by:
add in interface Collection<E>- Specified by:
add in interface Set<E>- Overrides:
add in class AbstractCollection<E>
- 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 Collection<E>- Specified by:
remove in interface Set<E>- Overrides:
remove in class AbstractCollection<E>
- 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 Collection<E>- Specified by:
clear in interface Set<E>- Overrides:
clear in class AbstractCollection<E>
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
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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