public
class
HashSet
extends AbstractSet<E>
implements
Set<E>,
Cloneable,
Serializable
| java.lang.Object | |||
| ↳ | java.util.AbstractCollection<E> | ||
| ↳ | java.util.AbstractSet<E> | ||
| ↳ | java.util.HashSet<E> | ||
 Known Direct Subclasses
|
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 hash 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 set:
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.
See also:
Public constructors | |
|---|---|
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, float loadFactor)
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor. |
|
HashSet(int initialCapacity)
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75). |
|
Public methods | |
|---|---|
boolean
|
add(E e)
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). |
Spliterator<E>
|
spliterator()
Creates a late-binding
and fail-fast |
Inherited methods | |
|---|---|
From
class
java.util.AbstractSet
| |
From
class
java.util.AbstractCollection
| |
From
class
java.lang.Object
| |
From
interface
java.util.Set
| |
From
interface
java.util.Collection
| |
From
interface
java.lang.Iterable
| |
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. 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 |
Collection:
the collection whose elements are to be placed into this set |
| Throws | |
|---|---|
NullPointerException |
if the specified collection is null |
Developers
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 |
int:
the initial capacity of the hash map |
loadFactor |
float:
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 (int initialCapacity)
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).
| Parameters | |
|---|---|
initialCapacity |
int:
the initial capacity of the hash table |
| Throws | |
|---|---|
IllegalArgumentException |
if the initial capacity is less than zero |
boolean add (E e)
Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.
| Parameters | |
|---|---|
e |
E:
element to be added to this set |
| Returns | |
|---|---|
boolean |
true if this set did not already contain the specified element |
void clear ()
Removes all of the elements from this set. The set will be empty after this call returns.
Object clone ()
Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.
| Returns | |
|---|---|
Object |
a shallow copy of this set |
boolean contains (Object o)
Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e)).
| Parameters | |
|---|---|
o |
Object:
element whose presence in this set is to be tested |
| Returns | |
|---|---|
boolean |
true if this set contains the specified element |
boolean isEmpty ()
Returns true if this set contains no elements.
| Returns | |
|---|---|
boolean |
true if this set contains no elements |
Iterator<E> iterator ()
Returns an iterator over the elements in this set. The elements are returned in no particular order.
| Returns | |
|---|---|
Iterator<E> |
an Iterator over the elements in this set |
See also:
boolean remove (Object o)
Removes the specified element from this set if it is present. More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if this set contains such an element. Returns true if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.)
| Parameters | |
|---|---|
o |
Object:
object to be removed from this set, if present |
| Returns | |
|---|---|
boolean |
true if the set contained the specified element |
int size ()
Returns the number of elements in this set (its cardinality).
| Returns | |
|---|---|
int |
the number of elements in this set (its cardinality) |
Spliterator<E> spliterator ()
Creates a late-binding
and fail-fast Spliterator over the elements in this
set.
The Spliterator reports SIZED and
DISTINCT. Overriding implementations should document
the reporting of additional characteristic values.
| Returns | |
|---|---|
Spliterator<E> |
a Spliterator over the elements in this set |