E
- the type of elements maintained by this setpublic 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 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.
Collection
, Set
, TreeSet
, HashMap
, Serialized Formpublic HashSet()
Constructs a new, empty set; the backing HashMap
instance has default initial capacity (16) and load factor (0.75).
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.
c
- the collection whose elements are to be placed into this setNullPointerException
- if the specified collection is nullpublic HashSet(int initialCapacity, float loadFactor)
Constructs a new, empty set; the backing HashMap
instance has the specified initial capacity and the specified load factor.
initialCapacity
- the initial capacity of the hash maploadFactor
- the load factor of the hash mapIllegalArgumentException
- if the initial capacity is less than zero, or if the load factor is nonpositivepublic HashSet(int initialCapacity)
Constructs a new, empty set; the backing HashMap
instance has the specified initial capacity and default load factor (0.75).
initialCapacity
- the initial capacity of the hash tableIllegalArgumentException
- if the initial capacity is less than zeropublic Iterator<E> iterator()
Returns an iterator over the elements in this set. The elements are returned in no particular order.
iterator
in interface Iterable<E>
iterator
in interface Collection<E>
iterator
in interface Set<E>
iterator
in class AbstractCollection<E>
ConcurrentModificationException
public int size()
Returns the number of elements in this set (its cardinality).
size
in interface Collection<E>
size
in interface Set<E>
size
in class AbstractCollection<E>
public boolean isEmpty()
Returns true
if this set contains no elements.
isEmpty
in interface Collection<E>
isEmpty
in interface Set<E>
isEmpty
in class AbstractCollection<E>
true
if this set contains no elementspublic 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))
.
contains
in interface Collection<E>
contains
in interface Set<E>
contains
in class AbstractCollection<E>
o
- element whose presence in this set is to be testedtrue
if this set contains the specified elementpublic 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
.
add
in interface Collection<E>
add
in interface Set<E>
add
in class AbstractCollection<E>
e
- element to be added to this settrue
if this set did not already contain the specified elementpublic 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.)
remove
in interface Collection<E>
remove
in interface Set<E>
remove
in class AbstractCollection<E>
o
- object to be removed from this set, if presenttrue
if the set contained the specified elementpublic void clear()
Removes all of the elements from this set. The set will be empty after this call returns.
clear
in interface Collection<E>
clear
in interface Set<E>
clear
in class AbstractCollection<E>
public Object clone()
Returns a shallow copy of this HashSet
instance: the elements themselves are not cloned.
public Spliterator<E> spliterator()
Creates a late-binding and fail-fast Spliterator
over the elements in this set.
The Spliterator
reports Spliterator.SIZED
and Spliterator.DISTINCT
. Overriding implementations should document the reporting of additional characteristic values.
spliterator
in interface Iterable<E>
spliterator
in interface Collection<E>
spliterator
in interface Set<E>
Spliterator
over the elements in this set
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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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