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HashSet

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:

Summary

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 Spliterator over the elements in this set.

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

Public constructors

HashSet

Added in API level 1
HashSet ()

Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).

HashSet

Added in API level 1
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

HashSet

Added in API level 1
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

Added in API level 1
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

Public methods

add

Added in API level 1
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

clear

Added in API level 1
void clear ()

Removes all of the elements from this set. The set will be empty after this call returns.

clone

Added in API level 1
Object clone ()

Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.

Returns
Object a shallow copy of this set

contains

Added in API level 1
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

isEmpty

Added in API level 1
boolean isEmpty ()

Returns true if this set contains no elements.

Returns
boolean true if this set contains no elements

iterator

Added in API level 1
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:

remove

Added in API level 1
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

size

Added in API level 1
int size ()

Returns the number of elements in this set (its cardinality).

Returns
int the number of elements in this set (its cardinality)

spliterator

Added in API level 24
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
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