OpenJDK 1.22 · java.util.HashSet · APIdia, the JavaDoc alternative

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.

Modifier and Type	Field and Description
pack-priv transient HashMap<E, Object>	map
pack-priv static final Object	PRESENT
pack-priv static final long	serialVersionUID

Constructor Summary

Access	Constructor and Description
public	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> the collection whose elements are to be placed into this set c) Constructs a new set containing the elements in the specified collection.
public	HashSet(int the initial capacity of the hash map initialCapacity, float the load factor of the hash map loadFactor) Constructs a new, empty set; the backing `HashMap` instance has the specified initial capacity and the specified load factor.
public	HashSet(int the initial capacity of the hash table initialCapacity) Constructs a new, empty set; the backing `HashMap` instance has the specified initial capacity and default load factor (0.75).
pack-priv	HashSet(int the initial capacity of the hash map initialCapacity, float the load factor of the hash map loadFactor, boolean ignored (distinguishes this constructor from other int, float constructor.) dummy) Constructs a new, empty linked hash set.

Method Summary

Modifier and Type	Method and Description
public boolean	Returns: `true` if this set did not already contain the specified element add(E element to be added to this set e) Overrides java.util.AbstractCollection.add. Implements java.util.Set.add, java.util.Collection.add. Adds the specified element to this set if it is not already present.
public void	clear() Overrides java.util.AbstractCollection.clear. Implements java.util.Set.clear, java.util.Collection.clear. Removes all of the elements from this set.
public Object	Returns: a shallow copy of this set clone() Overrides java.lang.Object.clone. Returns a shallow copy of this `HashSet` instance: the elements themselves are not cloned.
public boolean	Returns: `true` if this set contains the specified element contains(Object element whose presence in this set is to be tested o) Overrides java.util.AbstractCollection.contains. Implements java.util.Set.contains, java.util.Collection.contains. Returns `true` if this set contains the specified element.
public boolean	Returns: `true` if this set contains no elements isEmpty() Overrides java.util.AbstractCollection.isEmpty. Implements java.util.Set.isEmpty, java.util.Collection.isEmpty. Returns `true` if this set contains no elements.
public Iterator<E>	Returns: an Iterator over the elements in this set iterator() Implements abstract java.util.AbstractCollection.iterator. Implements java.util.Set.iterator, java.util.Collection.iterator. Returns an iterator over the elements in this set.
public static < the type of elements maintained by the new set T> HashSet<T>	Returns: the newly created set newHashSet(int the expected number of elements numElements) Creates a new, empty HashSet suitable for the expected number of elements.
private void	readObject(ObjectInputStream s) Reconstitute the `HashSet` instance from a stream (that is, deserialize it).
public boolean	Returns: `true` if the set contained the specified element remove(Object object to be removed from this set, if present o) Overrides java.util.AbstractCollection.remove. Implements java.util.Set.remove, java.util.Collection.remove. Removes the specified element from this set if it is present.
public int	Returns: the number of elements in this set (its cardinality) size() Implements abstract java.util.AbstractCollection.size. Implements java.util.Set.size, java.util.Collection.size. Returns the number of elements in this set (its cardinality).
public Spliterator<E>	Returns: a `Spliterator` over the elements in this set spliterator() Overrides default java.util.Set.spliterator, java.util.Collection.spliterator. Creates a late-binding and fail-fast `Spliterator` over the elements in this set.
public Object[]	toArray() Overrides java.util.AbstractCollection.toArray. Implements java.util.Set.toArray, java.util.Collection.toArray. Returns an array containing all of the elements in this set.
public <T> T[]	toArray(T[] the array into which the elements of this set are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose. a) Overrides java.util.AbstractCollection.toArray. Implements java.util.Set.toArray, java.util.Collection.toArray. Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.
private void	writeObject(ObjectOutputStream s) Save the state of this `HashSet` instance to a stream (that is, serialize it).

Inherited from java.util.AbstractSet:: equals hashCode removeAll

Field Detail

map	back to summary
pack-priv transient HashMap<E, Object> map

PRESENT	back to summary
pack-priv static final Object PRESENT

serialVersionUID	back to summary
pack-priv static final long serialVersionUID Annotations @Serial

Constructor Detail

HashSet	back to summary
public HashSet() Constructs a new, empty set; the backing `HashMap` instance has default initial capacity (16) and load factor (0.75).

HashSet back to summary

HashSet	back to summary
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:Collection<? extends E> the collection whose elements are to be placed into this set Exceptions NullPointerException: if the specified collection is null

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:Collection<? extends E>: the collection whose elements are to be placed into this set

Exceptions

NullPointerException:: if the specified collection is null

HashSet back to summary

HashSet	back to summary
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. API Note To create a `HashSet` with an initial capacity that accommodates an expected number of elements, use `newHashSet`. Parameters initialCapacity:int the initial capacity of the hash map loadFactor:float the load factor of the hash map Exceptions IllegalArgumentException: if the initial capacity is less than zero, or if the load factor is nonpositive

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.

API Note

To create a HashSet with an initial capacity that accommodates an expected number of elements, use newHashSet.

Parameters

initialCapacity:int: the initial capacity of the hash map
loadFactor:float: the load factor of the hash map

Exceptions

IllegalArgumentException:: if the initial capacity is less than zero, or if the load factor is nonpositive

HashSet back to summary

HashSet	back to summary
public HashSet(int initialCapacity) Constructs a new, empty set; the backing `HashMap` instance has the specified initial capacity and default load factor (0.75). API Note To create a `HashSet` with an initial capacity that accommodates an expected number of elements, use `newHashSet`. Parameters initialCapacity:int the initial capacity of the hash table Exceptions IllegalArgumentException: if the initial capacity is less than zero

public HashSet(int initialCapacity)

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

API Note

To create a HashSet with an initial capacity that accommodates an expected number of elements, use newHashSet.

Parameters

initialCapacity:int: the initial capacity of the hash table

Exceptions

IllegalArgumentException:: if the initial capacity is less than zero

HashSet	back to summary
pack-priv HashSet(int initialCapacity, float loadFactor, boolean dummy) Constructs a new, empty linked hash set. (This package private constructor is only used by LinkedHashSet.) The backing HashMap instance is a LinkedHashMap with 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 dummy:boolean ignored (distinguishes this constructor from other int, float constructor.) Exceptions IllegalArgumentException: if the initial capacity is less than zero, or if the load factor is nonpositive

HashSet

back to summary

pack-priv HashSet(int initialCapacity, float loadFactor, boolean dummy)

Constructs a new, empty linked hash set. (This package private constructor is only used by LinkedHashSet.) The backing HashMap instance is a LinkedHashMap with 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
dummy:boolean: ignored (distinguishes this constructor from other int, float constructor.)

Exceptions

IllegalArgumentException:: if the initial capacity is less than zero, or if the load factor is nonpositive

Method Detail

add back to summary

add	back to summary
public boolean add(E e) Overrides java.util.AbstractCollection.add. Implements java.util.Set.add, java.util.Collection.add. 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 `Objects.equals(e, 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

public boolean add(E e)

Overrides java.util.AbstractCollection.add.

Implements java.util.Set.add, java.util.Collection.add.

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 Objects.equals(e, 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	back to summary
public void clear() Overrides java.util.AbstractCollection.clear. Implements java.util.Set.clear, java.util.Collection.clear. Removes all of the elements from this set. The set will be empty after this call returns.

clear

back to summary

public void clear()

Overrides java.util.AbstractCollection.clear.

Implements java.util.Set.clear, java.util.Collection.clear.

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

clone back to summary

clone	back to summary
public Object clone() Overrides java.lang.Object.clone. Returns a shallow copy of this `HashSet` instance: the elements themselves are not cloned. Returns:Object a shallow copy of this set Annotations @SuppressWarnings:unchecked

public Object clone()

Overrides java.lang.Object.clone.

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

Returns:Object

a shallow copy of this set

Annotations

@SuppressWarnings:unchecked

contains back to summary

contains	back to summary
public boolean contains(Object o) Overrides java.util.AbstractCollection.contains. Implements java.util.Set.contains, java.util.Collection.contains. 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 `Objects.equals(o, e)`. Parameters o:Object element whose presence in this set is to be tested Returns:boolean `true` if this set contains the specified element

public boolean contains(Object o)

Overrides java.util.AbstractCollection.contains.

Implements java.util.Set.contains, java.util.Collection.contains.

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 Objects.equals(o, 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 back to summary

isEmpty	back to summary
public boolean isEmpty() Overrides java.util.AbstractCollection.isEmpty. Implements java.util.Set.isEmpty, java.util.Collection.isEmpty. Returns `true` if this set contains no elements. Returns:boolean `true` if this set contains no elements

public boolean isEmpty()

Overrides java.util.AbstractCollection.isEmpty.

Implements java.util.Set.isEmpty, java.util.Collection.isEmpty.

Returns true if this set contains no elements.

Returns:boolean: true if this set contains no elements

iterator back to summary

iterator	back to summary
public Iterator<E> iterator() Implements abstract java.util.AbstractCollection.iterator. Implements java.util.Set.iterator, java.util.Collection.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 `ConcurrentModificationException`

public Iterator<E> iterator()

Implements abstract java.util.AbstractCollection.iterator.

Implements java.util.Set.iterator, java.util.Collection.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: ConcurrentModificationException

newHashSet	back to summary
public static <T> HashSet<T> newHashSet(int numElements) Creates a new, empty HashSet suitable for the expected number of elements. The returned set uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of elements can be added without resizing the set. Parameters <T> the type of elements maintained by the new set numElements:int the expected number of elements Returns:HashSet<T> the newly created set Exceptions IllegalArgumentException: if numElements is negative Since 19

newHashSet

back to summary

public static <T> HashSet<T> newHashSet(int numElements)

Creates a new, empty HashSet suitable for the expected number of elements. The returned set uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of elements can be added without resizing the set.

Parameters

<T>: the type of elements maintained by the new set
numElements:int: the expected number of elements

Returns:HashSet<T>

the newly created set

Exceptions

IllegalArgumentException:: if numElements is negative

Since

readObject back to summary

readObject	back to summary
private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException Reconstitute the `HashSet` instance from a stream (that is, deserialize it). Annotations @Serial

private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException

Reconstitute the HashSet instance from a stream (that is, deserialize it).

Annotations

@Serial

remove back to summary

remove	back to summary
public boolean remove(Object o) Overrides java.util.AbstractCollection.remove. Implements java.util.Set.remove, java.util.Collection.remove. Removes the specified element from this set if it is present. More formally, removes an element `e` such that `Objects.equals(o, 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

public boolean remove(Object o)

Overrides java.util.AbstractCollection.remove.

Implements java.util.Set.remove, java.util.Collection.remove.

Removes the specified element from this set if it is present. More formally, removes an element e such that Objects.equals(o, 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	back to summary
public int size() Implements abstract java.util.AbstractCollection.size. Implements java.util.Set.size, java.util.Collection.size. Returns the number of elements in this set (its cardinality). Returns:int the number of elements in this set (its cardinality)

size

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public int size()

Implements abstract java.util.AbstractCollection.size.

Implements java.util.Set.size, java.util.Collection.size.

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

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

spliterator back to summary

spliterator	back to summary
public Spliterator<E> spliterator() Overrides default java.util.Set.spliterator, java.util.Collection.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. Returns:Spliterator<E> a `Spliterator` over the elements in this set Since 1.8

public Spliterator<E> spliterator()

Overrides default java.util.Set.spliterator, java.util.Collection.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.

Returns:Spliterator<E>: a Spliterator over the elements in this set
Since: 1.8

toArray	back to summary
public Object[] toArray() Overrides java.util.AbstractCollection.toArray. Implements java.util.Set.toArray, java.util.Collection.toArray. Doc from java.util.Set.toArray. Returns an array containing all of the elements in this set. If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array will be "safe" in that no references to it are maintained by this set. (In other words, this method must allocate a new array even if this set is backed by an array). The caller is thus free to modify the returned array. This method acts as bridge between array-based and collection-based APIs. Returns:Object[] an array containing all the elements in this set Annotations @Override

toArray

back to summary

public Object[] toArray()

Overrides java.util.AbstractCollection.toArray.

Implements java.util.Set.toArray, java.util.Collection.toArray.

Doc from java.util.Set.toArray.

Returns an array containing all of the elements in this set. If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

The returned array will be "safe" in that no references to it are maintained by this set. (In other words, this method must allocate a new array even if this set is backed by an array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs.

Returns:Object[]

an array containing all the elements in this set

Annotations

@Override

toArray back to summary

toArray	back to summary
public <T> T[] toArray(T[] a) Overrides java.util.AbstractCollection.toArray. Implements java.util.Set.toArray, java.util.Collection.toArray. Doc from java.util.Set.toArray. Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array. If the set fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this set. If this set fits in the specified array with room to spare (i.e., the array has more elements than this set), the element in the array immediately following the end of the set is set to `null`. (This is useful in determining the length of this set only if the caller knows that this set does not contain any null elements.) If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. Like the `toArray()` method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs. Suppose `x` is a set known to contain only strings. The following code can be used to dump the set into a newly allocated array of `String`: String[] y = x.toArray(new String[0]); Note that `toArray(new Object[0])` is identical in function to `toArray()`. Parameters <T> Doc from java.util.Collection.toArray. the component type of the array to contain the collection a:T[] the array into which the elements of this set are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose. Returns:T[] an array containing all the elements in this set Annotations @Override

public <T> T[] toArray(T[] a)

Overrides java.util.AbstractCollection.toArray.

Implements java.util.Set.toArray, java.util.Collection.toArray.

Doc from java.util.Set.toArray.

Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array. If the set fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this set.

If this set fits in the specified array with room to spare (i.e., the array has more elements than this set), the element in the array immediately following the end of the set is set to null. (This is useful in determining the length of this set only if the caller knows that this set does not contain any null elements.)

If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

Suppose x is a set known to contain only strings. The following code can be used to dump the set into a newly allocated array of String:

    String[] y = x.toArray(new String[0]);

Note that toArray(new Object[0]) is identical in function to toArray().

Parameters

<T>: Doc from java.util.Collection.toArray. the component type of the array to contain the collection
a:T[]: the array into which the elements of this set are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.

Returns:T[]

an array containing all the elements in this set

Annotations

@Override

writeObject back to summary

writeObject	back to summary
private void writeObject(ObjectOutputStream s) throws IOException Save the state of this `HashSet` instance to a stream (that is, serialize it). Annotations @Serial Serial data The capacity of the backing `HashMap` instance (int), and its load factor (float) are emitted, followed by the size of the set (the number of elements it contains) (int), followed by all of its elements (each an Object) in no particular order.

private void writeObject(ObjectOutputStream s) throws IOException

Save the state of this HashSet instance to a stream (that is, serialize it).

Annotations

@Serial

Serial data

The capacity of the backing HashMap instance (int), and its load factor (float) are emitted, followed by the size of the set (the number of elements it contains) (int), followed by all of its elements (each an Object) in no particular order.

public Class HashSet<E>

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