Added in API level 1

HashSet

open class HashSet<E : Any!> : AbstractSet<E>, Cloneable, Serializable, MutableSet<E>

kotlin.Any
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractSet<E>
		↳	java.util.HashSet

Known Direct Subclasses

LinkedHashSet

Hash table and linked list implementation of the Set interface, with well-defined encounter order.

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.

Summary

Public constructors
`HashSet()` Constructs a new, empty set; the backing `HashMap` instance has default initial capacity (16) and load factor (0.75).
`HashSet(initialCapacity: Int)` Constructs a new, empty set; the backing `HashMap` instance has the specified initial capacity and default load factor (0.75).
`HashSet(initialCapacity: Int, loadFactor: Float)` Constructs a new, empty set; the backing `HashMap` instance has the specified initial capacity and the specified load factor.
`HashSet(c: MutableCollection<out E>)` Constructs a new set containing the elements in the specified collection.

Public methods
open Boolean	`add(element: E)` Adds the specified element to this set if it is not already present.
open Unit	`clear()` Removes all of the elements from this set.
open Any	`clone()` Returns a shallow copy of this `HashSet` instance: the elements themselves are not cloned.
open Boolean	`contains(element: E)` Returns `true` if this set contains the specified element.
open Boolean	`contains(element: E)` Returns `true` if this set contains the specified element.
open Boolean	`isEmpty()` Returns `true` if this set contains no elements.
open MutableIterator<E>	`iterator()` Returns an iterator over the elements in this set.
open static HashSet<T>	`newHashSet(numElements: Int)` Creates a new, empty HashSet suitable for the expected number of elements.
open Boolean	`remove(element: E)` Removes the specified element from this set if it is present.
open Boolean	`remove(element: E)` Removes the specified element from this set if it is present.
open Spliterator<E>	`spliterator()` Creates a late-binding and fail-fast `Spliterator` over the elements in this set.
open Array<Any!>	`toArray()` Returns an array containing all of the elements in this collection.
open Array<T>	`toArray(a: Array<T>)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.

Inherited functions

From class AbstractCollection

`Boolean`	`add(element: E)` Ensures that this collection contains the specified element (optional operation). Returns `true` if this collection changed as a result of the call. (Returns `false` if this collection does not permit duplicates and already contains the specified element.) Collections that support this operation may place limitations on what elements may be added to this collection. In particular, some collections will refuse to add `null` elements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added. If a collection refuses to add a particular element for any reason other than that it already contains the element, it must throw an exception (rather than returning `false`). This preserves the invariant that a collection always contains the specified element after this call returns.
`Boolean`	`addAll(elements: Collection<E>)` Adds all of the elements in the specified collection to this collection (optional operation). The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this collection, and this collection is nonempty.) If the specified collection has a defined encounter order, processing of its elements generally occurs in that order.
`Unit`	`clear()` Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns.
`Boolean`	`contains(element: E?)` Returns `true` if this collection contains the specified element. More formally, returns `true` if and only if this collection contains at least one element `e` such that `Objects.equals(o, e)`.
`Boolean`	`containsAll(elements: Collection<E>)` Returns `true` if this collection contains all of the elements in the specified collection.
`Boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`MutableIterator<E>`	`iterator()` Returns an iterator over the elements contained in this collection.
`Boolean`	`remove(element: E?)` Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element `e` such that `Objects.equals(o, e)`, if this collection contains one or more such elements. Returns `true` if this collection contained the specified element (or equivalently, if this collection changed as a result of the call).
`Boolean`	`retainAll(elements: Collection<E>)` Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.
`Array<Any!>`	`toArray()` Returns an array containing all of the elements in this collection. If this collection 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's runtime component type is `Object`. The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array.
`Array<T>`	`toArray(a: Array<T>)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection 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 collection. If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to `null`. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain any `null` elements.) If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.
`String`	`toString()` Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets (`"[]"`). Adjacent elements are separated by the characters `", "` (comma and space). Elements are converted to strings as by `String.valueOf(Object)`.

From class AbstractSet

Boolean

equals(other: Any?)

Compares the specified object with this set for equality. Returns true if the given object is also a set, the two sets have the same size, and every member of the given set is contained in this set. This ensures that the equals method works properly across different implementations of the Set interface.

This implementation first checks if the specified object is this set; if so it returns true. Then, it checks if the specified object is a set whose size is identical to the size of this set; if not, it returns false. If so, it returns containsAll((Collection) o).

Int

hashCode()

Returns the hash code value for this set. The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the hash code of a null element is defined to be zero. This ensures that s1.equals(s2) implies that s1.hashCode()==s2.hashCode() for any two sets s1 and s2, as required by the general contract of Object.hashCode.

This implementation iterates over the set, calling the hashCode method on each element in the set, and adding up the results.

Boolean

removeAll(elements: Collection<E>)

Removes from this set all of its elements that are contained in the specified collection (optional operation). If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.

This implementation determines which is the smaller of this set and the specified collection, by invoking the size method on each. If this set has fewer elements, then the implementation iterates over this set, checking each element returned by the iterator in turn to see if it is contained in the specified collection. If it is so contained, it is removed from this set with the iterator's remove method. If the specified collection has fewer elements, then the implementation iterates over the specified collection, removing from this set each element returned by the iterator, using this set's remove method.

Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method.

Properties
open Int	`size` Returns the number of elements in this set (its cardinality).

Inherited properties

From class AbstractCollection

Int

size

Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

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(initialCapacity: Int)

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

Exceptions
`java.lang.IllegalArgumentException`	if the initial capacity is less than zero

HashSet

Added in API level 1

HashSet(
    initialCapacity: Int, 
    loadFactor: Float)

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

Exceptions
`java.lang.IllegalArgumentException`	if the initial capacity is less than zero, or if the load factor is nonpositive

HashSet

Added in API level 1

HashSet(c: MutableCollection<out E>)

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`	MutableCollection<out E>: the collection whose elements are to be placed into this set

Exceptions
`java.lang.NullPointerException`	if the specified collection is null

Public methods

add

Added in API level 1

open fun add(element: E): Boolean

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`	element to be added to this set

Return
`Boolean`	`true` if this set did not already contain the specified element

Exceptions
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this set
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this set
`java.lang.IllegalStateException`	if the element cannot be added at this time due to insertion restrictions
`java.lang.NullPointerException`	if the specified element is null and this set does not permit null elements
`java.lang.UnsupportedOperationException`	if the `add` operation is not supported by this set

clear

Added in API level 1

open fun clear(): Unit

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

Exceptions
`java.lang.UnsupportedOperationException`	if the `clear` method is not supported by this set

clone

Added in API level 1

open fun clone(): Any

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

Return
`Any`	a shallow copy of this set

Exceptions
`java.lang.CloneNotSupportedException`	if the object's class does not support the `Cloneable` interface. Subclasses that override the `clone` method can also throw this exception to indicate that an instance cannot be cloned.

contains

Added in API level 1

open fun contains(element: E): Boolean

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`	element whose presence in this set is to be tested

Return
`Boolean`	`true` if this set contains the specified element

Exceptions
`java.lang.ClassCastException`	if the type of the specified element is incompatible with this set (optional)
`java.lang.NullPointerException`	if the specified element is null and this set does not permit null elements (optional)

contains

Added in API level 1

open fun contains(element: E): Boolean

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`	element whose presence in this set is to be tested

Return
`Boolean`	`true` if this set contains the specified element

Exceptions
`java.lang.ClassCastException`	if the type of the specified element is incompatible with this set (optional)
`java.lang.NullPointerException`	if the specified element is null and this set does not permit null elements (optional)

isEmpty

Added in API level 1

open fun isEmpty(): Boolean

Returns true if this set contains no elements.

Return
`Boolean`	`true` if this set contains no elements

iterator

Added in API level 1

open fun iterator(): MutableIterator<E>

Returns an iterator over the elements in this set. The elements are returned in no particular order.

Return
`MutableIterator<E>`	an Iterator over the elements in this set

See Also

java.util.ConcurrentModificationException

newHashSet

Added in API level 35

open static fun <T : Any!> newHashSet(numElements: Int): HashSet<T>

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

Return
`HashSet<T>`	the newly created set

Exceptions
`java.lang.IllegalArgumentException`	if numElements is negative

remove

Added in API level 1

open fun remove(element: E): Boolean

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 to be removed from this set, if present

Return
`Boolean`	`true` if the set contained the specified element

Exceptions
`java.lang.ClassCastException`	if the type of the specified element is incompatible with this set (optional)
`java.lang.NullPointerException`	if the specified element is null and this set does not permit null elements (optional)
`java.lang.UnsupportedOperationException`	if the `remove` operation is not supported by this set

remove

Added in API level 1

open fun remove(element: E): Boolean

Parameters
`o`	object to be removed from this set, if present

Return
`Boolean`	`true` if the set contained the specified element

Exceptions
`java.lang.ClassCastException`	if the type of the specified element is incompatible with this set (optional)
`java.lang.NullPointerException`	if the specified element is null and this set does not permit null elements (optional)
`java.lang.UnsupportedOperationException`	if the `remove` operation is not supported by this set

spliterator

Added in API level 24

open fun spliterator(): Spliterator<E>

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.

Return
`Spliterator<E>`	a `Spliterator` over the elements in this set

toArray

Added in API level 1

open fun toArray(): Array<Any!>

Returns an array containing all of the elements in this collection. If this collection 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's runtime component type is Object.

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

Return
`Array<Any!>`	an array containing all the elements in this set

toArray

Added in API level 1

open fun <T : Any!> toArray(a: Array<T>): Array<T>

Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection 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 collection.

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

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

Parameters
`<T>`	the component type of the array to contain the collection
`a`	Array<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.

Return
`Array<T>`	an array containing all the elements in this set

Exceptions
`java.lang.ArrayStoreException`	if the runtime type of the specified array is not a supertype of the runtime type of every element in this set
`java.lang.NullPointerException`	if the specified array is null

Properties

size

Added in API level 1

open val size: Int

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

Return
`Int`	the number of elements in this set (its cardinality)