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

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

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

Constructs a new set containing the elements in the specified collection.

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(initialCapacity: Int)

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

Public methods
open Boolean
add(element: E)

Adds the specified element to this set if it is not already present.

open Unit

Removes all of the elements from this set.

open Any

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

Returns true if this set contains no elements.

open MutableIterator<E>

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>

Creates a late-binding and fail-fast Spliterator over the elements in this set.

open Array<Any!>

open Array<T>
toArray(a: Array<T>)

Inherited functions
Properties
open Int

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

Inherited properties

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(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

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(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

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.UnsupportedOperationException if the add operation is not supported by this set
java.lang.ClassCastException if the class of the specified element prevents it from being added to this set
java.lang.NullPointerException if the specified element is null and this set does not permit null elements
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

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

newHashSet

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

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

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!>
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>
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)