Added in API level 1

CopyOnWriteArraySet

open class CopyOnWriteArraySet<E : Any!> : AbstractSet<E>, Serializable

kotlin.Any
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractSet<E>
		↳	java.util.concurrent.CopyOnWriteArraySet

A Set that uses an internal CopyOnWriteArrayList for all of its operations. Thus, it shares the same basic properties:

It is best suited for applications in which set sizes generally stay small, read-only operations vastly outnumber mutative operations, and you need to prevent interference among threads during traversal.
It is thread-safe.
Mutative operations (add, set, remove, etc.) are expensive since they usually entail copying the entire underlying array.
Iterators do not support the mutative remove operation.
Traversal via iterators is fast and cannot encounter interference from other threads. Iterators rely on unchanging snapshots of the array at the time the iterators were constructed.

Sample Usage. The following code sketch uses a copy-on-write set to maintain a set of Handler objects that perform some action upon state updates.

<code>class Handler { void handle() { ... } }
 
  class X {
    private final CopyOnWriteArraySet&lt;Handler&gt; handlers
      = new CopyOnWriteArraySet&lt;&gt;();
    public void addHandler(Handler h) { handlers.add(h); }
 
    private long internalState;
    private synchronized void changeState() { internalState = ...; }
 
    public void update() {
      changeState();
      for (Handler handler : handlers)
        handler.handle();
    }
  }</code>

This class is a member of the Java Collections Framework.

Summary

Public constructors
`CopyOnWriteArraySet()` Creates an empty set.
`CopyOnWriteArraySet(c: MutableCollection<out E>!)` Creates a set containing all of the elements of the specified collection.

Public methods
open Boolean	`add(element: E)` Adds the specified element to this set if it is not already present.
open Boolean	`addAll(elements: Collection<E>)` Adds all of the elements in the specified collection to this set if they're not already present.
open Unit	`clear()` Removes all of the elements from this set.
open Boolean	`contains(element: E?)` Returns `true` if this set contains the specified element.
open Boolean	`containsAll(elements: Collection<E>)` Returns `true` if this set contains all of the elements of the specified collection.
open Boolean	`equals(other: Any?)` Compares the specified object with this set for equality.
open Unit	`forEach(action: Consumer<in E>)`
open Boolean	`isEmpty()` Returns `true` if this set contains no elements.
open MutableIterator<E>	`iterator()` Returns an iterator over the elements contained in this set in the order in which these elements were added.
open Boolean	`remove(element: E?)` Removes the specified element from this set if it is present.
open Boolean	`removeAll(elements: Collection<E>)` Removes from this set all of its elements that are contained in the specified collection.
open Boolean	`removeIf(filter: Predicate<in E>)`
open Boolean	`retainAll(elements: Collection<E>)` Retains only the elements in this set that are contained in the specified collection.
open Spliterator<E>	`spliterator()` Returns a `Spliterator` over the elements in this set in the order in which these elements were added.
open Array<Any!>	`toArray()` Returns an array containing all of the elements in this set.
open Array<T>	`toArray(a: Array<T>)` Returns an array containing all of the elements in this set; 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

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.

Properties
open Int	`size` Returns the number of elements in this set.

Inherited properties

From class AbstractCollection

Int

size

Public constructors

CopyOnWriteArraySet

Added in API level 1

CopyOnWriteArraySet()

Creates an empty set.

CopyOnWriteArraySet

Added in API level 1

CopyOnWriteArraySet(c: MutableCollection<out E>!)

Creates a set containing all of the elements of the specified collection.

Parameters
`c`	MutableCollection<out E>!: the collection of elements to initially contain

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 the 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

addAll

Added in API level 1

open fun addAll(elements: Collection<E>): Boolean

Adds all of the elements in the specified collection to this set if they're not already present. If the specified collection is also a set, the addAll operation effectively modifies this set so that its value is the union of the two sets. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

Parameters
`c`	collection containing elements to be added to this set

Return
`Boolean`	`true` if this set changed as a result of the call

Exceptions
`java.lang.UnsupportedOperationException`	if the `addAll` operation is not supported by this set
`java.lang.ClassCastException`	if the class of an element of the specified collection prevents it from being added to this set
`java.lang.NullPointerException`	if the specified collection is null
`java.lang.IllegalArgumentException`	if some property of an element of the specified collection prevents it from being added to this set
`java.lang.IllegalStateException`	if not all the elements can be added at this time due to insertion restrictions

See Also

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

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)

containsAll

Added in API level 1

open fun containsAll(elements: Collection<E>): Boolean

Returns true if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns true if it is a subset of this set.

Parameters
`c`	collection to be checked for containment in this set

Return
`Boolean`	`true` if this set contains all of the elements of the specified collection

Exceptions
`java.lang.ClassCastException`	if the types of one or more elements in the specified collection are incompatible with this set (optional)
`java.lang.NullPointerException`	if the specified collection is null

See Also

equals

Added in API level 1

open fun equals(other: Any?): Boolean

Compares the specified object with this set for equality. Returns true if the specified object is the same object as this object, or if it is also a Set and the elements returned by an java.util.Set#iterator() over the specified set are the same as the elements returned by an iterator over this set. More formally, the two iterators are considered to return the same elements if they return the same number of elements and for every element e1 returned by the iterator over the specified set, there is an element e2 returned by the iterator over this set such that Objects.equals(e1, e2).

Parameters
`obj`	the reference object with which to compare.
`o`	object to be compared for equality with this set

Return
`Boolean`	`true` if the specified object is equal to this set

forEach

Added in API level 24

open fun forEach(action: Consumer<in E>): Unit

Parameters
`action`	Consumer<in E>: The action to be performed for each element

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

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 contained in this set in the order in which these elements were added.

The returned iterator provides a snapshot of the state of the set when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove method.

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

removeAll

Added in API level 1

open fun removeAll(elements: Collection<E>): Boolean

Removes from this set all of its elements that are contained in the specified collection. 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.

Parameters
`c`	collection containing elements to be removed from this set

Return
`Boolean`	`true` if this set changed as a result of the call

Exceptions
`java.lang.UnsupportedOperationException`	if the `removeAll` operation is not supported by this set
`java.lang.ClassCastException`	if the class of an element of this set is incompatible with the specified collection (optional)
`java.lang.NullPointerException`	if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null

See Also

removeIf

Added in API level 24

open fun removeIf(filter: Predicate<in E>): Boolean

Parameters
`filter`	Predicate<in E>: a predicate which returns `true` for elements to be removed

Return
`Boolean`	`true` if any elements were removed

Exceptions
`java.lang.NullPointerException`	if the specified filter is null
`java.lang.UnsupportedOperationException`	if elements cannot be removed from this collection. Implementations may throw this exception if a matching element cannot be removed or if, in general, removal is not supported.

retainAll

Added in API level 1

open fun retainAll(elements: Collection<E>): Boolean

Retains only the elements in this set that are contained in the specified collection. In other words, removes from this set all of its elements that are not contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the intersection of the two sets.

Parameters
`c`	collection containing elements to be retained in this set

Return
`Boolean`	`true` if this set changed as a result of the call

Exceptions
`java.lang.UnsupportedOperationException`	if the `retainAll` operation is not supported by this set
`java.lang.ClassCastException`	if the class of an element of this set is incompatible with the specified collection (optional)
`java.lang.NullPointerException`	if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null

See Also

spliterator

Added in API level 24

open fun spliterator(): Spliterator<E>

Returns a Spliterator over the elements in this set in the order in which these elements were added.

The Spliterator reports Spliterator.IMMUTABLE, Spliterator.DISTINCT, Spliterator.SIZED, and Spliterator.SUBSIZED.

The spliterator provides a snapshot of the state of the set when the spliterator was constructed. No synchronization is needed while operating on the spliterator.

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 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.

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 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:

<code>String[] y = x.toArray(new String[0]);</code>

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

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.

Return
`Int`	the number of elements in this set