Added in API level 1

ConcurrentLinkedQueue

open class ConcurrentLinkedQueue<E : Any!> : AbstractQueue<E>, Queue<E>, Serializable

kotlin.Any
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractQueue<E>
		↳	java.util.concurrent.ConcurrentLinkedQueue

An unbounded thread-safe queue based on linked nodes. This queue orders elements FIFO (first-in-first-out). The head of the queue is that element that has been on the queue the longest time. The tail of the queue is that element that has been on the queue the shortest time. New elements are inserted at the tail of the queue, and the queue retrieval operations obtain elements at the head of the queue. A ConcurrentLinkedQueue is an appropriate choice when many threads will share access to a common collection. Like most other concurrent collection implementations, this class does not permit the use of null elements.

This implementation employs an efficient non-blocking algorithm based on one described in Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue Algorithms by Maged M. Michael and Michael L. Scott.

Iterators are weakly consistent, returning elements reflecting the state of the queue at some point at or since the creation of the iterator. They do not throw java.util.ConcurrentModificationException, and may proceed concurrently with other operations. Elements contained in the queue since the creation of the iterator will be returned exactly once.

Beware that, unlike in most collections, the size method is NOT a constant-time operation. Because of the asynchronous nature of these queues, determining the current number of elements requires a traversal of the elements, and so may report inaccurate results if this collection is modified during traversal.

Bulk operations that add, remove, or examine multiple elements, such as #addAll, removeIf or forEach, are not guaranteed to be performed atomically. For example, a forEach traversal concurrent with an addAll operation might observe only some of the added elements.

This class and its iterator implement all of the optional methods of the Queue and Iterator interfaces.

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentLinkedQueue happen-before actions subsequent to the access or removal of that element from the ConcurrentLinkedQueue in another thread.

This class is a member of the Java Collections Framework.

Summary

Public constructors
`ConcurrentLinkedQueue()` Creates a `ConcurrentLinkedQueue` that is initially empty.
`ConcurrentLinkedQueue(c: MutableCollection<out E>!)` Creates a `ConcurrentLinkedQueue` initially containing the elements of the given collection, added in traversal order of the collection's iterator.

Public methods
open Boolean	`add(element: E)` Inserts the specified element at the tail of this queue.
open Boolean	`addAll(elements: Collection<E>)` Appends all of the elements in the specified collection to the end of this queue, in the order that they are returned by the specified collection's iterator.
open Unit	`clear()` Removes all of the elements from this queue.
open Boolean	`contains(element: E?)` Returns `true` if this queue contains the specified element.
open Unit	`forEach(action: Consumer<in E>)`
open Boolean	`isEmpty()` Returns `true` if this queue contains no elements.
open MutableIterator<E>	`iterator()` Returns an iterator over the elements in this queue in proper sequence.
open Boolean	`offer(e: E)` Inserts the specified element at the tail of this queue.
open E?	`peek()` Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
open E?	`poll()` Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
open Boolean	`remove(element: E?)` Removes a single instance of the specified element from this queue, if it is present.
open Boolean	`removeAll(elements: Collection<E>)`
open Boolean	`removeIf(filter: Predicate<in E>)`
open Boolean	`retainAll(elements: Collection<E>)`
open Spliterator<E>	`spliterator()` Returns a `Spliterator` over the elements in this queue.
open Array<Any!>	`toArray()` Returns an array containing all of the elements in this queue, in proper sequence.
open Array<T>	`toArray(a: Array<T>)` Returns an array containing all of the elements in this queue, in proper sequence; the runtime type of the returned array is that of the specified array.
open String	`toString()` Returns a string representation of this collection.

Inherited functions

From class AbstractCollection

`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`	`removeAll(elements: Collection<E>)` Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.
`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 AbstractQueue

E

element()

Retrieves, but does not remove, the head of this queue. This method differs from peek only in that it throws an exception if this queue is empty.

This implementation returns the result of peek unless the queue is empty.

E

remove()

Retrieves and removes the head of this queue. This method differs from poll only in that it throws an exception if this queue is empty.

This implementation returns the result of poll unless the queue is empty.

From class Queue

Boolean

offer(e: E)

Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. When using a capacity-restricted queue, this method is generally preferable to add, which can fail to insert an element only by throwing an exception.

E?

peek()

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

E?

poll()

Retrieves and removes the head of this queue, or returns null if this queue is empty.

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

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

ConcurrentLinkedQueue

Added in API level 1

ConcurrentLinkedQueue()

Creates a ConcurrentLinkedQueue that is initially empty.

ConcurrentLinkedQueue

Added in API level 1

ConcurrentLinkedQueue(c: MutableCollection<out E>!)

Creates a ConcurrentLinkedQueue initially containing the elements of the given collection, added in traversal order of the collection's iterator.

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

Exceptions
`java.lang.NullPointerException`	if the specified collection or any of its elements are null

Public methods

add

Added in API level 1

open fun add(element: E): Boolean

Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never throw IllegalStateException or return false.

Parameters
`e`	the element to add

Return
`Boolean`	`true` (as specified by `Collection.add`)

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

addAll

Added in API level 1

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

Appends all of the elements in the specified collection to the end of this queue, in the order that they are returned by the specified collection's iterator. Attempts to addAll of a queue to itself result in IllegalArgumentException.

Parameters
`c`	the elements to be inserted into this queue

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

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

clear

Added in API level 1

open fun clear(): Unit

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

This implementation repeatedly invokes poll until it returns null.

Exceptions
`java.lang.UnsupportedOperationException`	if the `clear` operation is not supported by this collection

contains

Added in API level 1

open fun contains(element: E?): Boolean

Returns true if this queue contains the specified element. More formally, returns true if and only if this queue contains at least one element e such that o.equals(e).

Parameters
`o`	object to be checked for containment in this queue

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

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

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 queue contains no elements.

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

iterator

Added in API level 1

open fun iterator(): MutableIterator<E>

Returns an iterator over the elements in this queue in proper sequence. The elements will be returned in order from first (head) to last (tail).

The returned iterator is weakly consistent.

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

offer

Added in API level 1

open fun offer(e: E): Boolean

Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never return false.

Parameters
`e`	E: the element to add

Return
`Boolean`	`true` (as specified by `Queue.offer`)

Exceptions
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this queue
`java.lang.IllegalArgumentException`	if some property of this element prevents it from being added to this queue
`java.lang.NullPointerException`	if the specified element is null

peek

Added in API level 1

open fun peek(): E?

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

Return
`E?`	the head of this queue, or `null` if this queue is empty

poll

Added in API level 1

open fun poll(): E?

Retrieves and removes the head of this queue, or returns null if this queue is empty.

Return
`E?`	the head of this queue, or `null` if this queue is empty

remove

Added in API level 1

open fun remove(element: E?): Boolean

Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element e such that o.equals(e), if this queue contains one or more such elements. Returns true if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).

Parameters
`o`	element to be removed from this queue, if present

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

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

removeAll

Added in API level 1

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

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

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

Exceptions
`java.lang.ClassCastException`	if the types of one or more elements in this collection are incompatible with the specified collection (java.util.Collection#)
`java.lang.NullPointerException`	if this collection contains one or more null elements and the specified collection does not support null elements (java.util.Collection#) or if the specified collection is null
`java.lang.UnsupportedOperationException`	if the `removeAll` method is not supported by this collection

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

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

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

Exceptions
`java.lang.ClassCastException`	if the types of one or more elements in this collection are incompatible with the specified collection (java.util.Collection#)
`java.lang.NullPointerException`	if this collection contains one or more null elements and the specified collection does not permit null elements (java.util.Collection#) or if the specified collection is null
`java.lang.UnsupportedOperationException`	if the `retainAll` operation is not supported by this collection

spliterator

Added in API level 24

open fun spliterator(): Spliterator<E>

Returns a Spliterator over the elements in this queue.

The returned spliterator is weakly consistent.

The Spliterator reports Spliterator.CONCURRENT, Spliterator.ORDERED, and Spliterator.NONNULL.

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

toArray

Added in API level 1

open fun toArray(): Array<Any!>

Returns an array containing all of the elements in this queue, in proper sequence.

The returned array will be "safe" in that no references to it are maintained by this queue. (In other words, this method must allocate a new 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 of the elements in this queue

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 queue, in proper sequence; the runtime type of the returned array is that of the specified array. If the queue 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 queue.

If this queue fits in the specified array with room to spare (i.e., the array has more elements than this queue), the element in the array immediately following the end of the queue is set to null.

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 queue known to contain only strings. The following code can be used to dump the queue 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 the queue 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 of the elements in this queue

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 queue
`java.lang.NullPointerException`	if the specified array is null

toString

Added in API level 1

open fun toString(): String

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).

Return
`String`	a string representation of the object.

Properties

size

Added in API level 1

open val size: Int

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

Beware that, unlike in most collections, this method is NOT a constant-time operation. Because of the asynchronous nature of these queues, determining the current number of elements requires an O(n) traversal. Additionally, if elements are added or removed during execution of this method, the returned result may be inaccurate. Thus, this method is typically not very useful in concurrent applications.

Return
`Int`	the number of elements in this queue