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 , 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

Creates a ConcurrentLinkedQueue that is initially empty.

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

open Boolean
contains(element: E?)

Returns true if this queue contains the specified element.

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

open Boolean

Returns true if this queue contains no elements.

open MutableIterator<E>

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?

open E?

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>

Returns a Spliterator over the elements in this queue.

open Array<Any!>

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

Inherited functions
Properties
open Int

Returns the number of elements in this queue.

Inherited properties

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.UnsupportedOperationException if the add operation is not supported by this collection
java.lang.ClassCastException if the class of the specified element prevents it from being added to this queue
java.lang.NullPointerException if the specified element is null
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

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.UnsupportedOperationException if the addAll operation is not supported by this collection
java.lang.ClassCastException if the class of an element of the specified collection prevents it from being added to this collection
java.lang.NullPointerException if the specified collection or any of its elements are null
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

clear

Added in API level 1
open fun clear(): Unit
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 (optional)
java.lang.NullPointerException if the specified element is null and this collection does not permit null elements (optional)

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

peek

Added in API level 1
open fun peek(): E?
Return
E? the head of this queue, or null if this queue is empty

poll

Added in API level 1
open fun poll(): E?
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 (optional)
java.lang.NullPointerException if the specified element is null and this collection does not permit null elements (optional)
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.UnsupportedOperationException if the removeAll method is not supported by this collection
java.lang.ClassCastException if the types of one or more elements in this collection are incompatible with the specified collection (optional)
java.lang.NullPointerException if this collection contains one or more null elements and the specified collection does not support null elements (optional), or if the specified collection is null

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

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
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