Added in API level 1

SynchronousQueue

open class SynchronousQueue<E : Any!> : AbstractQueue<E>, BlockingQueue<E>, Serializable

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

A blocking queue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa. A synchronous queue does not have any internal capacity, not even a capacity of one. You cannot peek at a synchronous queue because an element is only present when you try to remove it; you cannot insert an element (using any method) unless another thread is trying to remove it; you cannot iterate as there is nothing to iterate. The head of the queue is the element that the first queued inserting thread is trying to add to the queue; if there is no such queued thread then no element is available for removal and poll() will return null. For purposes of other Collection methods (for example contains), a SynchronousQueue acts as an empty collection. This queue does not permit null elements.

Synchronous queues are similar to rendezvous channels used in CSP and Ada. They are well suited for handoff designs, in which an object running in one thread must sync up with an object running in another thread in order to hand it some information, event, or task.

This class supports an optional fairness policy for ordering waiting producer and consumer threads. By default, this ordering is not guaranteed. However, a queue constructed with fairness set to true grants threads access in FIFO order.

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

This class is a member of the Java Collections Framework.

Summary

Public constructors
`SynchronousQueue()` Creates a `SynchronousQueue` with nonfair access policy.
`SynchronousQueue(fair: Boolean)` Creates a `SynchronousQueue` with the specified fairness policy.

Public methods
open Unit	`clear()` Does nothing.
open Boolean	`contains(element: E?)` Always returns `false`.
open Boolean	`containsAll(elements: Collection<E>)` Returns `false` unless the given collection is empty.
open Int	`drainTo(c: MutableCollection<in E>!)`
open Int	`drainTo(c: MutableCollection<in E>!, maxElements: Int)`
open Boolean	`isEmpty()` Always returns `true`.
open MutableIterator<E>	`iterator()` Returns an empty iterator in which `hasNext` always returns `false`.
open Boolean	`offer(e: E)` Inserts the specified element into this queue, if another thread is waiting to receive it.
open Boolean	`offer(e: E, timeout: Long, unit: TimeUnit!)` Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.
open E?	`peek()` Always returns `null`.
open E?	`poll()` Retrieves and removes the head of this queue, if another thread is currently making an element available.
open E	`poll(timeout: Long, unit: TimeUnit!)` Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.
open Unit	`put(e: E)` Adds the specified element to this queue, waiting if necessary for another thread to receive it.
open Int	`remainingCapacity()` Always returns zero.
open Boolean	`remove(element: E?)` Always returns `false`.
open Boolean	`removeAll(elements: Collection<E>)` Always returns `false`.
open Boolean	`retainAll(elements: Collection<E>)` Always returns `false`.
open Spliterator<E>	`spliterator()` Returns an empty spliterator in which calls to `trySplit` always return `null`.
open E	`take()` Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
open Array<Any!>	`toArray()` Returns a zero-length array.
open Array<T>	`toArray(a: Array<T>)` Sets the zeroth element of the specified array to `null` (if the array has non-zero length) and returns it.
open String	`toString()` Always returns `"[]"`.

Inherited functions

From class AbstractQueue

`Boolean`	`add(element: E)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available. This implementation returns `true` if `offer` succeeds, else throws an `IllegalStateException`.
`Boolean`	`addAll(elements: Collection<E>)` Adds all of the elements in the specified collection to this queue. Attempts to addAll of a queue to itself result in `IllegalArgumentException`. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress. This implementation iterates over the specified collection, and adds each element returned by the iterator to this queue, in turn. A runtime exception encountered while trying to add an element (including, in particular, a `null` element) may result in only some of the elements having been successfully added when the associated exception is thrown.
`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 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 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` Always returns zero.

Inherited properties

From class AbstractCollection

Int

size

Public constructors

SynchronousQueue

Added in API level 1

SynchronousQueue()

Creates a SynchronousQueue with nonfair access policy.

SynchronousQueue

Added in API level 1

SynchronousQueue(fair: Boolean)

Creates a SynchronousQueue with the specified fairness policy.

Parameters
`fair`	Boolean: if true, waiting threads contend in FIFO order for access; otherwise the order is unspecified.

Public methods

clear

Added in API level 1

open fun clear(): Unit

Does nothing. A SynchronousQueue has no internal capacity.

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

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
`o`	the element

Return
`Boolean`	`false`

Exceptions
`java.lang.ClassCastException`	if the class of the specified element is incompatible with this queue (optional)
`java.lang.NullPointerException`	if the specified element is null (optional)

containsAll

Added in API level 1

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

Returns false unless the given collection is empty. A SynchronousQueue has no internal capacity.

Parameters
`c`	the collection

Return
`Boolean`	`false` unless given collection is empty

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

drainTo

Added in API level 1

open fun drainTo(c: MutableCollection<in E>!): Int

Parameters
`c`	MutableCollection<in E>!: the collection to transfer elements into

Return
`Int`	the number of elements transferred

Exceptions
`java.lang.UnsupportedOperationException`	if addition of elements is not supported by the specified collection
`java.lang.ClassCastException`	if the class of an element of this queue prevents it from being added to the specified collection
`java.lang.NullPointerException`	if the specified collection is null
`java.lang.IllegalArgumentException`	if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection

drainTo

Added in API level 1

open fun drainTo(
    c: MutableCollection<in E>!, 
    maxElements: Int
): Int

Parameters
`c`	MutableCollection<in E>!: the collection to transfer elements into
`maxElements`	Int: the maximum number of elements to transfer

Return
`Int`	the number of elements transferred

Exceptions
`java.lang.UnsupportedOperationException`	if addition of elements is not supported by the specified collection
`java.lang.ClassCastException`	if the class of an element of this queue prevents it from being added to the specified collection
`java.lang.NullPointerException`	if the specified collection is null
`java.lang.IllegalArgumentException`	if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection

isEmpty

Added in API level 1

open fun isEmpty(): Boolean

Always returns true. A SynchronousQueue has no internal capacity.

Return
`Boolean`	`true`

iterator

Added in API level 1

open fun iterator(): MutableIterator<E>

Returns an empty iterator in which hasNext always returns false.

Return
`MutableIterator<E>`	an empty iterator

offer

Added in API level 1

open fun offer(e: E): Boolean

Inserts the specified element into this queue, if another thread is waiting to receive it.

Parameters
`e`	E: the element to add

Return
`Boolean`	`true` if the element was added to this queue, else `false`

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 the specified element prevents it from being added to this queue

offer

Added in API level 1

open fun offer(
    e: E, 
    timeout: Long, 
    unit: TimeUnit!
): Boolean

Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.

Parameters
`e`	E: the element to add
`timeout`	Long: how long to wait before giving up, in units of `unit`
`unit`	TimeUnit!: a `TimeUnit` determining how to interpret the `timeout` parameter

Return
`Boolean`	`true` if successful, or `false` if the specified waiting time elapses before a consumer appears

Exceptions
`java.lang.InterruptedException`	if interrupted while waiting
`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 the specified element prevents it from being added to this queue

peek

Added in API level 1

open fun peek(): E?

Always returns null. A SynchronousQueue does not return elements unless actively waited on.

Return
`E?`	`null`

poll

Added in API level 1

open fun poll(): E?

Retrieves and removes the head of this queue, if another thread is currently making an element available.

Return
`E?`	the head of this queue, or `null` if no element is available

poll

Added in API level 1

open fun poll(
    timeout: Long, 
    unit: TimeUnit!
): E

Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.

Parameters
`timeout`	Long: how long to wait before giving up, in units of `unit`
`unit`	TimeUnit!: a `TimeUnit` determining how to interpret the `timeout` parameter

Return
`E`	the head of this queue, or `null` if the specified waiting time elapses before an element is present

Exceptions
`java.lang.InterruptedException`	if interrupted while waiting

put

Added in API level 1

open fun put(e: E): Unit

Adds the specified element to this queue, waiting if necessary for another thread to receive it.

Parameters
`e`	E: the element to add

Exceptions
`java.lang.InterruptedException`	if interrupted while waiting
`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 the specified element prevents it from being added to this queue

remainingCapacity

Added in API level 1

open fun remainingCapacity(): Int

Always returns zero. A SynchronousQueue has no internal capacity.

Return
`Int`	zero

remove

Added in API level 1

open fun remove(element: E?): Boolean

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
`o`	the element to remove

Return
`Boolean`	`false`

Exceptions
`java.lang.ClassCastException`	if the class of the specified element is incompatible with this queue (optional)
`java.lang.NullPointerException`	if the specified element is null (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

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
`c`	the collection

Return
`Boolean`	`false`

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 (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

retainAll

Added in API level 1

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

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
`c`	the collection

Return
`Boolean`	`false`

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 (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

spliterator

Added in API level 24

open fun spliterator(): Spliterator<E>

Returns an empty spliterator in which calls to trySplit always return null.

Return
`Spliterator<E>`	an empty spliterator

take

Added in API level 1

open fun take(): E

Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.

Return
`E`	the head of this queue

Exceptions
`java.lang.InterruptedException`	if interrupted while waiting

toArray

Added in API level 1

open fun toArray(): Array<Any!>

Returns a zero-length array.

Return
`Array<Any!>`	a zero-length array

toArray

Added in API level 1

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

Sets the zeroth element of the specified array to null (if the array has non-zero length) and returns it.

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

Return
`Array<T>`	the specified array

Exceptions
`java.lang.ArrayStoreException`	if the runtime type of any element in this collection is not assignable to the runtime component type of the specified array
`java.lang.NullPointerException`	if the specified array is null

toString

Added in API level 1

open fun toString(): String

Always returns "[]".

Return
`String`	`"[]"`

Properties

size

Added in API level 1

open val size: Int

Always returns zero. A SynchronousQueue has no internal capacity.

Return
`Int`	zero