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LinkedTransferQueue

public class LinkedTransferQueue
extends AbstractQueue<E> implements TransferQueue<E>, Serializable

java.lang.Object
   ↳ java.util.AbstractCollection<E>
     ↳ java.util.AbstractQueue<E>
       ↳ java.util.concurrent.LinkedTransferQueue<E>


An unbounded TransferQueue based on linked nodes. This queue orders elements FIFO (first-in-first-out) with respect to any given producer. The head of the queue is that element that has been on the queue the longest time for some producer. The tail of the queue is that element that has been on the queue the shortest time for some producer.

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. Additionally, the bulk operations addAll, removeAll, retainAll, containsAll, equals, and toArray are not guaranteed to be performed atomically. For example, an iterator operating concurrently with an addAll operation might view only some of the added elements.

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

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

Summary

Public constructors

LinkedTransferQueue()

Creates an initially empty LinkedTransferQueue.

LinkedTransferQueue(Collection<? extends E> c)

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

Public methods

boolean add(E e)

Inserts the specified element at the tail of this queue.

boolean contains(Object o)

Returns true if this queue contains the specified element.

int drainTo(Collection<? super E> c, int maxElements)

Removes at most the given number of available elements from this queue and adds them to the given collection.

int drainTo(Collection<? super E> c)

Removes all available elements from this queue and adds them to the given collection.

int getWaitingConsumerCount()

Returns an estimate of the number of consumers waiting to receive elements via BlockingQueue.take() or timed poll.

boolean hasWaitingConsumer()

Returns true if there is at least one consumer waiting to receive an element via BlockingQueue.take() or timed poll.

boolean isEmpty()

Returns true if this queue contains no elements.

Iterator<E> iterator()

Returns an iterator over the elements in this queue in proper sequence.

boolean offer(E e, long timeout, TimeUnit unit)

Inserts the specified element at the tail of this queue.

boolean offer(E e)

Inserts the specified element at the tail of this queue.

E peek()

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

E poll(long timeout, TimeUnit unit)

Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.

E poll()

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

void put(E e)

Inserts the specified element at the tail of this queue.

int remainingCapacity()

Always returns Integer.MAX_VALUE because a LinkedTransferQueue is not capacity constrained.

boolean remove(Object o)

Removes a single instance of the specified element from this queue, if it is present.

int size()

Returns the number of elements in this queue.

Spliterator<E> spliterator()

Returns a Spliterator over the elements in this queue.

E take()

Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

Object[] toArray()

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

<T> T[] toArray(T[] a)

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.

String toString()

Returns a string representation of this collection.

void transfer(E e)

Transfers the element to a consumer, waiting if necessary to do so.

boolean tryTransfer(E e)

Transfers the element to a waiting consumer immediately, if possible.

boolean tryTransfer(E e, long timeout, TimeUnit unit)

Transfers the element to a consumer if it is possible to do so before the timeout elapses.

Inherited methods

Public constructors

LinkedTransferQueue

added in API level 21
public LinkedTransferQueue ()

Creates an initially empty LinkedTransferQueue.

LinkedTransferQueue

added in API level 21
public LinkedTransferQueue (Collection<? extends E> c)

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

Parameters
c Collection: the collection of elements to initially contain

Throws
NullPointerException if the specified collection or any of its elements are null

Public methods

add

added in API level 21
public boolean add (E e)

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 E: the element to add

Returns
boolean true (as specified by Collection.add(E))

Throws
NullPointerException if the specified element is null

contains

added in API level 21
public boolean contains (Object o)

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: object to be checked for containment in this queue

Returns
boolean true if this queue contains the specified element

drainTo

added in API level 21
public int drainTo (Collection<? super E> c, 
                int maxElements)

Removes at most the given number of available elements from this queue and adds them to the given collection. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain 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.

Parameters
c Collection: the collection to transfer elements into

maxElements int: the maximum number of elements to transfer

Returns
int the number of elements transferred

Throws
NullPointerException
IllegalArgumentException

drainTo

added in API level 21
public int drainTo (Collection<? super E> c)

Removes all available elements from this queue and adds them to the given collection. This operation may be more efficient than repeatedly polling this queue. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain 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.

Parameters
c Collection: the collection to transfer elements into

Returns
int the number of elements transferred

Throws
NullPointerException
IllegalArgumentException

getWaitingConsumerCount

added in API level 21
public int getWaitingConsumerCount ()

Returns an estimate of the number of consumers waiting to receive elements via BlockingQueue.take() or timed poll. The return value is an approximation of a momentary state of affairs, that may be inaccurate if consumers have completed or given up waiting. The value may be useful for monitoring and heuristics, but not for synchronization control. Implementations of this method are likely to be noticeably slower than those for hasWaitingConsumer().

Returns
int the number of consumers waiting to receive elements

hasWaitingConsumer

added in API level 21
public boolean hasWaitingConsumer ()

Returns true if there is at least one consumer waiting to receive an element via BlockingQueue.take() or timed poll. The return value represents a momentary state of affairs.

Returns
boolean true if there is at least one waiting consumer

isEmpty

added in API level 21
public boolean isEmpty ()

Returns true if this queue contains no elements.

Returns
boolean true if this queue contains no elements

iterator

added in API level 21
public Iterator<E> iterator ()

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.

Returns
Iterator<E> an iterator over the elements in this queue in proper sequence

offer

added in API level 21
public boolean offer (E e, 
                long timeout, 
                TimeUnit unit)

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

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

Returns
boolean true (as specified by BlockingQueue.offer)

Throws
NullPointerException if the specified element is null

offer

added in API level 21
public boolean offer (E e)

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

Returns
boolean true (as specified by Queue.offer(E))

Throws
NullPointerException if the specified element is null

peek

added in API level 21
public E peek ()

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

Returns
E the head of this queue, or null if this queue is empty

poll

added in API level 21
public E poll (long timeout, 
                TimeUnit unit)

Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.

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

Returns
E the head of this queue, or null if the specified waiting time elapses before an element is available

Throws
InterruptedException

poll

added in API level 21
public E poll ()

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

Returns
E the head of this queue, or null if this queue is empty

put

added in API level 21
public void put (E e)

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

Parameters
e E: the element to add

Throws
NullPointerException if the specified element is null

remainingCapacity

added in API level 21
public int remainingCapacity ()

Always returns Integer.MAX_VALUE because a LinkedTransferQueue is not capacity constrained.

Returns
int Integer.MAX_VALUE (as specified by BlockingQueue.remainingCapacity)

remove

added in API level 21
public boolean remove (Object o)

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 Object: element to be removed from this queue, if present

Returns
boolean true if this queue changed as a result of the call

size

added in API level 21
public int size ()

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.

Returns
int the number of elements in this queue

spliterator

added in API level 24
public Spliterator<E> spliterator ()

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.

Implementation Note:
  • The Spliterator implements trySplit to permit limited parallelism.
Returns
Spliterator<E> a Spliterator over the elements in this queue

take

added in API level 21
public E take ()

Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

Returns
E the head of this queue

Throws
InterruptedException

toArray

added in API level 21
public Object[] toArray ()

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.

Returns
Object[] an array containing all of the elements in this queue

toArray

added in API level 21
public T[] toArray (T[] a)

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:

 String[] y = x.toArray(new String[0]);
Note that toArray(new Object[0]) is identical in function to toArray().

Parameters
a 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

Returns
T[] an array containing all of the elements in this queue

Throws
ArrayStoreException if the runtime type of the specified array is not a supertype of the runtime type of every element in this queue
NullPointerException if the specified array is null

toString

added in API level 21
public 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).

Returns
String a string representation of this collection

transfer

added in API level 21
public void transfer (E e)

Transfers the element to a consumer, waiting if necessary to do so.

More precisely, transfers the specified element immediately if there exists a consumer already waiting to receive it (in take() or timed poll), else inserts the specified element at the tail of this queue and waits until the element is received by a consumer.

Parameters
e E: the element to transfer

Throws
NullPointerException if the specified element is null
InterruptedException

tryTransfer

added in API level 21
public boolean tryTransfer (E e)

Transfers the element to a waiting consumer immediately, if possible.

More precisely, transfers the specified element immediately if there exists a consumer already waiting to receive it (in take() or timed poll), otherwise returning false without enqueuing the element.

Parameters
e E: the element to transfer

Returns
boolean true if the element was transferred, else false

Throws
NullPointerException if the specified element is null

tryTransfer

added in API level 21
public boolean tryTransfer (E e, 
                long timeout, 
                TimeUnit unit)

Transfers the element to a consumer if it is possible to do so before the timeout elapses.

More precisely, transfers the specified element immediately if there exists a consumer already waiting to receive it (in take() or timed poll), else inserts the specified element at the tail of this queue and waits until the element is received by a consumer, returning false if the specified wait time elapses before the element can be transferred.

Parameters
e E: the element to transfer

timeout long: how long to wait before giving up, in units of unit

unit TimeUnit: a TimeUnit determining how to interpret the timeout parameter

Returns
boolean true if successful, or false if the specified waiting time elapses before completion, in which case the element is not left enqueued

Throws
NullPointerException if the specified element is null
InterruptedException