Module java.base
Package java.util.concurrent

# Interface TransferQueue<E>

Type Parameters:
  `E` - the type of elements held in this queue

All Superinterfaces:
  `BlockingQueue<E>`, `Collection<E>`, `Iterable<E>`, `Queue<E>`

All Known Implementing Classes:
  `LinkedTransferQueue`

public interface TransferQueue<E> extends BlockingQueue<E>
A `BlockingQueue` in which producers may wait for consumers to receive elements. A `TransferQueue` may be useful for example in message passing applications in which producers sometimes (using method `transfer(E)`) await receipt of elements by consumers invoking `take` or `poll`, while at other times enqueue elements (via method `put`) without waiting for receipt. Non-blocking and time-out versions of `tryTransfer` are also available. A `TransferQueue` may also be queried, via `hasWaitingConsumer()`, whether there are any threads waiting for items, which is a converse analogy to a `peek` operation.

Like other blocking queues, a `TransferQueue` may be capacity bounded. If so, an attempted transfer operation may initially block waiting for available space, and/or subsequently block waiting for reception by a consumer. Note that in a queue with zero capacity, such as `SynchronousQueue`, `put` and `transfer` are effectively synonymous.

This interface is a member of the Java Collections Framework.

Since:
  1.7

- ## Method Summary

  Modifier and Type
  Method
  Description
  `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`.
  `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.

  ### Methods declared in interface java.util.concurrent.BlockingQueue

  `add, contains, drainTo, drainTo, offer, offer, poll, put, remainingCapacity, remove, take`

  ### Methods declared in interface java.util.Collection

  `addAll, clear, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray, toArray`

  ### Methods declared in interface java.lang.Iterable

  `forEach`

  ### Methods declared in interface java.util.Queue

  `element, peek, poll, remove`

- ## Method Details

  - ### tryTransfer

    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 `BlockingQueue.take()` or timed `poll`), otherwise returning `false` without enqueuing the element.

    Parameters:
      `e` - the element to transfer
    Returns:
      `true` if the element was transferred, else `false`
    Throws:
      `ClassCastException` - if the class of the specified element prevents it from being added to this queue
      `NullPointerException` - if the specified element is null
      `IllegalArgumentException` - if some property of the specified element prevents it from being added to this queue

  - ### transfer

    void transfer(E e) throws InterruptedException
    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 `BlockingQueue.take()` or timed `poll`), else waits until the element is received by a consumer.

    Parameters:
      `e` - the element to transfer
    Throws:
      `InterruptedException` - if interrupted while waiting, in which case the element is not left enqueued
      `ClassCastException` - if the class of the specified element prevents it from being added to this queue
      `NullPointerException` - if the specified element is null
      `IllegalArgumentException` - if some property of the specified element prevents it from being added to this queue

  - ### tryTransfer

    boolean tryTransfer(E e, long timeout, TimeUnit unit) throws InterruptedException
    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 `BlockingQueue.take()` or timed `poll`), else 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` - the element to transfer
      `timeout` - how long to wait before giving up, in units of `unit`
      `unit` - a `TimeUnit` determining how to interpret the `timeout` parameter
    Returns:
      `true` if successful, or `false` if the specified waiting time elapses before completion, in which case the element is not left enqueued
    Throws:
      `InterruptedException` - if interrupted while waiting, in which case the element is not left enqueued
      `ClassCastException` - if the class of the specified element prevents it from being added to this queue
      `NullPointerException` - if the specified element is null
      `IllegalArgumentException` - if some property of the specified element prevents it from being added to this queue

  - ### hasWaitingConsumer

    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:
      `true` if there is at least one waiting consumer

  - ### getWaitingConsumerCount

    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:
      the number of consumers waiting to receive elements