java.util.concurrent

## Class LinkedTransferQueue<E>

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

- Type Parameters:
    `E` - the type of elements held in this collection
  All Implemented Interfaces:
    Serializable, Iterable<E>, Collection<E>, BlockingQueue<E>, TransferQueue<E>, Queue<E>

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

  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.

  This class is a member of the Java Collections Framework.

  Since:
    1.7
  Author:
    Doug Lea
  See Also:
    Serialized Form

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `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.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `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)` Removes all available elements from this queue and adds them to the given collection.
    `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` | `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)` Inserts the specified element at the tail of this queue.
    `boolean` | `offer(E e, long timeout, TimeUnit unit)` 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()` Retrieves and removes 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.
    `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.
    `E` | `take()` Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
    `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 inherited from class java.util.AbstractQueue

      `addAll, clear, element, remove`

    - ### Methods inherited from class java.util.AbstractCollection

      `containsAll, removeAll, retainAll, toArray, toArray, toString`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.Queue

      `element, remove`

    - ### Methods inherited from interface java.util.Collection

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

    - ### Methods inherited from interface java.lang.Iterable

      `forEach`

- - ### Constructor Detail

    - #### LinkedTransferQueue

          public LinkedTransferQueue()

      Creates an initially empty `LinkedTransferQueue`.

    - #### LinkedTransferQueue

          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` - the collection of elements to initially contain
      Throws:
        `NullPointerException` - if the specified collection or any of its elements are null

  - ### Method Detail

    - #### put

          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.
      Specified by:
        `put` in interface `BlockingQueue<E>`
      Parameters:
        `e` - the element to add
      Throws:
        `NullPointerException` - if the specified element is null

    - #### offer

          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`.
      Specified by:
        `offer` in interface `BlockingQueue<E>`
      Parameters:
        `e` - the element to add
        `timeout` - how long to wait before giving up, in units of `unit`
        `unit` - a `TimeUnit` determining how to interpret the `timeout` parameter
      Returns:
        `true` (as specified by `BlockingQueue.offer`)
      Throws:
        `NullPointerException` - if the specified element is null

    - #### offer

          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`.
      Specified by:
        `offer` in interface `BlockingQueue<E>`
      Specified by:
        `offer` in interface `Queue<E>`
      Parameters:
        `e` - the element to add
      Returns:
        `true` (as specified by `Queue.offer(E)`)
      Throws:
        `NullPointerException` - if the specified element is null

    - #### add

          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`.
      Specified by:
        `add` in interface `Collection<E>`
      Specified by:
        `add` in interface `BlockingQueue<E>`
      Specified by:
        `add` in interface `Queue<E>`
      Overrides:
        `add` in class `AbstractQueue<E>`
      Parameters:
        `e` - the element to add
      Returns:
        `true` (as specified by `Collection.add(E)`)
      Throws:
        `NullPointerException` - if the specified element is null

    - #### tryTransfer

          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.

      Specified by:
        `tryTransfer` in interface `TransferQueue<E>`
      Parameters:
        `e` - the element to transfer
      Returns:
        `true` if the element was transferred, else `false`
      Throws:
        `NullPointerException` - if the specified element is null

    - #### transfer

          public 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 `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.

      Specified by:
        `transfer` in interface `TransferQueue<E>`
      Parameters:
        `e` - the element to transfer
      Throws:
        `NullPointerException` - if the specified element is null
        `InterruptedException` - if interrupted while waiting, in which case the element is not left enqueued

    - #### tryTransfer

          public 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 `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.

      Specified by:
        `tryTransfer` in interface `TransferQueue<E>`
      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:
        `NullPointerException` - if the specified element is null
        `InterruptedException` - if interrupted while waiting, in which case the element is not left enqueued

    - #### take

          public E take()
                 throws InterruptedException

      Description copied from interface: `BlockingQueue`
      Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
      Specified by:
        `take` in interface `BlockingQueue<E>`
      Returns:
        the head of this queue
      Throws:
        `InterruptedException` - if interrupted while waiting

    - #### poll

          public E poll(long timeout,
                        TimeUnit unit)
                 throws InterruptedException

      Description copied from interface: `BlockingQueue`
      Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
      Specified by:
        `poll` in interface `BlockingQueue<E>`
      Parameters:
        `timeout` - how long to wait before giving up, in units of `unit`
        `unit` - a `TimeUnit` determining how to interpret the `timeout` parameter
      Returns:
        the head of this queue, or `null` if the specified waiting time elapses before an element is available
      Throws:
        `InterruptedException` - if interrupted while waiting

    - #### poll

          public E poll()

      Description copied from interface: `Queue`
      Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
      Specified by:
        `poll` in interface `Queue<E>`
      Returns:
        the head of this queue, or `null` if this queue is empty

    - #### drainTo

          public int drainTo(Collection<? super E> c)

      Description copied from interface: `BlockingQueue`
      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.
      Specified by:
        `drainTo` in interface `BlockingQueue<E>`
      Parameters:
        `c` - the collection to transfer elements into
      Returns:
        the number of elements transferred
      Throws:
        `NullPointerException` - if the specified collection is null
        `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

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

      Description copied from interface: `BlockingQueue`
      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.
      Specified by:
        `drainTo` in interface `BlockingQueue<E>`
      Parameters:
        `c` - the collection to transfer elements into
        `maxElements` - the maximum number of elements to transfer
      Returns:
        the number of elements transferred
      Throws:
        `NullPointerException` - if the specified collection is null
        `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

    - #### iterator

          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 a "weakly consistent" iterator that will never throw `ConcurrentModificationException`, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

      Specified by:
        `iterator` in interface `Iterable<E>`
      Specified by:
        `iterator` in interface `Collection<E>`
      Specified by:
        `iterator` in class `AbstractCollection<E>`
      Returns:
        an iterator over the elements in this queue in proper sequence

    - #### peek

          public E peek()

      Description copied from interface: `Queue`
      Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
      Specified by:
        `peek` in interface `Queue<E>`
      Returns:
        the head of this queue, or `null` if this queue is empty

    - #### isEmpty

          public boolean isEmpty()

      Returns `true` if this queue contains no elements.
      Specified by:
        `isEmpty` in interface `Collection<E>`
      Overrides:
        `isEmpty` in class `AbstractCollection<E>`
      Returns:
        `true` if this queue contains no elements

    - #### hasWaitingConsumer

          public boolean hasWaitingConsumer()

      Description copied from interface: `TransferQueue`
      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.
      Specified by:
        `hasWaitingConsumer` in interface `TransferQueue<E>`
      Returns:
        `true` if there is at least one waiting consumer

    - #### size

          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.

      Specified by:
        `size` in interface `Collection<E>`
      Specified by:
        `size` in class `AbstractCollection<E>`
      Returns:
        the number of elements in this queue

    - #### getWaitingConsumerCount

          public int getWaitingConsumerCount()

      Description copied from interface: `TransferQueue`
      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 `TransferQueue.hasWaitingConsumer()`.
      Specified by:
        `getWaitingConsumerCount` in interface `TransferQueue<E>`
      Returns:
        the number of consumers waiting to receive elements

    - #### remove

          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).
      Specified by:
        `remove` in interface `Collection<E>`
      Specified by:
        `remove` in interface `BlockingQueue<E>`
      Overrides:
        `remove` in class `AbstractCollection<E>`
      Parameters:
        `o` - element to be removed from this queue, if present
      Returns:
        `true` if this queue changed as a result of the call

    - #### contains

          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)`.
      Specified by:
        `contains` in interface `Collection<E>`
      Specified by:
        `contains` in interface `BlockingQueue<E>`
      Overrides:
        `contains` in class `AbstractCollection<E>`
      Parameters:
        `o` - object to be checked for containment in this queue
      Returns:
        `true` if this queue contains the specified element

    - #### remainingCapacity

          public int remainingCapacity()

      Always returns `Integer.MAX_VALUE` because a `LinkedTransferQueue` is not capacity constrained.
      Specified by:
        `remainingCapacity` in interface `BlockingQueue<E>`
      Returns:
        `Integer.MAX_VALUE` (as specified by `BlockingQueue.remainingCapacity()`)

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