Module java.base
Package 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 queue

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.

Bulk operations that add, remove, or examine multiple elements, such as `AbstractQueue.addAll(java.util.Collection<? extends E>)`, `removeIf(java.util.function.Predicate<? super E>)` or `forEach(java.util.function.Consumer<? super E>)`, 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 `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
See Also:
  - Serialized Form

- ## Constructor Summary

  Constructors
  Constructor
  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

  Modifier and Type
  Method
  Description
  `boolean`
  `add(E e)`
  Inserts the specified element at the tail of this queue.
  `void`
  `clear()`
  Removes all of the elements from 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.
  `void`
  `forEach(Consumer<? super E> action)`
  Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
  `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.
  `boolean`
  `removeAll(Collection<?> c)`
  Removes all of this collection's elements that are also contained in the specified collection (optional operation).
  `boolean`
  `removeIf(Predicate<? super E> filter)`
  Removes all of the elements of this collection that satisfy the given predicate.
  `boolean`
  `retainAll(Collection<?> c)`
  Retains only the elements in this collection that are contained in the specified collection (optional operation).
  `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.
  `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 class java.util.AbstractQueue

  `addAll, clear, element, remove`

  ### Methods declared in class java.util.AbstractCollection

  `containsAll, toString`

  ### Methods declared in class java.lang.Object

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

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

  `addAll, containsAll, equals, hashCode, parallelStream, stream, toArray`

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

  `element, remove`

- ## Constructor Details

  - ### 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 Details

  - ### toArray

    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.

    Specified by:
      `toArray` in interface `Collection<E>`
    Overrides:
      `toArray` in class `AbstractCollection<E>`
    Returns:
      an array containing all of the elements in this queue

  - ### toArray

    public <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. 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()`.
    Specified by:
      `toArray` in interface `Collection<E>`
    Overrides:
      `toArray` in class `AbstractCollection<E>`
    Type Parameters:
      `T` - the component type of the array to contain the collection
    Parameters:
      `a` - 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:
      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

  - ### spliterator

    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`.

    Specified by:
      `spliterator` in interface `Collection<E>`
    Specified by:
      `spliterator` in interface `Iterable<E>`
    Implementation Note:
      The `Spliterator` implements `trySplit` to permit limited parallelism.
    Returns:
      a `Spliterator` over the elements in this queue
    Since:
      1.8

  - ### 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 `BlockingQueue<E>`
    Specified by:
      `add` in interface `Collection<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 weakly consistent.

    Specified by:
      `iterator` in interface `Collection<E>`
    Specified by:
      `iterator` in interface `Iterable<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>`
    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 `BlockingQueue<E>`
    Specified by:
      `remove` in interface `Collection<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 `BlockingQueue<E>`
    Specified by:
      `contains` in interface `Collection<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()`)

  - ### removeIf

    public boolean removeIf(Predicate<? super E> filter)
    Description copied from interface: `Collection`
    Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.
    Specified by:
      `removeIf` in interface `Collection<E>`
    Parameters:
      `filter` - a predicate which returns `true` for elements to be removed
    Returns:
      `true` if any elements were removed
    Throws:
      `NullPointerException` - if the specified filter is null

  - ### removeAll

    public boolean removeAll(Collection<?> c)
    Description copied from class: `AbstractCollection`
    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.
    Specified by:
      `removeAll` in interface `Collection<E>`
    Overrides:
      `removeAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection containing elements to be removed from this collection
    Returns:
      `true` if this collection changed as a result of the call
    Throws:
      `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
    See Also:
      - `AbstractCollection.remove(Object)`
      - `AbstractCollection.contains(Object)`

  - ### retainAll

    public boolean retainAll(Collection<?> c)
    Description copied from class: `AbstractCollection`
    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.
    Specified by:
      `retainAll` in interface `Collection<E>`
    Overrides:
      `retainAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection containing elements to be retained in this collection
    Returns:
      `true` if this collection changed as a result of the call
    Throws:
      `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
    See Also:
      - `AbstractCollection.remove(Object)`
      - `AbstractCollection.contains(Object)`

  - ### clear

    public void clear()
    Description copied from class: `AbstractQueue`
    Removes all of the elements from this queue. The queue will be empty after this call returns.

    This implementation repeatedly invokes `poll` until it returns `null`.

    Specified by:
      `clear` in interface `Collection<E>`
    Overrides:
      `clear` in class `AbstractQueue<E>`

  - ### forEach

    public void forEach(Consumer<? super E> action)
    Description copied from interface: `Iterable`
    Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.

    The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy.

    Specified by:
      `forEach` in interface `Iterable<E>`
    Parameters:
      `action` - The action to be performed for each element
    Throws:
      `NullPointerException` - if the specified action is null