SynchronousQueue (Java SE 12 & JDK 12 )
Keywords: java.util.concurrent.SynchronousQueue class

Module java.base
Package java.util.concurrent

## Class SynchronousQueue<E>

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

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

      public class SynchronousQueue<E>
      extends AbstractQueue<E>
      implements BlockingQueue<E>, Serializable

  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.

  Since:
    1.5
  See Also:
    Serialized Form

- - ### Constructor Summary

    Constructors
    Constructor | Description
    --- | ---
    `SynchronousQueue()` | Creates a `SynchronousQueue` with nonfair access policy.
    `SynchronousQueue​(boolean fair)` | Creates a `SynchronousQueue` with the specified fairness policy.

  - ### Method Summary

    Modifier and Type | Method | Description
    --- | --- | ---
    `void` | `clear()` | Does nothing.
    `boolean` | `contains​(Object o)` | Always returns `false`.
    `boolean` | `containsAll​(Collection<?> c)` | Returns `false` unless the given collection is empty.
    `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.
    `boolean` | `isEmpty()` | Always returns `true`.
    `Iterator<E>` | `iterator()` | Returns an empty iterator in which `hasNext` always returns `false`.
    `boolean` | `offer​(E e)` | Inserts the specified element into this queue, if another thread is waiting to receive it.
    `boolean` | `offer​(E e, long timeout, TimeUnit unit)` | Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.
    `E` | `peek()` | Always returns `null`.
    `E` | `poll()` | Retrieves and removes the head of this queue, if another thread is currently making an element available.
    `E` | `poll​(long timeout, TimeUnit unit)` | Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.
    `void` | `put​(E e)` | Adds the specified element to this queue, waiting if necessary for another thread to receive it.
    `int` | `remainingCapacity()` | Always returns zero.
    `boolean` | `remove​(Object o)` | Always returns `false`.
    `boolean` | `removeAll​(Collection<?> c)` | Always returns `false`.
    `boolean` | `retainAll​(Collection<?> c)` | Always returns `false`.
    `int` | `size()` | Always returns zero.
    `Spliterator<E>` | `spliterator()` | Returns an empty spliterator in which calls to `trySplit` always return `null`.
    `E` | `take()` | Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
    `Object[]` | `toArray()` | Returns a zero-length array.
    `<T> T[]` | `toArray​(T[] a)` | Sets the zeroth element of the specified array to `null` (if the array has non-zero length) and returns it.
    `String` | `toString()` | Always returns `"[]"`.

    - ### Methods declared in class java.util.AbstractQueue

      `add, addAll, element, remove`

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

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

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

      `add`

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

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

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

      `forEach`

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

      `element, remove`

- - ### Constructor Detail

    - #### SynchronousQueue

          public SynchronousQueue()

      Creates a `SynchronousQueue` with nonfair access policy.

    - #### SynchronousQueue

          public SynchronousQueue​(boolean fair)

      Creates a `SynchronousQueue` with the specified fairness policy.
      Parameters:
        `fair` - if true, waiting threads contend in FIFO order for access; otherwise the order is unspecified.

  - ### Method Detail

    - #### put

          public void put​(E e)
                   throws InterruptedException

      Adds the specified element to this queue, waiting if necessary for another thread to receive it.
      Specified by:
        `put` in interface `BlockingQueue<E>`
      Parameters:
        `e` - the element to add
      Throws:
        `InterruptedException` - if interrupted while waiting
        `NullPointerException` - if the specified element is null

    - #### offer

          public boolean offer​(E e,
                               long timeout,
                               TimeUnit unit)
                        throws InterruptedException

      Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.
      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` if successful, or `false` if the specified waiting time elapses before a consumer appears
      Throws:
        `InterruptedException` - if interrupted while waiting
        `NullPointerException` - if the specified element is null

    - #### offer

          public boolean offer​(E e)

      Inserts the specified element into this queue, if another thread is waiting to receive it.
      Specified by:
        `offer` in interface `BlockingQueue<E>`
      Specified by:
        `offer` in interface `Queue<E>`
      Parameters:
        `e` - the element to add
      Returns:
        `true` if the element was added to this queue, else `false`
      Throws:
        `NullPointerException` - if the specified element is null

    - #### take

          public E take()
                 throws InterruptedException

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

      Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.
      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 present
      Throws:
        `InterruptedException` - if interrupted while waiting

    - #### poll

          public E poll()

      Retrieves and removes the head of this queue, if another thread is currently making an element available.
      Specified by:
        `poll` in interface `Queue<E>`
      Returns:
        the head of this queue, or `null` if no element is available

    - #### isEmpty

          public boolean isEmpty()

      Always returns `true`. A `SynchronousQueue` has no internal capacity.
      Specified by:
        `isEmpty` in interface `Collection<E>`
      Overrides:
        `isEmpty` in class `AbstractCollection<E>`
      Returns:
        `true`

    - #### size

          public int size()

      Always returns zero. A `SynchronousQueue` has no internal capacity.
      Specified by:
        `size` in interface `Collection<E>`
      Returns:
        zero

    - #### remainingCapacity

          public int remainingCapacity()

      Always returns zero. A `SynchronousQueue` has no internal capacity.
      Specified by:
        `remainingCapacity` in interface `BlockingQueue<E>`
      Returns:
        zero

    - #### clear

          public void clear()

      Does nothing. A `SynchronousQueue` has no internal capacity.
      Specified by:
        `clear` in interface `Collection<E>`
      Overrides:
        `clear` in class `AbstractQueue<E>`

    - #### contains

          public boolean contains​(Object o)

      Always returns `false`. A `SynchronousQueue` has no internal capacity.
      Specified by:
        `contains` in interface `BlockingQueue<E>`
      Specified by:
        `contains` in interface `Collection<E>`
      Overrides:
        `contains` in class `AbstractCollection<E>`
      Parameters:
        `o` - the element
      Returns:
        `false`

    - #### remove

          public boolean remove​(Object o)

      Always returns `false`. A `SynchronousQueue` has no internal capacity.
      Specified by:
        `remove` in interface `BlockingQueue<E>`
      Specified by:
        `remove` in interface `Collection<E>`
      Overrides:
        `remove` in class `AbstractCollection<E>`
      Parameters:
        `o` - the element to remove
      Returns:
        `false`

    - #### containsAll

          public boolean containsAll​(Collection<?> c)

      Returns `false` unless the given collection is empty. A `SynchronousQueue` has no internal capacity.
      Specified by:
        `containsAll` in interface `Collection<E>`
      Overrides:
        `containsAll` in class `AbstractCollection<E>`
      Parameters:
        `c` - the collection
      Returns:
        `false` unless given collection is empty
      See Also:
        `AbstractCollection.contains(Object)`

    - #### removeAll

          public boolean removeAll​(Collection<?> c)

      Always returns `false`. A `SynchronousQueue` has no internal capacity.
      Specified by:
        `removeAll` in interface `Collection<E>`
      Overrides:
        `removeAll` in class `AbstractCollection<E>`
      Parameters:
        `c` - the collection
      Returns:
        `false`
      See Also:
        `AbstractCollection.remove(Object)`, `AbstractCollection.contains(Object)`

    - #### retainAll

          public boolean retainAll​(Collection<?> c)

      Always returns `false`. A `SynchronousQueue` has no internal capacity.
      Specified by:
        `retainAll` in interface `Collection<E>`
      Overrides:
        `retainAll` in class `AbstractCollection<E>`
      Parameters:
        `c` - the collection
      Returns:
        `false`
      See Also:
        `AbstractCollection.remove(Object)`, `AbstractCollection.contains(Object)`

    - #### peek

          public E peek()

      Always returns `null`. A `SynchronousQueue` does not return elements unless actively waited on.
      Specified by:
        `peek` in interface `Queue<E>`
      Returns:
        `null`

    - #### iterator

          public Iterator<E> iterator()

      Returns an empty iterator in which `hasNext` always returns `false`.
      Specified by:
        `iterator` in interface `Collection<E>`
      Specified by:
        `iterator` in interface `Iterable<E>`
      Specified by:
        `iterator` in class `AbstractCollection<E>`
      Returns:
        an empty iterator

    - #### spliterator

          public Spliterator<E> spliterator()

      Returns an empty spliterator in which calls to `trySplit` always return `null`.
      Specified by:
        `spliterator` in interface `Collection<E>`
      Specified by:
        `spliterator` in interface `Iterable<E>`
      Returns:
        an empty spliterator
      Since:
        1.8

    - #### toArray

          public Object[] toArray()

      Returns a zero-length array.
      Specified by:
        `toArray` in interface `Collection<E>`
      Overrides:
        `toArray` in class `AbstractCollection<E>`
      Returns:
        a zero-length array

    - #### toArray

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

      Sets the zeroth element of the specified array to `null` (if the array has non-zero length) and returns it.
      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
      Returns:
        the specified array
      Throws:
        `NullPointerException` - if the specified array is null

    - #### toString

          public String toString()

      Always returns `"[]"`.
      Overrides:
        `toString` in class `AbstractCollection<E>`
      Returns:
        `"[]"`

    - #### 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:
        `UnsupportedOperationException` - if addition of elements is not supported by the specified collection
        `ClassCastException` - if the class of an element of this queue prevents it from being added to the specified collection
        `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:
        `UnsupportedOperationException` - if addition of elements is not supported by the specified collection
        `ClassCastException` - if the class of an element of this queue prevents it from being added to the specified collection
        `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