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## 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 collection

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 `put` must wait for a `take`, 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 take it; you cannot add 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 thread is trying to add to the queue; if there are no queued threads then no element is being added and the head is `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. Fairness generally decreases throughput but reduces variability and avoids starvation.

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
`SynchronousQueue()` Creates a `SynchronousQueue` with nonfair access policy.
`SynchronousQueue(boolean fair)` Creates a `SynchronousQueue` with specified fairness policy.

Method Summary
`void` | `clear()` Does nothing.
`boolean` | `contains(Object o)` Always returns `false`.
`boolean` | `containsAll(Collection<?> c)` Returns `false` unless given collection is empty.
`int` | `drainTo(Collection<? super E> c)` Removes all available elements from this queue and adds them into 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 into the given collection.
`boolean` | `isEmpty()` Always returns `true`.
`Iterator<E>` | `iterator()` Returns an empty iterator in which `hasNext` always returns `false`.
`boolean` | `offer(E o)` Inserts the specified element into this queue, if another thread is waiting to receive it.
`boolean` | `offer(E o, 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 o)` 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.
`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 zeroeth element of the specified array to `null` (if the array has non-zero length) and returns it.

Methods inherited from class java.util.AbstractQueue
`add, addAll, element, remove`

Methods inherited from class java.util.AbstractCollection
`toString`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

Methods inherited from interface java.util.concurrent.BlockingQueue
`add`

Methods inherited from interface java.util.Queue
`element, remove`

Methods inherited from interface java.util.Collection
`addAll, equals, hashCode`

Constructor Detail

### SynchronousQueue

    public SynchronousQueue()

  Creates a `SynchronousQueue` with nonfair access policy.

### SynchronousQueue

    public SynchronousQueue(boolean fair)

  Creates a `SynchronousQueue` with specified fairness policy.

  Parameters:
    `fair` - if true, threads contend in FIFO order for access; otherwise the order is unspecified.

Method Detail

### put

    public void put(E o)
             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:
    `o` - the element to add
  Throws:
    `InterruptedException` - if interrupted while waiting.
    `NullPointerException` - if the specified element is `null`.

### offer

    public boolean offer(E o,
                         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:
    `o` - 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`.

### 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.

### offer

    public boolean offer(E o)

  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:
    `o` - the element to add.
  Returns:
    `true` if it was possible to add the element to this queue, else `false`
  Throws:
    `NullPointerException` - if the specified element is `null`

### 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>`
  Specified by:
    `size` in class `AbstractCollection<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 `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 `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 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 `Iterable<E>`
  Specified by:
    `iterator` in interface `Collection<E>`
  Specified by:
    `iterator` in class `AbstractCollection<E>`
  Returns:
    an empty iterator

### 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 zeroeth 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>`
  Parameters:
    `a` - the array
  Returns:
    the specified array

### drainTo

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

  Description copied from interface: `BlockingQueue`
  Removes all available elements from this queue and adds them into 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.

### 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 into 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.

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