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

All Methods Instance Methods Concrete Methods
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

Class SynchronousQueue<E>

Constructor Summary

Method Summary

Methods declared in class java.util.AbstractQueue

Methods declared in class java.lang.Object

Methods declared in interface java.util.concurrent.BlockingQueue

Methods declared in interface java.util.Collection

Methods declared in interface java.lang.Iterable

Methods declared in interface java.util.Queue

Constructor Detail

SynchronousQueue

SynchronousQueue

Method Detail

put

offer

offer

take

poll

poll

isEmpty

size

remainingCapacity

clear

contains

remove

containsAll

removeAll

retainAll

peek

iterator

spliterator

toArray

toArray

toString

drainTo

drainTo