SynchronousQueue

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

A {@linkplain BlockingQueue 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.

Public Constructor Summary

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

Public Method Summary

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, int maxElements) Removes at most the given number of available elements from this queue and adds them to the given collection.
int	drainTo(Collection<? super E> c) Removes all 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(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.
E	poll() Retrieves and removes the head of this queue, if another thread is currently making an element available.
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 Spliterator.trySplit() always return null.
E	take() Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
<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.
Object[]	toArray() Returns a zero-length array.
String	toString() Always returns "[]".

Inherited Method Summary

From class java.util.AbstractQueue

boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this queue.
void	clear() Removes all of the elements from this queue.
E	element() Retrieves, but does not remove, the head of this queue.
E	remove() Retrieves and removes the head of this queue.

From class java.util.AbstractCollection

boolean	add(E e) Ensures that this collection contains the specified element (optional operation). This implementation always throws an UnsupportedOperationException.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
void	clear() Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the Iterator.remove operation.
boolean	contains(Object o) Returns true if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
boolean	containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
boolean	isEmpty() Returns true if this collection contains no elements. This implementation returns size() == 0.
abstract Iterator<E>	iterator() Returns an iterator over the elements contained in this collection.
boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
abstract int	size() Returns the number of elements in this collection.
<T> T[]	toArray(T[] contents) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index 0.
Object[]	toArray() Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index 0.
String	toString() Returns a string representation of this collection.

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.util.Queue

abstract boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available.
abstract E	element() Retrieves, but does not remove, the head of this queue.
abstract boolean	offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
abstract E	peek() Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
abstract E	poll() Retrieves and removes the head of this queue, or returns null if this queue is empty.
abstract E	remove() Retrieves and removes the head of this queue.

From interface java.util.Collection

abstract boolean	add(E e) Ensures that this collection contains the specified element (optional operation).
abstract boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).
abstract void	clear() Removes all of the elements from this collection (optional operation).
abstract boolean	contains(Object o) Returns true if this collection contains the specified element.
abstract boolean	containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection.
abstract boolean	equals(Object o) Compares the specified object with this collection for equality.
abstract int	hashCode() Returns the hash code value for this collection.
abstract boolean	isEmpty() Returns true if this collection contains no elements.
abstract Iterator<E>	iterator() Returns an iterator over the elements in this collection.
Stream<E>	parallelStream() Returns a possibly parallel Stream with this collection as its source.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
abstract 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.
abstract boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
Spliterator<E>	spliterator() Creates a Spliterator over the elements in this collection.
Stream<E>	stream() Returns a sequential Stream with this collection as its source.
abstract <T> T[]	toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
abstract Object[]	toArray() Returns an array containing all of the elements in this collection.

From interface java.util.concurrent.BlockingQueue

abstract boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available.
abstract boolean	contains(Object o) Returns true if this queue contains the specified element.
abstract int	drainTo(Collection<? super E> c) Removes all available elements from this queue and adds them to the given collection.
abstract 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.
abstract boolean	offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and false if no space is currently available.
abstract boolean	offer(E e, long timeout, TimeUnit unit) Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
abstract 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.
abstract void	put(E e) Inserts the specified element into this queue, waiting if necessary for space to become available.
abstract int	remainingCapacity() Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or Integer.MAX_VALUE if there is no intrinsic limit.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this queue, if it is present.
abstract E	take() Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

From interface java.lang.Iterable

void	forEach(Consumer<? super T> action) Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
abstract Iterator<E>	iterator() Returns an iterator over elements of type T.
Spliterator<E>	spliterator() Creates a Spliterator over the elements described by this Iterable.

Public Constructors

public SynchronousQueue ()

Creates a SynchronousQueue with nonfair access policy.

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.

Public Methods

public void clear ()

Does nothing. A SynchronousQueue has no internal capacity.

public boolean contains (Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters

o	the element

Returns

• false

public boolean containsAll (Collection<?> c)

Returns false unless the given collection is empty. A SynchronousQueue has no internal capacity.

Parameters

c	the collection

Returns

• false unless given collection is empty

public 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. 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.

Parameters

c	the collection to transfer elements into
maxElements	the maximum number of elements to transfer

Returns

• the number of elements transferred

Throws

UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

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

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.

Parameters

c	the collection to transfer elements into

Returns

• the number of elements transferred

Throws

UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

public boolean isEmpty ()

Always returns true. A SynchronousQueue has no internal capacity.

Returns

• true

public Iterator<E> iterator ()

Returns an empty iterator in which hasNext always returns false.

Returns

• an empty iterator

public boolean offer (E e)

Inserts the specified element into this queue, if another thread is waiting to receive it.

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

public 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.

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
NullPointerException

public E peek ()

Always returns null. A SynchronousQueue does not return elements unless actively waited on.

Returns

• null

public 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.

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

public E poll ()

Retrieves and removes the head of this queue, if another thread is currently making an element available.

Returns

• the head of this queue, or null if no element is available

public void put (E e)

Adds the specified element to this queue, waiting if necessary for another thread to receive it.

Parameters

e	the element to add

Throws

InterruptedException
NullPointerException

public int remainingCapacity ()

Always returns zero. A SynchronousQueue has no internal capacity.

Returns

• zero

public boolean remove (Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters

o	the element to remove

Returns

• false

public boolean removeAll (Collection<?> c)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters

c	the collection

Returns

• false

public boolean retainAll (Collection<?> c)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters

c	the collection

Returns

• false

public int size ()

Always returns zero. A SynchronousQueue has no internal capacity.

Returns

• zero

public Spliterator<E> spliterator ()

Returns an empty spliterator in which calls to Spliterator.trySplit() always return null.

Returns

• an empty spliterator

public E take ()

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

Returns

• the head of this queue

Throws

InterruptedException

public T[] toArray (T[] a)

Sets the zeroth element of the specified array to null (if the array has non-zero length) and returns it.

Parameters

a	the array

Returns

• the specified array

Throws

NullPointerException	if the specified array is null

public Object[] toArray ()

Returns a zero-length array.

Returns

• a zero-length array

public String toString ()

Always returns "[]".

Returns

• "[]"