Added in API level 1

SynchronousQueue

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

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


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.

Summary

Public constructors

SynchronousQueue()

Creates a SynchronousQueue with nonfair access policy.

SynchronousQueue(boolean fair)

Creates a SynchronousQueue with the specified fairness policy.

Public methods

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 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 methods

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.

boolean add(E e)

Ensures that this collection contains the specified element (optional operation).

boolean addAll(Collection<? extends E> c)

Adds all of the elements in the specified collection to this collection (optional operation).

void clear()

Removes all of the elements from this collection (optional operation).

boolean contains(Object o)

Returns true if this collection contains the specified element.

boolean containsAll(Collection<?> c)

Returns true if this collection contains all of the elements in the specified collection.

boolean isEmpty()

Returns true if this collection contains no elements.

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).

boolean removeAll(Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection (optional operation).

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.

<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.

Object[] toArray()

Returns an array containing all of the elements in this collection.

String toString()

Returns a string representation of this collection.

Object clone()

Creates and returns a copy of this object.

boolean equals(Object obj)

Indicates whether some other object is "equal to" this one.

void finalize()

Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.

final Class<?> getClass()

Returns the runtime class of this Object.

int hashCode()

Returns a hash code value for the object.

final void notify()

Wakes up a single thread that is waiting on this object's monitor.

final void notifyAll()

Wakes up all threads that are waiting on this object's monitor.

String toString()

Returns a string representation of the object.

final void wait(long timeoutMillis, int nanos)

Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

final void wait(long timeoutMillis)

Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

final void wait()

Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

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.

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.

default 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).

default 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.

default Spliterator<E> spliterator()

Creates a Spliterator over the elements in this collection.

default 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.

default <T> T[] toArray(IntFunction<T[]> generator)

Returns an array containing all of the elements in this collection, using the provided generator function to allocate the returned array.

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.

default 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.

default Spliterator<E> spliterator()

Creates a Spliterator over the elements described by this Iterable.

Public constructors

SynchronousQueue

Added in API level 1
public SynchronousQueue ()

Creates a SynchronousQueue with nonfair access policy.

SynchronousQueue

Added in API level 1
public SynchronousQueue (boolean fair)

Creates a SynchronousQueue with the specified fairness policy.

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

Public methods

clear

Added in API level 1
public void clear ()

Does nothing. A SynchronousQueue has no internal capacity.

contains

Added in API level 1
public boolean contains (Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
o Object: the element

Returns
boolean false

containsAll

Added in API level 1
public boolean containsAll (Collection<?> c)

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

Parameters
c Collection: the collection

Returns
boolean false unless given collection is empty

drainTo

Added in API level 1
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 Collection: the collection to transfer elements into

maxElements int: the maximum number of elements to transfer

Returns
int the number of elements transferred

drainTo

Added in API level 1
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 Collection: the collection to transfer elements into

Returns
int the number of elements transferred

isEmpty

Added in API level 1
public boolean isEmpty ()

Always returns true. A SynchronousQueue has no internal capacity.

Returns
boolean true

iterator

Added in API level 1
public Iterator<E> iterator ()

Returns an empty iterator in which hasNext always returns false.

Returns
Iterator<E> an empty iterator

offer

Added in API level 1
public boolean offer (E e)

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

Parameters
e E: the element to add

Returns
boolean true if the element was added to this queue, else false

Throws
NullPointerException if the specified element is null

offer

Added in API level 1
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 E: the element to add

timeout long: how long to wait before giving up, in units of unit

unit TimeUnit: a TimeUnit determining how to interpret the timeout parameter

Returns
boolean true if successful, or false if the specified waiting time elapses before a consumer appears

peek

Added in API level 1
public E peek ()

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

Returns
E null

poll

Added in API level 1
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 long: how long to wait before giving up, in units of unit

unit TimeUnit: a TimeUnit determining how to interpret the timeout parameter

Returns
E the head of this queue, or null if the specified waiting time elapses before an element is present

poll

Added in API level 1
public E poll ()

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

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

put

Added in API level 1
public void put (E e)

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

Parameters
e E: the element to add

remainingCapacity

Added in API level 1
public int remainingCapacity ()

Always returns zero. A SynchronousQueue has no internal capacity.

Returns
int zero

remove

Added in API level 1
public boolean remove (Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
o Object: the element to remove

Returns
boolean false

removeAll

Added in API level 1
public boolean removeAll (Collection<?> c)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
c Collection: the collection

Returns
boolean false

retainAll

Added in API level 1
public boolean retainAll (Collection<?> c)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
c Collection: the collection

Returns
boolean false

size

Added in API level 1
public int size ()

Always returns zero. A SynchronousQueue has no internal capacity.

Returns
int zero

spliterator

Added in API level 24
public Spliterator<E> spliterator ()

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

Returns
Spliterator<E> an empty spliterator

take

Added in API level 1
public E take ()

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

Returns
E the head of this queue

toArray

Added in API level 1
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 T: the array

Returns
T[] the specified array

Throws
NullPointerException if the specified array is null

toArray

Added in API level 1
public Object[] toArray ()

Returns a zero-length array.

Returns
Object[] a zero-length array

toString

Added in API level 1
public String toString ()

Always returns "[]".

Returns
String "[]"