E
- the type of elements held in this collectionpublic 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.
public SynchronousQueue()
Creates a SynchronousQueue
with nonfair access policy.
public SynchronousQueue(boolean fair)
Creates a SynchronousQueue
with the specified fairness policy.
fair
- if true, waiting threads contend in FIFO order for access; otherwise the order is unspecified.public void put(E e) throws InterruptedException
Adds the specified element to this queue, waiting if necessary for another thread to receive it.
put
in interface BlockingQueue<E>
e
- the element to addInterruptedException
- if interrupted while waitingNullPointerException
- if the specified element is nullpublic 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.
offer
in interface BlockingQueue<E>
e
- the element to addtimeout
- how long to wait before giving up, in units of unit
unit
- a TimeUnit
determining how to interpret the timeout
parametertrue
if successful, or false
if the specified waiting time elapses before a consumer appearsInterruptedException
- if interrupted while waitingNullPointerException
- if the specified element is nullpublic boolean offer(E e)
Inserts the specified element into this queue, if another thread is waiting to receive it.
offer
in interface BlockingQueue<E>
offer
in interface Queue<E>
e
- the element to addtrue
if the element was added to this queue, else false
NullPointerException
- if the specified element is nullpublic E take() throws InterruptedException
Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
take
in interface BlockingQueue<E>
InterruptedException
- if interrupted while waitingpublic 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.
poll
in interface BlockingQueue<E>
timeout
- how long to wait before giving up, in units of unit
unit
- a TimeUnit
determining how to interpret the timeout
parameternull
if the specified waiting time elapses before an element is presentInterruptedException
- if interrupted while waitingpublic E poll()
Retrieves and removes the head of this queue, if another thread is currently making an element available.
poll
in interface Queue<E>
null
if no element is availablepublic boolean isEmpty()
Always returns true
. A SynchronousQueue
has no internal capacity.
isEmpty
in interface Collection<E>
isEmpty
in class AbstractCollection<E>
true
public int size()
Always returns zero. A SynchronousQueue
has no internal capacity.
size
in interface Collection<E>
size
in class AbstractCollection<E>
public int remainingCapacity()
Always returns zero. A SynchronousQueue
has no internal capacity.
remainingCapacity
in interface BlockingQueue<E>
public void clear()
Does nothing. A SynchronousQueue
has no internal capacity.
clear
in interface Collection<E>
clear
in class AbstractQueue<E>
public boolean contains(Object o)
Always returns false
. A SynchronousQueue
has no internal capacity.
contains
in interface Collection<E>
contains
in interface BlockingQueue<E>
contains
in class AbstractCollection<E>
o
- the elementfalse
public boolean remove(Object o)
Always returns false
. A SynchronousQueue
has no internal capacity.
remove
in interface Collection<E>
remove
in interface BlockingQueue<E>
remove
in class AbstractCollection<E>
o
- the element to removefalse
public boolean containsAll(Collection<?> c)
Returns false
unless the given collection is empty. A SynchronousQueue
has no internal capacity.
containsAll
in interface Collection<E>
containsAll
in class AbstractCollection<E>
c
- the collectionfalse
unless given collection is emptyAbstractCollection.contains(Object)
public boolean removeAll(Collection<?> c)
Always returns false
. A SynchronousQueue
has no internal capacity.
removeAll
in interface Collection<E>
removeAll
in class AbstractCollection<E>
c
- the collectionfalse
AbstractCollection.remove(Object)
, AbstractCollection.contains(Object)
public boolean retainAll(Collection<?> c)
Always returns false
. A SynchronousQueue
has no internal capacity.
retainAll
in interface Collection<E>
retainAll
in class AbstractCollection<E>
c
- the collectionfalse
AbstractCollection.remove(Object)
, AbstractCollection.contains(Object)
public E peek()
Always returns null
. A SynchronousQueue
does not return elements unless actively waited on.
public Iterator<E> iterator()
Returns an empty iterator in which hasNext
always returns false
.
iterator
in interface Iterable<E>
iterator
in interface Collection<E>
iterator
in class AbstractCollection<E>
public Spliterator<E> spliterator()
Returns an empty spliterator in which calls to Spliterator.trySplit()
always return null
.
spliterator
in interface Iterable<E>
spliterator
in interface Collection<E>
public Object[] toArray()
Returns a zero-length array.
toArray
in interface Collection<E>
toArray
in class AbstractCollection<E>
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.
toArray
in interface Collection<E>
toArray
in class AbstractCollection<E>
T
- the runtime type of the array to contain the collectiona
- the arrayNullPointerException
- if the specified array is nullpublic 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.
drainTo
in interface BlockingQueue<E>
c
- the collection to transfer elements intoUnsupportedOperationException
- if addition of elements is not supported by the specified collectionClassCastException
- if the class of an element of this queue prevents it from being added to the specified collectionNullPointerException
- if the specified collection is nullIllegalArgumentException
- if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collectionpublic 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.
drainTo
in interface BlockingQueue<E>
c
- the collection to transfer elements intomaxElements
- the maximum number of elements to transferUnsupportedOperationException
- if addition of elements is not supported by the specified collectionClassCastException
- if the class of an element of this queue prevents it from being added to the specified collectionNullPointerException
- if the specified collection is nullIllegalArgumentException
- 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
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