Class Overview
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.
Summary
| Public Constructors |
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SynchronousQueue()
Creates a SynchronousQueue with nonfair access policy.
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SynchronousQueue(boolean fair)
Creates a SynchronousQueue with the specified fairness policy.
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| Public Methods |
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void
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clear()
Does nothing.
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boolean
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contains(Object o)
Always returns false.
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boolean
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containsAll(Collection<?> c)
Returns false unless the given collection is empty.
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int
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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.
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int
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drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them
to the given collection.
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boolean
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isEmpty()
Always returns true.
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Iterator<E>
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iterator()
Returns an empty iterator in which hasNext always returns
false.
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boolean
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offer(E e)
Inserts the specified element into this queue, if another thread is
waiting to receive it.
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boolean
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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.
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E
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peek()
Always returns null.
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E
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poll()
Retrieves and removes the head of this queue, if another thread
is currently making an element available.
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E
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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.
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void
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put(E o)
Adds the specified element to this queue, waiting if necessary for
another thread to receive it.
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int
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remainingCapacity()
Always returns zero.
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boolean
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remove(Object o)
Always returns false.
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boolean
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removeAll(Collection<?> c)
Always returns false.
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boolean
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retainAll(Collection<?> c)
Always returns false.
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int
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size()
Always returns zero.
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E
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take()
Retrieves and removes the head of this queue, waiting if necessary
for another thread to insert it.
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<T>
T[]
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toArray(T[] a)
Sets the zeroeth element of the specified array to null
(if the array has non-zero length) and returns it.
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Object[]
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toArray()
Returns a zero-length array.
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[Expand]
Inherited Methods |
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From class
java.util.AbstractQueue
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boolean
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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.
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boolean
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addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this
queue.
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void
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clear()
Removes all of the elements from this queue.
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E
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element()
Retrieves, but does not remove, the head of this queue.
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E
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remove()
Retrieves and removes the head of this queue.
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From class
java.util.AbstractCollection
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boolean
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add(E object)
Attempts to add object to the contents of this
Collection (optional).
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boolean
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addAll(Collection<? extends E> collection)
Attempts to add all of the objects contained in collection
to the contents of this Collection (optional).
|
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void
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clear()
Removes all elements from this Collection, leaving it empty (optional).
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boolean
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contains(Object object)
Tests whether this Collection contains the specified object.
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boolean
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containsAll(Collection<?> collection)
Tests whether this Collection contains all objects contained in the
specified Collection.
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boolean
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isEmpty()
Returns if this Collection contains no elements.
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abstract
Iterator<E>
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iterator()
Returns an instance of Iterator that may be used to access the
objects contained by this Collection. |
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boolean
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remove(Object object)
Removes one instance of the specified object from this Collection if one
is contained (optional).
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boolean
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removeAll(Collection<?> collection)
Removes all occurrences in this Collection of each object in the
specified Collection (optional).
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boolean
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retainAll(Collection<?> collection)
Removes all objects from this Collection that are not also found in the
Collection passed (optional).
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abstract
int
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size()
Returns a count of how many objects this Collection contains.
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<T>
T[]
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toArray(T[] contents)
Returns an array containing all elements contained in this Collection.
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Object[]
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toArray()
Returns a new array containing all elements contained in this Collection.
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String
|
toString()
Returns the string representation of this Collection.
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From class
java.lang.Object
|
Object
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clone()
Creates and returns a copy of this Object.
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boolean
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equals(Object o)
Compares this instance with the specified object and indicates if they
are equal.
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void
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finalize()
Invoked when the garbage collector has detected that this instance is no longer reachable.
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final
Class<?>
|
getClass()
Returns the unique instance of Class that represents this
object's class. |
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int
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hashCode()
Returns an integer hash code for this object.
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final
void
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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.
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final
void
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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.
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String
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toString()
Returns a string containing a concise, human-readable description of this
object.
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final
void
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wait()
Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.
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final
void
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wait(long millis, 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.
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final
void
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wait(long millis)
Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the
specified timeout expires.
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From interface
java.lang.Iterable
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From interface
java.util.Collection
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abstract
boolean
|
add(E object)
Attempts to add object to the contents of this
Collection (optional).
|
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abstract
boolean
|
addAll(Collection<? extends E> collection)
Attempts to add all of the objects contained in Collection
to the contents of this Collection (optional).
|
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abstract
void
|
clear()
Removes all elements from this Collection, leaving it empty (optional).
|
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abstract
boolean
|
contains(Object object)
Tests whether this Collection contains the specified object.
|
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abstract
boolean
|
containsAll(Collection<?> collection)
Tests whether this Collection contains all objects contained in the
specified Collection.
|
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abstract
boolean
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equals(Object object)
Compares the argument to the receiver, and returns true if they represent
the same object using a class specific comparison.
|
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abstract
int
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hashCode()
Returns an integer hash code for the receiver.
|
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abstract
boolean
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isEmpty()
Returns if this Collection contains no elements.
|
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abstract
Iterator<E>
|
iterator()
Returns an instance of Iterator that may be used to access the
objects contained by this Collection. |
|
abstract
boolean
|
remove(Object object)
Removes one instance of the specified object from this Collection if one
is contained (optional).
|
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abstract
boolean
|
removeAll(Collection<?> collection)
Removes all occurrences in this Collection of each object in the
specified Collection (optional).
|
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abstract
boolean
|
retainAll(Collection<?> collection)
Removes all objects from this Collection that are not also found in the
Collection passed (optional).
|
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abstract
int
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size()
Returns a count of how many objects this Collection contains.
|
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abstract
<T>
T[]
|
toArray(T[] array)
Returns an array containing all elements contained in this Collection.
|
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abstract
Object[]
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toArray()
Returns a new array containing all elements contained in this Collection.
|
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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.
|
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abstract
E
|
element()
Retrieves, but does not remove, the head of this queue.
|
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abstract
boolean
|
offer(E e)
Inserts the specified element into this queue if it is possible to do
so immediately without violating capacity restrictions.
|
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abstract
E
|
peek()
Retrieves, but does not remove, the head of this queue,
or returns null if this queue is empty.
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abstract
E
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poll()
Retrieves and removes the head of this queue,
or returns null if this queue is empty.
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abstract
E
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remove()
Retrieves and removes the head of this queue.
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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.
|
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abstract
boolean
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contains(Object o)
Returns true if this queue contains the specified element.
|
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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, 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
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.
|
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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.
|
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abstract
void
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put(E e)
Inserts the specified element into this queue, waiting if necessary
for space to become available.
|
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abstract
int
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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.
|
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abstract
boolean
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remove(Object o)
Removes a single instance of the specified element from this queue,
if it is present.
|
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abstract
E
|
take()
Retrieves and removes the head of this queue, waiting if necessary
until an element becomes available.
|
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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.
public
boolean
containsAll
(Collection<?> c)
Returns false unless the given collection is empty.
A SynchronousQueue has no internal capacity.
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
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
public
boolean
isEmpty
()
Always returns true.
A SynchronousQueue has no internal capacity.
public
Iterator<E>
iterator
()
Returns an empty iterator in which hasNext always returns
false.
public
boolean
offer
(E e)
Inserts the specified element into this queue, if another thread is
waiting to receive it.
Returns
- true if the element was added to this queue, else
false
public
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.
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.
public
E
peek
()
Always returns null.
A SynchronousQueue does not return elements
unless actively waited on.
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
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.
public
void
put
(E o)
Adds the specified element to this queue, waiting if necessary for
another thread to receive it.
public
int
remainingCapacity
()
Always returns zero.
A SynchronousQueue has no internal capacity.
public
boolean
remove
(Object o)
Always returns false.
A SynchronousQueue has no internal capacity.
public
boolean
removeAll
(Collection<?> c)
Always returns false.
A SynchronousQueue has no internal capacity.
public
boolean
retainAll
(Collection<?> c)
Always returns false.
A SynchronousQueue has no internal capacity.
public
int
size
()
Always returns zero.
A SynchronousQueue has no internal capacity.
public
E
take
()
Retrieves and removes the head of this queue, waiting if necessary
for another thread to insert it.
public
T[]
toArray
(T[] a)
Sets the zeroeth element of the specified array to null
(if the array has non-zero length) and returns it.
public
Object[]
toArray
()
Returns a zero-length array.
