org.openimaj.util.concurrent

Interface BlockingDroppingQueue<E>

Type Parameters:

E - the type of elements held in this collection

All Superinterfaces:

Collection<E>, Iterable<E>, Queue<E>

All Known Implementing Classes:

ArrayBlockingDroppingQueue

public interface BlockingDroppingQueue<E>
extends Queue<E>

A Queue that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and drop the oldest element from the queue when storing a new element if space is not available for the new element.

BlockingDroppingQueue methods come in four forms, with different ways of handling operations that cannot be satisfied immediately, but may be satisfied at some point in the future: one throws an exception, the second returns a special value (either null, false or the dropped object, depending on the operation), the third blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up. These methods are summarized in the following table:

	Throws exception	Special value	Blocks	Times out
Insert	add(e)	offer(e), put(e)	not applicable	not applicable
Remove	remove()	poll()	take()	poll(time, unit)
Examine	element()	peek()	not applicable	not applicable

BlockingDroppingQueues keep track of statistics on the number of items that have been added to the queue and the number of items that have been dropped from the queue.

A BlockingDroppingQueue does not accept null elements. Implementations throw NullPointerException on attempts to add, put or offer a null. A null is used as a sentinel value to indicate failure of poll operations.

A BlockingDroppingQueue may be capacity bounded. At any given time it may have a remainingCapacity beyond which no additional elements can be put without blocking. A BlockingDroppingQueue without any intrinsic capacity constraints always reports a remaining capacity of Integer.MAX_VALUE.

BlockingDroppingQueue implementations are designed to be used primarily for producer-consumer queues, but additionally support the Collection interface. So, for example, it is possible to remove an arbitrary element from a queue using remove(x). However, such operations are in general not performed very efficiently, and are intended for only occasional use, such as when a queued message is cancelled.

BlockingDroppingQueue implementations are thread-safe. All queuing methods achieve their effects atomically using internal locks or other forms of concurrency control. However, the bulk Collection operations addAll, containsAll, retainAll and removeAll are not necessarily performed atomically unless specified otherwise in an implementation. So it is possible, for example, for addAll(c) to fail (throwing an exception) after adding only some of the elements in c.

A BlockingDroppingQueue does not intrinsically support any kind of "close" or "shutdown" operation to indicate that no more items will be added. The needs and usage of such features tend to be implementation-dependent. For example, a common tactic is for producers to insert special end-of-stream or poison objects, that are interpreted accordingly when taken by consumers.

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a BlockingDroppingQueue happen-before actions subsequent to the access or removal of that element from the BlockingDroppingQueue in another thread.

Author:

Jonathon Hare (jsh2@ecs.soton.ac.uk)

Method Summary

Methods

Modifier and Type	Method and Description
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	contains(Object o) Returns true if this queue contains the specified element.
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.
long	dropCount() Returns the total number of items that have been dropped by this BlockingDroppingQueue to make space for new elements within its lifetime.
long	insertCount() Returns the total number of items that have been inserted into this BlockingDroppingQueue within its lifetime.
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.
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.
E	put(E e) Inserts the specified element into this queue, dropping the oldest element to make space if necessary.
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.
boolean	remove(Object o) Removes a single instance of the specified element from this queue, if it is present.
E	take() Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

Methods inherited from interface java.util.Queue

element, peek, poll, remove

Methods inherited from interface java.util.Collection

addAll, clear, containsAll, equals, hashCode, isEmpty, iterator, removeAll, retainAll, size, toArray, toArray

Method Detail

add

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. When using a capacity-restricted queue, it is generally preferable to use offer.

Specified by:

add in interface Collection<E>

Specified by:

add in interface Queue<E>

Parameters:

e - the element to add

Returns:

true (as specified by Collection.add(E))

Throws:

IllegalStateException - if the element cannot be added at this time due to capacity restrictions

ClassCastException - if the class of the specified element prevents it from being added to this queue

NullPointerException - if the specified element is null

IllegalArgumentException - if some property of the specified element prevents it from being added to this queue

offer

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. When using a capacity-restricted queue, this method is generally preferable to add(E), which can fail to insert an element only by throwing an exception.

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:

ClassCastException - if the class of the specified element prevents it from being added to this queue

NullPointerException - if the specified element is null

IllegalArgumentException - if some property of the specified element prevents it from being added to this queue

put

E put(E e)
      throws InterruptedException

Inserts the specified element into this queue, dropping the oldest element to make space if necessary.

Parameters:

e - the element to add

Returns:

the element that was dropped to make space, or null if no element was dropped

Throws:

InterruptedException - if interrupted while waiting

ClassCastException - if the class of the specified element prevents it from being added to this queue

NullPointerException - if the specified element is null

IllegalArgumentException - if some property of the specified element prevents it from being added to this queue

take

E take()
       throws InterruptedException

Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

Returns:

the head of this queue

Throws:

InterruptedException - if interrupted while waiting

poll

E poll(long timeout,
     TimeUnit unit)
       throws InterruptedException

Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.

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 available

Throws:

InterruptedException - if interrupted while waiting

remainingCapacity

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.

Note that you cannot always tell if an attempt to insert an element will succeed by inspecting remainingCapacity because it may be the case that another thread is about to insert or remove an element.

Returns:

the remaining capacity

remove

boolean remove(Object o)

Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element e such that o.equals(e), if this queue contains one or more such elements. Returns true if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).

Specified by:

remove in interface Collection<E>

Parameters:

o - element to be removed from this queue, if present

Returns:

true if this queue changed as a result of the call

Throws:

ClassCastException - if the class of the specified element is incompatible with this queue (optional)

NullPointerException - if the specified element is null (optional)

contains

boolean contains(Object o)

Returns true if this queue contains the specified element. More formally, returns true if and only if this queue contains at least one element e such that o.equals(e).

Specified by:

contains in interface Collection<E>

Parameters:

o - object to be checked for containment in this queue

Returns:

true if this queue contains the specified element

Throws:

ClassCastException - if the class of the specified element is incompatible with this queue (optional)

NullPointerException - if the specified element is null (optional)

drainTo

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 - 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

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 - 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

insertCount

long insertCount()

Returns the total number of items that have been inserted into this BlockingDroppingQueue within its lifetime.

Returns:

the total number of items that have been inserted to the queue

dropCount

long dropCount()

Returns the total number of items that have been dropped by this BlockingDroppingQueue to make space for new elements within its lifetime.

Returns:

the total number of items that have been dropped from the queue