Added in API level 1

BlockingQueue

interface BlockingQueue<E : Any!> : Queue<E>

java.util.concurrent.BlockingQueue

Known Direct Subclasses

ArrayBlockingQueue, BlockingDeque, DelayQueue, LinkedBlockingQueue, PriorityBlockingQueue, SynchronousQueue, TransferQueue

ArrayBlockingQueue	A bounded blocking queue backed by an array.
BlockingDeque	A `Deque` that additionally supports blocking operations that wait for the deque to become non-empty when retrieving an element, and wait for space to become available in the deque when storing an element.
DelayQueue	An unbounded blocking queue of `Delayed` elements, in which an element can only be taken when its delay has expired.
LinkedBlockingQueue	An optionally-bounded blocking queue based on linked nodes.
PriorityBlockingQueue	An unbounded blocking queue that uses the same ordering rules as class `PriorityQueue` and supplies blocking retrieval operations.
SynchronousQueue	A blocking queue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa.
TransferQueue	A `BlockingQueue` in which producers may wait for consumers to receive elements.

Known Indirect Subclasses

LinkedBlockingDeque, LinkedTransferQueue

LinkedBlockingDeque	An optionally-bounded blocking deque based on linked nodes.
LinkedTransferQueue	An unbounded `TransferQueue` based on linked nodes.

A Queue that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.

BlockingQueue 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 or false, 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:

Summary of BlockingQueue methods

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

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

BlockingQueue 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.

BlockingQueue 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 BlockingQueue 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.

Usage example, based on a typical producer-consumer scenario. Note that a BlockingQueue can safely be used with multiple producers and multiple consumers.

<code>class Producer implements Runnable {
    private final BlockingQueue queue;
    Producer(BlockingQueue q) { queue = q; }
    public void run() {
      try {
        while (true) { queue.put(produce()); }
      } catch (InterruptedException ex) { ... handle ...}
    }
    Object produce() { ... }
  }
 
  class Consumer implements Runnable {
    private final BlockingQueue queue;
    Consumer(BlockingQueue q) { queue = q; }
    public void run() {
      try {
        while (true) { consume(queue.take()); }
      } catch (InterruptedException ex) { ... handle ...}
    }
    void consume(Object x) { ... }
  }
 
  class Setup {
    void main() {
      BlockingQueue q = new SomeQueueImplementation();
      Producer p = new Producer(q);
      Consumer c1 = new Consumer(q);
      Consumer c2 = new Consumer(q);
      new Thread(p).start();
      new Thread(c1).start();
      new Thread(c2).start();
    }
  }</code>

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

This interface is a member of the Java Collections Framework.

Summary

Public methods
abstract Boolean	`add(element: 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(element: E)` Returns `true` if this queue contains the specified element.
abstract Int	`drainTo(c: MutableCollection<in E>!)` Removes all available elements from this queue and adds them to the given collection.
abstract Int	`drainTo(c: MutableCollection<in E>!, maxElements: Int)` 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, timeout: Long, unit: TimeUnit!)` Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
abstract E	`poll(timeout: Long, unit: TimeUnit!)` Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
abstract Unit	`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(element: E)` 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.

Inherited functions

From class Queue

`E`	`element()` Retrieves, but does not remove, the head of this queue. This method differs from `peek` only in that it throws an exception if this queue is empty.
`E?`	`peek()` Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
`E?`	`poll()` Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
`E`	`remove()` Retrieves and removes the head of this queue. This method differs from `poll()` only in that it throws an exception if this queue is empty.

Public methods

add

Added in API level 1

abstract fun add(element: E): Boolean

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.

Parameters
`e`	the element to add

Return
`Boolean`	`true` (as specified by `Collection#add`)

Exceptions
`java.lang.UnsupportedOperationException`	if the `add` operation is not supported by this collection
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this queue
`java.lang.NullPointerException`	if the specified element is null
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this queue
`java.lang.IllegalStateException`	if the element cannot be added at this time due to capacity restrictions

contains

Added in API level 1

abstract fun contains(element: E): Boolean

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).

Parameters
`o`	object to be checked for containment in this queue

Return
`Boolean`	`true` if this queue contains the specified element

Exceptions
`java.lang.ClassCastException`	if the class of the specified element is incompatible with this queue (optional)
`java.lang.NullPointerException`	if the specified element is null (optional)

drainTo

Added in API level 1

abstract fun drainTo(c: MutableCollection<in E>!): Int

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`	MutableCollection<in E>!: the collection to transfer elements into

Return
`Int`	the number of elements transferred

Exceptions
`java.lang.UnsupportedOperationException`	if addition of elements is not supported by the specified collection
`java.lang.ClassCastException`	if the class of an element of this queue prevents it from being added to the specified collection
`java.lang.NullPointerException`	if the specified collection is null
`java.lang.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

Added in API level 1

abstract fun drainTo(
    c: MutableCollection<in E>!, 
    maxElements: Int
): Int

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`	MutableCollection<in E>!: the collection to transfer elements into
`maxElements`	Int: the maximum number of elements to transfer

Return
`Int`	the number of elements transferred

Exceptions
`java.lang.UnsupportedOperationException`	if addition of elements is not supported by the specified collection
`java.lang.ClassCastException`	if the class of an element of this queue prevents it from being added to the specified collection
`java.lang.NullPointerException`	if the specified collection is null
`java.lang.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

offer

Added in API level 1

abstract fun offer(e: E): Boolean

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, which can fail to insert an element only by throwing an exception.

Parameters
`e`	E: the element to add

Return
`Boolean`	`true` if the element was added to this queue, else `false`

Exceptions
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this queue
`java.lang.NullPointerException`	if the specified element is null
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this queue

offer

Added in API level 1

abstract fun offer(
    e: E, 
    timeout: Long, 
    unit: TimeUnit!
): Boolean

Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.

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

Return
`Boolean`	`true` if successful, or `false` if the specified waiting time elapses before space is available

Exceptions
`java.lang.InterruptedException`	if interrupted while waiting
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this queue
`java.lang.NullPointerException`	if the specified element is null
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this queue

poll

Added in API level 1

abstract fun poll(
    timeout: Long, 
    unit: TimeUnit!
): E

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`	Long: how long to wait before giving up, in units of `unit`
`unit`	TimeUnit!: a `TimeUnit` determining how to interpret the `timeout` parameter

Return
`E`	the head of this queue, or `null` if the specified waiting time elapses before an element is available

Exceptions
`java.lang.InterruptedException`	if interrupted while waiting

put

Added in API level 1

abstract fun put(e: E): Unit

Inserts the specified element into this queue, waiting if necessary for space to become available.

Parameters
`e`	E: the element to add

Exceptions
`java.lang.InterruptedException`	if interrupted while waiting
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this queue
`java.lang.NullPointerException`	if the specified element is null
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this queue

remainingCapacity

Added in API level 1

abstract fun remainingCapacity(): Int

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.

Return
`Int`	the remaining capacity

remove

Added in API level 1

abstract fun remove(element: E): Boolean

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).

Parameters
`o`	element to be removed from this queue, if present

Return
`Boolean`	`true` if this queue changed as a result of the call

Exceptions
`java.lang.ClassCastException`	if the class of the specified element is incompatible with this queue (optional)
`java.lang.NullPointerException`	if the specified element is null (optional)
`java.lang.UnsupportedOperationException`	if the remove operation is not supported by this collection

take

Added in API level 1

abstract fun take(): E

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

Return
`E`	the head of this queue

Exceptions
`java.lang.InterruptedException`	if interrupted while waiting