Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning {@code true} upon success and throwing an {@code IllegalStateException} if no space is currently available. When using a capacity-restricted queue, it is generally preferable to use {@link #offer(Object) offer}.
Returns {@code true} if this queue contains the specified element. More formally, returns {@code true} if and only if this queue contains at least one element {@code e} such that {@code o.equals(e)}.
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 {@code 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 {@code IllegalArgumentException}. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
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 {@code 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 {@code IllegalArgumentException}. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning {@code true} upon success and {@code false} if no space is currently available. When using a capacity-restricted queue, this method is generally preferable to {@link #add}, which can fail to insert an element only by throwing an exception.
Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
Inserts the specified element into this queue, waiting if necessary for space to become available.
Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or {@code Integer.MAX_VALUE} if there is no intrinsic limit.
Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element {@code e} such that {@code o.equals(e)}, if this queue contains one or more such elements. Returns {@code true} if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).
Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
A {@link 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.
<p>{@code 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 {@code null} or {@code 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:
<table class="plain"> <caption>Summary of BlockingQueue methods</caption> <tr> <td></td> <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th> <th scope="col" style="font-weight:normal; font-style:italic">Special value</th> <th scope="col" style="font-weight:normal; font-style:italic">Blocks</th> <th scope="col" style="font-weight:normal; font-style:italic">Times out</th> </tr> <tr> <th scope="row" style="text-align:left">Insert</th> <td>{@link #add(Object) add(e)}</td> <td>{@link #offer(Object) offer(e)}</td> <td>{@link #put(Object) put(e)}</td> <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td> </tr> <tr> <th scope="row" style="text-align:left">Remove</th> <td>{@link #remove() remove()}</td> <td>{@link #poll() poll()}</td> <td>{@link #take() take()}</td> <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td> </tr> <tr> <th scope="row" style="text-align:left">Examine</th> <td>{@link #element() element()}</td> <td>{@link #peek() peek()}</td> <td style="font-style: italic">not applicable</td> <td style="font-style: italic">not applicable</td> </tr> </table>
<p>A {@code BlockingQueue} does not accept {@code null} elements. Implementations throw {@code NullPointerException} on attempts to {@code add}, {@code put} or {@code offer} a {@code null}. A {@code null} is used as a sentinel value to indicate failure of {@code poll} operations.
<p>A {@code BlockingQueue} may be capacity bounded. At any given time it may have a {@code remainingCapacity} beyond which no additional elements can be {@code put} without blocking. A {@code BlockingQueue} without any intrinsic capacity constraints always reports a remaining capacity of {@code Integer.MAX_VALUE}.
<p>{@code BlockingQueue} implementations are designed to be used primarily for producer-consumer queues, but additionally support the {@link Collection} interface. So, for example, it is possible to remove an arbitrary element from a queue using {@code remove(x)}. However, such operations are in general <em>not</em> performed very efficiently, and are intended for only occasional use, such as when a queued message is cancelled.
<p>{@code BlockingQueue} implementations are thread-safe. All queuing methods achieve their effects atomically using internal locks or other forms of concurrency control. However, the <em>bulk</em> Collection operations {@code addAll}, {@code containsAll}, {@code retainAll} and {@code removeAll} are <em>not</em> necessarily performed atomically unless specified otherwise in an implementation. So it is possible, for example, for {@code addAll(c)} to fail (throwing an exception) after adding only some of the elements in {@code c}.
<p>A {@code BlockingQueue} does <em>not</em> 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 <em>end-of-stream</em> or <em>poison</em> objects, that are interpreted accordingly when taken by consumers.
<p> Usage example, based on a typical producer-consumer scenario. Note that a {@code BlockingQueue} can safely be used with multiple producers and multiple consumers. <pre> {@code class Producer implements Runnable { private final BlockingQueue queue; Producer(BlockingQueue q) { queue = q; } 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; } 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(); } }}</pre>
<p>Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a {@code BlockingQueue} <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> actions subsequent to the access or removal of that element from the {@code BlockingQueue} in another thread.
<p>This interface is a member of the <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> Java Collections Framework</a>.
@author Doug Lea @param (E) the type of elements held in this queue