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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent; |
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|
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import java.util.Collection; |
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import java.util.Queue; |
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|
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/** |
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* A {@link Queue} that additionally supports operations that wait for |
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* the queue to become non-empty when retrieving an element, and wait |
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* for space to become available in the queue when storing an element. |
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* |
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* <p>{@code BlockingQueue} methods come in four forms, with different ways |
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* of handling operations that cannot be satisfied immediately, but may be |
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* satisfied at some point in the future: |
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* one throws an exception, the second returns a special value (either |
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* {@code null} or {@code false}, depending on the operation), the third |
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* blocks the current thread indefinitely until the operation can succeed, |
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* and the fourth blocks for only a given maximum time limit before giving |
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* up. These methods are summarized in the following table: |
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* |
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* <table class="plain"> |
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* <caption>Summary of BlockingQueue methods</caption> |
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* <tr> |
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* <td></td> |
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* <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th> |
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* <th scope="col" style="font-weight:normal; font-style:italic">Special value</th> |
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* <th scope="col" style="font-weight:normal; font-style:italic">Blocks</th> |
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* <th scope="col" style="font-weight:normal; font-style:italic">Times out</th> |
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* </tr> |
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* <tr> |
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* <th scope="row" style="text-align:left">Insert</th> |
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* <td>{@link #add(Object) add(e)}</td> |
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* <td>{@link #offer(Object) offer(e)}</td> |
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* <td>{@link #put(Object) put(e)}</td> |
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* <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td> |
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* </tr> |
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* <tr> |
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* <th scope="row" style="text-align:left">Remove</th> |
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* <td>{@link #remove() remove()}</td> |
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* <td>{@link #poll() poll()}</td> |
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* <td>{@link #take() take()}</td> |
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* <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td> |
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* </tr> |
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* <tr> |
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* <th scope="row" style="text-align:left">Examine</th> |
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* <td>{@link #element() element()}</td> |
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* <td>{@link #peek() peek()}</td> |
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* <td style="font-style: italic">not applicable</td> |
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* <td style="font-style: italic">not applicable</td> |
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* </tr> |
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* </table> |
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* |
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* <p>A {@code BlockingQueue} does not accept {@code null} elements. |
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* Implementations throw {@code NullPointerException} on attempts |
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* to {@code add}, {@code put} or {@code offer} a {@code null}. A |
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* {@code null} is used as a sentinel value to indicate failure of |
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* {@code poll} operations. |
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* |
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* <p>A {@code BlockingQueue} may be capacity bounded. At any given |
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* time it may have a {@code remainingCapacity} beyond which no |
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* additional elements can be {@code put} without blocking. |
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* A {@code BlockingQueue} without any intrinsic capacity constraints always |
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* reports a remaining capacity of {@code Integer.MAX_VALUE}. |
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* |
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* <p>{@code BlockingQueue} implementations are designed to be used |
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* primarily for producer-consumer queues, but additionally support |
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* the {@link Collection} interface. So, for example, it is |
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* possible to remove an arbitrary element from a queue using |
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* {@code remove(x)}. However, such operations are in general |
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* <em>not</em> performed very efficiently, and are intended for only |
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* occasional use, such as when a queued message is cancelled. |
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* |
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* <p>{@code BlockingQueue} implementations are thread-safe. All |
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* queuing methods achieve their effects atomically using internal |
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* locks or other forms of concurrency control. However, the |
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* <em>bulk</em> Collection operations {@code addAll}, |
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* {@code containsAll}, {@code retainAll} and {@code removeAll} are |
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* <em>not</em> necessarily performed atomically unless specified |
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* otherwise in an implementation. So it is possible, for example, for |
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* {@code addAll(c)} to fail (throwing an exception) after adding |
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* only some of the elements in {@code c}. |
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* |
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* <p>A {@code BlockingQueue} does <em>not</em> intrinsically support |
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* any kind of "close" or "shutdown" operation to |
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* indicate that no more items will be added. The needs and usage of |
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* such features tend to be implementation-dependent. For example, a |
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* common tactic is for producers to insert special |
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* <em>end-of-stream</em> or <em>poison</em> objects, that are |
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* interpreted accordingly when taken by consumers. |
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* |
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* <p> |
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* Usage example, based on a typical producer-consumer scenario. |
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* Note that a {@code BlockingQueue} can safely be used with multiple |
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* producers and multiple consumers. |
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* <pre> {@code |
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* class Producer implements Runnable { |
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* private final BlockingQueue queue; |
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* Producer(BlockingQueue q) { queue = q; } |
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* public void run() { |
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* try { |
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* while (true) { queue.put(produce()); } |
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* } catch (InterruptedException ex) { ... handle ...} |
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* } |
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* Object produce() { ... } |
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* } |
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* |
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* class Consumer implements Runnable { |
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* private final BlockingQueue queue; |
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* Consumer(BlockingQueue q) { queue = q; } |
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* public void run() { |
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* try { |
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* while (true) { consume(queue.take()); } |
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* } catch (InterruptedException ex) { ... handle ...} |
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* } |
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* void consume(Object x) { ... } |
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* } |
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* |
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* class Setup { |
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* void main() { |
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* BlockingQueue q = new SomeQueueImplementation(); |
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* Producer p = new Producer(q); |
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* Consumer c1 = new Consumer(q); |
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* Consumer c2 = new Consumer(q); |
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* new Thread(p).start(); |
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* new Thread(c1).start(); |
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* new Thread(c2).start(); |
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* } |
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* }}</pre> |
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* |
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* <p>Memory consistency effects: As with other concurrent |
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* collections, actions in a thread prior to placing an object into a |
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* {@code BlockingQueue} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions subsequent to the access or removal of that element from |
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* the {@code BlockingQueue} in another thread. |
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* |
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* <p>This interface is a member of the |
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* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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* @param <E> the type of elements held in this queue |
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*/ |
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public interface BlockingQueue<E> extends Queue<E> { |
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/** |
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* Inserts the specified element into this queue if it is possible to do |
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* so immediately without violating capacity restrictions, returning |
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* {@code true} upon success and throwing an |
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* {@code IllegalStateException} if no space is currently available. |
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* When using a capacity-restricted queue, it is generally preferable to |
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* use {@link #offer(Object) offer}. |
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* |
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* @param e the element to add |
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* @return {@code true} (as specified by {@link Collection#add}) |
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* @throws IllegalStateException if the element cannot be added at this |
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* time due to capacity restrictions |
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* @throws ClassCastException if the class of the specified element |
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* prevents it from being added to this queue |
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* @throws NullPointerException if the specified element is null |
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* @throws IllegalArgumentException if some property of the specified |
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* element prevents it from being added to this queue |
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*/ |
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boolean add(E e); |
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|
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/** |
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* Inserts the specified element into this queue if it is possible to do |
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* so immediately without violating capacity restrictions, returning |
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* {@code true} upon success and {@code false} if no space is currently |
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* available. When using a capacity-restricted queue, this method is |
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* generally preferable to {@link #add}, which can fail to insert an |
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* element only by throwing an exception. |
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* |
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* @param e the element to add |
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* @return {@code true} if the element was added to this queue, else |
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* {@code false} |
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* @throws ClassCastException if the class of the specified element |
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* prevents it from being added to this queue |
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* @throws NullPointerException if the specified element is null |
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* @throws IllegalArgumentException if some property of the specified |
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* element prevents it from being added to this queue |
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*/ |
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boolean offer(E e); |
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|
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/** |
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* Inserts the specified element into this queue, waiting if necessary |
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* for space to become available. |
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* |
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* @param e the element to add |
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* @throws InterruptedException if interrupted while waiting |
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* @throws ClassCastException if the class of the specified element |
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* prevents it from being added to this queue |
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* @throws NullPointerException if the specified element is null |
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* @throws IllegalArgumentException if some property of the specified |
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* element prevents it from being added to this queue |
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*/ |
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void put(E e) throws InterruptedException; |
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|
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/** |
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* Inserts the specified element into this queue, waiting up to the |
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* specified wait time if necessary for space to become available. |
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* |
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* @param e the element to add |
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* @param timeout how long to wait before giving up, in units of |
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* {@code unit} |
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* @param unit a {@code TimeUnit} determining how to interpret the |
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* {@code timeout} parameter |
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* @return {@code true} if successful, or {@code false} if |
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* the specified waiting time elapses before space is available |
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* @throws InterruptedException if interrupted while waiting |
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* @throws ClassCastException if the class of the specified element |
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* prevents it from being added to this queue |
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* @throws NullPointerException if the specified element is null |
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* @throws IllegalArgumentException if some property of the specified |
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* element prevents it from being added to this queue |
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*/ |
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boolean offer(E e, long timeout, TimeUnit unit) |
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throws InterruptedException; |
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|
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/** |
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* Retrieves and removes the head of this queue, waiting if necessary |
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* until an element becomes available. |
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* |
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* @return the head of this queue |
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* @throws InterruptedException if interrupted while waiting |
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*/ |
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E take() throws InterruptedException; |
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|
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/** |
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* Retrieves and removes the head of this queue, waiting up to the |
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* specified wait time if necessary for an element to become available. |
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* |
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* @param timeout how long to wait before giving up, in units of |
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* {@code unit} |
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* @param unit a {@code TimeUnit} determining how to interpret the |
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* {@code timeout} parameter |
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* @return the head of this queue, or {@code null} if the |
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* specified waiting time elapses before an element is available |
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* @throws InterruptedException if interrupted while waiting |
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*/ |
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E poll(long timeout, TimeUnit unit) |
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throws InterruptedException; |
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|
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/** |
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* Returns the number of additional elements that this queue can ideally |
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* (in the absence of memory or resource constraints) accept without |
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* blocking, or {@code Integer.MAX_VALUE} if there is no intrinsic |
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* limit. |
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* |
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* <p>Note that you <em>cannot</em> always tell if an attempt to insert |
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* an element will succeed by inspecting {@code remainingCapacity} |
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* because it may be the case that another thread is about to |
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* insert or remove an element. |
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* |
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* @return the remaining capacity |
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*/ |
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int remainingCapacity(); |
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|
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/** |
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* Removes a single instance of the specified element from this queue, |
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* if it is present. More formally, removes an element {@code e} such |
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* that {@code o.equals(e)}, if this queue contains one or more such |
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* elements. |
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* Returns {@code true} if this queue contained the specified element |
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* (or equivalently, if this queue changed as a result of the call). |
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* |
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* @param o element to be removed from this queue, if present |
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* @return {@code true} if this queue changed as a result of the call |
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* @throws ClassCastException if the class of the specified element |
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* is incompatible with this queue |
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* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) |
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* @throws NullPointerException if the specified element is null |
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* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) |
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*/ |
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boolean remove(Object o); |
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|
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/** |
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* Returns {@code true} if this queue contains the specified element. |
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* More formally, returns {@code true} if and only if this queue contains |
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* at least one element {@code e} such that {@code o.equals(e)}. |
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* |
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* @param o object to be checked for containment in this queue |
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* @return {@code true} if this queue contains the specified element |
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* @throws ClassCastException if the class of the specified element |
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* is incompatible with this queue |
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* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) |
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* @throws NullPointerException if the specified element is null |
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* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) |
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*/ |
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boolean contains(Object o); |
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|
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/** |
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* Removes all available elements from this queue and adds them |
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* to the given collection. This operation may be more |
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* efficient than repeatedly polling this queue. A failure |
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* encountered while attempting to add elements to |
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* collection {@code c} may result in elements being in neither, |
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* either or both collections when the associated exception is |
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* thrown. Attempts to drain a queue to itself result in |
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* {@code IllegalArgumentException}. Further, the behavior of |
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* this operation is undefined if the specified collection is |
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* modified while the operation is in progress. |
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* |
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* @param c the collection to transfer elements into |
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* @return the number of elements transferred |
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* @throws UnsupportedOperationException if addition of elements |
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* is not supported by the specified collection |
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* @throws ClassCastException if the class of an element of this queue |
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* prevents it from being added to the specified collection |
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* @throws NullPointerException if the specified collection is null |
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* @throws IllegalArgumentException if the specified collection is this |
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* queue, or some property of an element of this queue prevents |
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* it from being added to the specified collection |
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*/ |
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int drainTo(Collection<? super E> c); |
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|
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/** |
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* Removes at most the given number of available elements from |
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* this queue and adds them to the given collection. A failure |
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* encountered while attempting to add elements to |
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* collection {@code c} may result in elements being in neither, |
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* either or both collections when the associated exception is |
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* thrown. Attempts to drain a queue to itself result in |
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* {@code IllegalArgumentException}. Further, the behavior of |
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* this operation is undefined if the specified collection is |
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* modified while the operation is in progress. |
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* |
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* @param c the collection to transfer elements into |
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* @param maxElements the maximum number of elements to transfer |
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* @return the number of elements transferred |
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* @throws UnsupportedOperationException if addition of elements |
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* is not supported by the specified collection |
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* @throws ClassCastException if the class of an element of this queue |
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* prevents it from being added to the specified collection |
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* @throws NullPointerException if the specified collection is null |
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* @throws IllegalArgumentException if the specified collection is this |
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* queue, or some property of an element of this queue prevents |
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* it from being added to the specified collection |
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*/ |
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int drainTo(Collection<? super E> c, int maxElements); |
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} |
