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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
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  /*
   * This file is available under and governed by the GNU General Public
   * License version 2 only, as published by the Free Software Foundation.
   * However, the following notice accompanied the original version of this
   * file:
   *
   * Written by Doug Lea with assistance from members of JCP JSR-166
   * Expert Group and released to the public domain, as explained at
   * http://creativecommons.org/publicdomain/zero/1.0/
   */
  
  package java.util.concurrent;
  
  import java.util.Collection;
  import java.util.Queue;
  
  // BEGIN android-note
  // removed link to collections framework docs from header
  // fixed framework docs link to "Collection#optional"
  // END android-note
  
  /**
   * 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 BORDER CELLPADDING=3 CELLSPACING=1>
   * <caption>Summary of BlockingQueue methods</caption>
   *  <tr>
   *    <td></td>
   *    <td ALIGN=CENTER><em>Throws exception</em></td>
   *    <td ALIGN=CENTER><em>Special value</em></td>
   *    <td ALIGN=CENTER><em>Blocks</em></td>
   *    <td ALIGN=CENTER><em>Times out</em></td>
   *  </tr>
   *  <tr>
   *    <td><b>Insert</b></td>
   *    <td>{@link #add add(e)}</td>
   *    <td>{@link #offer offer(e)}</td>
   *    <td>{@link #put put(e)}</td>
   *    <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td>
   *  </tr>
   *  <tr>
   *    <td><b>Remove</b></td>
   *    <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>
   *    <td><b>Examine</b></td>
   *    <td>{@link #element element()}</td>
   *    <td>{@link #peek peek()}</td>
   *    <td><em>not applicable</em></td>
   *    <td><em>not applicable</em></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 &quot;close&quot; or &quot;shutdown&quot; 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; }
   *   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();
   *   }
   * }}</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.
   *
   * @since 1.5
   * @author Doug Lea
   * @param <E> the type of elements held in this queue
   */
  public interface BlockingQueue<E> extends Queue<E> {
      /**
       * 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}.
       *
       * @param e the element to add
       * @return {@code true} (as specified by {@link Collection#add})
       * @throws IllegalStateException if the element cannot be added at this
       *         time due to capacity restrictions
       * @throws ClassCastException if the class of the specified element
       *         prevents it from being added to this queue
       * @throws NullPointerException if the specified element is null
       * @throws IllegalArgumentException if some property of the specified
       *         element prevents it from being added to this queue
       */
      boolean add(E e);
  
      /**
       * 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.
       *
       * @param e the element to add
       * @return {@code true} if the element was added to this queue, else
       *         {@code false}
       * @throws ClassCastException if the class of the specified element
       *         prevents it from being added to this queue
       * @throws NullPointerException if the specified element is null
       * @throws IllegalArgumentException if some property of the specified
       *         element prevents it from being added to this queue
       */
      boolean offer(E e);
  
      /**
       * Inserts the specified element into this queue, waiting if necessary
       * for space to become available.
       *
       * @param e the element to add
       * @throws InterruptedException if interrupted while waiting
       * @throws ClassCastException if the class of the specified element
       *         prevents it from being added to this queue
       * @throws NullPointerException if the specified element is null
       * @throws IllegalArgumentException if some property of the specified
       *         element prevents it from being added to this queue
       */
      void put(E e) throws InterruptedException;
  
      /**
       * Inserts the specified element into this queue, waiting up to the
       * specified wait time if necessary for space to become available.
       *
       * @param e the element to add
       * @param timeout how long to wait before giving up, in units of
       *        {@code unit}
       * @param unit a {@code TimeUnit} determining how to interpret the
       *        {@code timeout} parameter
       * @return {@code true} if successful, or {@code false} if
       *         the specified waiting time elapses before space is available
       * @throws InterruptedException if interrupted while waiting
       * @throws ClassCastException if the class of the specified element
       *         prevents it from being added to this queue
       * @throws NullPointerException if the specified element is null
       * @throws IllegalArgumentException if some property of the specified
       *         element prevents it from being added to this queue
       */
      boolean offer(E e, long timeout, TimeUnit unit)
          throws InterruptedException;
  
      /**
       * Retrieves and removes the head of this queue, waiting if necessary
       * until an element becomes available.
       *
       * @return the head of this queue
       * @throws InterruptedException if interrupted while waiting
       */
      E take() 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.
       *
       * @param timeout how long to wait before giving up, in units of
       *        {@code unit}
       * @param unit a {@code TimeUnit} determining how to interpret the
       *        {@code timeout} parameter
       * @return the head of this queue, or {@code null} if the
       *         specified waiting time elapses before an element is available
       * @throws InterruptedException if interrupted while waiting
       */
      E poll(long timeout, TimeUnit unit)
          throws InterruptedException;
  
      /**
       * 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.
       *
       * <p>Note that you <em>cannot</em> always tell if an attempt to insert
       * an element will succeed by inspecting {@code remainingCapacity}
       * because it may be the case that another thread is about to
       * insert or remove an element.
       *
       * @return the remaining capacity
       */
      int remainingCapacity();
  
      /**
       * 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).
       *
       * @param o element to be removed from this queue, if present
       * @return {@code 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
       * (<a href="../Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified element is null
       * (<a href="../Collection.html#optional-restrictions">optional</a>)
       */
      boolean remove(Object o);
  
      /**
       * 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)}.
       *
       * @param o object to be checked for containment in this queue
       * @return {@code true} if this queue contains the specified element
       * @throws ClassCastException if the class of the specified element
       *         is incompatible with this queue
       * (<a href="../Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified element is null
       * (<a href="../Collection.html#optional-restrictions">optional</a>)
       */
      boolean contains(Object o);
  
      /**
       * 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.
       *
       * @param c the collection to transfer elements into
       * @return the number of elements transferred
       * @throws UnsupportedOperationException if addition of elements
       *         is not supported by the specified collection
       * @throws ClassCastException if the class of an element of this queue
       *         prevents it from being added to the specified collection
       * @throws NullPointerException if the specified collection is null
       * @throws 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
       */
      int drainTo(Collection<? super E> c);
  
      /**
       * 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.
       *
       * @param c the collection to transfer elements into
       * @param maxElements the maximum number of elements to transfer
       * @return the number of elements transferred
       * @throws UnsupportedOperationException if addition of elements
       *         is not supported by the specified collection
       * @throws ClassCastException if the class of an element of this queue
       *         prevents it from being added to the specified collection
       * @throws NullPointerException if the specified collection is null
       * @throws 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
       */
      int drainTo(Collection<? super E> c, int maxElements);
  }
  