content: /*
   * 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
   * questions.
   */
  
  /*
   * 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.AbstractQueue;
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.Comparator;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.PriorityQueue;
  import java.util.Queue;
  import java.util.SortedSet;
  import java.util.Spliterator;
  import java.util.concurrent.locks.Condition;
  import java.util.concurrent.locks.ReentrantLock;
  import java.util.function.Consumer;
  
  // BEGIN android-note
  // removed link to collections framework docs
  // END android-note
  
  /**
   * An unbounded {@linkplain BlockingQueue blocking queue} that uses
   * the same ordering rules as class {@link PriorityQueue} and supplies
   * blocking retrieval operations.  While this queue is logically
   * unbounded, attempted additions may fail due to resource exhaustion
   * (causing {@code OutOfMemoryError}). This class does not permit
   * {@code null} elements.  A priority queue relying on {@linkplain
   * Comparable natural ordering} also does not permit insertion of
   * non-comparable objects (doing so results in
   * {@code ClassCastException}).
   *
   * <p>This class and its iterator implement all of the
   * <em>optional</em> methods of the {@link Collection} and {@link
   * Iterator} interfaces.  The Iterator provided in method {@link
   * #iterator()} is <em>not</em> guaranteed to traverse the elements of
   * the PriorityBlockingQueue in any particular order. If you need
   * ordered traversal, consider using
   * {@code Arrays.sort(pq.toArray())}.  Also, method {@code drainTo}
   * can be used to <em>remove</em> some or all elements in priority
   * order and place them in another collection.
   *
   * <p>Operations on this class make no guarantees about the ordering
   * of elements with equal priority. If you need to enforce an
   * ordering, you can define custom classes or comparators that use a
   * secondary key to break ties in primary priority values.  For
   * example, here is a class that applies first-in-first-out
   * tie-breaking to comparable elements. To use it, you would insert a
   * {@code new FIFOEntry(anEntry)} instead of a plain entry object.
   *
   * <pre> {@code
   * class FIFOEntry<E extends Comparable<? super E>>
   *     implements Comparable<FIFOEntry<E>> {
   *   static final AtomicLong seq = new AtomicLong(0);
   *   final long seqNum;
   *   final E entry;
   *   public FIFOEntry(E entry) {
   *     seqNum = seq.getAndIncrement();
   *     this.entry = entry;
   *   }
   *   public E getEntry() { return entry; }
   *   public int compareTo(FIFOEntry<E> other) {
   *     int res = entry.compareTo(other.entry);
   *     if (res == 0 && other.entry != this.entry)
   *       res = (seqNum < other.seqNum ? -1 : 1);
   *     return res;
   *   }
   * }}</pre>
   *
   * @since 1.5
   * @author Doug Lea
   * @param <E> the type of elements held in this queue
   */
  @SuppressWarnings("unchecked")
  public class PriorityBlockingQueue<E> extends AbstractQueue<E>
      implements BlockingQueue<E>, java.io.Serializable {
      private static final long serialVersionUID = 5595510919245408276L;
  
      /*
       * The implementation uses an array-based binary heap, with public
       * operations protected with a single lock. However, allocation
       * during resizing uses a simple spinlock (used only while not
       * holding main lock) in order to allow takes to operate
       * concurrently with allocation.  This avoids repeated
       * postponement of waiting consumers and consequent element
       * build-up. The need to back away from lock during allocation
       * makes it impossible to simply wrap delegated
       * java.util.PriorityQueue operations within a lock, as was done
       * in a previous version of this class. To maintain
       * interoperability, a plain PriorityQueue is still used during
       * serialization, which maintains compatibility at the expense of
       * transiently doubling overhead.
       */
  
      /**
       * Default array capacity.
       */
      private static final int DEFAULT_INITIAL_CAPACITY = 11;
  
      /**
       * The maximum size of array to allocate.
       * Some VMs reserve some header words in an array.
       * Attempts to allocate larger arrays may result in
       * OutOfMemoryError: Requested array size exceeds VM limit
       */
      private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
  
      /**
       * Priority queue represented as a balanced binary heap: the two
       * children of queue[n] are queue[2*n+1] and queue[2*(n+1)].  The
       * priority queue is ordered by comparator, or by the elements'
       * natural ordering, if comparator is null: For each node n in the
       * heap and each descendant d of n, n <= d.  The element with the
       * lowest value is in queue[0], assuming the queue is nonempty.
       */
      private transient Object[] queue;
  
      /**
       * The number of elements in the priority queue.
       */
      private transient int size;
  
      /**
       * The comparator, or null if priority queue uses elements'
       * natural ordering.
       */
      private transient Comparator<? super E> comparator;
  
      /**
       * Lock used for all public operations.
       */
      private final ReentrantLock lock;
  
      /**
       * Condition for blocking when empty.
       */
      private final Condition notEmpty;
  
      /**
       * Spinlock for allocation, acquired via CAS.
       */
      private transient volatile int allocationSpinLock;
  
      /**
       * A plain PriorityQueue used only for serialization,
       * to maintain compatibility with previous versions
       * of this class. Non-null only during serialization/deserialization.
       */
      private PriorityQueue<E> q;
  
      /**
       * Creates a {@code PriorityBlockingQueue} with the default
       * initial capacity (11) that orders its elements according to
       * their {@linkplain Comparable natural ordering}.
       */
      public PriorityBlockingQueue() {
          this(DEFAULT_INITIAL_CAPACITY, null);
      }
  
      /**
       * Creates a {@code PriorityBlockingQueue} with the specified
       * initial capacity that orders its elements according to their
       * {@linkplain Comparable natural ordering}.
       *
       * @param initialCapacity the initial capacity for this priority queue
       * @throws IllegalArgumentException if {@code initialCapacity} is less
       *         than 1
       */
      public PriorityBlockingQueue(int initialCapacity) {
          this(initialCapacity, null);
      }
  
      /**
       * Creates a {@code PriorityBlockingQueue} with the specified initial
       * capacity that orders its elements according to the specified
       * comparator.
       *
       * @param initialCapacity the initial capacity for this priority queue
       * @param  comparator the comparator that will be used to order this
       *         priority queue.  If {@code null}, the {@linkplain Comparable
       *         natural ordering} of the elements will be used.
       * @throws IllegalArgumentException if {@code initialCapacity} is less
       *         than 1
       */
      public PriorityBlockingQueue(int initialCapacity,
                                   Comparator<? super E> comparator) {
          if (initialCapacity < 1)
              throw new IllegalArgumentException();
          this.lock = new ReentrantLock();
          this.notEmpty = lock.newCondition();
          this.comparator = comparator;
          this.queue = new Object[initialCapacity];
      }
  
      /**
       * Creates a {@code PriorityBlockingQueue} containing the elements
       * in the specified collection.  If the specified collection is a
       * {@link SortedSet} or a {@link PriorityQueue}, this
       * priority queue will be ordered according to the same ordering.
       * Otherwise, this priority queue will be ordered according to the
       * {@linkplain Comparable natural ordering} of its elements.
       *
       * @param  c the collection whose elements are to be placed
       *         into this priority queue
       * @throws ClassCastException if elements of the specified collection
       *         cannot be compared to one another according to the priority
       *         queue's ordering
       * @throws NullPointerException if the specified collection or any
       *         of its elements are null
       */
      public PriorityBlockingQueue(Collection<? extends E> c) {
          this.lock = new ReentrantLock();
          this.notEmpty = lock.newCondition();
          boolean heapify = true; // true if not known to be in heap order
          boolean screen = true;  // true if must screen for nulls
          if (c instanceof SortedSet<?>) {
              SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
              this.comparator = (Comparator<? super E>) ss.comparator();
              heapify = false;
          }
          else if (c instanceof PriorityBlockingQueue<?>) {
              PriorityBlockingQueue<? extends E> pq =
                  (PriorityBlockingQueue<? extends E>) c;
              this.comparator = (Comparator<? super E>) pq.comparator();
              screen = false;
              if (pq.getClass() == PriorityBlockingQueue.class) // exact match
                  heapify = false;
          }
          Object[] a = c.toArray();
          int n = a.length;
          // If c.toArray incorrectly doesn't return Object[], copy it.
          if (a.getClass() != Object[].class)
              a = Arrays.copyOf(a, n, Object[].class);
          if (screen && (n == 1 || this.comparator != null)) {
              for (int i = 0; i < n; ++i)
                  if (a[i] == null)
                      throw new NullPointerException();
          }
          this.queue = a;
          this.size = n;
          if (heapify)
              heapify();
      }
  
      /**
       * Tries to grow array to accommodate at least one more element
       * (but normally expand by about 50%), giving up (allowing retry)
       * on contention (which we expect to be rare). Call only while
       * holding lock.
       *
       * @param array the heap array
       * @param oldCap the length of the array
       */
      private void tryGrow(Object[] array, int oldCap) {
          lock.unlock(); // must release and then re-acquire main lock
          Object[] newArray = null;
          if (allocationSpinLock == 0 &&
              U.compareAndSwapInt(this, ALLOCATIONSPINLOCK, 0, 1)) {
              try {
                  int newCap = oldCap + ((oldCap < 64) ?
                                         (oldCap + 2) : // grow faster if small
                                         (oldCap >> 1));
                  if (newCap - MAX_ARRAY_SIZE > 0) {    // possible overflow
                      int minCap = oldCap + 1;
                      if (minCap < 0 || minCap > MAX_ARRAY_SIZE)
                          throw new OutOfMemoryError();
                      newCap = MAX_ARRAY_SIZE;
                  }
                  if (newCap > oldCap && queue == array)
                      newArray = new Object[newCap];
              } finally {
                  allocationSpinLock = 0;
              }
          }
          if (newArray == null) // back off if another thread is allocating
              Thread.yield();
          lock.lock();
          if (newArray != null && queue == array) {
              queue = newArray;
              System.arraycopy(array, 0, newArray, 0, oldCap);
          }
      }
  
      /**
       * Mechanics for poll().  Call only while holding lock.
       */
      private E dequeue() {
          int n = size - 1;
          if (n < 0)
              return null;
          else {
              Object[] array = queue;
              E result = (E) array[0];
              E x = (E) array[n];
              array[n] = null;
              Comparator<? super E> cmp = comparator;
              if (cmp == null)
                  siftDownComparable(0, x, array, n);
              else
                  siftDownUsingComparator(0, x, array, n, cmp);
              size = n;
              return result;
          }
      }
  
      /**
       * Inserts item x at position k, maintaining heap invariant by
       * promoting x up the tree until it is greater than or equal to
       * its parent, or is the root.
       *
       * To simplify and speed up coercions and comparisons. the
       * Comparable and Comparator versions are separated into different
       * methods that are otherwise identical. (Similarly for siftDown.)
       * These methods are static, with heap state as arguments, to
       * simplify use in light of possible comparator exceptions.
       *
       * @param k the position to fill
       * @param x the item to insert
       * @param array the heap array
       */
      private static <T> void siftUpComparable(int k, T x, Object[] array) {
          Comparable<? super T> key = (Comparable<? super T>) x;
          while (k > 0) {
              int parent = (k - 1) >>> 1;
              Object e = array[parent];
              if (key.compareTo((T) e) >= 0)
                  break;
              array[k] = e;
              k = parent;
          }
          array[k] = key;
      }
  
      private static <T> void siftUpUsingComparator(int k, T x, Object[] array,
                                         Comparator<? super T> cmp) {
          while (k > 0) {
              int parent = (k - 1) >>> 1;
              Object e = array[parent];
              if (cmp.compare(x, (T) e) >= 0)
                  break;
              array[k] = e;
              k = parent;
          }
          array[k] = x;
      }
  
      /**
       * Inserts item x at position k, maintaining heap invariant by
       * demoting x down the tree repeatedly until it is less than or
       * equal to its children or is a leaf.
       *
       * @param k the position to fill
       * @param x the item to insert
       * @param array the heap array
       * @param n heap size
       */
      private static <T> void siftDownComparable(int k, T x, Object[] array,
                                                 int n) {
          if (n > 0) {
              Comparable<? super T> key = (Comparable<? super T>)x;
              int half = n >>> 1;           // loop while a non-leaf
              while (k < half) {
                  int child = (k << 1) + 1; // assume left child is least
                  Object c = array[child];
                  int right = child + 1;
                  if (right < n &&
                      ((Comparable<? super T>) c).compareTo((T) array[right]) > 0)
                      c = array[child = right];
                  if (key.compareTo((T) c) <= 0)
                      break;
                  array[k] = c;
                  k = child;
              }
              array[k] = key;
          }
      }
  
      private static <T> void siftDownUsingComparator(int k, T x, Object[] array,
                                                      int n,
                                                      Comparator<? super T> cmp) {
          if (n > 0) {
              int half = n >>> 1;
              while (k < half) {
                  int child = (k << 1) + 1;
                  Object c = array[child];
                  int right = child + 1;
                  if (right < n && cmp.compare((T) c, (T) array[right]) > 0)
                      c = array[child = right];
                  if (cmp.compare(x, (T) c) <= 0)
                      break;
                  array[k] = c;
                  k = child;
              }
              array[k] = x;
          }
      }
  
      /**
       * Establishes the heap invariant (described above) in the entire tree,
       * assuming nothing about the order of the elements prior to the call.
       */
      private void heapify() {
          Object[] array = queue;
          int n = size;
          int half = (n >>> 1) - 1;
          Comparator<? super E> cmp = comparator;
          if (cmp == null) {
              for (int i = half; i >= 0; i--)
                  siftDownComparable(i, (E) array[i], array, n);
          }
          else {
              for (int i = half; i >= 0; i--)
                  siftDownUsingComparator(i, (E) array[i], array, n, cmp);
          }
      }
  
      /**
       * Inserts the specified element into this priority queue.
       *
       * @param e the element to add
       * @return {@code true} (as specified by {@link Collection#add})
       * @throws ClassCastException if the specified element cannot be compared
       *         with elements currently in the priority queue according to the
       *         priority queue's ordering
       * @throws NullPointerException if the specified element is null
       */
      public boolean add(E e) {
          return offer(e);
      }
  
      /**
       * Inserts the specified element into this priority queue.
       * As the queue is unbounded, this method will never return {@code false}.
       *
       * @param e the element to add
       * @return {@code true} (as specified by {@link Queue#offer})
       * @throws ClassCastException if the specified element cannot be compared
       *         with elements currently in the priority queue according to the
       *         priority queue's ordering
       * @throws NullPointerException if the specified element is null
       */
      public boolean offer(E e) {
          if (e == null)
              throw new NullPointerException();
          final ReentrantLock lock = this.lock;
          lock.lock();
          int n, cap;
          Object[] array;
          while ((n = size) >= (cap = (array = queue).length))
              tryGrow(array, cap);
          try {
              Comparator<? super E> cmp = comparator;
              if (cmp == null)
                  siftUpComparable(n, e, array);
              else
                  siftUpUsingComparator(n, e, array, cmp);
              size = n + 1;
              notEmpty.signal();
          } finally {
              lock.unlock();
          }
          return true;
      }
  
      /**
       * Inserts the specified element into this priority queue.
       * As the queue is unbounded, this method will never block.
       *
       * @param e the element to add
       * @throws ClassCastException if the specified element cannot be compared
       *         with elements currently in the priority queue according to the
       *         priority queue's ordering
       * @throws NullPointerException if the specified element is null
       */
      public void put(E e) {
          offer(e); // never need to block
      }
  
      /**
       * Inserts the specified element into this priority queue.
       * As the queue is unbounded, this method will never block or
       * return {@code false}.
       *
       * @param e the element to add
       * @param timeout This parameter is ignored as the method never blocks
       * @param unit This parameter is ignored as the method never blocks
       * @return {@code true} (as specified by
       *  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
       * @throws ClassCastException if the specified element cannot be compared
       *         with elements currently in the priority queue according to the
       *         priority queue's ordering
       * @throws NullPointerException if the specified element is null
       */
      public boolean offer(E e, long timeout, TimeUnit unit) {
          return offer(e); // never need to block
      }
  
      public E poll() {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              return dequeue();
          } finally {
              lock.unlock();
          }
      }
  
      public E take() throws InterruptedException {
          final ReentrantLock lock = this.lock;
          lock.lockInterruptibly();
          E result;
          try {
              while ( (result = dequeue()) == null)
                  notEmpty.await();
          } finally {
              lock.unlock();
          }
          return result;
      }
  
      public E poll(long timeout, TimeUnit unit) throws InterruptedException {
          long nanos = unit.toNanos(timeout);
          final ReentrantLock lock = this.lock;
          lock.lockInterruptibly();
          E result;
          try {
              while ( (result = dequeue()) == null && nanos > 0)
                  nanos = notEmpty.awaitNanos(nanos);
          } finally {
              lock.unlock();
          }
          return result;
      }
  
      public E peek() {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              return (size == 0) ? null : (E) queue[0];
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * Returns the comparator used to order the elements in this queue,
       * or {@code null} if this queue uses the {@linkplain Comparable
       * natural ordering} of its elements.
       *
       * @return the comparator used to order the elements in this queue,
       *         or {@code null} if this queue uses the natural
       *         ordering of its elements
       */
      public Comparator<? super E> comparator() {
          return comparator;
      }
  
      public int size() {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              return size;
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * Always returns {@code Integer.MAX_VALUE} because
       * a {@code PriorityBlockingQueue} is not capacity constrained.
       * @return {@code Integer.MAX_VALUE} always
       */
      public int remainingCapacity() {
          return Integer.MAX_VALUE;
      }
  
      private int indexOf(Object o) {
          if (o != null) {
              Object[] array = queue;
              int n = size;
              for (int i = 0; i < n; i++)
                  if (o.equals(array[i]))
                      return i;
          }
          return -1;
      }
  
      /**
       * Removes the ith element from queue.
       */
      private void removeAt(int i) {
          Object[] array = queue;
          int n = size - 1;
          if (n == i) // removed last element
              array[i] = null;
          else {
              E moved = (E) array[n];
              array[n] = null;
              Comparator<? super E> cmp = comparator;
              if (cmp == null)
                  siftDownComparable(i, moved, array, n);
              else
                  siftDownUsingComparator(i, moved, array, n, cmp);
              if (array[i] == moved) {
                  if (cmp == null)
                      siftUpComparable(i, moved, array);
                  else
                      siftUpUsingComparator(i, moved, array, cmp);
              }
          }
          size = n;
      }
  
      /**
       * 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 and only 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
       */
      public boolean remove(Object o) {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              int i = indexOf(o);
              if (i == -1)
                  return false;
              removeAt(i);
              return true;
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * Identity-based version for use in Itr.remove.
       */
      void removeEQ(Object o) {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              Object[] array = queue;
              for (int i = 0, n = size; i < n; i++) {
                  if (o == array[i]) {
                      removeAt(i);
                      break;
                  }
              }
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * 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
       */
      public boolean contains(Object o) {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              return indexOf(o) != -1;
          } finally {
              lock.unlock();
          }
      }
  
      public String toString() {
          return Helpers.collectionToString(this);
      }
  
      /**
       * @throws UnsupportedOperationException {@inheritDoc}
       * @throws ClassCastException            {@inheritDoc}
       * @throws NullPointerException          {@inheritDoc}
       * @throws IllegalArgumentException      {@inheritDoc}
       */
      public int drainTo(Collection<? super E> c) {
          return drainTo(c, Integer.MAX_VALUE);
      }
  
      /**
       * @throws UnsupportedOperationException {@inheritDoc}
       * @throws ClassCastException            {@inheritDoc}
       * @throws NullPointerException          {@inheritDoc}
       * @throws IllegalArgumentException      {@inheritDoc}
       */
      public int drainTo(Collection<? super E> c, int maxElements) {
          if (c == null)
              throw new NullPointerException();
          if (c == this)
              throw new IllegalArgumentException();
          if (maxElements <= 0)
              return 0;
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              int n = Math.min(size, maxElements);
              for (int i = 0; i < n; i++) {
                  c.add((E) queue[0]); // In this order, in case add() throws.
                  dequeue();
              }
              return n;
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * Atomically removes all of the elements from this queue.
       * The queue will be empty after this call returns.
       */
      public void clear() {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              Object[] array = queue;
              int n = size;
              size = 0;
              for (int i = 0; i < n; i++)
                  array[i] = null;
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * Returns an array containing all of the elements in this queue.
       * The returned array elements are in no particular order.
       *
       * <p>The returned array will be "safe" in that no references to it are
       * maintained by this queue.  (In other words, this method must allocate
       * a new array).  The caller is thus free to modify the returned array.
       *
       * <p>This method acts as bridge between array-based and collection-based
       * APIs.
       *
       * @return an array containing all of the elements in this queue
       */
      public Object[] toArray() {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              return Arrays.copyOf(queue, size);
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * Returns an array containing all of the elements in this queue; the
       * runtime type of the returned array is that of the specified array.
       * The returned array elements are in no particular order.
       * If the queue fits in the specified array, it is returned therein.
       * Otherwise, a new array is allocated with the runtime type of the
       * specified array and the size of this queue.
       *
       * <p>If this queue fits in the specified array with room to spare
       * (i.e., the array has more elements than this queue), the element in
       * the array immediately following the end of the queue is set to
       * {@code null}.
       *
       * <p>Like the {@link #toArray()} method, this method acts as bridge between
       * array-based and collection-based APIs.  Further, this method allows
       * precise control over the runtime type of the output array, and may,
       * under certain circumstances, be used to save allocation costs.
       *
       * <p>Suppose {@code x} is a queue known to contain only strings.
       * The following code can be used to dump the queue into a newly
       * allocated array of {@code String}:
       *
       * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
       *
       * Note that {@code toArray(new Object[0])} is identical in function to
       * {@code toArray()}.
       *
       * @param a the array into which the elements of the queue are to
       *          be stored, if it is big enough; otherwise, a new array of the
       *          same runtime type is allocated for this purpose
       * @return an array containing all of the elements in this queue
       * @throws ArrayStoreException if the runtime type of the specified array
       *         is not a supertype of the runtime type of every element in
       *         this queue
       * @throws NullPointerException if the specified array is null
       */
      public <T> T[] toArray(T[] a) {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              int n = size;
              if (a.length < n)
                  // Make a new array of a's runtime type, but my contents:
                  return (T[]) Arrays.copyOf(queue, size, a.getClass());
              System.arraycopy(queue, 0, a, 0, n);
              if (a.length > n)
                  a[n] = null;
              return a;
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * Returns an iterator over the elements in this queue. The
       * iterator does not return the elements in any particular order.
       *
       * <p>The returned iterator is
       * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
       *
       * @return an iterator over the elements in this queue
       */
      public Iterator<E> iterator() {
          return new Itr(toArray());
      }
  
      /**
       * Snapshot iterator that works off copy of underlying q array.
       */
      final class Itr implements Iterator<E> {
          final Object[] array; // Array of all elements
          int cursor;           // index of next element to return
          int lastRet;          // index of last element, or -1 if no such
  
          Itr(Object[] array) {
              lastRet = -1;
              this.array = array;
          }
  
          public boolean hasNext() {
              return cursor < array.length;
          }
  
          public E next() {
              if (cursor >= array.length)
                  throw new NoSuchElementException();
              lastRet = cursor;
              return (E)array[cursor++];
          }
  
          public void remove() {
              if (lastRet < 0)
                  throw new IllegalStateException();
              removeEQ(array[lastRet]);
              lastRet = -1;
          }
      }
  
      /**
       * Saves this queue to a stream (that is, serializes it).
       *
       * For compatibility with previous version of this class, elements
       * are first copied to a java.util.PriorityQueue, which is then
       * serialized.
       *
       * @param s the stream
       * @throws java.io.IOException if an I/O error occurs
       */
      private void writeObject(java.io.ObjectOutputStream s)
          throws java.io.IOException {
          lock.lock();
          try {
              // avoid zero capacity argument
              q = new PriorityQueue<E>(Math.max(size, 1), comparator);
              q.addAll(this);
              s.defaultWriteObject();
          } finally {
              q = null;
              lock.unlock();
          }
      }
  
      /**
       * Reconstitutes this queue from a stream (that is, deserializes it).
       * @param s the stream
       * @throws ClassNotFoundException if the class of a serialized object
       *         could not be found
       * @throws java.io.IOException if an I/O error occurs
       */
      private void readObject(java.io.ObjectInputStream s)
          throws java.io.IOException, ClassNotFoundException {
          try {
              s.defaultReadObject();
              this.queue = new Object[q.size()];
              comparator = q.comparator();
              addAll(q);
          } finally {
              q = null;
          }
      }
  
      // Similar to Collections.ArraySnapshotSpliterator but avoids
      // commitment to toArray until needed
      static final class PBQSpliterator<E> implements Spliterator<E> {
          final PriorityBlockingQueue<E> queue;
          Object[] array;
          int index;
          int fence;
  
          PBQSpliterator(PriorityBlockingQueue<E> queue, Object[] array,
                         int index, int fence) {
              this.queue = queue;
              this.array = array;
              this.index = index;
              this.fence = fence;
          }
  
          final int getFence() {
              int hi;
              if ((hi = fence) < 0)
                  hi = fence = (array = queue.toArray()).length;
              return hi;
          }
  
          public PBQSpliterator<E> trySplit() {
              int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
              return (lo >= mid) ? null :
                  new PBQSpliterator<E>(queue, array, lo, index = mid);
          }
  
          @SuppressWarnings("unchecked")
          public void forEachRemaining(Consumer<? super E> action) {
              Object[] a; int i, hi; // hoist accesses and checks from loop
              if (action == null)
                  throw new NullPointerException();
              if ((a = array) == null)
                  fence = (a = queue.toArray()).length;
              if ((hi = fence) <= a.length &&
                  (i = index) >= 0 && i < (index = hi)) {
                  do { action.accept((E)a[i]); } while (++i < hi);
              }
          }
  
          public boolean tryAdvance(Consumer<? super E> action) {
              if (action == null)
                  throw new NullPointerException();
              if (getFence() > index && index >= 0) {
                  @SuppressWarnings("unchecked") E e = (E) array[index++];
                  action.accept(e);
                  return true;
              }
              return false;
          }
  
          public long estimateSize() { return (long)(getFence() - index); }
  
          public int characteristics() {
              return Spliterator.NONNULL | Spliterator.SIZED | Spliterator.SUBSIZED;
          }
      }
  
      /**
       * Returns a {@link Spliterator} over the elements in this queue.
       *
       * <p>The returned spliterator is
       * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
       *
       * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
       * {@link Spliterator#NONNULL}.
       *
       * @implNote
       * The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}.
       *
       * @return a {@code Spliterator} over the elements in this queue
       * @since 1.8
       */
      public Spliterator<E> spliterator() {
          return new PBQSpliterator<E>(this, null, 0, -1);
      }
  
      // Unsafe mechanics
      private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
      private static final long ALLOCATIONSPINLOCK;
      static {
          try {
              ALLOCATIONSPINLOCK = U.objectFieldOffset
                  (PriorityBlockingQueue.class.getDeclaredField("allocationSpinLock"));
          } catch (ReflectiveOperationException e) {
              throw new Error(e);
          }
      }
  }
  