content: /*
   * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
   * 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.
   */
  
  package java.util;
  
  import java.util.function.Consumer;
  
  /**
   * An unbounded priority {@linkplain Queue queue} based on a priority heap.
   * The elements of the priority queue are ordered according to their
   * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
   * provided at queue construction time, depending on which constructor is
   * used.  A priority queue does not permit {@code null} elements.
   * A priority queue relying on natural ordering also does not permit
   * insertion of non-comparable objects (doing so may result in
   * {@code ClassCastException}).
   *
   * <p>The <em>head</em> of this queue is the <em>least</em> element
   * with respect to the specified ordering.  If multiple elements are
   * tied for least value, the head is one of those elements -- ties are
   * broken arbitrarily.  The queue retrieval operations {@code poll},
   * {@code remove}, {@code peek}, and {@code element} access the
   * element at the head of the queue.
   *
   * <p>A priority queue is unbounded, but has an internal
   * <i>capacity</i> governing the size of an array used to store the
   * elements on the queue.  It is always at least as large as the queue
   * size.  As elements are added to a priority queue, its capacity
   * grows automatically.  The details of the growth policy are not
   * specified.
   *
   * <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 priority queue in any particular order. If you need ordered
   * traversal, consider using {@code Arrays.sort(pq.toArray())}.
   *
   * <p><strong>Note that this implementation is not synchronized.</strong>
   * Multiple threads should not access a {@code PriorityQueue}
   * instance concurrently if any of the threads modifies the queue.
   * Instead, use the thread-safe {@link
   * java.util.concurrent.PriorityBlockingQueue} class.
   *
   * <p>Implementation note: this implementation provides
   * O(log(n)) time for the enqueuing and dequeuing methods
   * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
   * linear time for the {@code remove(Object)} and {@code contains(Object)}
   * methods; and constant time for the retrieval methods
   * ({@code peek}, {@code element}, and {@code size}).
   *
   * <p>This class is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @since 1.5
   * @author Josh Bloch, Doug Lea
   * @param <E> the type of elements held in this queue
   */
  public class PriorityQueue<E> extends AbstractQueue<E>
      implements java.io.Serializable {
  
      private static final long serialVersionUID = -7720805057305804111L;
  
      private static final int DEFAULT_INITIAL_CAPACITY = 11;
  
      /**
       * 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.
       */
      transient Object[] queue; // non-private to simplify nested class access
  
      /**
       * The number of elements in the priority queue.
       */
      int size;
  
      /**
       * The comparator, or null if priority queue uses elements'
       * natural ordering.
       */
      private final Comparator<? super E> comparator;
  
      /**
       * The number of times this priority queue has been
       * <i>structurally modified</i>.  See AbstractList for gory details.
       */
      transient int modCount;     // non-private to simplify nested class access
  
      /**
       * Creates a {@code PriorityQueue} with the default initial
       * capacity (11) that orders its elements according to their
       * {@linkplain Comparable natural ordering}.
       */
      public PriorityQueue() {
          this(DEFAULT_INITIAL_CAPACITY, null);
      }
  
      /**
       * Creates a {@code PriorityQueue} 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 PriorityQueue(int initialCapacity) {
          this(initialCapacity, null);
      }
  
      /**
       * Creates a {@code PriorityQueue} with the default initial capacity and
       * whose elements are ordered according to the specified comparator.
       *
       * @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.
       * @since 1.8
       */
      public PriorityQueue(Comparator<? super E> comparator) {
          this(DEFAULT_INITIAL_CAPACITY, comparator);
      }
  
      /**
       * Creates a {@code PriorityQueue} 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 PriorityQueue(int initialCapacity,
                           Comparator<? super E> comparator) {
          // Note: This restriction of at least one is not actually needed,
          // but continues for 1.5 compatibility
          if (initialCapacity < 1)
              throw new IllegalArgumentException();
          this.queue = new Object[initialCapacity];
          this.comparator = comparator;
      }
  
      /**
       * Creates a {@code PriorityQueue} containing the elements in the
       * specified collection.  If the specified collection is an instance of
       * a {@link SortedSet} or is another {@code 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
       */
      @SuppressWarnings("unchecked")
      public PriorityQueue(Collection<? extends E> c) {
          if (c instanceof SortedSet<?>) {
              SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
              this.comparator = (Comparator<? super E>) ss.comparator();
              initElementsFromCollection(ss);
          }
          else if (c instanceof PriorityQueue<?>) {
              PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
              this.comparator = (Comparator<? super E>) pq.comparator();
              initFromPriorityQueue(pq);
          }
          else {
              this.comparator = null;
              initFromCollection(c);
          }
      }
  
      /**
       * Creates a {@code PriorityQueue} containing the elements in the
       * specified priority queue.  This priority queue will be
       * ordered according to the same ordering as the given priority
       * queue.
       *
       * @param  c the priority queue whose elements are to be placed
       *         into this priority queue
       * @throws ClassCastException if elements of {@code c} cannot be
       *         compared to one another according to {@code c}'s
       *         ordering
       * @throws NullPointerException if the specified priority queue or any
       *         of its elements are null
       */
      @SuppressWarnings("unchecked")
      public PriorityQueue(PriorityQueue<? extends E> c) {
          this.comparator = (Comparator<? super E>) c.comparator();
          initFromPriorityQueue(c);
      }
  
      /**
       * Creates a {@code PriorityQueue} containing the elements in the
       * specified sorted set.   This priority queue will be ordered
       * according to the same ordering as the given sorted set.
       *
       * @param  c the sorted set whose elements are to be placed
       *         into this priority queue
       * @throws ClassCastException if elements of the specified sorted
       *         set cannot be compared to one another according to the
       *         sorted set's ordering
       * @throws NullPointerException if the specified sorted set or any
       *         of its elements are null
       */
      @SuppressWarnings("unchecked")
      public PriorityQueue(SortedSet<? extends E> c) {
          this.comparator = (Comparator<? super E>) c.comparator();
          initElementsFromCollection(c);
      }
  
      private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
          if (c.getClass() == PriorityQueue.class) {
              this.queue = c.toArray();
              this.size = c.size();
          } else {
              initFromCollection(c);
          }
      }
  
      private void initElementsFromCollection(Collection<? extends E> c) {
          Object[] a = c.toArray();
          // If c.toArray incorrectly doesn't return Object[], copy it.
          if (a.getClass() != Object[].class)
              a = Arrays.copyOf(a, a.length, Object[].class);
          int len = a.length;
          if (len == 1 || this.comparator != null)
              for (Object e : a)
                  if (e == null)
                      throw new NullPointerException();
          this.queue = a;
          this.size = a.length;
      }
  
      /**
       * Initializes queue array with elements from the given Collection.
       *
       * @param c the collection
       */
      private void initFromCollection(Collection<? extends E> c) {
          initElementsFromCollection(c);
          heapify();
      }
  
      /**
       * 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;
  
      /**
       * Increases the capacity of the array.
       *
       * @param minCapacity the desired minimum capacity
       */
      private void grow(int minCapacity) {
          int oldCapacity = queue.length;
          // Double size if small; else grow by 50%
          int newCapacity = oldCapacity + ((oldCapacity < 64) ?
                                           (oldCapacity + 2) :
                                           (oldCapacity >> 1));
          // overflow-conscious code
          if (newCapacity - MAX_ARRAY_SIZE > 0)
              newCapacity = hugeCapacity(minCapacity);
          queue = Arrays.copyOf(queue, newCapacity);
      }
  
      private static int hugeCapacity(int minCapacity) {
          if (minCapacity < 0) // overflow
              throw new OutOfMemoryError();
          return (minCapacity > MAX_ARRAY_SIZE) ?
              Integer.MAX_VALUE :
              MAX_ARRAY_SIZE;
      }
  
      /**
       * Inserts the specified element into this priority queue.
       *
       * @return {@code true} (as specified by {@link Collection#add})
       * @throws ClassCastException if the specified element cannot be
       *         compared with elements currently in this 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.
       *
       * @return {@code true} (as specified by {@link Queue#offer})
       * @throws ClassCastException if the specified element cannot be
       *         compared with elements currently in this 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();
          modCount++;
          int i = size;
          if (i >= queue.length)
              grow(i + 1);
          size = i + 1;
          if (i == 0)
              queue[0] = e;
          else
              siftUp(i, e);
          return true;
      }
  
      @SuppressWarnings("unchecked")
      public E peek() {
          return (size == 0) ? null : (E) queue[0];
      }
  
      private int indexOf(Object o) {
          if (o != null) {
              for (int i = 0; i < size; i++)
                  if (o.equals(queue[i]))
                      return i;
          }
          return -1;
      }
  
      /**
       * 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) {
          int i = indexOf(o);
          if (i == -1)
              return false;
          else {
              removeAt(i);
              return true;
          }
      }
  
      /**
       * Version of remove using reference equality, not equals.
       * Needed by iterator.remove.
       *
       * @param o element to be removed from this queue, if present
       * @return {@code true} if removed
       */
      boolean removeEq(Object o) {
          for (int i = 0; i < size; i++) {
              if (o == queue[i]) {
                  removeAt(i);
                  return true;
              }
          }
          return false;
      }
  
      /**
       * 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) {
          return indexOf(o) >= 0;
      }
  
      /**
       * Returns an array containing all of the elements in this queue.
       * The 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() {
          return Arrays.copyOf(queue, size);
      }
  
      /**
       * 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 the queue fits in the specified array with room to spare
       * (i.e., the array has more elements than the queue), the element in
       * the array immediately following the end of the collection 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
       */
      @SuppressWarnings("unchecked")
      public <T> T[] toArray(T[] a) {
          final int size = this.size;
          if (a.length < size)
              // 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, size);
          if (a.length > size)
              a[size] = null;
          return a;
      }
  
      /**
       * Returns an iterator over the elements in this queue. The iterator
       * does not return the elements in any particular order.
       *
       * @return an iterator over the elements in this queue
       */
      public Iterator<E> iterator() {
          return new Itr();
      }
  
      private final class Itr implements Iterator<E> {
          /**
           * Index (into queue array) of element to be returned by
           * subsequent call to next.
           */
          private int cursor;
  
          /**
           * Index of element returned by most recent call to next,
           * unless that element came from the forgetMeNot list.
           * Set to -1 if element is deleted by a call to remove.
           */
          private int lastRet = -1;
  
          /**
           * A queue of elements that were moved from the unvisited portion of
           * the heap into the visited portion as a result of "unlucky" element
           * removals during the iteration.  (Unlucky element removals are those
           * that require a siftup instead of a siftdown.)  We must visit all of
           * the elements in this list to complete the iteration.  We do this
           * after we've completed the "normal" iteration.
           *
           * We expect that most iterations, even those involving removals,
           * will not need to store elements in this field.
           */
          private ArrayDeque<E> forgetMeNot;
  
          /**
           * Element returned by the most recent call to next iff that
           * element was drawn from the forgetMeNot list.
           */
          private E lastRetElt;
  
          /**
           * The modCount value that the iterator believes that the backing
           * Queue should have.  If this expectation is violated, the iterator
           * has detected concurrent modification.
           */
          private int expectedModCount = modCount;
  
          public boolean hasNext() {
              return cursor < size ||
                  (forgetMeNot != null && !forgetMeNot.isEmpty());
          }
  
          @SuppressWarnings("unchecked")
          public E next() {
              if (expectedModCount != modCount)
                  throw new ConcurrentModificationException();
              if (cursor < size)
                  return (E) queue[lastRet = cursor++];
              if (forgetMeNot != null) {
                  lastRet = -1;
                  lastRetElt = forgetMeNot.poll();
                  if (lastRetElt != null)
                      return lastRetElt;
              }
              throw new NoSuchElementException();
          }
  
          public void remove() {
              if (expectedModCount != modCount)
                  throw new ConcurrentModificationException();
              if (lastRet != -1) {
                  E moved = PriorityQueue.this.removeAt(lastRet);
                  lastRet = -1;
                  if (moved == null)
                      cursor--;
                  else {
                      if (forgetMeNot == null)
                          forgetMeNot = new ArrayDeque<>();
                      forgetMeNot.add(moved);
                  }
              } else if (lastRetElt != null) {
                  PriorityQueue.this.removeEq(lastRetElt);
                  lastRetElt = null;
              } else {
                  throw new IllegalStateException();
              }
              expectedModCount = modCount;
          }
      }
  
      public int size() {
          return size;
      }
  
      /**
       * Removes all of the elements from this priority queue.
       * The queue will be empty after this call returns.
       */
      public void clear() {
          modCount++;
          for (int i = 0; i < size; i++)
              queue[i] = null;
          size = 0;
      }
  
      @SuppressWarnings("unchecked")
      public E poll() {
          if (size == 0)
              return null;
          int s = --size;
          modCount++;
          E result = (E) queue[0];
          E x = (E) queue[s];
          queue[s] = null;
          if (s != 0)
              siftDown(0, x);
          return result;
      }
  
      /**
       * Removes the ith element from queue.
       *
       * Normally this method leaves the elements at up to i-1,
       * inclusive, untouched.  Under these circumstances, it returns
       * null.  Occasionally, in order to maintain the heap invariant,
       * it must swap a later element of the list with one earlier than
       * i.  Under these circumstances, this method returns the element
       * that was previously at the end of the list and is now at some
       * position before i. This fact is used by iterator.remove so as to
       * avoid missing traversing elements.
       */
      @SuppressWarnings("unchecked")
      E removeAt(int i) {
          // assert i >= 0 && i < size;
          modCount++;
          int s = --size;
          if (s == i) // removed last element
              queue[i] = null;
          else {
              E moved = (E) queue[s];
              queue[s] = null;
              siftDown(i, moved);
              if (queue[i] == moved) {
                  siftUp(i, moved);
                  if (queue[i] != moved)
                      return moved;
              }
          }
          return null;
      }
  
      /**
       * 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.)
       *
       * @param k the position to fill
       * @param x the item to insert
       */
      private void siftUp(int k, E x) {
          if (comparator != null)
              siftUpUsingComparator(k, x);
          else
              siftUpComparable(k, x);
      }
  
      @SuppressWarnings("unchecked")
      private void siftUpComparable(int k, E x) {
          Comparable<? super E> key = (Comparable<? super E>) x;
          while (k > 0) {
              int parent = (k - 1) >>> 1;
              Object e = queue[parent];
              if (key.compareTo((E) e) >= 0)
                  break;
              queue[k] = e;
              k = parent;
          }
          queue[k] = key;
      }
  
      @SuppressWarnings("unchecked")
      private void siftUpUsingComparator(int k, E x) {
          while (k > 0) {
              int parent = (k - 1) >>> 1;
              Object e = queue[parent];
              if (comparator.compare(x, (E) e) >= 0)
                  break;
              queue[k] = e;
              k = parent;
          }
          queue[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
       */
      private void siftDown(int k, E x) {
          if (comparator != null)
              siftDownUsingComparator(k, x);
          else
              siftDownComparable(k, x);
      }
  
      @SuppressWarnings("unchecked")
      private void siftDownComparable(int k, E x) {
          Comparable<? super E> key = (Comparable<? super E>)x;
          int half = size >>> 1;        // loop while a non-leaf
          while (k < half) {
              int child = (k << 1) + 1; // assume left child is least
              Object c = queue[child];
              int right = child + 1;
              if (right < size &&
                  ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
                  c = queue[child = right];
              if (key.compareTo((E) c) <= 0)
                  break;
              queue[k] = c;
              k = child;
          }
          queue[k] = key;
      }
  
      @SuppressWarnings("unchecked")
      private void siftDownUsingComparator(int k, E x) {
          int half = size >>> 1;
          while (k < half) {
              int child = (k << 1) + 1;
              Object c = queue[child];
              int right = child + 1;
              if (right < size &&
                  comparator.compare((E) c, (E) queue[right]) > 0)
                  c = queue[child = right];
              if (comparator.compare(x, (E) c) <= 0)
                  break;
              queue[k] = c;
              k = child;
          }
          queue[k] = x;
      }
  
      /**
       * Establishes the heap invariant (described above) in the entire tree,
       * assuming nothing about the order of the elements prior to the call.
       */
      @SuppressWarnings("unchecked")
      private void heapify() {
          for (int i = (size >>> 1) - 1; i >= 0; i--)
              siftDown(i, (E) queue[i]);
      }
  
      /**
       * Returns the comparator used to order the elements in this
       * queue, or {@code null} if this queue is sorted according to
       * the {@linkplain Comparable natural ordering} of its elements.
       *
       * @return the comparator used to order this queue, or
       *         {@code null} if this queue is sorted according to the
       *         natural ordering of its elements
       */
      public Comparator<? super E> comparator() {
          return comparator;
      }
  
      /**
       * Saves this queue to a stream (that is, serializes it).
       *
       * @serialData The length of the array backing the instance is
       *             emitted (int), followed by all of its elements
       *             (each an {@code Object}) in the proper order.
       * @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 {
          // Write out element count, and any hidden stuff
          s.defaultWriteObject();
  
          // Write out array length, for compatibility with 1.5 version
          s.writeInt(Math.max(2, size + 1));
  
          // Write out all elements in the "proper order".
          for (int i = 0; i < size; i++)
              s.writeObject(queue[i]);
      }
  
      /**
       * Reconstitutes the {@code PriorityQueue} instance 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 {
          // Read in size, and any hidden stuff
          s.defaultReadObject();
  
          // Read in (and discard) array length
          s.readInt();
  
          queue = new Object[size];
  
          // Read in all elements.
          for (int i = 0; i < size; i++)
              queue[i] = s.readObject();
  
          // Elements are guaranteed to be in "proper order", but the
          // spec has never explained what that might be.
          heapify();
      }
  
      /**
       * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
       * and <em>fail-fast</em> {@link Spliterator} over the elements in this
       * queue.
       *
       * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
       * {@link Spliterator#SUBSIZED}, and {@link Spliterator#NONNULL}.
       * Overriding implementations should document the reporting of additional
       * characteristic values.
       *
       * @return a {@code Spliterator} over the elements in this queue
       * @since 1.8
       */
      public final Spliterator<E> spliterator() {
          return new PriorityQueueSpliterator<>(this, 0, -1, 0);
      }
  
      static final class PriorityQueueSpliterator<E> implements Spliterator<E> {
          /*
           * This is very similar to ArrayList Spliterator, except for
           * extra null checks.
           */
          private final PriorityQueue<E> pq;
          private int index;            // current index, modified on advance/split
          private int fence;            // -1 until first use
          private int expectedModCount; // initialized when fence set
  
          /** Creates new spliterator covering the given range. */
          PriorityQueueSpliterator(PriorityQueue<E> pq, int origin, int fence,
                                   int expectedModCount) {
              this.pq = pq;
              this.index = origin;
              this.fence = fence;
              this.expectedModCount = expectedModCount;
          }
  
          private int getFence() { // initialize fence to size on first use
              int hi;
              if ((hi = fence) < 0) {
                  expectedModCount = pq.modCount;
                  hi = fence = pq.size;
              }
              return hi;
          }
  
          public PriorityQueueSpliterator<E> trySplit() {
              int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
              return (lo >= mid) ? null :
                  new PriorityQueueSpliterator<>(pq, lo, index = mid,
                                                 expectedModCount);
          }
  
          @SuppressWarnings("unchecked")
          public void forEachRemaining(Consumer<? super E> action) {
              int i, hi, mc; // hoist accesses and checks from loop
              PriorityQueue<E> q; Object[] a;
              if (action == null)
                  throw new NullPointerException();
              if ((q = pq) != null && (a = q.queue) != null) {
                  if ((hi = fence) < 0) {
                      mc = q.modCount;
                      hi = q.size;
                  }
                  else
                      mc = expectedModCount;
                  if ((i = index) >= 0 && (index = hi) <= a.length) {
                      for (E e;; ++i) {
                          if (i < hi) {
                              if ((e = (E) a[i]) == null) // must be CME
                                  break;
                              action.accept(e);
                          }
                          else if (q.modCount != mc)
                              break;
                          else
                              return;
                      }
                  }
              }
              throw new ConcurrentModificationException();
          }
  
          public boolean tryAdvance(Consumer<? super E> action) {
              if (action == null)
                  throw new NullPointerException();
              int hi = getFence(), lo = index;
              if (lo >= 0 && lo < hi) {
                  index = lo + 1;
                  @SuppressWarnings("unchecked") E e = (E)pq.queue[lo];
                  if (e == null)
                      throw new ConcurrentModificationException();
                  action.accept(e);
                  if (pq.modCount != expectedModCount)
                      throw new ConcurrentModificationException();
                  return true;
              }
              return false;
          }
  
          public long estimateSize() {
              return (long) (getFence() - index);
          }
  
          public int characteristics() {
              return Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.NONNULL;
          }
      }
  }
  