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1    /*
2     * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
3     * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4     *
5     * This code is free software; you can redistribute it and/or modify it
6     * under the terms of the GNU General Public License version 2 only, as
7     * published by the Free Software Foundation.  Oracle designates this
8     * particular file as subject to the "Classpath" exception as provided
9     * by Oracle in the LICENSE file that accompanied this code.
10     *
11     * This code is distributed in the hope that it will be useful, but WITHOUT
12     * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13     * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14     * version 2 for more details (a copy is included in the LICENSE file that
15     * accompanied this code).
16     *
17     * You should have received a copy of the GNU General Public License version
18     * 2 along with this work; if not, write to the Free Software Foundation,
19     * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20     *
21     * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22     * or visit www.oracle.com if you need additional information or have any
23     * questions.
24     */
25    
26  package java.util;  package java.util;
27    
28  import java.util.*;  import java.util.function.Consumer;
29    
30  /**  /**
31   * An unbounded (resizable) priority queue based on a priority   * An unbounded priority {@linkplain Queue queue} based on a priority heap.
32   * heap.The take operation returns the least element with respect to   * The elements of the priority queue are ordered according to their
33   * the given ordering. (If more than one element is tied for least   * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
34   * value, one of them is arbitrarily chosen to be returned -- no   * provided at queue construction time, depending on which constructor is
35   * guarantees are made for ordering across ties.) Ordering follows the   * used.  A priority queue does not permit {@code null} elements.
36   * java.util.Collection conventions: Either the elements must be   * A priority queue relying on natural ordering also does not permit
37   * Comparable, or a Comparator must be supplied. Comparison failures   * insertion of non-comparable objects (doing so may result in
38   * throw ClassCastExceptions during insertions and extractions.   * {@code ClassCastException}).
39   **/   *
40  public class PriorityQueue<E> extends AbstractCollection<E> implements Queue<E> {   * <p>The <em>head</em> of this queue is the <em>least</em> element
41      public PriorityQueue(int initialCapacity) {}   * with respect to the specified ordering.  If multiple elements are
42      public PriorityQueue(int initialCapacity, Comparator comparator) {}   * tied for least value, the head is one of those elements -- ties are
43     * broken arbitrarily.  The queue retrieval operations {@code poll},
44     * {@code remove}, {@code peek}, and {@code element} access the
45     * element at the head of the queue.
46     *
47     * <p>A priority queue is unbounded, but has an internal
48     * <i>capacity</i> governing the size of an array used to store the
49     * elements on the queue.  It is always at least as large as the queue
50     * size.  As elements are added to a priority queue, its capacity
51     * grows automatically.  The details of the growth policy are not
52     * specified.
53     *
54     * <p>This class and its iterator implement all of the
55     * <em>optional</em> methods of the {@link Collection} and {@link
56     * Iterator} interfaces.  The Iterator provided in method {@link
57     * #iterator()} and the Spliterator provided in method {@link #spliterator()}
58     * are <em>not</em> guaranteed to traverse the elements of
59     * the priority queue in any particular order. If you need ordered
60     * traversal, consider using {@code Arrays.sort(pq.toArray())}.
61     *
62     * <p><strong>Note that this implementation is not synchronized.</strong>
63     * Multiple threads should not access a {@code PriorityQueue}
64     * instance concurrently if any of the threads modifies the queue.
65     * Instead, use the thread-safe {@link
66     * java.util.concurrent.PriorityBlockingQueue} class.
67     *
68     * <p>Implementation note: this implementation provides
69     * O(log(n)) time for the enqueuing and dequeuing methods
70     * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
71     * linear time for the {@code remove(Object)} and {@code contains(Object)}
72     * methods; and constant time for the retrieval methods
73     * ({@code peek}, {@code element}, and {@code size}).
74     *
75     * <p>This class is a member of the
76     * <a href="{@docRoot}/../technotes/guides/collections/index.html">
77     * Java Collections Framework</a>.
78     *
79     * @since 1.5
80     * @author Josh Bloch, Doug Lea
81     * @param <E> the type of elements held in this queue
82     */
83    public class PriorityQueue<E> extends AbstractQueue<E>
84        implements java.io.Serializable {
85    
86      public PriorityQueue(int initialCapacity, Collection initialElements) {}      private static final long serialVersionUID = -7720805057305804111L;
87    
88      public PriorityQueue(int initialCapacity, Comparator comparator, Collection initialElements) {}      private static final int DEFAULT_INITIAL_CAPACITY = 11;
89    
90      public boolean add(E x) {      /**
91          return false;       * Priority queue represented as a balanced binary heap: the two
92         * children of queue[n] are queue[2*n+1] and queue[2*(n+1)].  The
93         * priority queue is ordered by comparator, or by the elements'
94         * natural ordering, if comparator is null: For each node n in the
95         * heap and each descendant d of n, n <= d.  The element with the
96         * lowest value is in queue[0], assuming the queue is nonempty.
97         */
98        transient Object[] queue; // non-private to simplify nested class access
99    
100        /**
101         * The number of elements in the priority queue.
102         */
103        int size;
104    
105        /**
106         * The comparator, or null if priority queue uses elements'
107         * natural ordering.
108         */
109        private final Comparator<? super E> comparator;
110    
111        /**
112         * The number of times this priority queue has been
113         * <i>structurally modified</i>.  See AbstractList for gory details.
114         */
115        transient int modCount;     // non-private to simplify nested class access
116    
117        /**
118         * Creates a {@code PriorityQueue} with the default initial
119         * capacity (11) that orders its elements according to their
120         * {@linkplain Comparable natural ordering}.
121         */
122        public PriorityQueue() {
123            this(DEFAULT_INITIAL_CAPACITY, null);
124        }
125    
126        /**
127         * Creates a {@code PriorityQueue} with the specified initial
128         * capacity that orders its elements according to their
129         * {@linkplain Comparable natural ordering}.
130         *
131         * @param initialCapacity the initial capacity for this priority queue
132         * @throws IllegalArgumentException if {@code initialCapacity} is less
133         *         than 1
134         */
135        public PriorityQueue(int initialCapacity) {
136            this(initialCapacity, null);
137        }
138    
139        /**
140         * Creates a {@code PriorityQueue} with the default initial capacity and
141         * whose elements are ordered according to the specified comparator.
142         *
143         * @param  comparator the comparator that will be used to order this
144         *         priority queue.  If {@code null}, the {@linkplain Comparable
145         *         natural ordering} of the elements will be used.
146         * @since 1.8
147         */
148        public PriorityQueue(Comparator<? super E> comparator) {
149            this(DEFAULT_INITIAL_CAPACITY, comparator);
150        }
151    
152        /**
153         * Creates a {@code PriorityQueue} with the specified initial capacity
154         * that orders its elements according to the specified comparator.
155         *
156         * @param  initialCapacity the initial capacity for this priority queue
157         * @param  comparator the comparator that will be used to order this
158         *         priority queue.  If {@code null}, the {@linkplain Comparable
159         *         natural ordering} of the elements will be used.
160         * @throws IllegalArgumentException if {@code initialCapacity} is
161         *         less than 1
162         */
163        public PriorityQueue(int initialCapacity,
164                             Comparator<? super E> comparator) {
165            // Note: This restriction of at least one is not actually needed,
166            // but continues for 1.5 compatibility
167            if (initialCapacity < 1)
168                throw new IllegalArgumentException();
169            this.queue = new Object[initialCapacity];
170            this.comparator = comparator;
171        }
172    
173        /**
174         * Creates a {@code PriorityQueue} containing the elements in the
175         * specified collection.  If the specified collection is an instance of
176         * a {@link SortedSet} or is another {@code PriorityQueue}, this
177         * priority queue will be ordered according to the same ordering.
178         * Otherwise, this priority queue will be ordered according to the
179         * {@linkplain Comparable natural ordering} of its elements.
180         *
181         * @param  c the collection whose elements are to be placed
182         *         into this priority queue
183         * @throws ClassCastException if elements of the specified collection
184         *         cannot be compared to one another according to the priority
185         *         queue's ordering
186         * @throws NullPointerException if the specified collection or any
187         *         of its elements are null
188         */
189        @SuppressWarnings("unchecked")
190        public PriorityQueue(Collection<? extends E> c) {
191            if (c instanceof SortedSet<?>) {
192                SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
193                this.comparator = (Comparator<? super E>) ss.comparator();
194                initElementsFromCollection(ss);
195            }
196            else if (c instanceof PriorityQueue<?>) {
197                PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
198                this.comparator = (Comparator<? super E>) pq.comparator();
199                initFromPriorityQueue(pq);
200            }
201            else {
202                this.comparator = null;
203                initFromCollection(c);
204            }
205        }
206    
207        /**
208         * Creates a {@code PriorityQueue} containing the elements in the
209         * specified priority queue.  This priority queue will be
210         * ordered according to the same ordering as the given priority
211         * queue.
212         *
213         * @param  c the priority queue whose elements are to be placed
214         *         into this priority queue
215         * @throws ClassCastException if elements of {@code c} cannot be
216         *         compared to one another according to {@code c}'s
217         *         ordering
218         * @throws NullPointerException if the specified priority queue or any
219         *         of its elements are null
220         */
221        @SuppressWarnings("unchecked")
222        public PriorityQueue(PriorityQueue<? extends E> c) {
223            this.comparator = (Comparator<? super E>) c.comparator();
224            initFromPriorityQueue(c);
225        }
226    
227        /**
228         * Creates a {@code PriorityQueue} containing the elements in the
229         * specified sorted set.   This priority queue will be ordered
230         * according to the same ordering as the given sorted set.
231         *
232         * @param  c the sorted set whose elements are to be placed
233         *         into this priority queue
234         * @throws ClassCastException if elements of the specified sorted
235         *         set cannot be compared to one another according to the
236         *         sorted set's ordering
237         * @throws NullPointerException if the specified sorted set or any
238         *         of its elements are null
239         */
240        @SuppressWarnings("unchecked")
241        public PriorityQueue(SortedSet<? extends E> c) {
242            this.comparator = (Comparator<? super E>) c.comparator();
243            initElementsFromCollection(c);
244        }
245    
246        private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
247            if (c.getClass() == PriorityQueue.class) {
248                this.queue = c.toArray();
249                this.size = c.size();
250            } else {
251                initFromCollection(c);
252            }
253        }
254    
255        private void initElementsFromCollection(Collection<? extends E> c) {
256            Object[] a = c.toArray();
257            // If c.toArray incorrectly doesn't return Object[], copy it.
258            if (a.getClass() != Object[].class)
259                a = Arrays.copyOf(a, a.length, Object[].class);
260            int len = a.length;
261            if (len == 1 || this.comparator != null)
262                for (Object e : a)
263                    if (e == null)
264                        throw new NullPointerException();
265            this.queue = a;
266            this.size = a.length;
267        }
268    
269        /**
270         * Initializes queue array with elements from the given Collection.
271         *
272         * @param c the collection
273         */
274        private void initFromCollection(Collection<? extends E> c) {
275            initElementsFromCollection(c);
276            heapify();
277        }
278    
279        /**
280         * The maximum size of array to allocate.
281         * Some VMs reserve some header words in an array.
282         * Attempts to allocate larger arrays may result in
283         * OutOfMemoryError: Requested array size exceeds VM limit
284         */
285        private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
286    
287        /**
288         * Increases the capacity of the array.
289         *
290         * @param minCapacity the desired minimum capacity
291         */
292        private void grow(int minCapacity) {
293            int oldCapacity = queue.length;
294            // Double size if small; else grow by 50%
295            int newCapacity = oldCapacity + ((oldCapacity < 64) ?
296                                             (oldCapacity + 2) :
297                                             (oldCapacity >> 1));
298            // overflow-conscious code
299            if (newCapacity - MAX_ARRAY_SIZE > 0)
300                newCapacity = hugeCapacity(minCapacity);
301            queue = Arrays.copyOf(queue, newCapacity);
302        }
303    
304        private static int hugeCapacity(int minCapacity) {
305            if (minCapacity < 0) // overflow
306                throw new OutOfMemoryError();
307            return (minCapacity > MAX_ARRAY_SIZE) ?
308                Integer.MAX_VALUE :
309                MAX_ARRAY_SIZE;
310      }      }
311      public boolean offer(E x) {  
312        /**
313         * Inserts the specified element into this priority queue.
314         *
315         * @return {@code true} (as specified by {@link Collection#add})
316         * @throws ClassCastException if the specified element cannot be
317         *         compared with elements currently in this priority queue
318         *         according to the priority queue's ordering
319         * @throws NullPointerException if the specified element is null
320         */
321        public boolean add(E e) {
322            return offer(e);
323        }
324    
325        /**
326         * Inserts the specified element into this priority queue.
327         *
328         * @return {@code true} (as specified by {@link Queue#offer})
329         * @throws ClassCastException if the specified element cannot be
330         *         compared with elements currently in this priority queue
331         *         according to the priority queue's ordering
332         * @throws NullPointerException if the specified element is null
333         */
334        public boolean offer(E e) {
335            if (e == null)
336                throw new NullPointerException();
337            modCount++;
338            int i = size;
339            if (i >= queue.length)
340                grow(i + 1);
341            siftUp(i, e);
342            size = i + 1;
343            return true;
344        }
345    
346        @SuppressWarnings("unchecked")
347        public E peek() {
348            return (size == 0) ? null : (E) queue[0];
349        }
350    
351        private int indexOf(Object o) {
352            if (o != null) {
353                for (int i = 0; i < size; i++)
354                    if (o.equals(queue[i]))
355                        return i;
356            }
357            return -1;
358        }
359    
360        /**
361         * Removes a single instance of the specified element from this queue,
362         * if it is present.  More formally, removes an element {@code e} such
363         * that {@code o.equals(e)}, if this queue contains one or more such
364         * elements.  Returns {@code true} if and only if this queue contained
365         * the specified element (or equivalently, if this queue changed as a
366         * result of the call).
367         *
368         * @param o element to be removed from this queue, if present
369         * @return {@code true} if this queue changed as a result of the call
370         */
371        public boolean remove(Object o) {
372            int i = indexOf(o);
373            if (i == -1)
374          return false;          return false;
375            else {
376                removeAt(i);
377                return true;
378            }
379        }
380    
381        /**
382         * Version of remove using reference equality, not equals.
383         * Needed by iterator.remove.
384         *
385         * @param o element to be removed from this queue, if present
386         * @return {@code true} if removed
387         */
388        boolean removeEq(Object o) {
389            for (int i = 0; i < size; i++) {
390                if (o == queue[i]) {
391                    removeAt(i);
392                    return true;
393                }
394      }      }
     public boolean remove(Object x) {  
395          return false;          return false;
396      }      }
397    
398      public E remove() {      /**
399          return null;       * Returns {@code true} if this queue contains the specified element.
400         * More formally, returns {@code true} if and only if this queue contains
401         * at least one element {@code e} such that {@code o.equals(e)}.
402         *
403         * @param o object to be checked for containment in this queue
404         * @return {@code true} if this queue contains the specified element
405         */
406        public boolean contains(Object o) {
407            return indexOf(o) >= 0;
408        }
409    
410        /**
411         * Returns an array containing all of the elements in this queue.
412         * The elements are in no particular order.
413         *
414         * <p>The returned array will be "safe" in that no references to it are
415         * maintained by this queue.  (In other words, this method must allocate
416         * a new array).  The caller is thus free to modify the returned array.
417         *
418         * <p>This method acts as bridge between array-based and collection-based
419         * APIs.
420         *
421         * @return an array containing all of the elements in this queue
422         */
423        public Object[] toArray() {
424            return Arrays.copyOf(queue, size);
425        }
426    
427        /**
428         * Returns an array containing all of the elements in this queue; the
429         * runtime type of the returned array is that of the specified array.
430         * The returned array elements are in no particular order.
431         * If the queue fits in the specified array, it is returned therein.
432         * Otherwise, a new array is allocated with the runtime type of the
433         * specified array and the size of this queue.
434         *
435         * <p>If the queue fits in the specified array with room to spare
436         * (i.e., the array has more elements than the queue), the element in
437         * the array immediately following the end of the collection is set to
438         * {@code null}.
439         *
440         * <p>Like the {@link #toArray()} method, this method acts as bridge between
441         * array-based and collection-based APIs.  Further, this method allows
442         * precise control over the runtime type of the output array, and may,
443         * under certain circumstances, be used to save allocation costs.
444         *
445         * <p>Suppose {@code x} is a queue known to contain only strings.
446         * The following code can be used to dump the queue into a newly
447         * allocated array of {@code String}:
448         *
449         * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
450         *
451         * Note that {@code toArray(new Object[0])} is identical in function to
452         * {@code toArray()}.
453         *
454         * @param a the array into which the elements of the queue are to
455         *          be stored, if it is big enough; otherwise, a new array of the
456         *          same runtime type is allocated for this purpose.
457         * @return an array containing all of the elements in this queue
458         * @throws ArrayStoreException if the runtime type of the specified array
459         *         is not a supertype of the runtime type of every element in
460         *         this queue
461         * @throws NullPointerException if the specified array is null
462         */
463        @SuppressWarnings("unchecked")
464        public <T> T[] toArray(T[] a) {
465            final int size = this.size;
466            if (a.length < size)
467                // Make a new array of a's runtime type, but my contents:
468                return (T[]) Arrays.copyOf(queue, size, a.getClass());
469            System.arraycopy(queue, 0, a, 0, size);
470            if (a.length > size)
471                a[size] = null;
472            return a;
473      }      }
474    
475        /**
476         * Returns an iterator over the elements in this queue. The iterator
477         * does not return the elements in any particular order.
478         *
479         * @return an iterator over the elements in this queue
480         */
481      public Iterator<E> iterator() {      public Iterator<E> iterator() {
482        return null;          return new Itr();
483      }      }
484    
485      public E element() {      private final class Itr implements Iterator<E> {
486          return null;          /**
487             * Index (into queue array) of element to be returned by
488             * subsequent call to next.
489             */
490            private int cursor;
491    
492            /**
493             * Index of element returned by most recent call to next,
494             * unless that element came from the forgetMeNot list.
495             * Set to -1 if element is deleted by a call to remove.
496             */
497            private int lastRet = -1;
498    
499            /**
500             * A queue of elements that were moved from the unvisited portion of
501             * the heap into the visited portion as a result of "unlucky" element
502             * removals during the iteration.  (Unlucky element removals are those
503             * that require a siftup instead of a siftdown.)  We must visit all of
504             * the elements in this list to complete the iteration.  We do this
505             * after we've completed the "normal" iteration.
506             *
507             * We expect that most iterations, even those involving removals,
508             * will not need to store elements in this field.
509             */
510            private ArrayDeque<E> forgetMeNot;
511    
512            /**
513             * Element returned by the most recent call to next iff that
514             * element was drawn from the forgetMeNot list.
515             */
516            private E lastRetElt;
517    
518            /**
519             * The modCount value that the iterator believes that the backing
520             * Queue should have.  If this expectation is violated, the iterator
521             * has detected concurrent modification.
522             */
523            private int expectedModCount = modCount;
524    
525            Itr() {}                        // prevent access constructor creation
526    
527            public boolean hasNext() {
528                return cursor < size ||
529                    (forgetMeNot != null && !forgetMeNot.isEmpty());
530      }      }
531      public E poll() {  
532          return null;          @SuppressWarnings("unchecked")
533            public E next() {
534                if (expectedModCount != modCount)
535                    throw new ConcurrentModificationException();
536                if (cursor < size)
537                    return (E) queue[lastRet = cursor++];
538                if (forgetMeNot != null) {
539                    lastRet = -1;
540                    lastRetElt = forgetMeNot.poll();
541                    if (lastRetElt != null)
542                        return lastRetElt;
543      }      }
544      public E peek() {              throw new NoSuchElementException();
         return null;  
545      }      }
546    
547      public boolean isEmpty() {          public void remove() {
548          return false;              if (expectedModCount != modCount)
549                    throw new ConcurrentModificationException();
550                if (lastRet != -1) {
551                    E moved = PriorityQueue.this.removeAt(lastRet);
552                    lastRet = -1;
553                    if (moved == null)
554                        cursor--;
555                    else {
556                        if (forgetMeNot == null)
557                            forgetMeNot = new ArrayDeque<>();
558                        forgetMeNot.add(moved);
559                    }
560                } else if (lastRetElt != null) {
561                    PriorityQueue.this.removeEq(lastRetElt);
562                    lastRetElt = null;
563                } else {
564                    throw new IllegalStateException();
565                }
566                expectedModCount = modCount;
567            }
568      }      }
569    
570      public int size() {      public int size() {
571          return 0;          return size;
572      }      }
573      public Object[] toArray() {  
574        /**
575         * Removes all of the elements from this priority queue.
576         * The queue will be empty after this call returns.
577         */
578        public void clear() {
579            modCount++;
580            for (int i = 0; i < size; i++)
581                queue[i] = null;
582            size = 0;
583        }
584    
585        @SuppressWarnings("unchecked")
586        public E poll() {
587            if (size == 0)
588          return null;          return null;
589            int s = --size;
590            modCount++;
591            E result = (E) queue[0];
592            E x = (E) queue[s];
593            queue[s] = null;
594            if (s != 0)
595                siftDown(0, x);
596            return result;
597      }      }
598    
599      public <T> T[] toArray(T[] array) {      /**
600         * Removes the ith element from queue.
601         *
602         * Normally this method leaves the elements at up to i-1,
603         * inclusive, untouched.  Under these circumstances, it returns
604         * null.  Occasionally, in order to maintain the heap invariant,
605         * it must swap a later element of the list with one earlier than
606         * i.  Under these circumstances, this method returns the element
607         * that was previously at the end of the list and is now at some
608         * position before i. This fact is used by iterator.remove so as to
609         * avoid missing traversing elements.
610         */
611        @SuppressWarnings("unchecked")
612        E removeAt(int i) {
613            // assert i >= 0 && i < size;
614            modCount++;
615            int s = --size;
616            if (s == i) // removed last element
617                queue[i] = null;
618            else {
619                E moved = (E) queue[s];
620                queue[s] = null;
621                siftDown(i, moved);
622                if (queue[i] == moved) {
623                    siftUp(i, moved);
624                    if (queue[i] != moved)
625                        return moved;
626                }
627            }
628          return null;          return null;
629      }      }
630    
631        /**
632         * Inserts item x at position k, maintaining heap invariant by
633         * promoting x up the tree until it is greater than or equal to
634         * its parent, or is the root.
635         *
636         * To simplify and speed up coercions and comparisons, the
637         * Comparable and Comparator versions are separated into different
638         * methods that are otherwise identical. (Similarly for siftDown.)
639         *
640         * @param k the position to fill
641         * @param x the item to insert
642         */
643        private void siftUp(int k, E x) {
644            if (comparator != null)
645                siftUpUsingComparator(k, x);
646            else
647                siftUpComparable(k, x);
648        }
649    
650        @SuppressWarnings("unchecked")
651        private void siftUpComparable(int k, E x) {
652            Comparable<? super E> key = (Comparable<? super E>) x;
653            while (k > 0) {
654                int parent = (k - 1) >>> 1;
655                Object e = queue[parent];
656                if (key.compareTo((E) e) >= 0)
657                    break;
658                queue[k] = e;
659                k = parent;
660            }
661            queue[k] = key;
662        }
663    
664        @SuppressWarnings("unchecked")
665        private void siftUpUsingComparator(int k, E x) {
666            while (k > 0) {
667                int parent = (k - 1) >>> 1;
668                Object e = queue[parent];
669                if (comparator.compare(x, (E) e) >= 0)
670                    break;
671                queue[k] = e;
672                k = parent;
673            }
674            queue[k] = x;
675        }
676    
677        /**
678         * Inserts item x at position k, maintaining heap invariant by
679         * demoting x down the tree repeatedly until it is less than or
680         * equal to its children or is a leaf.
681         *
682         * @param k the position to fill
683         * @param x the item to insert
684         */
685        private void siftDown(int k, E x) {
686            if (comparator != null)
687                siftDownUsingComparator(k, x);
688            else
689                siftDownComparable(k, x);
690        }
691    
692        @SuppressWarnings("unchecked")
693        private void siftDownComparable(int k, E x) {
694            Comparable<? super E> key = (Comparable<? super E>)x;
695            int half = size >>> 1;        // loop while a non-leaf
696            while (k < half) {
697                int child = (k << 1) + 1; // assume left child is least
698                Object c = queue[child];
699                int right = child + 1;
700                if (right < size &&
701                    ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
702                    c = queue[child = right];
703                if (key.compareTo((E) c) <= 0)
704                    break;
705                queue[k] = c;
706                k = child;
707            }
708            queue[k] = key;
709        }
710    
711        @SuppressWarnings("unchecked")
712        private void siftDownUsingComparator(int k, E x) {
713            int half = size >>> 1;
714            while (k < half) {
715                int child = (k << 1) + 1;
716                Object c = queue[child];
717                int right = child + 1;
718                if (right < size &&
719                    comparator.compare((E) c, (E) queue[right]) > 0)
720                    c = queue[child = right];
721                if (comparator.compare(x, (E) c) <= 0)
722                    break;
723                queue[k] = c;
724                k = child;
725            }
726            queue[k] = x;
727        }
728    
729        /**
730         * Establishes the heap invariant (described above) in the entire tree,
731         * assuming nothing about the order of the elements prior to the call.
732         * This classic algorithm due to Floyd (1964) is known to be O(size).
733         */
734        @SuppressWarnings("unchecked")
735        private void heapify() {
736            final Object[] es = queue;
737            final int half = (size >>> 1) - 1;
738            if (comparator == null)
739                for (int i = half; i >= 0; i--)
740                    siftDownComparable(i, (E) es[i]);
741            else
742                for (int i = half; i >= 0; i--)
743                    siftDownUsingComparator(i, (E) es[i]);
744        }
745    
746        /**
747         * Returns the comparator used to order the elements in this
748         * queue, or {@code null} if this queue is sorted according to
749         * the {@linkplain Comparable natural ordering} of its elements.
750         *
751         * @return the comparator used to order this queue, or
752         *         {@code null} if this queue is sorted according to the
753         *         natural ordering of its elements
754         */
755        public Comparator<? super E> comparator() {
756            return comparator;
757        }
758    
759        /**
760         * Saves this queue to a stream (that is, serializes it).
761         *
762         * @param s the stream
763         * @throws java.io.IOException if an I/O error occurs
764         * @serialData The length of the array backing the instance is
765         *             emitted (int), followed by all of its elements
766         *             (each an {@code Object}) in the proper order.
767         */
768        private void writeObject(java.io.ObjectOutputStream s)
769            throws java.io.IOException {
770            // Write out element count, and any hidden stuff
771            s.defaultWriteObject();
772    
773            // Write out array length, for compatibility with 1.5 version
774            s.writeInt(Math.max(2, size + 1));
775    
776            // Write out all elements in the "proper order".
777            for (int i = 0; i < size; i++)
778                s.writeObject(queue[i]);
779        }
780    
781        /**
782         * Reconstitutes the {@code PriorityQueue} instance from a stream
783         * (that is, deserializes it).
784         *
785         * @param s the stream
786         * @throws ClassNotFoundException if the class of a serialized object
787         *         could not be found
788         * @throws java.io.IOException if an I/O error occurs
789         */
790        private void readObject(java.io.ObjectInputStream s)
791            throws java.io.IOException, ClassNotFoundException {
792            // Read in size, and any hidden stuff
793            s.defaultReadObject();
794    
795            // Read in (and discard) array length
796            s.readInt();
797    
798            queue = new Object[size];
799    
800            // Read in all elements.
801            for (int i = 0; i < size; i++)
802                queue[i] = s.readObject();
803    
804            // Elements are guaranteed to be in "proper order", but the
805            // spec has never explained what that might be.
806            heapify();
807        }
808    
809        /**
810         * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
811         * and <em>fail-fast</em> {@link Spliterator} over the elements in this
812         * queue. The spliterator does not traverse elements in any particular order
813         * (the {@link Spliterator#ORDERED ORDERED} characteristic is not reported).
814         *
815         * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
816         * {@link Spliterator#SUBSIZED}, and {@link Spliterator#NONNULL}.
817         * Overriding implementations should document the reporting of additional
818         * characteristic values.
819         *
820         * @return a {@code Spliterator} over the elements in this queue
821         * @since 1.8
822         */
823        public final Spliterator<E> spliterator() {
824            return new PriorityQueueSpliterator(0, -1, 0);
825        }
826    
827        final class PriorityQueueSpliterator implements Spliterator<E> {
828            private int index;            // current index, modified on advance/split
829            private int fence;            // -1 until first use
830            private int expectedModCount; // initialized when fence set
831    
832            /** Creates new spliterator covering the given range. */
833            PriorityQueueSpliterator(int origin, int fence, int expectedModCount) {
834                this.index = origin;
835                this.fence = fence;
836                this.expectedModCount = expectedModCount;
837            }
838    
839            private int getFence() { // initialize fence to size on first use
840                int hi;
841                if ((hi = fence) < 0) {
842                    expectedModCount = modCount;
843                    hi = fence = size;
844                }
845                return hi;
846            }
847    
848            public PriorityQueueSpliterator trySplit() {
849                int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
850                return (lo >= mid) ? null :
851                    new PriorityQueueSpliterator(lo, index = mid, expectedModCount);
852            }
853    
854            @SuppressWarnings("unchecked")
855            public void forEachRemaining(Consumer<? super E> action) {
856                if (action == null)
857                    throw new NullPointerException();
858                if (fence < 0) { fence = size; expectedModCount = modCount; }
859                final Object[] a = queue;
860                int i, hi; E e;
861                for (i = index, index = hi = fence; i < hi; i++) {
862                    if ((e = (E) a[i]) == null)
863                        break;      // must be CME
864                    action.accept(e);
865                }
866                if (modCount != expectedModCount)
867                    throw new ConcurrentModificationException();
868            }
869    
870            @SuppressWarnings("unchecked")
871            public boolean tryAdvance(Consumer<? super E> action) {
872                if (action == null)
873                    throw new NullPointerException();
874                if (fence < 0) { fence = size; expectedModCount = modCount; }
875                int i;
876                if ((i = index) < fence) {
877                    index = i + 1;
878                    E e;
879                    if ((e = (E) queue[i]) == null
880                        || modCount != expectedModCount)
881                        throw new ConcurrentModificationException();
882                    action.accept(e);
883                    return true;
884                }
885                return false;
886            }
887    
888            public long estimateSize() {
889                return getFence() - index;
890            }
891    
892            public int characteristics() {
893                return Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.NONNULL;
894            }
895        }
896  }  }

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