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
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19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
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23 * questions.
24 */
25
26 package java.util;
27
28 import java.util.function.Consumer;
29
30 /**
31 * An unbounded priority {@linkplain Queue queue} based on a priority heap.
32 * The elements of the priority queue are ordered according to their
33 * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
34 * provided at queue construction time, depending on which constructor is
35 * used. A priority queue does not permit {@code null} elements.
36 * A priority queue relying on natural ordering also does not permit
37 * insertion of non-comparable objects (doing so may result in
38 * {@code ClassCastException}).
39 *
40 * <p>The <em>head</em> of this queue is the <em>least</em> element
41 * with respect to the specified ordering. If multiple elements are
42 * 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()} is <em>not</em> guaranteed to traverse the elements of
58 * the priority queue in any particular order. If you need ordered
59 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
60 *
61 * <p><strong>Note that this implementation is not synchronized.</strong>
62 * Multiple threads should not access a {@code PriorityQueue}
63 * instance concurrently if any of the threads modifies the queue.
64 * Instead, use the thread-safe {@link
65 * java.util.concurrent.PriorityBlockingQueue} class.
66 *
67 * <p>Implementation note: this implementation provides
68 * O(log(n)) time for the enqueing and dequeing methods
69 * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
70 * linear time for the {@code remove(Object)} and {@code contains(Object)}
71 * methods; and constant time for the retrieval methods
72 * ({@code peek}, {@code element}, and {@code size}).
73 *
74 * <p>This class is a member of the
75 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
76 * Java Collections Framework</a>.
77 *
78 * @since 1.5
79 * @author Josh Bloch, Doug Lea
80 * @param <E> the type of elements held in this collection
81 */
82 public class PriorityQueue<E> extends AbstractQueue<E>
83 implements java.io.Serializable {
84
85 private static final long serialVersionUID = -7720805057305804111L;
86
87 private static final int DEFAULT_INITIAL_CAPACITY = 11;
88
89 /**
90 * Priority queue represented as a balanced binary heap: the two
91 * children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
92 * priority queue is ordered by comparator, or by the elements'
93 * natural ordering, if comparator is null: For each node n in the
94 * heap and each descendant d of n, n <= d. The element with the
95 * lowest value is in queue[0], assuming the queue is nonempty.
96 */
97 transient Object[] queue; // non-private to simplify nested class access
98
99 /**
100 * The number of elements in the priority queue.
101 */
102 private int size = 0;
103
104 /**
105 * The comparator, or null if priority queue uses elements'
106 * natural ordering.
107 */
108 private final Comparator<? super E> comparator;
109
110 /**
111 * The number of times this priority queue has been
112 * <i>structurally modified</i>. See AbstractList for gory details.
113 */
114 transient int modCount = 0; // non-private to simplify nested class access
115
116 /**
117 * Creates a {@code PriorityQueue} with the default initial
118 * capacity (11) that orders its elements according to their
119 * {@linkplain Comparable natural ordering}.
120 */
121 public PriorityQueue() {
122 this(DEFAULT_INITIAL_CAPACITY, null);
123 }
124
125 /**
126 * Creates a {@code PriorityQueue} with the specified initial
127 * capacity that orders its elements according to their
128 * {@linkplain Comparable natural ordering}.
129 *
130 * @param initialCapacity the initial capacity for this priority queue
131 * @throws IllegalArgumentException if {@code initialCapacity} is less
132 * than 1
133 */
134 public PriorityQueue(int initialCapacity) {
135 this(initialCapacity, null);
136 }
137
138 /**
139 * Creates a {@code PriorityQueue} with the default initial capacity
140 * that orders its elements according to the specified comparator.
141 *
142 * @param comparator the comparator that will be used to order this
143 * priority queue. If {@code null}, the {@linkplain Comparable
144 * natural ordering} of the elements will be used.
145 * @since 1.8
146 */
147 public PriorityQueue(Comparator<? super E> comparator) {
148 this(DEFAULT_INITIAL_CAPACITY, comparator);
149 }
150
151 /**
152 * Creates a {@code PriorityQueue} with the specified initial capacity
153 * that orders its elements according to the specified comparator.
154 *
155 * @param initialCapacity the initial capacity for this priority queue
156 * @param comparator the comparator that will be used to order this
157 * priority queue. If {@code null}, the {@linkplain Comparable
158 * natural ordering} of the elements will be used.
159 * @throws IllegalArgumentException if {@code initialCapacity} is
160 * less than 1
161 */
162 public PriorityQueue(int initialCapacity,
163 Comparator<? super E> comparator) {
164 // Note: This restriction of at least one is not actually needed,
165 // but continues for 1.5 compatibility
166 if (initialCapacity < 1)
167 throw new IllegalArgumentException();
168 this.queue = new Object[initialCapacity];
169 this.comparator = comparator;
170 }
171
172 /**
173 * Creates a {@code PriorityQueue} containing the elements in the
174 * specified collection. If the specified collection is an instance of
175 * a {@link SortedSet} or is another {@code PriorityQueue}, this
176 * priority queue will be ordered according to the same ordering.
177 * Otherwise, this priority queue will be ordered according to the
178 * {@linkplain Comparable natural ordering} of its elements.
179 *
180 * @param c the collection whose elements are to be placed
181 * into this priority queue
182 * @throws ClassCastException if elements of the specified collection
183 * cannot be compared to one another according to the priority
184 * queue's ordering
185 * @throws NullPointerException if the specified collection or any
186 * of its elements are null
187 */
188 @SuppressWarnings("unchecked")
189 public PriorityQueue(Collection<? extends E> c) {
190 if (c instanceof SortedSet<?>) {
191 SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
192 this.comparator = (Comparator<? super E>) ss.comparator();
193 initElementsFromCollection(ss);
194 }
195 else if (c instanceof PriorityQueue<?>) {
196 PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
197 this.comparator = (Comparator<? super E>) pq.comparator();
198 initFromPriorityQueue(pq);
199 }
200 else {
201 this.comparator = null;
202 initFromCollection(c);
203 }
204 }
205
206 /**
207 * Creates a {@code PriorityQueue} containing the elements in the
208 * specified priority queue. This priority queue will be
209 * ordered according to the same ordering as the given priority
210 * queue.
211 *
212 * @param c the priority queue whose elements are to be placed
213 * into this priority queue
214 * @throws ClassCastException if elements of {@code c} cannot be
215 * compared to one another according to {@code c}'s
216 * ordering
217 * @throws NullPointerException if the specified priority queue or any
218 * of its elements are null
219 */
220 @SuppressWarnings("unchecked")
221 public PriorityQueue(PriorityQueue<? extends E> c) {
222 this.comparator = (Comparator<? super E>) c.comparator();
223 initFromPriorityQueue(c);
224 }
225
226 /**
227 * Creates a {@code PriorityQueue} containing the elements in the
228 * specified sorted set. This priority queue will be ordered
229 * according to the same ordering as the given sorted set.
230 *
231 * @param c the sorted set whose elements are to be placed
232 * into this priority queue
233 * @throws ClassCastException if elements of the specified sorted
234 * set cannot be compared to one another according to the
235 * sorted set's ordering
236 * @throws NullPointerException if the specified sorted set or any
237 * of its elements are null
238 */
239 @SuppressWarnings("unchecked")
240 public PriorityQueue(SortedSet<? extends E> c) {
241 this.comparator = (Comparator<? super E>) c.comparator();
242 initElementsFromCollection(c);
243 }
244
245 private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
246 if (c.getClass() == PriorityQueue.class) {
247 this.queue = c.toArray();
248 this.size = c.size();
249 } else {
250 initFromCollection(c);
251 }
252 }
253
254 private void initElementsFromCollection(Collection<? extends E> c) {
255 Object[] a = c.toArray();
256 // If c.toArray incorrectly doesn't return Object[], copy it.
257 if (a.getClass() != Object[].class)
258 a = Arrays.copyOf(a, a.length, Object[].class);
259 int len = a.length;
260 if (len == 1 || this.comparator != null)
261 for (int i = 0; i < len; i++)
262 if (a[i] == null)
263 throw new NullPointerException();
264 this.queue = a;
265 this.size = a.length;
266 }
267
268 /**
269 * Initializes queue array with elements from the given Collection.
270 *
271 * @param c the collection
272 */
273 private void initFromCollection(Collection<? extends E> c) {
274 initElementsFromCollection(c);
275 heapify();
276 }
277
278 /**
279 * The maximum size of array to allocate.
280 * Some VMs reserve some header words in an array.
281 * Attempts to allocate larger arrays may result in
282 * OutOfMemoryError: Requested array size exceeds VM limit
283 */
284 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
285
286 /**
287 * Increases the capacity of the array.
288 *
289 * @param minCapacity the desired minimum capacity
290 */
291 private void grow(int minCapacity) {
292 int oldCapacity = queue.length;
293 // Double size if small; else grow by 50%
294 int newCapacity = oldCapacity + ((oldCapacity < 64) ?
295 (oldCapacity + 2) :
296 (oldCapacity >> 1));
297 // overflow-conscious code
298 if (newCapacity - MAX_ARRAY_SIZE > 0)
299 newCapacity = hugeCapacity(minCapacity);
300 queue = Arrays.copyOf(queue, newCapacity);
301 }
302
303 private static int hugeCapacity(int minCapacity) {
304 if (minCapacity < 0) // overflow
305 throw new OutOfMemoryError();
306 return (minCapacity > MAX_ARRAY_SIZE) ?
307 Integer.MAX_VALUE :
308 MAX_ARRAY_SIZE;
309 }
310
311 /**
312 * Inserts the specified element into this priority queue.
313 *
314 * @return {@code true} (as specified by {@link Collection#add})
315 * @throws ClassCastException if the specified element cannot be
316 * compared with elements currently in this priority queue
317 * according to the priority queue's ordering
318 * @throws NullPointerException if the specified element is null
319 */
320 public boolean add(E e) {
321 return offer(e);
322 }
323
324 /**
325 * Inserts the specified element into this priority queue.
326 *
327 * @return {@code true} (as specified by {@link Queue#offer})
328 * @throws ClassCastException if the specified element cannot be
329 * compared with elements currently in this priority queue
330 * according to the priority queue's ordering
331 * @throws NullPointerException if the specified element is null
332 */
333 public boolean offer(E e) {
334 if (e == null)
335 throw new NullPointerException();
336 modCount++;
337 int i = size;
338 if (i >= queue.length)
339 grow(i + 1);
340 size = i + 1;
341 if (i == 0)
342 queue[0] = e;
343 else
344 siftUp(i, e);
345 return true;
346 }
347
348 @SuppressWarnings("unchecked")
349 public E peek() {
350 return (size == 0) ? null : (E) queue[0];
351 }
352
353 private int indexOf(Object o) {
354 if (o != null) {
355 for (int i = 0; i < size; i++)
356 if (o.equals(queue[i]))
357 return i;
358 }
359 return -1;
360 }
361
362 /**
363 * Removes a single instance of the specified element from this queue,
364 * if it is present. More formally, removes an element {@code e} such
365 * that {@code o.equals(e)}, if this queue contains one or more such
366 * elements. Returns {@code true} if and only if this queue contained
367 * the specified element (or equivalently, if this queue changed as a
368 * result of the call).
369 *
370 * @param o element to be removed from this queue, if present
371 * @return {@code true} if this queue changed as a result of the call
372 */
373 public boolean remove(Object o) {
374 int i = indexOf(o);
375 if (i == -1)
376 return false;
377 else {
378 removeAt(i);
379 return true;
380 }
381 }
382
383 /**
384 * Version of remove using reference equality, not equals.
385 * Needed by iterator.remove.
386 *
387 * @param o element to be removed from this queue, if present
388 * @return {@code true} if removed
389 */
390 boolean removeEq(Object o) {
391 for (int i = 0; i < size; i++) {
392 if (o == queue[i]) {
393 removeAt(i);
394 return true;
395 }
396 }
397 return false;
398 }
399
400 /**
401 * Returns {@code true} if this queue contains the specified element.
402 * More formally, returns {@code true} if and only if this queue contains
403 * at least one element {@code e} such that {@code o.equals(e)}.
404 *
405 * @param o object to be checked for containment in this queue
406 * @return {@code true} if this queue contains the specified element
407 */
408 public boolean contains(Object o) {
409 return indexOf(o) != -1;
410 }
411
412 /**
413 * Returns an array containing all of the elements in this queue.
414 * The elements are in no particular order.
415 *
416 * <p>The returned array will be "safe" in that no references to it are
417 * maintained by this queue. (In other words, this method must allocate
418 * a new array). The caller is thus free to modify the returned array.
419 *
420 * <p>This method acts as bridge between array-based and collection-based
421 * APIs.
422 *
423 * @return an array containing all of the elements in this queue
424 */
425 public Object[] toArray() {
426 return Arrays.copyOf(queue, size);
427 }
428
429 /**
430 * Returns an array containing all of the elements in this queue; the
431 * runtime type of the returned array is that of the specified array.
432 * The returned array elements are in no particular order.
433 * If the queue fits in the specified array, it is returned therein.
434 * Otherwise, a new array is allocated with the runtime type of the
435 * specified array and the size of this queue.
436 *
437 * <p>If the queue fits in the specified array with room to spare
438 * (i.e., the array has more elements than the queue), the element in
439 * the array immediately following the end of the collection is set to
440 * {@code null}.
441 *
442 * <p>Like the {@link #toArray()} method, this method acts as bridge between
443 * array-based and collection-based APIs. Further, this method allows
444 * precise control over the runtime type of the output array, and may,
445 * under certain circumstances, be used to save allocation costs.
446 *
447 * <p>Suppose {@code x} is a queue known to contain only strings.
448 * The following code can be used to dump the queue into a newly
449 * allocated array of {@code String}:
450 *
451 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
452 *
453 * Note that {@code toArray(new Object[0])} is identical in function to
454 * {@code toArray()}.
455 *
456 * @param a the array into which the elements of the queue are to
457 * be stored, if it is big enough; otherwise, a new array of the
458 * same runtime type is allocated for this purpose.
459 * @return an array containing all of the elements in this queue
460 * @throws ArrayStoreException if the runtime type of the specified array
461 * is not a supertype of the runtime type of every element in
462 * this queue
463 * @throws NullPointerException if the specified array is null
464 */
465 @SuppressWarnings("unchecked")
466 public <T> T[] toArray(T[] a) {
467 final int size = this.size;
468 if (a.length < size)
469 // Make a new array of a's runtime type, but my contents:
470 return (T[]) Arrays.copyOf(queue, size, a.getClass());
471 System.arraycopy(queue, 0, a, 0, size);
472 if (a.length > size)
473 a[size] = null;
474 return a;
475 }
476
477 /**
478 * Returns an iterator over the elements in this queue. The iterator
479 * does not return the elements in any particular order.
480 *
481 * @return an iterator over the elements in this queue
482 */
483 public Iterator<E> iterator() {
484 return new Itr();
485 }
486
487 private final class Itr implements Iterator<E> {
488 /**
489 * Index (into queue array) of element to be returned by
490 * subsequent call to next.
491 */
492 private int cursor = 0;
493
494 /**
495 * Index of element returned by most recent call to next,
496 * unless that element came from the forgetMeNot list.
497 * Set to -1 if element is deleted by a call to remove.
498 */
499 private int lastRet = -1;
500
501 /**
502 * A queue of elements that were moved from the unvisited portion of
503 * the heap into the visited portion as a result of "unlucky" element
504 * removals during the iteration. (Unlucky element removals are those
505 * that require a siftup instead of a siftdown.) We must visit all of
506 * the elements in this list to complete the iteration. We do this
507 * after we've completed the "normal" iteration.
508 *
509 * We expect that most iterations, even those involving removals,
510 * will not need to store elements in this field.
511 */
512 private ArrayDeque<E> forgetMeNot = null;
513
514 /**
515 * Element returned by the most recent call to next iff that
516 * element was drawn from the forgetMeNot list.
517 */
518 private E lastRetElt = null;
519
520 /**
521 * The modCount value that the iterator believes that the backing
522 * Queue should have. If this expectation is violated, the iterator
523 * has detected concurrent modification.
524 */
525 private int expectedModCount = modCount;
526
527 public boolean hasNext() {
528 return cursor < size ||
529 (forgetMeNot != null && !forgetMeNot.isEmpty());
530 }
531
532 @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 throw new NoSuchElementException();
545 }
546
547 public void remove() {
548 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() {
571 return size;
572 }
573
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;
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 /**
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 private 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;
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 */
733 @SuppressWarnings("unchecked")
734 private void heapify() {
735 for (int i = (size >>> 1) - 1; i >= 0; i--)
736 siftDown(i, (E) queue[i]);
737 }
738
739 /**
740 * Returns the comparator used to order the elements in this
741 * queue, or {@code null} if this queue is sorted according to
742 * the {@linkplain Comparable natural ordering} of its elements.
743 *
744 * @return the comparator used to order this queue, or
745 * {@code null} if this queue is sorted according to the
746 * natural ordering of its elements
747 */
748 public Comparator<? super E> comparator() {
749 return comparator;
750 }
751
752 /**
753 * Saves this queue to a stream (that is, serializes it).
754 *
755 * @serialData The length of the array backing the instance is
756 * emitted (int), followed by all of its elements
757 * (each an {@code Object}) in the proper order.
758 * @param s the stream
759 */
760 private void writeObject(java.io.ObjectOutputStream s)
761 throws java.io.IOException {
762 // Write out element count, and any hidden stuff
763 s.defaultWriteObject();
764
765 // Write out array length, for compatibility with 1.5 version
766 s.writeInt(Math.max(2, size + 1));
767
768 // Write out all elements in the "proper order".
769 for (int i = 0; i < size; i++)
770 s.writeObject(queue[i]);
771 }
772
773 /**
774 * Reconstitutes the {@code PriorityQueue} instance from a stream
775 * (that is, deserializes it).
776 *
777 * @param s the stream
778 */
779 private void readObject(java.io.ObjectInputStream s)
780 throws java.io.IOException, ClassNotFoundException {
781 // Read in size, and any hidden stuff
782 s.defaultReadObject();
783
784 // Read in (and discard) array length
785 s.readInt();
786
787 queue = new Object[size];
788
789 // Read in all elements.
790 for (int i = 0; i < size; i++)
791 queue[i] = s.readObject();
792
793 // Elements are guaranteed to be in "proper order", but the
794 // spec has never explained what that might be.
795 heapify();
796 }
797
798 public final Spliterator<E> spliterator() {
799 return new PriorityQueueSpliterator<E>(this, 0, -1, 0);
800 }
801
802 static final class PriorityQueueSpliterator<E> implements Spliterator<E> {
803 /*
804 * This is very similar to ArrayList Spliterator, except for
805 * extra null checks.
806 */
807 private final PriorityQueue<E> pq;
808 private int index; // current index, modified on advance/split
809 private int fence; // -1 until first use
810 private int expectedModCount; // initialized when fence set
811
812 /** Creates new spliterator covering the given range */
813 PriorityQueueSpliterator(PriorityQueue<E> pq, int origin, int fence,
814 int expectedModCount) {
815 this.pq = pq;
816 this.index = origin;
817 this.fence = fence;
818 this.expectedModCount = expectedModCount;
819 }
820
821 private int getFence() { // initialize fence to size on first use
822 int hi;
823 if ((hi = fence) < 0) {
824 expectedModCount = pq.modCount;
825 hi = fence = pq.size;
826 }
827 return hi;
828 }
829
830 public PriorityQueueSpliterator<E> trySplit() {
831 int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
832 return (lo >= mid) ? null :
833 new PriorityQueueSpliterator<E>(pq, lo, index = mid,
834 expectedModCount);
835 }
836
837 @SuppressWarnings("unchecked")
838 public void forEachRemaining(Consumer<? super E> action) {
839 int i, hi, mc; // hoist accesses and checks from loop
840 PriorityQueue<E> q; Object[] a;
841 if (action == null)
842 throw new NullPointerException();
843 if ((q = pq) != null && (a = q.queue) != null) {
844 if ((hi = fence) < 0) {
845 mc = q.modCount;
846 hi = q.size;
847 }
848 else
849 mc = expectedModCount;
850 if ((i = index) >= 0 && (index = hi) <= a.length) {
851 for (E e;; ++i) {
852 if (i < hi) {
853 if ((e = (E) a[i]) == null) // must be CME
854 break;
855 action.accept(e);
856 }
857 else if (q.modCount != mc)
858 break;
859 else
860 return;
861 }
862 }
863 }
864 throw new ConcurrentModificationException();
865 }
866
867 public boolean tryAdvance(Consumer<? super E> action) {
868 if (action == null)
869 throw new NullPointerException();
870 int hi = getFence(), lo = index;
871 if (lo >= 0 && lo < hi) {
872 index = lo + 1;
873 @SuppressWarnings("unchecked") E e = (E)pq.queue[lo];
874 if (e == null)
875 throw new ConcurrentModificationException();
876 action.accept(e);
877 if (pq.modCount != expectedModCount)
878 throw new ConcurrentModificationException();
879 return true;
880 }
881 return false;
882 }
883
884 public long estimateSize() {
885 return (long) (getFence() - index);
886 }
887
888 public int characteristics() {
889 return Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.NONNULL;
890 }
891 }
892 }