1 /*
2 * Copyright (c) 2003, 2019, 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 *
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24 */
25
26 package java.util;
27
28 import java.util.function.Consumer;
29 import java.util.function.Predicate;
30 import jdk.internal.access.SharedSecrets;
31 import jdk.internal.util.ArraysSupport;
32
33 /**
34 * An unbounded priority {@linkplain Queue queue} based on a priority heap.
35 * The elements of the priority queue are ordered according to their
36 * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
37 * provided at queue construction time, depending on which constructor is
38 * used. A priority queue does not permit {@code null} elements.
39 * A priority queue relying on natural ordering also does not permit
40 * insertion of non-comparable objects (doing so may result in
41 * {@code ClassCastException}).
42 *
43 * <p>The <em>head</em> of this queue is the <em>least</em> element
44 * with respect to the specified ordering. If multiple elements are
45 * tied for least value, the head is one of those elements -- ties are
46 * broken arbitrarily. The queue retrieval operations {@code poll},
47 * {@code remove}, {@code peek}, and {@code element} access the
48 * element at the head of the queue.
49 *
50 * <p>A priority queue is unbounded, but has an internal
51 * <i>capacity</i> governing the size of an array used to store the
52 * elements on the queue. It is always at least as large as the queue
53 * size. As elements are added to a priority queue, its capacity
54 * grows automatically. The details of the growth policy are not
55 * specified.
56 *
57 * <p>This class and its iterator implement all of the
58 * <em>optional</em> methods of the {@link Collection} and {@link
59 * Iterator} interfaces. The Iterator provided in method {@link
60 * #iterator()} and the Spliterator provided in method {@link #spliterator()}
61 * are <em>not</em> guaranteed to traverse the elements of
62 * the priority queue in any particular order. If you need ordered
63 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
64 *
65 * <p><strong>Note that this implementation is not synchronized.</strong>
66 * Multiple threads should not access a {@code PriorityQueue}
67 * instance concurrently if any of the threads modifies the queue.
68 * Instead, use the thread-safe {@link
69 * java.util.concurrent.PriorityBlockingQueue} class.
70 *
71 * <p>Implementation note: this implementation provides
72 * O(log(n)) time for the enqueuing and dequeuing methods
73 * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
74 * linear time for the {@code remove(Object)} and {@code contains(Object)}
75 * methods; and constant time for the retrieval methods
76 * ({@code peek}, {@code element}, and {@code size}).
77 *
78 * <p>This class is a member of the
79 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
80 * Java Collections Framework</a>.
81 *
82 * @since 1.5
83 * @author Josh Bloch, Doug Lea
84 * @param <E> the type of elements held in this queue
85 */
86 @SuppressWarnings("unchecked")
87 public class PriorityQueue<E> extends AbstractQueue<E>
88 implements java.io.Serializable {
89
90 @java.io.Serial
91 private static final long serialVersionUID = -7720805057305804111L;
92
93 private static final int DEFAULT_INITIAL_CAPACITY = 11;
94
95 /**
96 * Priority queue represented as a balanced binary heap: the two
97 * children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
98 * priority queue is ordered by comparator, or by the elements'
99 * natural ordering, if comparator is null: For each node n in the
100 * heap and each descendant d of n, n <= d. The element with the
101 * lowest value is in queue[0], assuming the queue is nonempty.
102 */
103 transient Object[] queue; // non-private to simplify nested class access
104
105 /**
106 * The number of elements in the priority queue.
107 */
108 int size;
109
110 /**
111 * The comparator, or null if priority queue uses elements'
112 * natural ordering.
113 */
114 @SuppressWarnings("serial") // Not statically typed as Serializable
115 private final Comparator<? super E> comparator;
116
117 /**
118 * The number of times this priority queue has been
119 * <i>structurally modified</i>. See AbstractList for gory details.
120 */
121 transient int modCount; // non-private to simplify nested class access
122
123 /**
124 * Creates a {@code PriorityQueue} with the default initial
125 * capacity (11) that orders its elements according to their
126 * {@linkplain Comparable natural ordering}.
127 */
128 public PriorityQueue() {
129 this(DEFAULT_INITIAL_CAPACITY, null);
130 }
131
132 /**
133 * Creates a {@code PriorityQueue} with the specified initial
134 * capacity that orders its elements according to their
135 * {@linkplain Comparable natural ordering}.
136 *
137 * @param initialCapacity the initial capacity for this priority queue
138 * @throws IllegalArgumentException if {@code initialCapacity} is less
139 * than 1
140 */
141 public PriorityQueue(int initialCapacity) {
142 this(initialCapacity, null);
143 }
144
145 /**
146 * Creates a {@code PriorityQueue} with the default initial capacity and
147 * whose elements are ordered according to the specified comparator.
148 *
149 * @param comparator the comparator that will be used to order this
150 * priority queue. If {@code null}, the {@linkplain Comparable
151 * natural ordering} of the elements will be used.
152 * @since 1.8
153 */
154 public PriorityQueue(Comparator<? super E> comparator) {
155 this(DEFAULT_INITIAL_CAPACITY, comparator);
156 }
157
158 /**
159 * Creates a {@code PriorityQueue} with the specified initial capacity
160 * that orders its elements according to the specified comparator.
161 *
162 * @param initialCapacity the initial capacity for this priority queue
163 * @param comparator the comparator that will be used to order this
164 * priority queue. If {@code null}, the {@linkplain Comparable
165 * natural ordering} of the elements will be used.
166 * @throws IllegalArgumentException if {@code initialCapacity} is
167 * less than 1
168 */
169 public PriorityQueue(int initialCapacity,
170 Comparator<? super E> comparator) {
171 // Note: This restriction of at least one is not actually needed,
172 // but continues for 1.5 compatibility
173 if (initialCapacity < 1)
174 throw new IllegalArgumentException();
175 this.queue = new Object[initialCapacity];
176 this.comparator = comparator;
177 }
178
179 /**
180 * Creates a {@code PriorityQueue} containing the elements in the
181 * specified collection. If the specified collection is an instance of
182 * a {@link SortedSet} or is another {@code PriorityQueue}, this
183 * priority queue will be ordered according to the same ordering.
184 * Otherwise, this priority queue will be ordered according to the
185 * {@linkplain Comparable natural ordering} of its elements.
186 *
187 * @param c the collection whose elements are to be placed
188 * into this priority queue
189 * @throws ClassCastException if elements of the specified collection
190 * cannot be compared to one another according to the priority
191 * queue's ordering
192 * @throws NullPointerException if the specified collection or any
193 * of its elements are null
194 */
195 public PriorityQueue(Collection<? extends E> c) {
196 if (c instanceof SortedSet<?>) {
197 SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
198 this.comparator = (Comparator<? super E>) ss.comparator();
199 initElementsFromCollection(ss);
200 }
201 else if (c instanceof PriorityQueue<?>) {
202 PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
203 this.comparator = (Comparator<? super E>) pq.comparator();
204 initFromPriorityQueue(pq);
205 }
206 else {
207 this.comparator = null;
208 initFromCollection(c);
209 }
210 }
211
212 /**
213 * Creates a {@code PriorityQueue} containing the elements in the
214 * specified priority queue. This priority queue will be
215 * ordered according to the same ordering as the given priority
216 * queue.
217 *
218 * @param c the priority queue whose elements are to be placed
219 * into this priority queue
220 * @throws ClassCastException if elements of {@code c} cannot be
221 * compared to one another according to {@code c}'s
222 * ordering
223 * @throws NullPointerException if the specified priority queue or any
224 * of its elements are null
225 */
226 public PriorityQueue(PriorityQueue<? extends E> c) {
227 this.comparator = (Comparator<? super E>) c.comparator();
228 initFromPriorityQueue(c);
229 }
230
231 /**
232 * Creates a {@code PriorityQueue} containing the elements in the
233 * specified sorted set. This priority queue will be ordered
234 * according to the same ordering as the given sorted set.
235 *
236 * @param c the sorted set whose elements are to be placed
237 * into this priority queue
238 * @throws ClassCastException if elements of the specified sorted
239 * set cannot be compared to one another according to the
240 * sorted set's ordering
241 * @throws NullPointerException if the specified sorted set or any
242 * of its elements are null
243 */
244 public PriorityQueue(SortedSet<? extends E> c) {
245 this.comparator = (Comparator<? super E>) c.comparator();
246 initElementsFromCollection(c);
247 }
248
249 /** Ensures that queue[0] exists, helping peek() and poll(). */
250 private static Object[] ensureNonEmpty(Object[] es) {
251 return (es.length > 0) ? es : new Object[1];
252 }
253
254 private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
255 if (c.getClass() == PriorityQueue.class) {
256 this.queue = ensureNonEmpty(c.toArray());
257 this.size = c.size();
258 } else {
259 initFromCollection(c);
260 }
261 }
262
263 private void initElementsFromCollection(Collection<? extends E> c) {
264 Object[] es = c.toArray();
265 int len = es.length;
266 // If c.toArray incorrectly doesn't return Object[], copy it.
267 if (es.getClass() != Object[].class)
268 es = Arrays.copyOf(es, len, Object[].class);
269 if (len == 1 || this.comparator != null)
270 for (Object e : es)
271 if (e == null)
272 throw new NullPointerException();
273 this.queue = ensureNonEmpty(es);
274 this.size = len;
275 }
276
277 /**
278 * Initializes queue array with elements from the given Collection.
279 *
280 * @param c the collection
281 */
282 private void initFromCollection(Collection<? extends E> c) {
283 initElementsFromCollection(c);
284 heapify();
285 }
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 = ArraysSupport.newLength(oldCapacity,
296 minCapacity - oldCapacity, /* minimum growth */
297 oldCapacity < 64 ? oldCapacity + 2 : oldCapacity >> 1
298 /* preferred growth */);
299 queue = Arrays.copyOf(queue, newCapacity);
300 }
301
302 /**
303 * Inserts the specified element into this priority queue.
304 *
305 * @return {@code true} (as specified by {@link Collection#add})
306 * @throws ClassCastException if the specified element cannot be
307 * compared with elements currently in this priority queue
308 * according to the priority queue's ordering
309 * @throws NullPointerException if the specified element is null
310 */
311 public boolean add(E e) {
312 return offer(e);
313 }
314
315 /**
316 * Inserts the specified element into this priority queue.
317 *
318 * @return {@code true} (as specified by {@link Queue#offer})
319 * @throws ClassCastException if the specified element cannot be
320 * compared with elements currently in this priority queue
321 * according to the priority queue's ordering
322 * @throws NullPointerException if the specified element is null
323 */
324 public boolean offer(E e) {
325 if (e == null)
326 throw new NullPointerException();
327 modCount++;
328 int i = size;
329 if (i >= queue.length)
330 grow(i + 1);
331 siftUp(i, e);
332 size = i + 1;
333 return true;
334 }
335
336 public E peek() {
337 return (E) queue[0];
338 }
339
340 private int indexOf(Object o) {
341 if (o != null) {
342 final Object[] es = queue;
343 for (int i = 0, n = size; i < n; i++)
344 if (o.equals(es[i]))
345 return i;
346 }
347 return -1;
348 }
349
350 /**
351 * Removes a single instance of the specified element from this queue,
352 * if it is present. More formally, removes an element {@code e} such
353 * that {@code o.equals(e)}, if this queue contains one or more such
354 * elements. Returns {@code true} if and only if this queue contained
355 * the specified element (or equivalently, if this queue changed as a
356 * result of the call).
357 *
358 * @param o element to be removed from this queue, if present
359 * @return {@code true} if this queue changed as a result of the call
360 */
361 public boolean remove(Object o) {
362 int i = indexOf(o);
363 if (i == -1)
364 return false;
365 else {
366 removeAt(i);
367 return true;
368 }
369 }
370
371 /**
372 * Identity-based version for use in Itr.remove.
373 *
374 * @param o element to be removed from this queue, if present
375 */
376 void removeEq(Object o) {
377 final Object[] es = queue;
378 for (int i = 0, n = size; i < n; i++) {
379 if (o == es[i]) {
380 removeAt(i);
381 break;
382 }
383 }
384 }
385
386 /**
387 * Returns {@code true} if this queue contains the specified element.
388 * More formally, returns {@code true} if and only if this queue contains
389 * at least one element {@code e} such that {@code o.equals(e)}.
390 *
391 * @param o object to be checked for containment in this queue
392 * @return {@code true} if this queue contains the specified element
393 */
394 public boolean contains(Object o) {
395 return indexOf(o) >= 0;
396 }
397
398 /**
399 * Returns an array containing all of the elements in this queue.
400 * The elements are in no particular order.
401 *
402 * <p>The returned array will be "safe" in that no references to it are
403 * maintained by this queue. (In other words, this method must allocate
404 * a new array). The caller is thus free to modify the returned array.
405 *
406 * <p>This method acts as bridge between array-based and collection-based
407 * APIs.
408 *
409 * @return an array containing all of the elements in this queue
410 */
411 public Object[] toArray() {
412 return Arrays.copyOf(queue, size);
413 }
414
415 /**
416 * Returns an array containing all of the elements in this queue; the
417 * runtime type of the returned array is that of the specified array.
418 * The returned array elements are in no particular order.
419 * If the queue fits in the specified array, it is returned therein.
420 * Otherwise, a new array is allocated with the runtime type of the
421 * specified array and the size of this queue.
422 *
423 * <p>If the queue fits in the specified array with room to spare
424 * (i.e., the array has more elements than the queue), the element in
425 * the array immediately following the end of the collection is set to
426 * {@code null}.
427 *
428 * <p>Like the {@link #toArray()} method, this method acts as bridge between
429 * array-based and collection-based APIs. Further, this method allows
430 * precise control over the runtime type of the output array, and may,
431 * under certain circumstances, be used to save allocation costs.
432 *
433 * <p>Suppose {@code x} is a queue known to contain only strings.
434 * The following code can be used to dump the queue into a newly
435 * allocated array of {@code String}:
436 *
437 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
438 *
439 * Note that {@code toArray(new Object[0])} is identical in function to
440 * {@code toArray()}.
441 *
442 * @param a the array into which the elements of the queue are to
443 * be stored, if it is big enough; otherwise, a new array of the
444 * same runtime type is allocated for this purpose.
445 * @return an array containing all of the elements in this queue
446 * @throws ArrayStoreException if the runtime type of the specified array
447 * is not a supertype of the runtime type of every element in
448 * this queue
449 * @throws NullPointerException if the specified array is null
450 */
451 public <T> T[] toArray(T[] a) {
452 final int size = this.size;
453 if (a.length < size)
454 // Make a new array of a's runtime type, but my contents:
455 return (T[]) Arrays.copyOf(queue, size, a.getClass());
456 System.arraycopy(queue, 0, a, 0, size);
457 if (a.length > size)
458 a[size] = null;
459 return a;
460 }
461
462 /**
463 * Returns an iterator over the elements in this queue. The iterator
464 * does not return the elements in any particular order.
465 *
466 * @return an iterator over the elements in this queue
467 */
468 public Iterator<E> iterator() {
469 return new Itr();
470 }
471
472 private final class Itr implements Iterator<E> {
473 /**
474 * Index (into queue array) of element to be returned by
475 * subsequent call to next.
476 */
477 private int cursor;
478
479 /**
480 * Index of element returned by most recent call to next,
481 * unless that element came from the forgetMeNot list.
482 * Set to -1 if element is deleted by a call to remove.
483 */
484 private int lastRet = -1;
485
486 /**
487 * A queue of elements that were moved from the unvisited portion of
488 * the heap into the visited portion as a result of "unlucky" element
489 * removals during the iteration. (Unlucky element removals are those
490 * that require a siftup instead of a siftdown.) We must visit all of
491 * the elements in this list to complete the iteration. We do this
492 * after we've completed the "normal" iteration.
493 *
494 * We expect that most iterations, even those involving removals,
495 * will not need to store elements in this field.
496 */
497 private ArrayDeque<E> forgetMeNot;
498
499 /**
500 * Element returned by the most recent call to next iff that
501 * element was drawn from the forgetMeNot list.
502 */
503 private E lastRetElt;
504
505 /**
506 * The modCount value that the iterator believes that the backing
507 * Queue should have. If this expectation is violated, the iterator
508 * has detected concurrent modification.
509 */
510 private int expectedModCount = modCount;
511
512 Itr() {} // prevent access constructor creation
513
514 public boolean hasNext() {
515 return cursor < size ||
516 (forgetMeNot != null && !forgetMeNot.isEmpty());
517 }
518
519 public E next() {
520 if (expectedModCount != modCount)
521 throw new ConcurrentModificationException();
522 if (cursor < size)
523 return (E) queue[lastRet = cursor++];
524 if (forgetMeNot != null) {
525 lastRet = -1;
526 lastRetElt = forgetMeNot.poll();
527 if (lastRetElt != null)
528 return lastRetElt;
529 }
530 throw new NoSuchElementException();
531 }
532
533 public void remove() {
534 if (expectedModCount != modCount)
535 throw new ConcurrentModificationException();
536 if (lastRet != -1) {
537 E moved = PriorityQueue.this.removeAt(lastRet);
538 lastRet = -1;
539 if (moved == null)
540 cursor--;
541 else {
542 if (forgetMeNot == null)
543 forgetMeNot = new ArrayDeque<>();
544 forgetMeNot.add(moved);
545 }
546 } else if (lastRetElt != null) {
547 PriorityQueue.this.removeEq(lastRetElt);
548 lastRetElt = null;
549 } else {
550 throw new IllegalStateException();
551 }
552 expectedModCount = modCount;
553 }
554 }
555
556 public int size() {
557 return size;
558 }
559
560 /**
561 * Removes all of the elements from this priority queue.
562 * The queue will be empty after this call returns.
563 */
564 public void clear() {
565 modCount++;
566 final Object[] es = queue;
567 for (int i = 0, n = size; i < n; i++)
568 es[i] = null;
569 size = 0;
570 }
571
572 public E poll() {
573 final Object[] es;
574 final E result;
575
576 if ((result = (E) ((es = queue)[0])) != null) {
577 modCount++;
578 final int n;
579 final E x = (E) es[(n = --size)];
580 es[n] = null;
581 if (n > 0) {
582 final Comparator<? super E> cmp;
583 if ((cmp = comparator) == null)
584 siftDownComparable(0, x, es, n);
585 else
586 siftDownUsingComparator(0, x, es, n, cmp);
587 }
588 }
589 return result;
590 }
591
592 /**
593 * Removes the ith element from queue.
594 *
595 * Normally this method leaves the elements at up to i-1,
596 * inclusive, untouched. Under these circumstances, it returns
597 * null. Occasionally, in order to maintain the heap invariant,
598 * it must swap a later element of the list with one earlier than
599 * i. Under these circumstances, this method returns the element
600 * that was previously at the end of the list and is now at some
601 * position before i. This fact is used by iterator.remove so as to
602 * avoid missing traversing elements.
603 */
604 E removeAt(int i) {
605 // assert i >= 0 && i < size;
606 final Object[] es = queue;
607 modCount++;
608 int s = --size;
609 if (s == i) // removed last element
610 es[i] = null;
611 else {
612 E moved = (E) es[s];
613 es[s] = null;
614 siftDown(i, moved);
615 if (es[i] == moved) {
616 siftUp(i, moved);
617 if (es[i] != moved)
618 return moved;
619 }
620 }
621 return null;
622 }
623
624 /**
625 * Inserts item x at position k, maintaining heap invariant by
626 * promoting x up the tree until it is greater than or equal to
627 * its parent, or is the root.
628 *
629 * To simplify and speed up coercions and comparisons, the
630 * Comparable and Comparator versions are separated into different
631 * methods that are otherwise identical. (Similarly for siftDown.)
632 *
633 * @param k the position to fill
634 * @param x the item to insert
635 */
636 private void siftUp(int k, E x) {
637 if (comparator != null)
638 siftUpUsingComparator(k, x, queue, comparator);
639 else
640 siftUpComparable(k, x, queue);
641 }
642
643 private static <T> void siftUpComparable(int k, T x, Object[] es) {
644 Comparable<? super T> key = (Comparable<? super T>) x;
645 while (k > 0) {
646 int parent = (k - 1) >>> 1;
647 Object e = es[parent];
648 if (key.compareTo((T) e) >= 0)
649 break;
650 es[k] = e;
651 k = parent;
652 }
653 es[k] = key;
654 }
655
656 private static <T> void siftUpUsingComparator(
657 int k, T x, Object[] es, Comparator<? super T> cmp) {
658 while (k > 0) {
659 int parent = (k - 1) >>> 1;
660 Object e = es[parent];
661 if (cmp.compare(x, (T) e) >= 0)
662 break;
663 es[k] = e;
664 k = parent;
665 }
666 es[k] = x;
667 }
668
669 /**
670 * Inserts item x at position k, maintaining heap invariant by
671 * demoting x down the tree repeatedly until it is less than or
672 * equal to its children or is a leaf.
673 *
674 * @param k the position to fill
675 * @param x the item to insert
676 */
677 private void siftDown(int k, E x) {
678 if (comparator != null)
679 siftDownUsingComparator(k, x, queue, size, comparator);
680 else
681 siftDownComparable(k, x, queue, size);
682 }
683
684 private static <T> void siftDownComparable(int k, T x, Object[] es, int n) {
685 // assert n > 0;
686 Comparable<? super T> key = (Comparable<? super T>)x;
687 int half = n >>> 1; // loop while a non-leaf
688 while (k < half) {
689 int child = (k << 1) + 1; // assume left child is least
690 Object c = es[child];
691 int right = child + 1;
692 if (right < n &&
693 ((Comparable<? super T>) c).compareTo((T) es[right]) > 0)
694 c = es[child = right];
695 if (key.compareTo((T) c) <= 0)
696 break;
697 es[k] = c;
698 k = child;
699 }
700 es[k] = key;
701 }
702
703 private static <T> void siftDownUsingComparator(
704 int k, T x, Object[] es, int n, Comparator<? super T> cmp) {
705 // assert n > 0;
706 int half = n >>> 1;
707 while (k < half) {
708 int child = (k << 1) + 1;
709 Object c = es[child];
710 int right = child + 1;
711 if (right < n && cmp.compare((T) c, (T) es[right]) > 0)
712 c = es[child = right];
713 if (cmp.compare(x, (T) c) <= 0)
714 break;
715 es[k] = c;
716 k = child;
717 }
718 es[k] = x;
719 }
720
721 /**
722 * Establishes the heap invariant (described above) in the entire tree,
723 * assuming nothing about the order of the elements prior to the call.
724 * This classic algorithm due to Floyd (1964) is known to be O(size).
725 */
726 private void heapify() {
727 final Object[] es = queue;
728 int n = size, i = (n >>> 1) - 1;
729 final Comparator<? super E> cmp;
730 if ((cmp = comparator) == null)
731 for (; i >= 0; i--)
732 siftDownComparable(i, (E) es[i], es, n);
733 else
734 for (; i >= 0; i--)
735 siftDownUsingComparator(i, (E) es[i], es, n, cmp);
736 }
737
738 /**
739 * Returns the comparator used to order the elements in this
740 * queue, or {@code null} if this queue is sorted according to
741 * the {@linkplain Comparable natural ordering} of its elements.
742 *
743 * @return the comparator used to order this queue, or
744 * {@code null} if this queue is sorted according to the
745 * natural ordering of its elements
746 */
747 public Comparator<? super E> comparator() {
748 return comparator;
749 }
750
751 /**
752 * Saves this queue to a stream (that is, serializes it).
753 *
754 * @param s the stream
755 * @throws java.io.IOException if an I/O error occurs
756 * @serialData The length of the array backing the instance is
757 * emitted (int), followed by all of its elements
758 * (each an {@code Object}) in the proper order.
759 */
760 @java.io.Serial
761 private void writeObject(java.io.ObjectOutputStream s)
762 throws java.io.IOException {
763 // Write out element count, and any hidden stuff
764 s.defaultWriteObject();
765
766 // Write out array length, for compatibility with 1.5 version
767 s.writeInt(Math.max(2, size + 1));
768
769 // Write out all elements in the "proper order".
770 final Object[] es = queue;
771 for (int i = 0, n = size; i < n; i++)
772 s.writeObject(es[i]);
773 }
774
775 /**
776 * Reconstitutes the {@code PriorityQueue} instance from a stream
777 * (that is, deserializes it).
778 *
779 * @param s the stream
780 * @throws ClassNotFoundException if the class of a serialized object
781 * could not be found
782 * @throws java.io.IOException if an I/O error occurs
783 */
784 @java.io.Serial
785 private void readObject(java.io.ObjectInputStream s)
786 throws java.io.IOException, ClassNotFoundException {
787 // Read in size, and any hidden stuff
788 s.defaultReadObject();
789
790 // Read in (and discard) array length
791 s.readInt();
792
793 SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size);
794 final Object[] es = queue = new Object[Math.max(size, 1)];
795
796 // Read in all elements.
797 for (int i = 0, n = size; i < n; i++)
798 es[i] = s.readObject();
799
800 // Elements are guaranteed to be in "proper order", but the
801 // spec has never explained what that might be.
802 heapify();
803 }
804
805 /**
806 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
807 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
808 * queue. The spliterator does not traverse elements in any particular order
809 * (the {@link Spliterator#ORDERED ORDERED} characteristic is not reported).
810 *
811 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
812 * {@link Spliterator#SUBSIZED}, and {@link Spliterator#NONNULL}.
813 * Overriding implementations should document the reporting of additional
814 * characteristic values.
815 *
816 * @return a {@code Spliterator} over the elements in this queue
817 * @since 1.8
818 */
819 public final Spliterator<E> spliterator() {
820 return new PriorityQueueSpliterator(0, -1, 0);
821 }
822
823 final class PriorityQueueSpliterator implements Spliterator<E> {
824 private int index; // current index, modified on advance/split
825 private int fence; // -1 until first use
826 private int expectedModCount; // initialized when fence set
827
828 /** Creates new spliterator covering the given range. */
829 PriorityQueueSpliterator(int origin, int fence, int expectedModCount) {
830 this.index = origin;
831 this.fence = fence;
832 this.expectedModCount = expectedModCount;
833 }
834
835 private int getFence() { // initialize fence to size on first use
836 int hi;
837 if ((hi = fence) < 0) {
838 expectedModCount = modCount;
839 hi = fence = size;
840 }
841 return hi;
842 }
843
844 public PriorityQueueSpliterator trySplit() {
845 int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
846 return (lo >= mid) ? null :
847 new PriorityQueueSpliterator(lo, index = mid, expectedModCount);
848 }
849
850 public void forEachRemaining(Consumer<? super E> action) {
851 if (action == null)
852 throw new NullPointerException();
853 if (fence < 0) { fence = size; expectedModCount = modCount; }
854 final Object[] es = queue;
855 int i, hi; E e;
856 for (i = index, index = hi = fence; i < hi; i++) {
857 if ((e = (E) es[i]) == null)
858 break; // must be CME
859 action.accept(e);
860 }
861 if (modCount != expectedModCount)
862 throw new ConcurrentModificationException();
863 }
864
865 public boolean tryAdvance(Consumer<? super E> action) {
866 if (action == null)
867 throw new NullPointerException();
868 if (fence < 0) { fence = size; expectedModCount = modCount; }
869 int i;
870 if ((i = index) < fence) {
871 index = i + 1;
872 E e;
873 if ((e = (E) queue[i]) == null
874 || modCount != expectedModCount)
875 throw new ConcurrentModificationException();
876 action.accept(e);
877 return true;
878 }
879 return false;
880 }
881
882 public long estimateSize() {
883 return getFence() - index;
884 }
885
886 public int characteristics() {
887 return Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.NONNULL;
888 }
889 }
890
891 /**
892 * @throws NullPointerException {@inheritDoc}
893 */
894 public boolean removeIf(Predicate<? super E> filter) {
895 Objects.requireNonNull(filter);
896 return bulkRemove(filter);
897 }
898
899 /**
900 * @throws NullPointerException {@inheritDoc}
901 */
902 public boolean removeAll(Collection<?> c) {
903 Objects.requireNonNull(c);
904 return bulkRemove(e -> c.contains(e));
905 }
906
907 /**
908 * @throws NullPointerException {@inheritDoc}
909 */
910 public boolean retainAll(Collection<?> c) {
911 Objects.requireNonNull(c);
912 return bulkRemove(e -> !c.contains(e));
913 }
914
915 // A tiny bit set implementation
916
917 private static long[] nBits(int n) {
918 return new long[((n - 1) >> 6) + 1];
919 }
920 private static void setBit(long[] bits, int i) {
921 bits[i >> 6] |= 1L << i;
922 }
923 private static boolean isClear(long[] bits, int i) {
924 return (bits[i >> 6] & (1L << i)) == 0;
925 }
926
927 /** Implementation of bulk remove methods. */
928 private boolean bulkRemove(Predicate<? super E> filter) {
929 final int expectedModCount = ++modCount;
930 final Object[] es = queue;
931 final int end = size;
932 int i;
933 // Optimize for initial run of survivors
934 for (i = 0; i < end && !filter.test((E) es[i]); i++)
935 ;
936 if (i >= end) {
937 if (modCount != expectedModCount)
938 throw new ConcurrentModificationException();
939 return false;
940 }
941 // Tolerate predicates that reentrantly access the collection for
942 // read (but writers still get CME), so traverse once to find
943 // elements to delete, a second pass to physically expunge.
944 final int beg = i;
945 final long[] deathRow = nBits(end - beg);
946 deathRow[0] = 1L; // set bit 0
947 for (i = beg + 1; i < end; i++)
948 if (filter.test((E) es[i]))
949 setBit(deathRow, i - beg);
950 if (modCount != expectedModCount)
951 throw new ConcurrentModificationException();
952 int w = beg;
953 for (i = beg; i < end; i++)
954 if (isClear(deathRow, i - beg))
955 es[w++] = es[i];
956 for (i = size = w; i < end; i++)
957 es[i] = null;
958 heapify();
959 return true;
960 }
961
962 /**
963 * @throws NullPointerException {@inheritDoc}
964 */
965 public void forEach(Consumer<? super E> action) {
966 Objects.requireNonNull(action);
967 final int expectedModCount = modCount;
968 final Object[] es = queue;
969 for (int i = 0, n = size; i < n; i++)
970 action.accept((E) es[i]);
971 if (expectedModCount != modCount)
972 throw new ConcurrentModificationException();
973 }
974 }