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