1 /*
2 * Copyright (c) 2003, 2010, 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 /**
29 * An unbounded priority {@linkplain Queue queue} based on a priority heap.
30 * The elements of the priority queue are ordered according to their
31 * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
32 * provided at queue construction time, depending on which constructor is
33 * used. A priority queue does not permit {@code null} elements.
34 * A priority queue relying on natural ordering also does not permit
35 * insertion of non-comparable objects (doing so may result in
36 * {@code ClassCastException}).
37 *
38 * <p>The <em>head</em> of this queue is the <em>least</em> element
39 * with respect to the specified ordering. If multiple elements are
40 * tied for least value, the head is one of those elements -- ties are
41 * broken arbitrarily. The queue retrieval operations {@code poll},
42 * {@code remove}, {@code peek}, and {@code element} access the
43 * element at the head of the queue.
44 *
45 * <p>A priority queue is unbounded, but has an internal
46 * <i>capacity</i> governing the size of an array used to store the
47 * elements on the queue. It is always at least as large as the queue
48 * size. As elements are added to a priority queue, its capacity
49 * grows automatically. The details of the growth policy are not
50 * specified.
51 *
52 * <p>This class and its iterator implement all of the
53 * <em>optional</em> methods of the {@link Collection} and {@link
54 * Iterator} interfaces. The Iterator provided in method {@link
55 * #iterator()} is <em>not</em> guaranteed to traverse the elements of
56 * the priority queue in any particular order. If you need ordered
57 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
58 *
59 * <p> <strong>Note that this implementation is not synchronized.</strong>
60 * Multiple threads should not access a {@code PriorityQueue}
61 * instance concurrently if any of the threads modifies the queue.
62 * Instead, use the thread-safe {@link
63 * java.util.concurrent.PriorityBlockingQueue} class.
64 *
65 * <p>Implementation note: this implementation provides
66 * O(log(n)) time for the enqueing and dequeing methods
67 * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
68 * linear time for the {@code remove(Object)} and {@code contains(Object)}
69 * methods; and constant time for the retrieval methods
70 * ({@code peek}, {@code element}, and {@code size}).
71 *
72 * <p>This class is a member of the
73 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
74 * Java Collections Framework</a>.
75 *
76 * @since 1.5
77 * @author Josh Bloch, Doug Lea
78 * @param <E> the type of elements held in this collection
79 */
80 public class PriorityQueue<E> extends AbstractQueue<E>
81 implements java.io.Serializable {
82
83 private static final long serialVersionUID = -7720805057305804111L;
84
85 private static final int DEFAULT_INITIAL_CAPACITY = 11;
86
87 /**
88 * Priority queue represented as a balanced binary heap: the two
89 * children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
90 * priority queue is ordered by comparator, or by the elements'
91 * natural ordering, if comparator is null: For each node n in the
92 * heap and each descendant d of n, n <= d. The element with the
93 * lowest value is in queue[0], assuming the queue is nonempty.
94 */
95 private transient Object[] queue;
96
97 /**
98 * The number of elements in the priority queue.
99 */
100 private int size = 0;
101
102 /**
103 * The comparator, or null if priority queue uses elements'
104 * natural ordering.
105 */
106 private final Comparator<? super E> comparator;
107
108 /**
109 * The number of times this priority queue has been
110 * <i>structurally modified</i>. See AbstractList for gory details.
111 */
112 private transient int modCount = 0;
113
114 /**
115 * Creates a {@code PriorityQueue} with the default initial
116 * capacity (11) that orders its elements according to their
117 * {@linkplain Comparable natural ordering}.
118 */
119 public PriorityQueue() {
120 this(DEFAULT_INITIAL_CAPACITY, null);
121 }
122
123 /**
124 * Creates a {@code PriorityQueue} with the specified initial
125 * capacity that orders its elements according to their
126 * {@linkplain Comparable natural ordering}.
127 *
128 * @param initialCapacity the initial capacity for this priority queue
129 * @throws IllegalArgumentException if {@code initialCapacity} is less
130 * than 1
131 */
132 public PriorityQueue(int initialCapacity) {
133 this(initialCapacity, null);
134 }
135
136 /**
137 * Creates a {@code PriorityQueue} with the specified initial capacity
138 * that orders its elements according to the specified comparator.
139 *
140 * @param initialCapacity the initial capacity for this priority queue
141 * @param comparator the comparator that will be used to order this
142 * priority queue. If {@code null}, the {@linkplain Comparable
143 * natural ordering} of the elements will be used.
144 * @throws IllegalArgumentException if {@code initialCapacity} is
145 * less than 1
146 */
147 public PriorityQueue(int initialCapacity,
148 Comparator<? super E> comparator) {
149 // Note: This restriction of at least one is not actually needed,
150 // but continues for 1.5 compatibility
151 if (initialCapacity < 1)
152 throw new IllegalArgumentException();
153 this.queue = new Object[initialCapacity];
154 this.comparator = comparator;
155 }
156
157 /**
158 * Creates a {@code PriorityQueue} containing the elements in the
159 * specified collection. If the specified collection is an instance of
160 * a {@link SortedSet} or is another {@code PriorityQueue}, this
161 * priority queue will be ordered according to the same ordering.
162 * Otherwise, this priority queue will be ordered according to the
163 * {@linkplain Comparable natural ordering} of its elements.
164 *
165 * @param c the collection whose elements are to be placed
166 * into this priority queue
167 * @throws ClassCastException if elements of the specified collection
168 * cannot be compared to one another according to the priority
169 * queue's ordering
170 * @throws NullPointerException if the specified collection or any
171 * of its elements are null
172 */
173 @SuppressWarnings("unchecked")
174 public PriorityQueue(Collection<? extends E> c) {
175 if (c instanceof SortedSet<?>) {
176 SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
177 this.comparator = (Comparator<? super E>) ss.comparator();
178 initElementsFromCollection(ss);
179 }
180 else if (c instanceof PriorityQueue<?>) {
181 PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
182 this.comparator = (Comparator<? super E>) pq.comparator();
183 initFromPriorityQueue(pq);
184 }
185 else {
186 this.comparator = null;
187 initFromCollection(c);
188 }
189 }
190
191 /**
192 * Creates a {@code PriorityQueue} containing the elements in the
193 * specified priority queue. This priority queue will be
194 * ordered according to the same ordering as the given priority
195 * queue.
196 *
197 * @param c the priority queue whose elements are to be placed
198 * into this priority queue
199 * @throws ClassCastException if elements of {@code c} cannot be
200 * compared to one another according to {@code c}'s
201 * ordering
202 * @throws NullPointerException if the specified priority queue or any
203 * of its elements are null
204 */
205 @SuppressWarnings("unchecked")
206 public PriorityQueue(PriorityQueue<? extends E> c) {
207 this.comparator = (Comparator<? super E>) c.comparator();
208 initFromPriorityQueue(c);
209 }
210
211 /**
212 * Creates a {@code PriorityQueue} containing the elements in the
213 * specified sorted set. This priority queue will be ordered
214 * according to the same ordering as the given sorted set.
215 *
216 * @param c the sorted set whose elements are to be placed
217 * into this priority queue
218 * @throws ClassCastException if elements of the specified sorted
219 * set cannot be compared to one another according to the
220 * sorted set's ordering
221 * @throws NullPointerException if the specified sorted set or any
222 * of its elements are null
223 */
224 @SuppressWarnings("unchecked")
225 public PriorityQueue(SortedSet<? extends E> c) {
226 this.comparator = (Comparator<? super E>) c.comparator();
227 initElementsFromCollection(c);
228 }
229
230 private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
231 if (c.getClass() == PriorityQueue.class) {
232 this.queue = c.toArray();
233 this.size = c.size();
234 } else {
235 initFromCollection(c);
236 }
237 }
238
239 private void initElementsFromCollection(Collection<? extends E> c) {
240 Object[] a = c.toArray();
241 // If c.toArray incorrectly doesn't return Object[], copy it.
242 if (a.getClass() != Object[].class)
243 a = Arrays.copyOf(a, a.length, Object[].class);
244 int len = a.length;
245 if (len == 1 || this.comparator != null)
246 for (int i = 0; i < len; i++)
247 if (a[i] == null)
248 throw new NullPointerException();
249 this.queue = a;
250 this.size = a.length;
251 }
252
253 /**
254 * Initializes queue array with elements from the given Collection.
255 *
256 * @param c the collection
257 */
258 private void initFromCollection(Collection<? extends E> c) {
259 initElementsFromCollection(c);
260 heapify();
261 }
262
263 /**
264 * The maximum size of array to allocate.
265 * Some VMs reserve some header words in an array.
266 * Attempts to allocate larger arrays may result in
267 * OutOfMemoryError: Requested array size exceeds VM limit
268 */
269 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
270
271 /**
272 * Increases the capacity of the array.
273 *
274 * @param minCapacity the desired minimum capacity
275 */
276 private void grow(int minCapacity) {
277 int oldCapacity = queue.length;
278 // Double size if small; else grow by 50%
279 int newCapacity = oldCapacity + ((oldCapacity < 64) ?
280 (oldCapacity + 2) :
281 (oldCapacity >> 1));
282 // overflow-conscious code
283 if (newCapacity - MAX_ARRAY_SIZE > 0)
284 newCapacity = hugeCapacity(minCapacity);
285 queue = Arrays.copyOf(queue, newCapacity);
286 }
287
288 private static int hugeCapacity(int minCapacity) {
289 if (minCapacity < 0) // overflow
290 throw new OutOfMemoryError();
291 return (minCapacity > MAX_ARRAY_SIZE) ?
292 Integer.MAX_VALUE :
293 MAX_ARRAY_SIZE;
294 }
295
296 /**
297 * Inserts the specified element into this priority queue.
298 *
299 * @return {@code true} (as specified by {@link Collection#add})
300 * @throws ClassCastException if the specified element cannot be
301 * compared with elements currently in this priority queue
302 * according to the priority queue's ordering
303 * @throws NullPointerException if the specified element is null
304 */
305 public boolean add(E e) {
306 return offer(e);
307 }
308
309 /**
310 * Inserts the specified element into this priority queue.
311 *
312 * @return {@code true} (as specified by {@link Queue#offer})
313 * @throws ClassCastException if the specified element cannot be
314 * compared with elements currently in this priority queue
315 * according to the priority queue's ordering
316 * @throws NullPointerException if the specified element is null
317 */
318 public boolean offer(E e) {
319 if (e == null)
320 throw new NullPointerException();
321 modCount++;
322 int i = size;
323 if (i >= queue.length)
324 grow(i + 1);
325 size = i + 1;
326 if (i == 0)
327 queue[0] = e;
328 else
329 siftUp(i, e);
330 return true;
331 }
332
333 public E peek() {
334 if (size == 0)
335 return null;
336 return (E) queue[0];
337 }
338
339 private int indexOf(Object o) {
340 if (o != null) {
341 for (int i = 0; i < size; i++)
342 if (o.equals(queue[i]))
343 return i;
344 }
345 return -1;
346 }
347
348 /**
349 * Removes a single instance of the specified element from this queue,
350 * if it is present. More formally, removes an element {@code e} such
351 * that {@code o.equals(e)}, if this queue contains one or more such
352 * elements. Returns {@code true} if and only if this queue contained
353 * the specified element (or equivalently, if this queue changed as a
354 * result of the call).
355 *
356 * @param o element to be removed from this queue, if present
357 * @return {@code true} if this queue changed as a result of the call
358 */
359 public boolean remove(Object o) {
360 int i = indexOf(o);
361 if (i == -1)
362 return false;
363 else {
364 removeAt(i);
365 return true;
366 }
367 }
368
369 /**
370 * Version of remove using reference equality, not equals.
371 * Needed by iterator.remove.
372 *
373 * @param o element to be removed from this queue, if present
374 * @return {@code true} if removed
375 */
376 boolean removeEq(Object o) {
377 for (int i = 0; i < size; i++) {
378 if (o == queue[i]) {
379 removeAt(i);
380 return true;
381 }
382 }
383 return false;
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) != -1;
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 <tt>x</tt> 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 <tt>String</tt>:
436 *
437 * <pre>
438 * String[] y = x.toArray(new String[0]);</pre>
439 *
440 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
441 * <tt>toArray()</tt>.
442 *
443 * @param a the array into which the elements of the queue are to
444 * be stored, if it is big enough; otherwise, a new array of the
445 * same runtime type is allocated for this purpose.
446 * @return an array containing all of the elements in this queue
447 * @throws ArrayStoreException if the runtime type of the specified array
448 * is not a supertype of the runtime type of every element in
449 * this queue
450 * @throws NullPointerException if the specified array is null
451 */
452 @SuppressWarnings("unchecked")
453 public <T> T[] toArray(T[] a) {
454 if (a.length < size)
455 // Make a new array of a's runtime type, but my contents:
456 return (T[]) Arrays.copyOf(queue, size, a.getClass());
457 System.arraycopy(queue, 0, a, 0, size);
458 if (a.length > size)
459 a[size] = null;
460 return a;
461 }
462
463 /**
464 * Returns an iterator over the elements in this queue. The iterator
465 * does not return the elements in any particular order.
466 *
467 * @return an iterator over the elements in this queue
468 */
469 public Iterator<E> iterator() {
470 return new Itr();
471 }
472
473 private final class Itr implements Iterator<E> {
474 /**
475 * Index (into queue array) of element to be returned by
476 * subsequent call to next.
477 */
478 private int cursor = 0;
479
480 /**
481 * Index of element returned by most recent call to next,
482 * unless that element came from the forgetMeNot list.
483 * Set to -1 if element is deleted by a call to remove.
484 */
485 private int lastRet = -1;
486
487 /**
488 * A queue of elements that were moved from the unvisited portion of
489 * the heap into the visited portion as a result of "unlucky" element
490 * removals during the iteration. (Unlucky element removals are those
491 * that require a siftup instead of a siftdown.) We must visit all of
492 * the elements in this list to complete the iteration. We do this
493 * after we've completed the "normal" iteration.
494 *
495 * We expect that most iterations, even those involving removals,
496 * will not need to store elements in this field.
497 */
498 private ArrayDeque<E> forgetMeNot = null;
499
500 /**
501 * Element returned by the most recent call to next iff that
502 * element was drawn from the forgetMeNot list.
503 */
504 private E lastRetElt = null;
505
506 /**
507 * The modCount value that the iterator believes that the backing
508 * Queue should have. If this expectation is violated, the iterator
509 * has detected concurrent modification.
510 */
511 private int expectedModCount = modCount;
512
513 public boolean hasNext() {
514 return cursor < size ||
515 (forgetMeNot != null && !forgetMeNot.isEmpty());
516 }
517
518 @SuppressWarnings("unchecked")
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 for (int i = 0; i < size; i++)
567 queue[i] = null;
568 size = 0;
569 }
570
571 public E poll() {
572 if (size == 0)
573 return null;
574 int s = --size;
575 modCount++;
576 @SuppressWarnings("unchecked")
577 E result = (E) queue[0];
578 @SuppressWarnings("unchecked")
579 E x = (E) queue[s];
580 queue[s] = null;
581 if (s != 0)
582 siftDown(0, x);
583 return result;
584 }
585
586 /**
587 * Removes the ith element from queue.
588 *
589 * Normally this method leaves the elements at up to i-1,
590 * inclusive, untouched. Under these circumstances, it returns
591 * null. Occasionally, in order to maintain the heap invariant,
592 * it must swap a later element of the list with one earlier than
593 * i. Under these circumstances, this method returns the element
594 * that was previously at the end of the list and is now at some
595 * position before i. This fact is used by iterator.remove so as to
596 * avoid missing traversing elements.
597 */
598 private E removeAt(int i) {
599 assert i >= 0 && i < size;
600 modCount++;
601 int s = --size;
602 if (s == i) // removed last element
603 queue[i] = null;
604 else {
605 @SuppressWarnings("unchecked")
606 E moved = (E) queue[s];
607 queue[s] = null;
608 siftDown(i, moved);
609 if (queue[i] == moved) {
610 siftUp(i, moved);
611 if (queue[i] != moved)
612 return moved;
613 }
614 }
615 return null;
616 }
617
618 /**
619 * Inserts item x at position k, maintaining heap invariant by
620 * promoting x up the tree until it is greater than or equal to
621 * its parent, or is the root.
622 *
623 * To simplify and speed up coercions and comparisons. the
624 * Comparable and Comparator versions are separated into different
625 * methods that are otherwise identical. (Similarly for siftDown.)
626 *
627 * @param k the position to fill
628 * @param x the item to insert
629 */
630 private void siftUp(int k, E x) {
631 if (comparator != null)
632 siftUpUsingComparator(k, x);
633 else
634 siftUpComparable(k, x);
635 }
636
637 @SuppressWarnings("unchecked")
638 private void siftUpComparable(int k, E x) {
639 Comparable<? super E> key = (Comparable<? super E>) x;
640 while (k > 0) {
641 int parent = (k - 1) >>> 1;
642 Object e = queue[parent];
643 if (key.compareTo((E) e) >= 0)
644 break;
645 queue[k] = e;
646 k = parent;
647 }
648 queue[k] = key;
649 }
650
651 private void siftUpUsingComparator(int k, E x) {
652 while (k > 0) {
653 int parent = (k - 1) >>> 1;
654 @SuppressWarnings("unchecked")
655 E e = (E) queue[parent];
656 if (comparator.compare(x, e) >= 0)
657 break;
658 queue[k] = e;
659 k = parent;
660 }
661 queue[k] = x;
662 }
663
664 /**
665 * Inserts item x at position k, maintaining heap invariant by
666 * demoting x down the tree repeatedly until it is less than or
667 * equal to its children or is a leaf.
668 *
669 * @param k the position to fill
670 * @param x the item to insert
671 */
672 private void siftDown(int k, E x) {
673 if (comparator != null)
674 siftDownUsingComparator(k, x);
675 else
676 siftDownComparable(k, x);
677 }
678
679 @SuppressWarnings("unchecked")
680 private void siftDownComparable(int k, E x) {
681 Comparable<? super E> key = (Comparable<? super E>)x;
682 int half = size >>> 1; // loop while a non-leaf
683 while (k < half) {
684 int child = (k << 1) + 1; // assume left child is least
685 Object c = queue[child];
686 int right = child + 1;
687 if (right < size &&
688 ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
689 c = queue[child = right];
690 if (key.compareTo((E) c) <= 0)
691 break;
692 queue[k] = c;
693 k = child;
694 }
695 queue[k] = key;
696 }
697
698 @SuppressWarnings("unchecked")
699 private void siftDownUsingComparator(int k, E x) {
700 int half = size >>> 1;
701 while (k < half) {
702 int child = (k << 1) + 1;
703 Object c = queue[child];
704 int right = child + 1;
705 if (right < size &&
706 comparator.compare((E) c, (E) queue[right]) > 0)
707 c = queue[child = right];
708 if (comparator.compare(x, (E) c) <= 0)
709 break;
710 queue[k] = c;
711 k = child;
712 }
713 queue[k] = x;
714 }
715
716 /**
717 * Establishes the heap invariant (described above) in the entire tree,
718 * assuming nothing about the order of the elements prior to the call.
719 */
720 @SuppressWarnings("unchecked")
721 private void heapify() {
722 for (int i = (size >>> 1) - 1; i >= 0; i--)
723 siftDown(i, (E) queue[i]);
724 }
725
726 /**
727 * Returns the comparator used to order the elements in this
728 * queue, or {@code null} if this queue is sorted according to
729 * the {@linkplain Comparable natural ordering} of its elements.
730 *
731 * @return the comparator used to order this queue, or
732 * {@code null} if this queue is sorted according to the
733 * natural ordering of its elements
734 */
735 public Comparator<? super E> comparator() {
736 return comparator;
737 }
738
739 /**
740 * Saves the state of the instance to a stream (that
741 * is, serializes it).
742 *
743 * @serialData The length of the array backing the instance is
744 * emitted (int), followed by all of its elements
745 * (each an {@code Object}) in the proper order.
746 * @param s the stream
747 */
748 private void writeObject(java.io.ObjectOutputStream s)
749 throws java.io.IOException{
750 // Write out element count, and any hidden stuff
751 s.defaultWriteObject();
752
753 // Write out array length, for compatibility with 1.5 version
754 s.writeInt(Math.max(2, size + 1));
755
756 // Write out all elements in the "proper order".
757 for (int i = 0; i < size; i++)
758 s.writeObject(queue[i]);
759 }
760
761 /**
762 * Reconstitutes the {@code PriorityQueue} instance from a stream
763 * (that is, deserializes it).
764 *
765 * @param s the stream
766 */
767 private void readObject(java.io.ObjectInputStream s)
768 throws java.io.IOException, ClassNotFoundException {
769 // Read in size, and any hidden stuff
770 s.defaultReadObject();
771
772 // Read in (and discard) array length
773 s.readInt();
774
775 queue = new Object[size];
776
777 // Read in all elements.
778 for (int i = 0; i < size; i++)
779 queue[i] = s.readObject();
780
781 // Elements are guaranteed to be in "proper order", but the
782 // spec has never explained what that might be.
783 heapify();
784 }
785 }