| PriorityQueue.java |
1 /*
2 * %W% %E%
3 *
4 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
5 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6 */
7
8 package java.util;
9
10 /**
11 * An unbounded priority {@linkplain Queue queue} based on a priority heap.
12 * The elements of the priority queue are ordered according to their
13 * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
14 * provided at queue construction time, depending on which constructor is
15 * used. A priority queue does not permit {@code null} elements.
16 * A priority queue relying on natural ordering also does not permit
17 * insertion of non-comparable objects (doing so may result in
18 * {@code ClassCastException}).
19 *
20 * <p>The <em>head</em> of this queue is the <em>least</em> element
21 * with respect to the specified ordering. If multiple elements are
22 * tied for least value, the head is one of those elements -- ties are
23 * broken arbitrarily. The queue retrieval operations {@code poll},
24 * {@code remove}, {@code peek}, and {@code element} access the
25 * element at the head of the queue.
26 *
27 * <p>A priority queue is unbounded, but has an internal
28 * <i>capacity</i> governing the size of an array used to store the
29 * elements on the queue. It is always at least as large as the queue
30 * size. As elements are added to a priority queue, its capacity
31 * grows automatically. The details of the growth policy are not
32 * specified.
33 *
34 * <p>This class and its iterator implement all of the
35 * <em>optional</em> methods of the {@link Collection} and {@link
36 * Iterator} interfaces. The Iterator provided in method {@link
37 * #iterator()} is <em>not</em> guaranteed to traverse the elements of
38 * the priority queue in any particular order. If you need ordered
39 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
40 *
41 * <p> <strong>Note that this implementation is not synchronized.</strong>
42 * Multiple threads should not access a {@code PriorityQueue}
43 * instance concurrently if any of the threads modifies the queue.
44 * Instead, use the thread-safe {@link
45 * java.util.concurrent.PriorityBlockingQueue} class.
46 *
47 * <p>Implementation note: this implementation provides
48 * O(log(n)) time for the enqueing and dequeing methods
49 * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
50 * linear time for the {@code remove(Object)} and {@code contains(Object)}
51 * methods; and constant time for the retrieval methods
52 * ({@code peek}, {@code element}, and {@code size}).
53 *
54 * <p>This class is a member of the
55 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
56 * Java Collections Framework</a>.
57 *
58 * @since 1.5
59 * @version %I%, %G%
60 * @author Josh Bloch, Doug Lea
61 * @param <E> the type of elements held in this collection
62 */
63 public class PriorityQueue<E> extends AbstractQueue<E>
64 implements java.io.Serializable {
65
66 private static final long serialVersionUID = -7720805057305804111L;
67
68 private static final int DEFAULT_INITIAL_CAPACITY = 11;
69
70 /**
71 * Priority queue represented as a balanced binary heap: the two
72 * children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
73 * priority queue is ordered by comparator, or by the elements'
74 * natural ordering, if comparator is null: For each node n in the
75 * heap and each descendant d of n, n <= d. The element with the
76 * lowest value is in queue[0], assuming the queue is nonempty.
77 */
78 private transient Object[] queue;
79
80 /**
81 * The number of elements in the priority queue.
82 */
83 private int size = 0;
84
85 /**
86 * The comparator, or null if priority queue uses elements'
87 * natural ordering.
88 */
89 private final Comparator<? super E> comparator;
90
91 /**
92 * The number of times this priority queue has been
93 * <i>structurally modified</i>. See AbstractList for gory details.
94 */
95 private transient int modCount = 0;
96
97 /**
98 * Creates a {@code PriorityQueue} with the default initial
99 * capacity (11) that orders its elements according to their
100 * {@linkplain Comparable natural ordering}.
101 */
102 public PriorityQueue() {
103 this(DEFAULT_INITIAL_CAPACITY, null);
104 }
105
106 /**
107 * Creates a {@code PriorityQueue} with the specified initial
108 * capacity that orders its elements according to their
109 * {@linkplain Comparable natural ordering}.
110 *
111 * @param initialCapacity the initial capacity for this priority queue
112 * @throws IllegalArgumentException if {@code initialCapacity} is less
113 * than 1
114 */
115 public PriorityQueue(int initialCapacity) {
116 this(initialCapacity, null);
117 }
118
119 /**
120 * Creates a {@code PriorityQueue} with the specified initial capacity
121 * that orders its elements according to the specified comparator.
122 *
123 * @param initialCapacity the initial capacity for this priority queue
124 * @param comparator the comparator that will be used to order this
125 * priority queue. If {@code null}, the {@linkplain Comparable
126 * natural ordering} of the elements will be used.
127 * @throws IllegalArgumentException if {@code initialCapacity} is
128 * less than 1
129 */
130 public PriorityQueue(int initialCapacity,
131 Comparator<? super E> comparator) {
132 // Note: This restriction of at least one is not actually needed,
133 // but continues for 1.5 compatibility
134 if (initialCapacity < 1)
135 throw new IllegalArgumentException();
136 this.queue = new Object[initialCapacity];
137 this.comparator = comparator;
138 }
139
140 /**
141 * Creates a {@code PriorityQueue} containing the elements in the
142 * specified collection. If the specified collection is an instance of
143 * a {@link SortedSet} or is another {@code PriorityQueue}, this
144 * priority queue will be ordered according to the same ordering.
145 * Otherwise, this priority queue will be ordered according to the
146 * {@linkplain Comparable natural ordering} of its elements.
147 *
148 * @param c the collection whose elements are to be placed
149 * into this priority queue
150 * @throws ClassCastException if elements of the specified collection
151 * cannot be compared to one another according to the priority
152 * queue's ordering
153 * @throws NullPointerException if the specified collection or any
154 * of its elements are null
155 */
156 public PriorityQueue(Collection<? extends E> c) {
157 initFromCollection(c);
158 if (c instanceof SortedSet)
159 comparator = (Comparator<? super E>)
160 ((SortedSet<? extends E>)c).comparator();
161 else if (c instanceof PriorityQueue)
162 comparator = (Comparator<? super E>)
163 ((PriorityQueue<? extends E>)c).comparator();
164 else {
165 comparator = null;
166 heapify();
167 }
168 }
169
170 /**
171 * Creates a {@code PriorityQueue} containing the elements in the
172 * specified priority queue. This priority queue will be
173 * ordered according to the same ordering as the given priority
174 * queue.
175 *
176 * @param c the priority queue whose elements are to be placed
177 * into this priority queue
178 * @throws ClassCastException if elements of {@code c} cannot be
179 * compared to one another according to {@code c}'s
180 * ordering
181 * @throws NullPointerException if the specified priority queue or any
182 * of its elements are null
183 */
184 public PriorityQueue(PriorityQueue<? extends E> c) {
185 comparator = (Comparator<? super E>)c.comparator();
186 initFromCollection(c);
187 }
188
189 /**
190 * Creates a {@code PriorityQueue} containing the elements in the
191 * specified sorted set. This priority queue will be ordered
192 * according to the same ordering as the given sorted set.
193 *
194 * @param c the sorted set whose elements are to be placed
195 * into this priority queue
196 * @throws ClassCastException if elements of the specified sorted
197 * set cannot be compared to one another according to the
198 * sorted set's ordering
199 * @throws NullPointerException if the specified sorted set or any
200 * of its elements are null
201 */
202 public PriorityQueue(SortedSet<? extends E> c) {
203 comparator = (Comparator<? super E>)c.comparator();
204 initFromCollection(c);
205 }
206
207 /**
208 * Initializes queue array with elements from the given Collection.
209 *
210 * @param c the collection
211 */
212 private void initFromCollection(Collection<? extends E> c) {
213 Object[] a = c.toArray();
214 // If c.toArray incorrectly doesn't return Object[], copy it.
215 if (a.getClass() != Object[].class)
216 a = Arrays.copyOf(a, a.length, Object[].class);
217 queue = a;
218 size = a.length;
219 }
220
221 /**
222 * Increases the capacity of the array.
223 *
224 * @param minCapacity the desired minimum capacity
225 */
226 private void grow(int minCapacity) {
227 if (minCapacity < 0) // overflow
228 throw new OutOfMemoryError();
229 int oldCapacity = queue.length;
230 // Double size if small; else grow by 50%
231 int newCapacity = ((oldCapacity < 64)?
232 ((oldCapacity + 1) * 2):
233 ((oldCapacity / 2) * 3));
234 if (newCapacity < 0) // overflow
235 newCapacity = Integer.MAX_VALUE;
236 if (newCapacity < minCapacity)
237 newCapacity = minCapacity;
238 queue = Arrays.copyOf(queue, newCapacity);
239 }
240
241 /**
242 * Inserts the specified element into this priority queue.
243 *
244 * @return {@code true} (as specified by {@link Collection#add})
245 * @throws ClassCastException if the specified element cannot be
246 * compared with elements currently in this priority queue
247 * according to the priority queue's ordering
248 * @throws NullPointerException if the specified element is null
249 */
250 public boolean add(E e) {
251 return offer(e);
252 }
253
254 /**
255 * Inserts the specified element into this priority queue.
256 *
257 * @return {@code true} (as specified by {@link Queue#offer})
258 * @throws ClassCastException if the specified element cannot be
259 * compared with elements currently in this priority queue
260 * according to the priority queue's ordering
261 * @throws NullPointerException if the specified element is null
262 */
263 public boolean offer(E e) {
264 if (e == null)
265 throw new NullPointerException();
266 modCount++;
267 int i = size;
268 if (i >= queue.length)
269 grow(i + 1);
270 size = i + 1;
271 if (i == 0)
272 queue[0] = e;
273 else
274 siftUp(i, e);
275 return true;
276 }
277
278 public E peek() {
279 if (size == 0)
280 return null;
281 return (E) queue[0];
282 }
283
284 private int indexOf(Object o) {
285 if (o != null) {
286 for (int i = 0; i < size; i++)
287 if (o.equals(queue[i]))
288 return i;
289 }
290 return -1;
291 }
292
293 /**
294 * Removes a single instance of the specified element from this queue,
295 * if it is present. More formally, removes an element {@code e} such
296 * that {@code o.equals(e)}, if this queue contains one or more such
297 * elements. Returns {@code true} if and only if this queue contained
298 * the specified element (or equivalently, if this queue changed as a
299 * result of the call).
300 *
301 * @param o element to be removed from this queue, if present
302 * @return {@code true} if this queue changed as a result of the call
303 */
304 public boolean remove(Object o) {
305 int i = indexOf(o);
306 if (i == -1)
307 return false;
308 else {
309 removeAt(i);
310 return true;
311 }
312 }
313
314 /**
315 * Version of remove using reference equality, not equals.
316 * Needed by iterator.remove.
317 *
318 * @param o element to be removed from this queue, if present
319 * @return {@code true} if removed
320 */
321 boolean removeEq(Object o) {
322 for (int i = 0; i < size; i++) {
323 if (o == queue[i]) {
324 removeAt(i);
325 return true;
326 }
327 }
328 return false;
329 }
330
331 /**
332 * Returns {@code true} if this queue contains the specified element.
333 * More formally, returns {@code true} if and only if this queue contains
334 * at least one element {@code e} such that {@code o.equals(e)}.
335 *
336 * @param o object to be checked for containment in this queue
337 * @return {@code true} if this queue contains the specified element
338 */
339 public boolean contains(Object o) {
340 return indexOf(o) != -1;
341 }
342
343 /**
344 * Returns an array containing all of the elements in this queue.
345 * The elements are in no particular order.
346 *
347 * <p>The returned array will be "safe" in that no references to it are
348 * maintained by this queue. (In other words, this method must allocate
349 * a new array). The caller is thus free to modify the returned array.
350 *
351 * <p>This method acts as bridge between array-based and collection-based
352 * APIs.
353 *
354 * @return an array containing all of the elements in this queue
355 */
356 public Object[] toArray() {
357 return Arrays.copyOf(queue, size);
358 }
359
360 /**
361 * Returns an array containing all of the elements in this queue; the
362 * runtime type of the returned array is that of the specified array.
363 * The returned array elements are in no particular order.
364 * If the queue fits in the specified array, it is returned therein.
365 * Otherwise, a new array is allocated with the runtime type of the
366 * specified array and the size of this queue.
367 *
368 * <p>If the queue fits in the specified array with room to spare
369 * (i.e., the array has more elements than the queue), the element in
370 * the array immediately following the end of the collection is set to
371 * {@code null}.
372 *
373 * <p>Like the {@link #toArray()} method, this method acts as bridge between
374 * array-based and collection-based APIs. Further, this method allows
375 * precise control over the runtime type of the output array, and may,
376 * under certain circumstances, be used to save allocation costs.
377 *
378 * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
379 * The following code can be used to dump the queue into a newly
380 * allocated array of <tt>String</tt>:
381 *
382 * <pre>
383 * String[] y = x.toArray(new String[0]);</pre>
384 *
385 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
386 * <tt>toArray()</tt>.
387 *
388 * @param a the array into which the elements of the queue are to
389 * be stored, if it is big enough; otherwise, a new array of the
390 * same runtime type is allocated for this purpose.
391 * @return an array containing all of the elements in this queue
392 * @throws ArrayStoreException if the runtime type of the specified array
393 * is not a supertype of the runtime type of every element in
394 * this queue
395 * @throws NullPointerException if the specified array is null
396 */
397 public <T> T[] toArray(T[] a) {
398 if (a.length < size)
399 // Make a new array of a's runtime type, but my contents:
400 return (T[]) Arrays.copyOf(queue, size, a.getClass());
401 System.arraycopy(queue, 0, a, 0, size);
402 if (a.length > size)
403 a[size] = null;
404 return a;
405 }
406
407 /**
408 * Returns an iterator over the elements in this queue. The iterator
409 * does not return the elements in any particular order.
410 *
411 * @return an iterator over the elements in this queue
412 */
413 public Iterator<E> iterator() {
414 return new Itr();
415 }
416
417 private final class Itr implements Iterator<E> {
418 /**
419 * Index (into queue array) of element to be returned by
420 * subsequent call to next.
421 */
422 private int cursor = 0;
423
424 /**
425 * Index of element returned by most recent call to next,
426 * unless that element came from the forgetMeNot list.
427 * Set to -1 if element is deleted by a call to remove.
428 */
429 private int lastRet = -1;
430
431 /**
432 * A queue of elements that were moved from the unvisited portion of
433 * the heap into the visited portion as a result of "unlucky" element
434 * removals during the iteration. (Unlucky element removals are those
435 * that require a siftup instead of a siftdown.) We must visit all of
436 * the elements in this list to complete the iteration. We do this
437 * after we've completed the "normal" iteration.
438 *
439 * We expect that most iterations, even those involving removals,
440 * will not need to store elements in this field.
441 */
442 private ArrayDeque<E> forgetMeNot = null;
443
444 /**
445 * Element returned by the most recent call to next iff that
446 * element was drawn from the forgetMeNot list.
447 */
448 private E lastRetElt = null;
449
450 /**
451 * The modCount value that the iterator believes that the backing
452 * Queue should have. If this expectation is violated, the iterator
453 * has detected concurrent modification.
454 */
455 private int expectedModCount = modCount;
456
457 public boolean hasNext() {
458 return cursor < size ||
459 (forgetMeNot != null && !forgetMeNot.isEmpty());
460 }
461
462 public E next() {
463 if (expectedModCount != modCount)
464 throw new ConcurrentModificationException();
465 if (cursor < size)
466 return (E) queue[lastRet = cursor++];
467 if (forgetMeNot != null) {
468 lastRet = -1;
469 lastRetElt = forgetMeNot.poll();
470 if (lastRetElt != null)
471 return lastRetElt;
472 }
473 throw new NoSuchElementException();
474 }
475
476 public void remove() {
477 if (expectedModCount != modCount)
478 throw new ConcurrentModificationException();
479 if (lastRet != -1) {
480 E moved = PriorityQueue.this.removeAt(lastRet);
481 lastRet = -1;
482 if (moved == null)
483 cursor--;
484 else {
485 if (forgetMeNot == null)
486 forgetMeNot = new ArrayDeque<E>();
487 forgetMeNot.add(moved);
488 }
489 } else if (lastRetElt != null) {
490 PriorityQueue.this.removeEq(lastRetElt);
491 lastRetElt = null;
492 } else {
493 throw new IllegalStateException();
494 }
495 expectedModCount = modCount;
496 }
497 }
498
499 public int size() {
500 return size;
501 }
502
503 /**
504 * Removes all of the elements from this priority queue.
505 * The queue will be empty after this call returns.
506 */
507 public void clear() {
508 modCount++;
509 for (int i = 0; i < size; i++)
510 queue[i] = null;
511 size = 0;
512 }
513
514 public E poll() {
515 if (size == 0)
516 return null;
517 int s = --size;
518 modCount++;
519 E result = (E) queue[0];
520 E x = (E) queue[s];
521 queue[s] = null;
522 if (s != 0)
523 siftDown(0, x);
524 return result;
525 }
526
527 /**
528 * Removes the ith element from queue.
529 *
530 * Normally this method leaves the elements at up to i-1,
531 * inclusive, untouched. Under these circumstances, it returns
532 * null. Occasionally, in order to maintain the heap invariant,
533 * it must swap a later element of the list with one earlier than
534 * i. Under these circumstances, this method returns the element
535 * that was previously at the end of the list and is now at some
536 * position before i. This fact is used by iterator.remove so as to
537 * avoid missing traversing elements.
538 */
539 private E removeAt(int i) {
540 assert i >= 0 && i < size;
541 modCount++;
542 int s = --size;
543 if (s == i) // removed last element
544 queue[i] = null;
545 else {
546 E moved = (E) queue[s];
547 queue[s] = null;
548 siftDown(i, moved);
549 if (queue[i] == moved) {
550 siftUp(i, moved);
551 if (queue[i] != moved)
552 return moved;
553 }
554 }
555 return null;
556 }
557
558 /**
559 * Inserts item x at position k, maintaining heap invariant by
560 * promoting x up the tree until it is greater than or equal to
561 * its parent, or is the root.
562 *
563 * To simplify and speed up coercions and comparisons. the
564 * Comparable and Comparator versions are separated into different
565 * methods that are otherwise identical. (Similarly for siftDown.)
566 *
567 * @param k the position to fill
568 * @param x the item to insert
569 */
570 private void siftUp(int k, E x) {
571 if (comparator != null)
572 siftUpUsingComparator(k, x);
573 else
574 siftUpComparable(k, x);
575 }
576
577 private void siftUpComparable(int k, E x) {
578 Comparable<? super E> key = (Comparable<? super E>) x;
579 while (k > 0) {
580 int parent = (k - 1) >>> 1;
581 Object e = queue[parent];
582 if (key.compareTo((E) e) >= 0)
583 break;
584 queue[k] = e;
585 k = parent;
586 }
587 queue[k] = key;
588 }
589
590 private void siftUpUsingComparator(int k, E x) {
591 while (k > 0) {
592 int parent = (k - 1) >>> 1;
593 Object e = queue[parent];
594 if (comparator.compare(x, (E) e) >= 0)
595 break;
596 queue[k] = e;
597 k = parent;
598 }
599 queue[k] = x;
600 }
601
602 /**
603 * Inserts item x at position k, maintaining heap invariant by
604 * demoting x down the tree repeatedly until it is less than or
605 * equal to its children or is a leaf.
606 *
607 * @param k the position to fill
608 * @param x the item to insert
609 */
610 private void siftDown(int k, E x) {
611 if (comparator != null)
612 siftDownUsingComparator(k, x);
613 else
614 siftDownComparable(k, x);
615 }
616
617 private void siftDownComparable(int k, E x) {
618 Comparable<? super E> key = (Comparable<? super E>)x;
619 int half = size >>> 1; // loop while a non-leaf
620 while (k < half) {
621 int child = (k << 1) + 1; // assume left child is least
622 Object c = queue[child];
623 int right = child + 1;
624 if (right < size &&
625 ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
626 c = queue[child = right];
627 if (key.compareTo((E) c) <= 0)
628 break;
629 queue[k] = c;
630 k = child;
631 }
632 queue[k] = key;
633 }
634
635 private void siftDownUsingComparator(int k, E x) {
636 int half = size >>> 1;
637 while (k < half) {
638 int child = (k << 1) + 1;
639 Object c = queue[child];
640 int right = child + 1;
641 if (right < size &&
642 comparator.compare((E) c, (E) queue[right]) > 0)
643 c = queue[child = right];
644 if (comparator.compare(x, (E) c) <= 0)
645 break;
646 queue[k] = c;
647 k = child;
648 }
649 queue[k] = x;
650 }
651
652 /**
653 * Establishes the heap invariant (described above) in the entire tree,
654 * assuming nothing about the order of the elements prior to the call.
655 */
656 private void heapify() {
657 for (int i = (size >>> 1) - 1; i >= 0; i--)
658 siftDown(i, (E) queue[i]);
659 }
660
661 /**
662 * Returns the comparator used to order the elements in this
663 * queue, or {@code null} if this queue is sorted according to
664 * the {@linkplain Comparable natural ordering} of its elements.
665 *
666 * @return the comparator used to order this queue, or
667 * {@code null} if this queue is sorted according to the
668 * natural ordering of its elements
669 */
670 public Comparator<? super E> comparator() {
671 return comparator;
672 }
673
674 /**
675 * Saves the state of the instance to a stream (that
676 * is, serializes it).
677 *
678 * @serialData The length of the array backing the instance is
679 * emitted (int), followed by all of its elements
680 * (each an {@code Object}) in the proper order.
681 * @param s the stream
682 */
683 private void writeObject(java.io.ObjectOutputStream s)
684 throws java.io.IOException{
685 // Write out element count, and any hidden stuff
686 s.defaultWriteObject();
687
688 // Write out array length, for compatibility with 1.5 version
689 s.writeInt(Math.max(2, size + 1));
690
691 // Write out all elements in the "proper order".
692 for (int i = 0; i < size; i++)
693 s.writeObject(queue[i]);
694 }
695
696 /**
697 * Reconstitutes the {@code PriorityQueue} instance from a stream
698 * (that is, deserializes it).
699 *
700 * @param s the stream
701 */
702 private void readObject(java.io.ObjectInputStream s)
703 throws java.io.IOException, ClassNotFoundException {
704 // Read in size, and any hidden stuff
705 s.defaultReadObject();
706
707 // Read in (and discard) array length
708 s.readInt();
709
710 queue = new Object[size];
711
712 // Read in all elements.
713 for (int i = 0; i < size; i++)
714 queue[i] = s.readObject();
715
716 // Elements are guaranteed to be in "proper order", but the
717 // spec has never explained what that might be.
718 heapify();
719 }
720 }
721