001 /*
002 * Copyright 2003-2006 Sun Microsystems, Inc. All Rights Reserved.
003 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004 *
005 * This code is free software; you can redistribute it and/or modify it
006 * under the terms of the GNU General Public License version 2 only, as
007 * published by the Free Software Foundation. Sun designates this
008 * particular file as subject to the "Classpath" exception as provided
009 * by Sun in the LICENSE file that accompanied this code.
010 *
011 * This code is distributed in the hope that it will be useful, but WITHOUT
012 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
014 * version 2 for more details (a copy is included in the LICENSE file that
015 * accompanied this code).
016 *
017 * You should have received a copy of the GNU General Public License version
018 * 2 along with this work; if not, write to the Free Software Foundation,
019 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020 *
021 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
022 * CA 95054 USA or visit www.sun.com if you need additional information or
023 * have any questions.
024 */
025
026 package java.util;
027
028 /**
029 * An unbounded priority {@linkplain Queue queue} based on a priority heap.
030 * The elements of the priority queue are ordered according to their
031 * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
032 * provided at queue construction time, depending on which constructor is
033 * used. A priority queue does not permit {@code null} elements.
034 * A priority queue relying on natural ordering also does not permit
035 * insertion of non-comparable objects (doing so may result in
036 * {@code ClassCastException}).
037 *
038 * <p>The <em>head</em> of this queue is the <em>least</em> element
039 * with respect to the specified ordering. If multiple elements are
040 * tied for least value, the head is one of those elements -- ties are
041 * broken arbitrarily. The queue retrieval operations {@code poll},
042 * {@code remove}, {@code peek}, and {@code element} access the
043 * element at the head of the queue.
044 *
045 * <p>A priority queue is unbounded, but has an internal
046 * <i>capacity</i> governing the size of an array used to store the
047 * elements on the queue. It is always at least as large as the queue
048 * size. As elements are added to a priority queue, its capacity
049 * grows automatically. The details of the growth policy are not
050 * specified.
051 *
052 * <p>This class and its iterator implement all of the
053 * <em>optional</em> methods of the {@link Collection} and {@link
054 * Iterator} interfaces. The Iterator provided in method {@link
055 * #iterator()} is <em>not</em> guaranteed to traverse the elements of
056 * the priority queue in any particular order. If you need ordered
057 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
058 *
059 * <p> <strong>Note that this implementation is not synchronized.</strong>
060 * Multiple threads should not access a {@code PriorityQueue}
061 * instance concurrently if any of the threads modifies the queue.
062 * Instead, use the thread-safe {@link
063 * java.util.concurrent.PriorityBlockingQueue} class.
064 *
065 * <p>Implementation note: this implementation provides
066 * O(log(n)) time for the enqueing and dequeing methods
067 * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
068 * linear time for the {@code remove(Object)} and {@code contains(Object)}
069 * methods; and constant time for the retrieval methods
070 * ({@code peek}, {@code element}, and {@code size}).
071 *
072 * <p>This class is a member of the
073 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
074 * Java Collections Framework</a>.
075 *
076 * @since 1.5
077 * @version 1.22, 05/05/07
078 * @author Josh Bloch, Doug Lea
079 * @param <E> the type of elements held in this collection
080 */
081 public class PriorityQueue<E> extends AbstractQueue<E> implements
082 java.io.Serializable {
083
084 private static final long serialVersionUID = -7720805057305804111L;
085
086 private static final int DEFAULT_INITIAL_CAPACITY = 11;
087
088 /**
089 * Priority queue represented as a balanced binary heap: the two
090 * children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
091 * priority queue is ordered by comparator, or by the elements'
092 * natural ordering, if comparator is null: For each node n in the
093 * heap and each descendant d of n, n <= d. The element with the
094 * lowest value is in queue[0], assuming the queue is nonempty.
095 */
096 private transient Object[] queue;
097
098 /**
099 * The number of elements in the priority queue.
100 */
101 private int size = 0;
102
103 /**
104 * The comparator, or null if priority queue uses elements'
105 * natural ordering.
106 */
107 private final Comparator<? super E> comparator;
108
109 /**
110 * The number of times this priority queue has been
111 * <i>structurally modified</i>. See AbstractList for gory details.
112 */
113 private transient int modCount = 0;
114
115 /**
116 * Creates a {@code PriorityQueue} with the default initial
117 * capacity (11) that orders its elements according to their
118 * {@linkplain Comparable natural ordering}.
119 */
120 public PriorityQueue() {
121 this (DEFAULT_INITIAL_CAPACITY, null);
122 }
123
124 /**
125 * Creates a {@code PriorityQueue} with the specified initial
126 * capacity that orders its elements according to their
127 * {@linkplain Comparable natural ordering}.
128 *
129 * @param initialCapacity the initial capacity for this priority queue
130 * @throws IllegalArgumentException if {@code initialCapacity} is less
131 * than 1
132 */
133 public PriorityQueue(int initialCapacity) {
134 this (initialCapacity, null);
135 }
136
137 /**
138 * Creates a {@code PriorityQueue} with the specified initial capacity
139 * that orders its elements according to the specified comparator.
140 *
141 * @param initialCapacity the initial capacity for this priority queue
142 * @param comparator the comparator that will be used to order this
143 * priority queue. If {@code null}, the {@linkplain Comparable
144 * natural ordering} of the elements will be used.
145 * @throws IllegalArgumentException if {@code initialCapacity} is
146 * less than 1
147 */
148 public PriorityQueue(int initialCapacity,
149 Comparator<? super E> comparator) {
150 // Note: This restriction of at least one is not actually needed,
151 // but continues for 1.5 compatibility
152 if (initialCapacity < 1)
153 throw new IllegalArgumentException();
154 this .queue = new Object[initialCapacity];
155 this .comparator = comparator;
156 }
157
158 /**
159 * Creates a {@code PriorityQueue} containing the elements in the
160 * specified collection. If the specified collection is an instance of
161 * a {@link SortedSet} or is another {@code PriorityQueue}, this
162 * priority queue will be ordered according to the same ordering.
163 * Otherwise, this priority queue will be ordered according to the
164 * {@linkplain Comparable natural ordering} of its elements.
165 *
166 * @param c the collection whose elements are to be placed
167 * into this priority queue
168 * @throws ClassCastException if elements of the specified collection
169 * cannot be compared to one another according to the priority
170 * queue's ordering
171 * @throws NullPointerException if the specified collection or any
172 * of its elements are null
173 */
174 public PriorityQueue(Collection<? extends E> c) {
175 initFromCollection(c);
176 if (c instanceof SortedSet)
177 comparator = (Comparator<? super E>) ((SortedSet<? extends E>) c)
178 .comparator();
179 else if (c instanceof PriorityQueue)
180 comparator = (Comparator<? super E>) ((PriorityQueue<? extends E>) c)
181 .comparator();
182 else {
183 comparator = null;
184 heapify();
185 }
186 }
187
188 /**
189 * Creates a {@code PriorityQueue} containing the elements in the
190 * specified priority queue. This priority queue will be
191 * ordered according to the same ordering as the given priority
192 * queue.
193 *
194 * @param c the priority queue whose elements are to be placed
195 * into this priority queue
196 * @throws ClassCastException if elements of {@code c} cannot be
197 * compared to one another according to {@code c}'s
198 * ordering
199 * @throws NullPointerException if the specified priority queue or any
200 * of its elements are null
201 */
202 public PriorityQueue(PriorityQueue<? extends E> c) {
203 comparator = (Comparator<? super E>) c.comparator();
204 initFromCollection(c);
205 }
206
207 /**
208 * Creates a {@code PriorityQueue} containing the elements in the
209 * specified sorted set. This priority queue will be ordered
210 * according to the same ordering as the given sorted set.
211 *
212 * @param c the sorted set whose elements are to be placed
213 * into this priority queue
214 * @throws ClassCastException if elements of the specified sorted
215 * set cannot be compared to one another according to the
216 * sorted set's ordering
217 * @throws NullPointerException if the specified sorted set or any
218 * of its elements are null
219 */
220 public PriorityQueue(SortedSet<? extends E> c) {
221 comparator = (Comparator<? super E>) c.comparator();
222 initFromCollection(c);
223 }
224
225 /**
226 * Initializes queue array with elements from the given Collection.
227 *
228 * @param c the collection
229 */
230 private void initFromCollection(Collection<? extends E> c) {
231 Object[] a = c.toArray();
232 // If c.toArray incorrectly doesn't return Object[], copy it.
233 if (a.getClass() != Object[].class)
234 a = Arrays.copyOf(a, a.length, Object[].class);
235 queue = a;
236 size = a.length;
237 }
238
239 /**
240 * Increases the capacity of the array.
241 *
242 * @param minCapacity the desired minimum capacity
243 */
244 private void grow(int minCapacity) {
245 if (minCapacity < 0) // overflow
246 throw new OutOfMemoryError();
247 int oldCapacity = queue.length;
248 // Double size if small; else grow by 50%
249 int newCapacity = ((oldCapacity < 64) ? ((oldCapacity + 1) * 2)
250 : ((oldCapacity / 2) * 3));
251 if (newCapacity < 0) // overflow
252 newCapacity = Integer.MAX_VALUE;
253 if (newCapacity < minCapacity)
254 newCapacity = minCapacity;
255 queue = Arrays.copyOf(queue, newCapacity);
256 }
257
258 /**
259 * Inserts the specified element into this priority queue.
260 *
261 * @return {@code true} (as specified by {@link Collection#add})
262 * @throws ClassCastException if the specified element cannot be
263 * compared with elements currently in this priority queue
264 * according to the priority queue's ordering
265 * @throws NullPointerException if the specified element is null
266 */
267 public boolean add(E e) {
268 return offer(e);
269 }
270
271 /**
272 * Inserts the specified element into this priority queue.
273 *
274 * @return {@code true} (as specified by {@link Queue#offer})
275 * @throws ClassCastException if the specified element cannot be
276 * compared with elements currently in this priority queue
277 * according to the priority queue's ordering
278 * @throws NullPointerException if the specified element is null
279 */
280 public boolean offer(E e) {
281 if (e == null)
282 throw new NullPointerException();
283 modCount++;
284 int i = size;
285 if (i >= queue.length)
286 grow(i + 1);
287 size = i + 1;
288 if (i == 0)
289 queue[0] = e;
290 else
291 siftUp(i, e);
292 return true;
293 }
294
295 public E peek() {
296 if (size == 0)
297 return null;
298 return (E) queue[0];
299 }
300
301 private int indexOf(Object o) {
302 if (o != null) {
303 for (int i = 0; i < size; i++)
304 if (o.equals(queue[i]))
305 return i;
306 }
307 return -1;
308 }
309
310 /**
311 * Removes a single instance of the specified element from this queue,
312 * if it is present. More formally, removes an element {@code e} such
313 * that {@code o.equals(e)}, if this queue contains one or more such
314 * elements. Returns {@code true} if and only if this queue contained
315 * the specified element (or equivalently, if this queue changed as a
316 * result of the call).
317 *
318 * @param o element to be removed from this queue, if present
319 * @return {@code true} if this queue changed as a result of the call
320 */
321 public boolean remove(Object o) {
322 int i = indexOf(o);
323 if (i == -1)
324 return false;
325 else {
326 removeAt(i);
327 return true;
328 }
329 }
330
331 /**
332 * Version of remove using reference equality, not equals.
333 * Needed by iterator.remove.
334 *
335 * @param o element to be removed from this queue, if present
336 * @return {@code true} if removed
337 */
338 boolean removeEq(Object o) {
339 for (int i = 0; i < size; i++) {
340 if (o == queue[i]) {
341 removeAt(i);
342 return true;
343 }
344 }
345 return false;
346 }
347
348 /**
349 * Returns {@code true} if this queue contains the specified element.
350 * More formally, returns {@code true} if and only if this queue contains
351 * at least one element {@code e} such that {@code o.equals(e)}.
352 *
353 * @param o object to be checked for containment in this queue
354 * @return {@code true} if this queue contains the specified element
355 */
356 public boolean contains(Object o) {
357 return indexOf(o) != -1;
358 }
359
360 /**
361 * Returns an array containing all of the elements in this queue.
362 * The elements are in no particular order.
363 *
364 * <p>The returned array will be "safe" in that no references to it are
365 * maintained by this queue. (In other words, this method must allocate
366 * a new array). The caller is thus free to modify the returned array.
367 *
368 * <p>This method acts as bridge between array-based and collection-based
369 * APIs.
370 *
371 * @return an array containing all of the elements in this queue
372 */
373 public Object[] toArray() {
374 return Arrays.copyOf(queue, size);
375 }
376
377 /**
378 * Returns an array containing all of the elements in this queue; the
379 * runtime type of the returned array is that of the specified array.
380 * The returned array elements are in no particular order.
381 * If the queue fits in the specified array, it is returned therein.
382 * Otherwise, a new array is allocated with the runtime type of the
383 * specified array and the size of this queue.
384 *
385 * <p>If the queue fits in the specified array with room to spare
386 * (i.e., the array has more elements than the queue), the element in
387 * the array immediately following the end of the collection is set to
388 * {@code null}.
389 *
390 * <p>Like the {@link #toArray()} method, this method acts as bridge between
391 * array-based and collection-based APIs. Further, this method allows
392 * precise control over the runtime type of the output array, and may,
393 * under certain circumstances, be used to save allocation costs.
394 *
395 * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
396 * The following code can be used to dump the queue into a newly
397 * allocated array of <tt>String</tt>:
398 *
399 * <pre>
400 * String[] y = x.toArray(new String[0]);</pre>
401 *
402 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
403 * <tt>toArray()</tt>.
404 *
405 * @param a the array into which the elements of the queue are to
406 * be stored, if it is big enough; otherwise, a new array of the
407 * same runtime type is allocated for this purpose.
408 * @return an array containing all of the elements in this queue
409 * @throws ArrayStoreException if the runtime type of the specified array
410 * is not a supertype of the runtime type of every element in
411 * this queue
412 * @throws NullPointerException if the specified array is null
413 */
414 public <T> T[] toArray(T[] a) {
415 if (a.length < size)
416 // Make a new array of a's runtime type, but my contents:
417 return (T[]) Arrays.copyOf(queue, size, a.getClass());
418 System.arraycopy(queue, 0, a, 0, size);
419 if (a.length > size)
420 a[size] = null;
421 return a;
422 }
423
424 /**
425 * Returns an iterator over the elements in this queue. The iterator
426 * does not return the elements in any particular order.
427 *
428 * @return an iterator over the elements in this queue
429 */
430 public Iterator<E> iterator() {
431 return new Itr();
432 }
433
434 private final class Itr implements Iterator<E> {
435 /**
436 * Index (into queue array) of element to be returned by
437 * subsequent call to next.
438 */
439 private int cursor = 0;
440
441 /**
442 * Index of element returned by most recent call to next,
443 * unless that element came from the forgetMeNot list.
444 * Set to -1 if element is deleted by a call to remove.
445 */
446 private int lastRet = -1;
447
448 /**
449 * A queue of elements that were moved from the unvisited portion of
450 * the heap into the visited portion as a result of "unlucky" element
451 * removals during the iteration. (Unlucky element removals are those
452 * that require a siftup instead of a siftdown.) We must visit all of
453 * the elements in this list to complete the iteration. We do this
454 * after we've completed the "normal" iteration.
455 *
456 * We expect that most iterations, even those involving removals,
457 * will not need to store elements in this field.
458 */
459 private ArrayDeque<E> forgetMeNot = null;
460
461 /**
462 * Element returned by the most recent call to next iff that
463 * element was drawn from the forgetMeNot list.
464 */
465 private E lastRetElt = null;
466
467 /**
468 * The modCount value that the iterator believes that the backing
469 * Queue should have. If this expectation is violated, the iterator
470 * has detected concurrent modification.
471 */
472 private int expectedModCount = modCount;
473
474 public boolean hasNext() {
475 return cursor < size
476 || (forgetMeNot != null && !forgetMeNot.isEmpty());
477 }
478
479 public E next() {
480 if (expectedModCount != modCount)
481 throw new ConcurrentModificationException();
482 if (cursor < size)
483 return (E) queue[lastRet = cursor++];
484 if (forgetMeNot != null) {
485 lastRet = -1;
486 lastRetElt = forgetMeNot.poll();
487 if (lastRetElt != null)
488 return lastRetElt;
489 }
490 throw new NoSuchElementException();
491 }
492
493 public void remove() {
494 if (expectedModCount != modCount)
495 throw new ConcurrentModificationException();
496 if (lastRet != -1) {
497 E moved = PriorityQueue.this .removeAt(lastRet);
498 lastRet = -1;
499 if (moved == null)
500 cursor--;
501 else {
502 if (forgetMeNot == null)
503 forgetMeNot = new ArrayDeque<E>();
504 forgetMeNot.add(moved);
505 }
506 } else if (lastRetElt != null) {
507 PriorityQueue.this .removeEq(lastRetElt);
508 lastRetElt = null;
509 } else {
510 throw new IllegalStateException();
511 }
512 expectedModCount = modCount;
513 }
514 }
515
516 public int size() {
517 return size;
518 }
519
520 /**
521 * Removes all of the elements from this priority queue.
522 * The queue will be empty after this call returns.
523 */
524 public void clear() {
525 modCount++;
526 for (int i = 0; i < size; i++)
527 queue[i] = null;
528 size = 0;
529 }
530
531 public E poll() {
532 if (size == 0)
533 return null;
534 int s = --size;
535 modCount++;
536 E result = (E) queue[0];
537 E x = (E) queue[s];
538 queue[s] = null;
539 if (s != 0)
540 siftDown(0, x);
541 return result;
542 }
543
544 /**
545 * Removes the ith element from queue.
546 *
547 * Normally this method leaves the elements at up to i-1,
548 * inclusive, untouched. Under these circumstances, it returns
549 * null. Occasionally, in order to maintain the heap invariant,
550 * it must swap a later element of the list with one earlier than
551 * i. Under these circumstances, this method returns the element
552 * that was previously at the end of the list and is now at some
553 * position before i. This fact is used by iterator.remove so as to
554 * avoid missing traversing elements.
555 */
556 private E removeAt(int i) {
557 assert i >= 0 && i < size;
558 modCount++;
559 int s = --size;
560 if (s == i) // removed last element
561 queue[i] = null;
562 else {
563 E moved = (E) queue[s];
564 queue[s] = null;
565 siftDown(i, moved);
566 if (queue[i] == moved) {
567 siftUp(i, moved);
568 if (queue[i] != moved)
569 return moved;
570 }
571 }
572 return null;
573 }
574
575 /**
576 * Inserts item x at position k, maintaining heap invariant by
577 * promoting x up the tree until it is greater than or equal to
578 * its parent, or is the root.
579 *
580 * To simplify and speed up coercions and comparisons. the
581 * Comparable and Comparator versions are separated into different
582 * methods that are otherwise identical. (Similarly for siftDown.)
583 *
584 * @param k the position to fill
585 * @param x the item to insert
586 */
587 private void siftUp(int k, E x) {
588 if (comparator != null)
589 siftUpUsingComparator(k, x);
590 else
591 siftUpComparable(k, x);
592 }
593
594 private void siftUpComparable(int k, E x) {
595 Comparable<? super E> key = (Comparable<? super E>) x;
596 while (k > 0) {
597 int parent = (k - 1) >>> 1;
598 Object e = queue[parent];
599 if (key.compareTo((E) e) >= 0)
600 break;
601 queue[k] = e;
602 k = parent;
603 }
604 queue[k] = key;
605 }
606
607 private void siftUpUsingComparator(int k, E x) {
608 while (k > 0) {
609 int parent = (k - 1) >>> 1;
610 Object e = queue[parent];
611 if (comparator.compare(x, (E) e) >= 0)
612 break;
613 queue[k] = e;
614 k = parent;
615 }
616 queue[k] = x;
617 }
618
619 /**
620 * Inserts item x at position k, maintaining heap invariant by
621 * demoting x down the tree repeatedly until it is less than or
622 * equal to its children or is a leaf.
623 *
624 * @param k the position to fill
625 * @param x the item to insert
626 */
627 private void siftDown(int k, E x) {
628 if (comparator != null)
629 siftDownUsingComparator(k, x);
630 else
631 siftDownComparable(k, x);
632 }
633
634 private void siftDownComparable(int k, E x) {
635 Comparable<? super E> key = (Comparable<? super E>) x;
636 int half = size >>> 1; // loop while a non-leaf
637 while (k < half) {
638 int child = (k << 1) + 1; // assume left child is least
639 Object c = queue[child];
640 int right = child + 1;
641 if (right < size
642 && ((Comparable<? super E>) c)
643 .compareTo((E) queue[right]) > 0)
644 c = queue[child = right];
645 if (key.compareTo((E) c) <= 0)
646 break;
647 queue[k] = c;
648 k = child;
649 }
650 queue[k] = key;
651 }
652
653 private void siftDownUsingComparator(int k, E x) {
654 int half = size >>> 1;
655 while (k < half) {
656 int child = (k << 1) + 1;
657 Object c = queue[child];
658 int right = child + 1;
659 if (right < size
660 && comparator.compare((E) c, (E) queue[right]) > 0)
661 c = queue[child = right];
662 if (comparator.compare(x, (E) c) <= 0)
663 break;
664 queue[k] = c;
665 k = child;
666 }
667 queue[k] = x;
668 }
669
670 /**
671 * Establishes the heap invariant (described above) in the entire tree,
672 * assuming nothing about the order of the elements prior to the call.
673 */
674 private void heapify() {
675 for (int i = (size >>> 1) - 1; i >= 0; i--)
676 siftDown(i, (E) queue[i]);
677 }
678
679 /**
680 * Returns the comparator used to order the elements in this
681 * queue, or {@code null} if this queue is sorted according to
682 * the {@linkplain Comparable natural ordering} of its elements.
683 *
684 * @return the comparator used to order this queue, or
685 * {@code null} if this queue is sorted according to the
686 * natural ordering of its elements
687 */
688 public Comparator<? super E> comparator() {
689 return comparator;
690 }
691
692 /**
693 * Saves the state of the instance to a stream (that
694 * is, serializes it).
695 *
696 * @serialData The length of the array backing the instance is
697 * emitted (int), followed by all of its elements
698 * (each an {@code Object}) in the proper order.
699 * @param s the stream
700 */
701 private void writeObject(java.io.ObjectOutputStream s)
702 throws java.io.IOException {
703 // Write out element count, and any hidden stuff
704 s.defaultWriteObject();
705
706 // Write out array length, for compatibility with 1.5 version
707 s.writeInt(Math.max(2, size + 1));
708
709 // Write out all elements in the "proper order".
710 for (int i = 0; i < size; i++)
711 s.writeObject(queue[i]);
712 }
713
714 /**
715 * Reconstitutes the {@code PriorityQueue} instance from a stream
716 * (that is, deserializes it).
717 *
718 * @param s the stream
719 */
720 private void readObject(java.io.ObjectInputStream s)
721 throws java.io.IOException, ClassNotFoundException {
722 // Read in size, and any hidden stuff
723 s.defaultReadObject();
724
725 // Read in (and discard) array length
726 s.readInt();
727
728 queue = new Object[size];
729
730 // Read in all elements.
731 for (int i = 0; i < size; i++)
732 queue[i] = s.readObject();
733
734 // Elements are guaranteed to be in "proper order", but the
735 // spec has never explained what that might be.
736 heapify();
737 }
738 }
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