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1 : dl 1.38 /*
2 : jsr166 1.67 * Copyright 2003-2006 Sun Microsystems, Inc. All Rights Reserved.
3 :     * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 : dl 1.38 *
5 : jsr166 1.67 * 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. Sun designates this
8 :     * particular file as subject to the "Classpath" exception as provided
9 :     * by Sun 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 :     *
17 :     * You should have received a copy of the GNU General Public License version
18 :     * 2 along with this work; if not, write to the Free Software Foundation,
19 :     * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 :     *
21 :     * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
22 :     * CA 95054 USA or visit www.sun.com if you need additional information or
23 :     * have any questions.
24 : dl 1.38 */
25 :    
26 :     package java.util;
27 : tim 1.1
28 :     /**
29 : jsr166 1.63 * 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 : dl 1.40 *
38 : dl 1.41 * <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 : jsr166 1.63 * broken arbitrarily. The queue retrieval operations {@code poll},
42 :     * {@code remove}, {@code peek}, and {@code element} access the
43 : dl 1.42 * element at the head of the queue.
44 : tim 1.14 *
45 : dl 1.41 * <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 : dl 1.40 * 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 : tim 1.2 *
52 : dl 1.50 * <p>This class and its iterator implement all of the
53 :     * <em>optional</em> methods of the {@link Collection} and {@link
54 : dl 1.52 * 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 : jsr166 1.63 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
58 : dl 1.29 *
59 :     * <p> <strong>Note that this implementation is not synchronized.</strong>
60 : jsr166 1.63 * 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 : dholmes 1.34 * java.util.concurrent.PriorityBlockingQueue} class.
64 : dl 1.29 *
65 : jsr166 1.63 * <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 : tim 1.2 *
72 :     * <p>This class is a member of the
73 : jsr166 1.65 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
74 : tim 1.2 * Java Collections Framework</a>.
75 : jsr166 1.63 *
76 : dl 1.7 * @since 1.5
77 : jsr166 1.62 * @version %I%, %G%
78 : jsr166 1.63 * @author Josh Bloch, Doug Lea
79 : dl 1.45 * @param <E> the type of elements held in this collection
80 : tim 1.2 */
81 :     public class PriorityQueue<E> extends AbstractQueue<E>
82 : dl 1.47 implements java.io.Serializable {
83 : dholmes 1.11
84 : dl 1.31 private static final long serialVersionUID = -7720805057305804111L;
85 : dl 1.30
86 : tim 1.2 private static final int DEFAULT_INITIAL_CAPACITY = 11;
87 : tim 1.1
88 : tim 1.2 /**
89 : dl 1.55 * Priority queue represented as a balanced binary heap: the two
90 :     * children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
91 :     * priority queue is ordered by comparator, or by the elements'
92 :     * natural ordering, if comparator is null: For each node n in the
93 :     * heap and each descendant d of n, n <= d. The element with the
94 :     * lowest value is in queue[0], assuming the queue is nonempty.
95 : tim 1.2 */
96 : tim 1.16 private transient Object[] queue;
97 : tim 1.1
98 : tim 1.2 /**
99 :     * The number of elements in the priority queue.
100 :     */
101 :     private int size = 0;
102 : tim 1.1
103 : tim 1.2 /**
104 :     * The comparator, or null if priority queue uses elements'
105 :     * natural ordering.
106 :     */
107 : tim 1.16 private final Comparator<? super E> comparator;
108 : tim 1.2
109 :     /**
110 :     * The number of times this priority queue has been
111 :     * <i>structurally modified</i>. See AbstractList for gory details.
112 :     */
113 : dl 1.5 private transient int modCount = 0;
114 : tim 1.2
115 :     /**
116 : jsr166 1.63 * Creates a {@code PriorityQueue} with the default initial
117 : dl 1.52 * capacity (11) that orders its elements according to their
118 :     * {@linkplain Comparable natural ordering}.
119 : tim 1.2 */
120 :     public PriorityQueue() {
121 : dholmes 1.11 this(DEFAULT_INITIAL_CAPACITY, null);
122 : tim 1.1 }
123 : tim 1.2
124 :     /**
125 : jsr166 1.63 * Creates a {@code PriorityQueue} with the specified initial
126 : dl 1.52 * capacity that orders its elements according to their
127 :     * {@linkplain Comparable natural ordering}.
128 : tim 1.2 *
129 : dl 1.52 * @param initialCapacity the initial capacity for this priority queue
130 : jsr166 1.63 * @throws IllegalArgumentException if {@code initialCapacity} is less
131 :     * than 1
132 : tim 1.2 */
133 :     public PriorityQueue(int initialCapacity) {
134 :     this(initialCapacity, null);
135 : tim 1.1 }
136 : tim 1.2
137 :     /**
138 : jsr166 1.63 * Creates a {@code PriorityQueue} with the specified initial capacity
139 : tim 1.2 * that orders its elements according to the specified comparator.
140 :     *
141 : dl 1.52 * @param initialCapacity the initial capacity for this priority queue
142 : jsr166 1.63 * @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 : dl 1.52 * less than 1
147 : tim 1.2 */
148 : dl 1.52 public PriorityQueue(int initialCapacity,
149 : dholmes 1.23 Comparator<? super E> comparator) {
150 : dl 1.55 // Note: This restriction of at least one is not actually needed,
151 :     // but continues for 1.5 compatibility
152 : tim 1.2 if (initialCapacity < 1)
153 : dholmes 1.15 throw new IllegalArgumentException();
154 : dl 1.55 this.queue = new Object[initialCapacity];
155 : tim 1.2 this.comparator = comparator;
156 : tim 1.1 }
157 : jsr166 1.56
158 : dl 1.22 /**
159 : jsr166 1.63 * 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 : tim 1.2 *
166 : dl 1.52 * @param c the collection whose elements are to be placed
167 :     * into this priority queue
168 : tim 1.2 * @throws ClassCastException if elements of the specified collection
169 :     * cannot be compared to one another according to the priority
170 : dl 1.52 * queue's ordering
171 :     * @throws NullPointerException if the specified collection or any
172 :     * of its elements are null
173 : tim 1.2 */
174 : tim 1.16 public PriorityQueue(Collection<? extends E> c) {
175 : dl 1.55 initFromCollection(c);
176 : jsr166 1.56 if (c instanceof SortedSet)
177 : dl 1.55 comparator = (Comparator<? super E>)
178 :     ((SortedSet<? extends E>)c).comparator();
179 : jsr166 1.56 else if (c instanceof PriorityQueue)
180 : dl 1.55 comparator = (Comparator<? super E>)
181 :     ((PriorityQueue<? extends E>)c).comparator();
182 :     else {
183 : tim 1.2 comparator = null;
184 : dl 1.55 heapify();
185 : tim 1.2 }
186 : dl 1.22 }
187 :    
188 :     /**
189 : jsr166 1.63 * Creates a {@code PriorityQueue} containing the elements in the
190 : dl 1.55 * specified priority queue. This priority queue will be
191 : dl 1.52 * 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 : jsr166 1.63 * @throws ClassCastException if elements of {@code c} cannot be
197 :     * compared to one another according to {@code c}'s
198 : dl 1.52 * ordering
199 :     * @throws NullPointerException if the specified priority queue or any
200 :     * of its elements are null
201 : dl 1.22 */
202 :     public PriorityQueue(PriorityQueue<? extends E> c) {
203 :     comparator = (Comparator<? super E>)c.comparator();
204 : dl 1.55 initFromCollection(c);
205 : dl 1.22 }
206 : dholmes 1.18
207 : dl 1.22 /**
208 : jsr166 1.63 * Creates a {@code PriorityQueue} containing the elements in the
209 :     * specified sorted set. This priority queue will be ordered
210 : dl 1.52 * according to the same ordering as the given sorted set.
211 :     *
212 :     * @param c the sorted set whose elements are to be placed
213 : jsr166 1.63 * into this priority queue
214 : dl 1.52 * @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 : dl 1.22 */
220 :     public PriorityQueue(SortedSet<? extends E> c) {
221 :     comparator = (Comparator<? super E>)c.comparator();
222 : dl 1.55 initFromCollection(c);
223 : tim 1.1 }
224 :    
225 : dl 1.22 /**
226 : jsr166 1.63 * Initializes queue array with elements from the given Collection.
227 :     *
228 : dl 1.55 * @param c the collection
229 : dl 1.22 */
230 : dl 1.55 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 : jsr166 1.56 }
238 : dl 1.55
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 : dl 1.22 throw new OutOfMemoryError();
247 : dl 1.55 int oldCapacity = queue.length;
248 :     // Double size if small; else grow by 50%
249 :     int newCapacity = ((oldCapacity < 64)?
250 :     ((oldCapacity + 1) * 2):
251 : dl 1.58 ((oldCapacity / 2) * 3));
252 :     if (newCapacity < 0) // overflow
253 :     newCapacity = Integer.MAX_VALUE;
254 : dl 1.55 if (newCapacity < minCapacity)
255 :     newCapacity = minCapacity;
256 :     queue = Arrays.copyOf(queue, newCapacity);
257 : dl 1.22 }
258 : dl 1.36
259 : tim 1.2 /**
260 : dl 1.42 * Inserts the specified element into this priority queue.
261 : tim 1.2 *
262 : jsr166 1.63 * @return {@code true} (as specified by {@link Collection#add})
263 : dl 1.52 * @throws ClassCastException if the specified element cannot be
264 :     * compared with elements currently in this priority queue
265 :     * according to the priority queue's ordering
266 :     * @throws NullPointerException if the specified element is null
267 : tim 1.2 */
268 : dl 1.52 public boolean add(E e) {
269 :     return offer(e);
270 :     }
271 :    
272 :     /**
273 :     * Inserts the specified element into this priority queue.
274 :     *
275 : jsr166 1.63 * @return {@code true} (as specified by {@link Queue#offer})
276 : dl 1.52 * @throws ClassCastException if the specified element cannot be
277 :     * compared with elements currently in this priority queue
278 :     * according to the priority queue's ordering
279 :     * @throws NullPointerException if the specified element is null
280 :     */
281 :     public boolean offer(E e) {
282 :     if (e == null)
283 : dholmes 1.11 throw new NullPointerException();
284 :     modCount++;
285 : dl 1.55 int i = size;
286 :     if (i >= queue.length)
287 :     grow(i + 1);
288 :     size = i + 1;
289 :     if (i == 0)
290 :     queue[0] = e;
291 :     else
292 :     siftUp(i, e);
293 : dholmes 1.11 return true;
294 :     }
295 :    
296 : dl 1.40 public E peek() {
297 : tim 1.2 if (size == 0)
298 :     return null;
299 : dl 1.55 return (E) queue[0];
300 : tim 1.1 }
301 :    
302 : dl 1.52 private int indexOf(Object o) {
303 : dl 1.55 if (o != null) {
304 :     for (int i = 0; i < size; i++)
305 :     if (o.equals(queue[i]))
306 :     return i;
307 :     }
308 : dl 1.52 return -1;
309 :     }
310 :    
311 :     /**
312 :     * Removes a single instance of the specified element from this queue,
313 : jsr166 1.63 * if it is present. More formally, removes an element {@code e} such
314 :     * that {@code o.equals(e)}, if this queue contains one or more such
315 :     * elements. Returns {@code true} if and only if this queue contained
316 :     * the specified element (or equivalently, if this queue changed as a
317 :     * result of the call).
318 : dl 1.52 *
319 :     * @param o element to be removed from this queue, if present
320 : jsr166 1.63 * @return {@code true} if this queue changed as a result of the call
321 : dl 1.52 */
322 :     public boolean remove(Object o) {
323 :     int i = indexOf(o);
324 :     if (i == -1)
325 :     return false;
326 :     else {
327 :     removeAt(i);
328 :     return true;
329 :     }
330 :     }
331 : dholmes 1.11
332 : jsr166 1.56 /**
333 : dl 1.55 * Version of remove using reference equality, not equals.
334 : jsr166 1.59 * Needed by iterator.remove.
335 : jsr166 1.56 *
336 : dl 1.55 * @param o element to be removed from this queue, if present
337 : jsr166 1.63 * @return {@code true} if removed
338 : dl 1.55 */
339 :     boolean removeEq(Object o) {
340 :     for (int i = 0; i < size; i++) {
341 :     if (o == queue[i]) {
342 :     removeAt(i);
343 :     return true;
344 :     }
345 :     }
346 :     return false;
347 :     }
348 :    
349 : dholmes 1.11 /**
350 : jsr166 1.63 * Returns {@code true} if this queue contains the specified element.
351 :     * More formally, returns {@code true} if and only if this queue contains
352 :     * at least one element {@code e} such that {@code o.equals(e)}.
353 : dholmes 1.23 *
354 : dl 1.52 * @param o object to be checked for containment in this queue
355 : jsr166 1.63 * @return {@code true} if this queue contains the specified element
356 : dholmes 1.11 */
357 : dl 1.52 public boolean contains(Object o) {
358 :     return indexOf(o) != -1;
359 : tim 1.14 }
360 : dholmes 1.11
361 : dl 1.49 /**
362 : jsr166 1.63 * Returns an array containing all of the elements in this queue.
363 : dl 1.52 * The elements are in no particular order.
364 :     *
365 :     * <p>The returned array will be "safe" in that no references to it are
366 : jsr166 1.63 * maintained by this queue. (In other words, this method must allocate
367 : dl 1.52 * a new array). The caller is thus free to modify the returned array.
368 :     *
369 : jsr166 1.63 * <p>This method acts as bridge between array-based and collection-based
370 :     * APIs.
371 :     *
372 : jsr166 1.59 * @return an array containing all of the elements in this queue
373 : dl 1.49 */
374 : dl 1.52 public Object[] toArray() {
375 : dl 1.55 return Arrays.copyOf(queue, size);
376 : dl 1.52 }
377 : tim 1.2
378 : dl 1.52 /**
379 : jsr166 1.63 * Returns an array containing all of the elements in this queue; the
380 :     * runtime type of the returned array is that of the specified array.
381 :     * The returned array elements are in no particular order.
382 :     * If the queue fits in the specified array, it is returned therein.
383 :     * Otherwise, a new array is allocated with the runtime type of the
384 :     * specified array and the size of this queue.
385 : dl 1.52 *
386 :     * <p>If the queue fits in the specified array with room to spare
387 :     * (i.e., the array has more elements than the queue), the element in
388 :     * the array immediately following the end of the collection is set to
389 : jsr166 1.63 * {@code null}.
390 :     *
391 :     * <p>Like the {@link #toArray()} method, this method acts as bridge between
392 :     * array-based and collection-based APIs. Further, this method allows
393 :     * precise control over the runtime type of the output array, and may,
394 :     * under certain circumstances, be used to save allocation costs.
395 :     *
396 :     * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
397 :     * The following code can be used to dump the queue into a newly
398 :     * allocated array of <tt>String</tt>:
399 :     *
400 :     * <pre>
401 :     * String[] y = x.toArray(new String[0]);</pre>
402 :     *
403 :     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
404 :     * <tt>toArray()</tt>.
405 : dl 1.52 *
406 :     * @param a the array into which the elements of the queue are to
407 :     * be stored, if it is big enough; otherwise, a new array of the
408 :     * same runtime type is allocated for this purpose.
409 : jsr166 1.63 * @return an array containing all of the elements in this queue
410 : dl 1.52 * @throws ArrayStoreException if the runtime type of the specified array
411 :     * is not a supertype of the runtime type of every element in
412 :     * this queue
413 :     * @throws NullPointerException if the specified array is null
414 :     */
415 :     public <T> T[] toArray(T[] a) {
416 :     if (a.length < size)
417 :     // Make a new array of a's runtime type, but my contents:
418 : dl 1.55 return (T[]) Arrays.copyOf(queue, size, a.getClass());
419 :     System.arraycopy(queue, 0, a, 0, size);
420 : dl 1.52 if (a.length > size)
421 :     a[size] = null;
422 :     return a;
423 : tim 1.1 }
424 : tim 1.2
425 : dholmes 1.23 /**
426 :     * Returns an iterator over the elements in this queue. The iterator
427 :     * does not return the elements in any particular order.
428 :     *
429 : dl 1.52 * @return an iterator over the elements in this queue
430 : dholmes 1.23 */
431 : tim 1.2 public Iterator<E> iterator() {
432 : dl 1.7 return new Itr();
433 : tim 1.2 }
434 :    
435 : dl 1.55 private final class Itr implements Iterator<E> {
436 : dl 1.7 /**
437 :     * Index (into queue array) of element to be returned by
438 : tim 1.2 * subsequent call to next.
439 : dl 1.7 */
440 : dl 1.55 private int cursor = 0;
441 : tim 1.2
442 : dl 1.7 /**
443 : dl 1.36 * Index of element returned by most recent call to next,
444 :     * unless that element came from the forgetMeNot list.
445 : dl 1.55 * Set to -1 if element is deleted by a call to remove.
446 : dl 1.7 */
447 : dl 1.55 private int lastRet = -1;
448 : dl 1.7
449 :     /**
450 : dl 1.55 * A queue of elements that were moved from the unvisited portion of
451 : dl 1.36 * the heap into the visited portion as a result of "unlucky" element
452 :     * removals during the iteration. (Unlucky element removals are those
453 : dl 1.55 * that require a siftup instead of a siftdown.) We must visit all of
454 : dl 1.36 * the elements in this list to complete the iteration. We do this
455 :     * after we've completed the "normal" iteration.
456 :     *
457 :     * We expect that most iterations, even those involving removals,
458 : jsr166 1.63 * will not need to store elements in this field.
459 : dl 1.36 */
460 : dl 1.55 private ArrayDeque<E> forgetMeNot = null;
461 : dl 1.36
462 :     /**
463 :     * Element returned by the most recent call to next iff that
464 :     * element was drawn from the forgetMeNot list.
465 :     */
466 : dl 1.55 private E lastRetElt = null;
467 :    
468 :     /**
469 :     * The modCount value that the iterator believes that the backing
470 : jsr166 1.63 * Queue should have. If this expectation is violated, the iterator
471 : dl 1.55 * has detected concurrent modification.
472 :     */
473 :     private int expectedModCount = modCount;
474 : dl 1.35
475 : dl 1.7 public boolean hasNext() {
476 : jsr166 1.56 return cursor < size ||
477 : dl 1.55 (forgetMeNot != null && !forgetMeNot.isEmpty());
478 : dl 1.7 }
479 :    
480 :     public E next() {
481 : dl 1.55 if (expectedModCount != modCount)
482 :     throw new ConcurrentModificationException();
483 : jsr166 1.56 if (cursor < size)
484 : dl 1.55 return (E) queue[lastRet = cursor++];
485 :     if (forgetMeNot != null) {
486 :     lastRet = -1;
487 :     lastRetElt = forgetMeNot.poll();
488 : jsr166 1.56 if (lastRetElt != null)
489 : dl 1.55 return lastRetElt;
490 : dl 1.36 }
491 : dl 1.55 throw new NoSuchElementException();
492 : dl 1.7 }
493 : tim 1.2
494 : dl 1.7 public void remove() {
495 : dl 1.55 if (expectedModCount != modCount)
496 :     throw new ConcurrentModificationException();
497 :     if (lastRet != -1) {
498 : dl 1.36 E moved = PriorityQueue.this.removeAt(lastRet);
499 : dl 1.55 lastRet = -1;
500 : jsr166 1.56 if (moved == null)
501 : dl 1.36 cursor--;
502 : dl 1.55 else {
503 : dl 1.36 if (forgetMeNot == null)
504 : dl 1.55 forgetMeNot = new ArrayDeque<E>();
505 : dl 1.36 forgetMeNot.add(moved);
506 : jsr166 1.56 }
507 : jsr166 1.63 } else if (lastRetElt != null) {
508 : dl 1.55 PriorityQueue.this.removeEq(lastRetElt);
509 : dl 1.36 lastRetElt = null;
510 : jsr166 1.63 } else {
511 :     throw new IllegalStateException();
512 :     }
513 : tim 1.2 expectedModCount = modCount;
514 : dl 1.7 }
515 : tim 1.2 }
516 :    
517 : tim 1.1 public int size() {
518 : tim 1.2 return size;
519 : tim 1.1 }
520 : tim 1.2
521 :     /**
522 : dl 1.52 * Removes all of the elements from this priority queue.
523 : dl 1.49 * The queue will be empty after this call returns.
524 : tim 1.2 */
525 :     public void clear() {
526 :     modCount++;
527 : dl 1.55 for (int i = 0; i < size; i++)
528 : tim 1.2 queue[i] = null;
529 :     size = 0;
530 :     }
531 :    
532 : dl 1.40 public E poll() {
533 : dl 1.36 if (size == 0)
534 : dl 1.40 return null;
535 : dl 1.55 int s = --size;
536 : dl 1.36 modCount++;
537 : jsr166 1.63 E result = (E) queue[0];
538 :     E x = (E) queue[s];
539 : dl 1.55 queue[s] = null;
540 :     if (s != 0)
541 :     siftDown(0, x);
542 : dl 1.36 return result;
543 :     }
544 :    
545 :     /**
546 : dl 1.55 * Removes the ith element from queue.
547 : tim 1.2 *
548 : dl 1.55 * Normally this method leaves the elements at up to i-1,
549 :     * inclusive, untouched. Under these circumstances, it returns
550 :     * null. Occasionally, in order to maintain the heap invariant,
551 :     * it must swap a later element of the list with one earlier than
552 :     * i. Under these circumstances, this method returns the element
553 :     * that was previously at the end of the list and is now at some
554 :     * position before i. This fact is used by iterator.remove so as to
555 : jsr166 1.63 * avoid missing traversing elements.
556 : tim 1.2 */
557 : dl 1.52 private E removeAt(int i) {
558 : dl 1.55 assert i >= 0 && i < size;
559 : tim 1.2 modCount++;
560 : dl 1.55 int s = --size;
561 :     if (s == i) // removed last element
562 :     queue[i] = null;
563 :     else {
564 :     E moved = (E) queue[s];
565 : jsr166 1.56 queue[s] = null;
566 : dl 1.55 siftDown(i, moved);
567 : dl 1.36 if (queue[i] == moved) {
568 : dl 1.55 siftUp(i, moved);
569 : dl 1.36 if (queue[i] != moved)
570 :     return moved;
571 :     }
572 : dl 1.35 }
573 : dl 1.36 return null;
574 : tim 1.1 }
575 :    
576 : tim 1.2 /**
577 : dl 1.55 * Inserts item x at position k, maintaining heap invariant by
578 :     * promoting x up the tree until it is greater than or equal to
579 :     * its parent, or is the root.
580 :     *
581 :     * To simplify and speed up coercions and comparisons. the
582 :     * Comparable and Comparator versions are separated into different
583 :     * methods that are otherwise identical. (Similarly for siftDown.)
584 : jsr166 1.56 *
585 : dl 1.55 * @param k the position to fill
586 :     * @param x the item to insert
587 :     */
588 :     private void siftUp(int k, E x) {
589 : jsr166 1.56 if (comparator != null)
590 : dl 1.55 siftUpUsingComparator(k, x);
591 :     else
592 :     siftUpComparable(k, x);
593 :     }
594 :    
595 :     private void siftUpComparable(int k, E x) {
596 :     Comparable<? super E> key = (Comparable<? super E>) x;
597 :     while (k > 0) {
598 :     int parent = (k - 1) >>> 1;
599 :     Object e = queue[parent];
600 : jsr166 1.63 if (key.compareTo((E) e) >= 0)
601 : dl 1.55 break;
602 :     queue[k] = e;
603 :     k = parent;
604 :     }
605 :     queue[k] = key;
606 :     }
607 :    
608 :     private void siftUpUsingComparator(int k, E x) {
609 :     while (k > 0) {
610 :     int parent = (k - 1) >>> 1;
611 :     Object e = queue[parent];
612 : jsr166 1.63 if (comparator.compare(x, (E) e) >= 0)
613 : dl 1.55 break;
614 :     queue[k] = e;
615 :     k = parent;
616 :     }
617 :     queue[k] = x;
618 :     }
619 :    
620 :     /**
621 :     * Inserts item x at position k, maintaining heap invariant by
622 :     * demoting x down the tree repeatedly until it is less than or
623 :     * equal to its children or is a leaf.
624 :     *
625 :     * @param k the position to fill
626 :     * @param x the item to insert
627 :     */
628 :     private void siftDown(int k, E x) {
629 : jsr166 1.56 if (comparator != null)
630 : dl 1.55 siftDownUsingComparator(k, x);
631 :     else
632 :     siftDownComparable(k, x);
633 :     }
634 :    
635 :     private void siftDownComparable(int k, E x) {
636 :     Comparable<? super E> key = (Comparable<? super E>)x;
637 :     int half = size >>> 1; // loop while a non-leaf
638 :     while (k < half) {
639 :     int child = (k << 1) + 1; // assume left child is least
640 :     Object c = queue[child];
641 :     int right = child + 1;
642 :     if (right < size &&
643 : jsr166 1.63 ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
644 : dl 1.55 c = queue[child = right];
645 : jsr166 1.63 if (key.compareTo((E) c) <= 0)
646 : dl 1.55 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 : jsr166 1.63 comparator.compare((E) c, (E) queue[right]) > 0)
661 : jsr166 1.56 c = queue[child = right];
662 : jsr166 1.63 if (comparator.compare(x, (E) c) <= 0)
663 : dl 1.55 break;
664 :     queue[k] = c;
665 :     k = child;
666 : tim 1.2 }
667 : dl 1.55 queue[k] = x;
668 : dl 1.36 }
669 : dl 1.35
670 : dl 1.36 /**
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 : jsr166 1.56 for (int i = (size >>> 1) - 1; i >= 0; i--)
676 : jsr166 1.63 siftDown(i, (E) queue[i]);
677 : tim 1.2 }
678 :    
679 : dholmes 1.23 /**
680 : dl 1.52 * Returns the comparator used to order the elements in this
681 : jsr166 1.63 * queue, or {@code null} if this queue is sorted according to
682 : dl 1.52 * the {@linkplain Comparable natural ordering} of its elements.
683 :     *
684 :     * @return the comparator used to order this queue, or
685 : jsr166 1.63 * {@code null} if this queue is sorted according to the
686 :     * natural ordering of its elements
687 : dholmes 1.23 */
688 : tim 1.16 public Comparator<? super E> comparator() {
689 : tim 1.2 return comparator;
690 :     }
691 : dl 1.5
692 :     /**
693 : jsr166 1.63 * Saves the state of the instance to a stream (that
694 :     * is, serializes it).
695 : dl 1.5 *
696 :     * @serialData The length of the array backing the instance is
697 : jsr166 1.63 * emitted (int), followed by all of its elements
698 :     * (each an {@code Object}) in the proper order.
699 : dl 1.7 * @param s the stream
700 : dl 1.5 */
701 : dl 1.22 private void writeObject(java.io.ObjectOutputStream s)
702 : dl 1.5 throws java.io.IOException{
703 : dl 1.7 // Write out element count, and any hidden stuff
704 :     s.defaultWriteObject();
705 : dl 1.5
706 : jsr166 1.63 // Write out array length, for compatibility with 1.5 version
707 :     s.writeInt(Math.max(2, size + 1));
708 : dl 1.5
709 : jsr166 1.64 // Write out all elements in the "proper order".
710 : jsr166 1.63 for (int i = 0; i < size; i++)
711 : dl 1.5 s.writeObject(queue[i]);
712 :     }
713 :    
714 :     /**
715 : jsr166 1.63 * Reconstitutes the {@code PriorityQueue} instance from a stream
716 :     * (that is, deserializes it).
717 :     *
718 : dl 1.7 * @param s the stream
719 : dl 1.5 */
720 : dl 1.22 private void readObject(java.io.ObjectInputStream s)
721 : dl 1.5 throws java.io.IOException, ClassNotFoundException {
722 : dl 1.7 // Read in size, and any hidden stuff
723 :     s.defaultReadObject();
724 : dl 1.5
725 : jsr166 1.63 // Read in (and discard) array length
726 :     s.readInt();
727 :    
728 :     queue = new Object[size];
729 : dl 1.5
730 : jsr166 1.64 // Read in all elements.
731 : jsr166 1.63 for (int i = 0; i < size; i++)
732 :     queue[i] = s.readObject();
733 : jsr166 1.64
734 :     // Elements are guaranteed to be in "proper order", but the
735 :     // spec has never explained what that might be.
736 :     heapify();
737 : dl 1.5 }
738 : tim 1.1 }

Doug Lea
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