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30 * An unbounded priority {@linkplain Queue queue} based on a priority heap.
31 * The elements of the priority queue are ordered according to their
32 * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
33 * provided at queue construction time, depending on which constructor is
34 * used. A priority queue does not permit {@code null} elements.
35 * A priority queue relying on natural ordering also does not permit
36 * insertion of non-comparable objects (doing so may result in
37 * {@code ClassCastException}).
39 * <p>The <em>head</em> of this queue is the <em>least</em> element
40 * with respect to the specified ordering. If multiple elements are
41 * tied for least value, the head is one of those elements -- ties are
42 * broken arbitrarily. The queue retrieval operations {@code poll},
43 * {@code remove}, {@code peek}, and {@code element} access the
44 * element at the head of the queue.
46 * <p>A priority queue is unbounded, but has an internal
47 * <i>capacity</i> governing the size of an array used to store the
48 * elements on the queue. It is always at least as large as the queue
49 * size. As elements are added to a priority queue, its capacity
50 * grows automatically. The details of the growth policy are not
53 * <p>This class and its iterator implement all of the
54 * <em>optional</em> methods of the {@link Collection} and {@link
55 * Iterator} interfaces. The Iterator provided in method {@link
56 * #iterator()} is <em>not</em> guaranteed to traverse the elements of
57 * the priority queue in any particular order. If you need ordered
58 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
60 * <p> <strong>Note that this implementation is not synchronized.</strong>
61 * Multiple threads should not access a {@code PriorityQueue}
62 * instance concurrently if any of the threads modifies the queue.
63 * Instead, use the thread-safe {@link
64 * java.util.concurrent.PriorityBlockingQueue} class.
66 * <p>Implementation note: this implementation provides
67 * O(log(n)) time for the enqueing and dequeing methods
68 * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
69 * linear time for the {@code remove(Object)} and {@code contains(Object)}
70 * methods; and constant time for the retrieval methods
71 * ({@code peek}, {@code element}, and {@code size}).
73 * <p>This class is a member of the
74 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
75 * Java Collections Framework</a>.
78 * @author Josh Bloch, Doug Lea
79 * @param <E> the type of elements held in this collection
81 public class PriorityQueue<E> extends AbstractQueue<E>
82 implements java.io.Serializable {
84 private static final long serialVersionUID = -7720805057305804111L;
86 private static final int DEFAULT_INITIAL_CAPACITY = 11;
89 * 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.
96 private transient Object[] queue;
99 * The number of elements in the priority queue.
101 private int size = 0;
104 * The comparator, or null if priority queue uses elements'
107 private final Comparator<? super E> comparator;
110 * The number of times this priority queue has been
111 * <i>structurally modified</i>. See AbstractList for gory details.
113 private transient int modCount = 0;
116 * Creates a {@code PriorityQueue} with the default initial
117 * capacity (11) that orders its elements according to their
118 * {@linkplain Comparable natural ordering}.
120 public PriorityQueue() {
121 this(DEFAULT_INITIAL_CAPACITY, null);
125 * Creates a {@code PriorityQueue} with the specified initial
126 * capacity that orders its elements according to their
127 * {@linkplain Comparable natural ordering}.
129 * @param initialCapacity the initial capacity for this priority queue
130 * @throws IllegalArgumentException if {@code initialCapacity} is less
133 public PriorityQueue(int initialCapacity) {
134 this(initialCapacity, null);
138 * Creates a {@code PriorityQueue} with the specified initial capacity
139 * that orders its elements according to the specified comparator.
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
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;
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.
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
171 * @throws NullPointerException if the specified collection or any
172 * of its elements are null
174 @SuppressWarnings("unchecked")
175 public PriorityQueue(Collection<? extends E> c) {
176 if (c instanceof SortedSet<?>) {
177 SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
178 this.comparator = (Comparator<? super E>) ss.comparator();
179 initElementsFromCollection(ss);
181 else if (c instanceof PriorityQueue<?>) {
182 PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
183 this.comparator = (Comparator<? super E>) pq.comparator();
184 initFromPriorityQueue(pq);
187 this.comparator = null;
188 initFromCollection(c);
193 * Creates a {@code PriorityQueue} containing the elements in the
194 * specified priority queue. This priority queue will be
195 * ordered according to the same ordering as the given priority
198 * @param c the priority queue whose elements are to be placed
199 * into this priority queue
200 * @throws ClassCastException if elements of {@code c} cannot be
201 * compared to one another according to {@code c}'s
203 * @throws NullPointerException if the specified priority queue or any
204 * of its elements are null
206 @SuppressWarnings("unchecked")
207 public PriorityQueue(PriorityQueue<? extends E> c) {
208 this.comparator = (Comparator<? super E>) c.comparator();
209 initFromPriorityQueue(c);
213 * Creates a {@code PriorityQueue} containing the elements in the
214 * specified sorted set. This priority queue will be ordered
215 * according to the same ordering as the given sorted set.
217 * @param c the sorted set whose elements are to be placed
218 * into this priority queue
219 * @throws ClassCastException if elements of the specified sorted
220 * set cannot be compared to one another according to the
221 * sorted set's ordering
222 * @throws NullPointerException if the specified sorted set or any
223 * of its elements are null
225 @SuppressWarnings("unchecked")
226 public PriorityQueue(SortedSet<? extends E> c) {
227 this.comparator = (Comparator<? super E>) c.comparator();
228 initElementsFromCollection(c);
231 private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
232 if (c.getClass() == PriorityQueue.class) {
233 this.queue = c.toArray();
234 this.size = c.size();
236 initFromCollection(c);
240 private void initElementsFromCollection(Collection<? extends E> c) {
241 Object[] a = c.toArray();
242 // If c.toArray incorrectly doesn't return Object[], copy it.
243 if (a.getClass() != Object[].class)
244 a = Arrays.copyOf(a, a.length, Object[].class);
246 if (len == 1 || this.comparator != null)
247 for (int i = 0; i < len; i++)
249 throw new NullPointerException();
251 this.size = a.length;
255 * Initializes queue array with elements from the given Collection.
257 * @param c the collection
259 private void initFromCollection(Collection<? extends E> c) {
260 initElementsFromCollection(c);
265 * The maximum size of array to allocate.
266 * Some VMs reserve some header words in an array.
267 * Attempts to allocate larger arrays may result in
268 * OutOfMemoryError: Requested array size exceeds VM limit
270 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
273 * Increases the capacity of the array.
275 * @param minCapacity the desired minimum capacity
277 private void grow(int minCapacity) {
278 int oldCapacity = queue.length;
279 // Double size if small; else grow by 50%
280 int newCapacity = oldCapacity + ((oldCapacity < 64) ?
283 // overflow-conscious code
284 if (newCapacity - MAX_ARRAY_SIZE > 0)
285 newCapacity = hugeCapacity(minCapacity);
286 queue = Arrays.copyOf(queue, newCapacity);
289 private static int hugeCapacity(int minCapacity) {
290 if (minCapacity < 0) // overflow
291 throw new OutOfMemoryError();
292 return (minCapacity > MAX_ARRAY_SIZE) ?
298 * Inserts the specified element into this priority queue.
300 * @return {@code true} (as specified by {@link Collection#add})
301 * @throws ClassCastException if the specified element cannot be
302 * compared with elements currently in this priority queue
303 * according to the priority queue's ordering
304 * @throws NullPointerException if the specified element is null
306 public boolean add(E e) {
311 * Inserts the specified element into this priority queue.
313 * @return {@code true} (as specified by {@link Queue#offer})
314 * @throws ClassCastException if the specified element cannot be
315 * compared with elements currently in this priority queue
316 * according to the priority queue's ordering
317 * @throws NullPointerException if the specified element is null
319 public boolean offer(E e) {
321 throw new NullPointerException();
324 if (i >= queue.length)
340 private int indexOf(Object o) {
342 for (int i = 0; i < size; i++)
343 if (o.equals(queue[i]))
350 * Removes a single instance of the specified element from this queue,
351 * if it is present. More formally, removes an element {@code e} such
352 * that {@code o.equals(e)}, if this queue contains one or more such
353 * elements. Returns {@code true} if and only if this queue contained
354 * the specified element (or equivalently, if this queue changed as a
355 * result of the call).
357 * @param o element to be removed from this queue, if present
358 * @return {@code true} if this queue changed as a result of the call
360 public boolean remove(Object o) {
371 * Version of remove using reference equality, not equals.
372 * Needed by iterator.remove.
374 * @param o element to be removed from this queue, if present
375 * @return {@code true} if removed
377 boolean removeEq(Object o) {
378 for (int i = 0; i < size; i++) {
388 * Returns {@code true} if this queue contains the specified element.
389 * More formally, returns {@code true} if and only if this queue contains
390 * at least one element {@code e} such that {@code o.equals(e)}.
392 * @param o object to be checked for containment in this queue
393 * @return {@code true} if this queue contains the specified element
395 public boolean contains(Object o) {
396 return indexOf(o) != -1;
400 * Returns an array containing all of the elements in this queue.
401 * The elements are in no particular order.
403 * <p>The returned array will be "safe" in that no references to it are
404 * maintained by this queue. (In other words, this method must allocate
405 * a new array). The caller is thus free to modify the returned array.
407 * <p>This method acts as bridge between array-based and collection-based
410 * @return an array containing all of the elements in this queue
412 public Object[] toArray() {
413 return Arrays.copyOf(queue, size);
417 * Returns an array containing all of the elements in this queue; the
418 * runtime type of the returned array is that of the specified array.
419 * The returned array elements are in no particular order.
420 * If the queue fits in the specified array, it is returned therein.
421 * Otherwise, a new array is allocated with the runtime type of the
422 * specified array and the size of this queue.
424 * <p>If the queue fits in the specified array with room to spare
425 * (i.e., the array has more elements than the queue), the element in
426 * the array immediately following the end of the collection is set to
429 * <p>Like the {@link #toArray()} method, this method acts as bridge between
430 * array-based and collection-based APIs. Further, this method allows
431 * precise control over the runtime type of the output array, and may,
432 * under certain circumstances, be used to save allocation costs.
434 * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
435 * The following code can be used to dump the queue into a newly
436 * allocated array of <tt>String</tt>:
439 * String[] y = x.toArray(new String[0]);</pre>
441 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
442 * <tt>toArray()</tt>.
444 * @param a the array into which the elements of the queue are to
445 * be stored, if it is big enough; otherwise, a new array of the
446 * same runtime type is allocated for this purpose.
447 * @return an array containing all of the elements in this queue
448 * @throws ArrayStoreException if the runtime type of the specified array
449 * is not a supertype of the runtime type of every element in
451 * @throws NullPointerException if the specified array is null
453 public <T> T[] toArray(T[] a) {
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);
464 * Returns an iterator over the elements in this queue. The iterator
465 * does not return the elements in any particular order.
467 * @return an iterator over the elements in this queue
469 public Iterator<E> iterator() {
473 private final class Itr implements Iterator<E> {
475 * Index (into queue array) of element to be returned by
476 * subsequent call to next.
478 private int cursor = 0;
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.
485 private int lastRet = -1;
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.
495 * We expect that most iterations, even those involving removals,
496 * will not need to store elements in this field.
498 private ArrayDeque<E> forgetMeNot = null;
501 * Element returned by the most recent call to next iff that
502 * element was drawn from the forgetMeNot list.
504 private E lastRetElt = null;
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.
511 private int expectedModCount = modCount;
513 public boolean hasNext() {
514 return cursor < size ||
515 (forgetMeNot != null && !forgetMeNot.isEmpty());
519 if (expectedModCount != modCount)
520 throw new ConcurrentModificationException();
522 return (E) queue[lastRet = cursor++];
523 if (forgetMeNot != null) {
525 lastRetElt = forgetMeNot.poll();
526 if (lastRetElt != null)
529 throw new NoSuchElementException();
532 public void remove() {
533 if (expectedModCount != modCount)
534 throw new ConcurrentModificationException();
536 E moved = PriorityQueue.this.removeAt(lastRet);
541 if (forgetMeNot == null)
542 forgetMeNot = new ArrayDeque<>();
543 forgetMeNot.add(moved);
545 } else if (lastRetElt != null) {
546 PriorityQueue.this.removeEq(lastRetElt);
549 throw new IllegalStateException();
551 expectedModCount = modCount;
560 * Removes all of the elements from this priority queue.
561 * The queue will be empty after this call returns.
563 public void clear() {
565 for (int i = 0; i < size; i++)
575 E result = (E) queue[0];
584 * Removes the ith element from queue.
586 * Normally this method leaves the elements at up to i-1,
587 * inclusive, untouched. Under these circumstances, it returns
588 * null. Occasionally, in order to maintain the heap invariant,
589 * it must swap a later element of the list with one earlier than
590 * i. Under these circumstances, this method returns the element
591 * that was previously at the end of the list and is now at some
592 * position before i. This fact is used by iterator.remove so as to
593 * avoid missing traversing elements.
595 private E removeAt(int i) {
596 assert i >= 0 && i < size;
599 if (s == i) // removed last element
602 E moved = (E) queue[s];
605 if (queue[i] == moved) {
607 if (queue[i] != moved)
615 * Inserts item x at position k, maintaining heap invariant by
616 * promoting x up the tree until it is greater than or equal to
617 * its parent, or is the root.
619 * To simplify and speed up coercions and comparisons. the
620 * Comparable and Comparator versions are separated into different
621 * methods that are otherwise identical. (Similarly for siftDown.)
623 * @param k the position to fill
624 * @param x the item to insert
626 private void siftUp(int k, E x) {
627 if (comparator != null)
628 siftUpUsingComparator(k, x);
630 siftUpComparable(k, x);
633 private void siftUpComparable(int k, E x) {
634 Comparable<? super E> key = (Comparable<? super E>) x;
636 int parent = (k - 1) >>> 1;
637 Object e = queue[parent];
638 if (key.compareTo((E) e) >= 0)
646 private void siftUpUsingComparator(int k, E x) {
648 int parent = (k - 1) >>> 1;
649 Object e = queue[parent];
650 if (comparator.compare(x, (E) e) >= 0)
659 * Inserts item x at position k, maintaining heap invariant by
660 * demoting x down the tree repeatedly until it is less than or
661 * equal to its children or is a leaf.
663 * @param k the position to fill
664 * @param x the item to insert
666 private void siftDown(int k, E x) {
667 if (comparator != null)
668 siftDownUsingComparator(k, x);
670 siftDownComparable(k, x);
673 private void siftDownComparable(int k, E x) {
674 Comparable<? super E> key = (Comparable<? super E>)x;
675 int half = size >>> 1; // loop while a non-leaf
677 int child = (k << 1) + 1; // assume left child is least
678 Object c = queue[child];
679 int right = child + 1;
681 ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
682 c = queue[child = right];
683 if (key.compareTo((E) c) <= 0)
691 private void siftDownUsingComparator(int k, E x) {
692 int half = size >>> 1;
694 int child = (k << 1) + 1;
695 Object c = queue[child];
696 int right = child + 1;
698 comparator.compare((E) c, (E) queue[right]) > 0)
699 c = queue[child = right];
700 if (comparator.compare(x, (E) c) <= 0)
709 * Establishes the heap invariant (described above) in the entire tree,
710 * assuming nothing about the order of the elements prior to the call.
712 private void heapify() {
713 for (int i = (size >>> 1) - 1; i >= 0; i--)
714 siftDown(i, (E) queue[i]);
718 * Returns the comparator used to order the elements in this
719 * queue, or {@code null} if this queue is sorted according to
720 * the {@linkplain Comparable natural ordering} of its elements.
722 * @return the comparator used to order this queue, or
723 * {@code null} if this queue is sorted according to the
724 * natural ordering of its elements
726 public Comparator<? super E> comparator() {