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1    /*
2     * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
3     * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4     *
5     * This code is free software; you can redistribute it and/or modify it
6     * under the terms of the GNU General Public License version 2 only, as
7     * published by the Free Software Foundation.  Oracle designates this
8     * particular file as subject to the "Classpath" exception as provided
9     * by Oracle in the LICENSE file that accompanied this code.
10     *
11     * This code is distributed in the hope that it will be useful, but WITHOUT
12     * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13     * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14     * version 2 for more details (a copy is included in the LICENSE file that
15     * accompanied this code).
16     *
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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22     * or visit www.oracle.com if you need additional information or have any
23     * questions.
24     */
25    
26   package java.util;   package java.util;
27    
28    import java.util.function.Consumer;
29    
30  /**  /**
31   * An unbounded priority queue based on a priority heap.  This queue orders   * An unbounded priority {@linkplain Queue queue} based on a priority heap.
32   * elements according to an order specified at construction time, which is   * The elements of the priority queue are ordered according to their
33   * specified in the same manner as {@link TreeSet} and {@link TreeMap}: elements are ordered   * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
34   * either according to their <i>natural order</i> (see {@link Comparable}), or   * provided at queue construction time, depending on which constructor is
35   * according to a {@link Comparator}, depending on which constructor is used.   * used.  A priority queue does not permit {@code null} elements.
36   * The {@link #peek}, {@link #poll}, and {@link #remove} methods return the   * A priority queue relying on natural ordering also does not permit
37   * minimal element with respect to the specified ordering.  If multiple   * insertion of non-comparable objects (doing so may result in
38   * elements are tied for least value, no guarantees are made as to   * {@code ClassCastException}).
39   * which of these elements is returned.   *
40   *   * <p>The <em>head</em> of this queue is the <em>least</em> element
41   * <p>A priority queue has a <i>capacity</i>.  The capacity is the size of   * with respect to the specified ordering.  If multiple elements are
42   * the array used internally to store the elements on the queue.  It is always at least   * tied for least value, the head is one of those elements -- ties are
43   * as large as the queue size.  As elements are added to a priority queue,   * broken arbitrarily.  The queue retrieval operations {@code poll},
44   * its capacity grows automatically.  The details of the growth policy are not   * {@code remove}, {@code peek}, and {@code element} access the
45     * element at the head of the queue.
46     *
47     * <p>A priority queue is unbounded, but has an internal
48     * <i>capacity</i> governing the size of an array used to store the
49     * elements on the queue.  It is always at least as large as the queue
50     * size.  As elements are added to a priority queue, its capacity
51     * grows automatically.  The details of the growth policy are not
52   * specified.   * specified.
53   *   *
54   *<p>Implementation note: this implementation provides O(log(n)) time for   * <p>This class and its iterator implement all of the
55   * the insertion methods (<tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt>)   * <em>optional</em> methods of the {@link Collection} and {@link
56   * methods; linear time for the <tt>remove(Object)</tt> and   * Iterator} interfaces.  The Iterator provided in method {@link
57   * <tt>contains(Object)</tt> methods; and constant time for the retrieval methods (<tt>peek</tt>,   * #iterator()} and the Spliterator provided in method {@link #spliterator()}
58   * <tt>element</tt>, and <tt>size</tt>).   * are <em>not</em> guaranteed to traverse the elements of
59     * the priority queue in any particular order. If you need ordered
60     * traversal, consider using {@code Arrays.sort(pq.toArray())}.
61     *
62     * <p><strong>Note that this implementation is not synchronized.</strong>
63     * Multiple threads should not access a {@code PriorityQueue}
64     * instance concurrently if any of the threads modifies the queue.
65     * Instead, use the thread-safe {@link
66     * java.util.concurrent.PriorityBlockingQueue} class.
67     *
68     * <p>Implementation note: this implementation provides
69     * O(log(n)) time for the enqueuing and dequeuing methods
70     * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
71     * linear time for the {@code remove(Object)} and {@code contains(Object)}
72     * methods; and constant time for the retrieval methods
73     * ({@code peek}, {@code element}, and {@code size}).
74   *   *
75   * <p>This class is a member of the   * <p>This class is a member of the
76   * <a href="{@docRoot}/../guide/collections/index.html">   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
77   * Java Collections Framework</a>.   * Java Collections Framework</a>.
78     *
79     * @since 1.5
80     * @author Josh Bloch, Doug Lea
81     * @param <E> the type of elements held in this queue
82   */   */
83  public class PriorityQueue<E> extends AbstractQueue<E>  public class PriorityQueue<E> extends AbstractQueue<E>
84                                implements Queue<E>      implements java.io.Serializable {
85  {  
86        private static final long serialVersionUID = -7720805057305804111L;
87    
88      private static final int DEFAULT_INITIAL_CAPACITY = 11;      private static final int DEFAULT_INITIAL_CAPACITY = 11;
89    
90      /**      /**
91       * Priority queue represented as a balanced binary heap: the two children       * Priority queue represented as a balanced binary heap: the two
92       * of queue[n] are queue[2*n] and queue[2*n + 1].  The priority queue is       * children of queue[n] are queue[2*n+1] and queue[2*(n+1)].  The
93       * ordered by comparator, or by the elements' natural ordering, if       * priority queue is ordered by comparator, or by the elements'
94       * comparator is null:  For each node n in the heap and each descendant d       * natural ordering, if comparator is null: For each node n in the
95       * of n, n <= d.       * heap and each descendant d of n, n <= d.  The element with the
96       *       * lowest value is in queue[0], assuming the queue is nonempty.
      * The element with the lowest value is in queue[1], assuming the queue is  
      * nonempty.  (A one-based array is used in preference to the traditional  
      * zero-based array to simplify parent and child calculations.)  
      *  
      * queue.length must be >= 2, even if size == 0.  
97       */       */
98      private transient E[] queue;      transient Object[] queue; // non-private to simplify nested class access
99    
100      /**      /**
101       * The number of elements in the priority queue.       * The number of elements in the priority queue.
102       */       */
103      private int size = 0;      int size;
104    
105      /**      /**
106       * The comparator, or null if priority queue uses elements'       * The comparator, or null if priority queue uses elements'
107       * natural ordering.       * natural ordering.
108       */       */
109      private final Comparator<E> comparator;      private final Comparator<? super E> comparator;
110    
111      /**      /**
112       * The number of times this priority queue has been       * The number of times this priority queue has been
113       * <i>structurally modified</i>.  See AbstractList for gory details.       * <i>structurally modified</i>.  See AbstractList for gory details.
114       */       */
115      private transient int modCount = 0;      transient int modCount;     // non-private to simplify nested class access
116    
117      /**      /**
118       * Create a new priority queue with the default initial capacity (11)       * Creates a {@code PriorityQueue} with the default initial
119       * that orders its elements according to their natural ordering (using <tt>Comparable</tt>.)       * capacity (11) that orders its elements according to their
120         * {@linkplain Comparable natural ordering}.
121       */       */
122      public PriorityQueue() {      public PriorityQueue() {
123          this(DEFAULT_INITIAL_CAPACITY);          this(DEFAULT_INITIAL_CAPACITY, null);
124      }      }
125    
126      /**      /**
127       * Create a new priority queue with the specified initial capacity       * Creates a {@code PriorityQueue} with the specified initial
128       * that orders its elements according to their natural ordering (using <tt>Comparable</tt>.)       * capacity that orders its elements according to their
129         * {@linkplain Comparable natural ordering}.
130       *       *
131       * @param initialCapacity the initial capacity for this priority queue.       * @param initialCapacity the initial capacity for this priority queue
132         * @throws IllegalArgumentException if {@code initialCapacity} is less
133         *         than 1
134       */       */
135      public PriorityQueue(int initialCapacity) {      public PriorityQueue(int initialCapacity) {
136          this(initialCapacity, null);          this(initialCapacity, null);
137      }      }
138    
139      /**      /**
140       * Create a new priority queue with the specified initial capacity (11)       * Creates a {@code PriorityQueue} with the default initial capacity and
141       * that orders its elements according to the specified comparator.       * whose elements are ordered according to the specified comparator.
142       *       *
143       * @param initialCapacity the initial capacity for this priority queue.       * @param  comparator the comparator that will be used to order this
144       * @param comparator the comparator used to order this priority queue.       *         priority queue.  If {@code null}, the {@linkplain Comparable
145         *         natural ordering} of the elements will be used.
146         * @since 1.8
147       */       */
148      public PriorityQueue(int initialCapacity, Comparator<E> comparator) {      public PriorityQueue(Comparator<? super E> comparator) {
149            this(DEFAULT_INITIAL_CAPACITY, comparator);
150        }
151    
152        /**
153         * Creates a {@code PriorityQueue} with the specified initial capacity
154         * that orders its elements according to the specified comparator.
155         *
156         * @param  initialCapacity the initial capacity for this priority queue
157         * @param  comparator the comparator that will be used to order this
158         *         priority queue.  If {@code null}, the {@linkplain Comparable
159         *         natural ordering} of the elements will be used.
160         * @throws IllegalArgumentException if {@code initialCapacity} is
161         *         less than 1
162         */
163        public PriorityQueue(int initialCapacity,
164                             Comparator<? super E> comparator) {
165            // Note: This restriction of at least one is not actually needed,
166            // but continues for 1.5 compatibility
167          if (initialCapacity < 1)          if (initialCapacity < 1)
168              initialCapacity = 1;              throw new IllegalArgumentException();
169          queue = new E[initialCapacity + 1];          this.queue = new Object[initialCapacity];
170          this.comparator = comparator;          this.comparator = comparator;
171      }      }
172    
173      /**      /**
174       * Create a new priority queue containing the elements in the specified       * Creates a {@code PriorityQueue} containing the elements in the
175       * collection.  The priority queue has an initial capacity of 110% of the       * specified collection.  If the specified collection is an instance of
176       * size of the specified collection. If the specified collection       * a {@link SortedSet} or is another {@code PriorityQueue}, this
177       * implements the {@link Sorted} interface, the priority queue will be       * priority queue will be ordered according to the same ordering.
178       * sorted according to the same comparator, or according to its elements'       * Otherwise, this priority queue will be ordered according to the
179       * natural order if the collection is sorted according to its elements'       * {@linkplain Comparable natural ordering} of its elements.
      * natural order.  If the specified collection does not implement  
      * <tt>Sorted</tt>, the priority queue is ordered according to  
      * its elements' natural order.  
180       *       *
181       * @param initialElements the collection whose elements are to be placed       * @param  c the collection whose elements are to be placed
182       *        into this priority queue.       *         into this priority queue
183       * @throws ClassCastException if elements of the specified collection       * @throws ClassCastException if elements of the specified collection
184       *         cannot be compared to one another according to the priority       *         cannot be compared to one another according to the priority
185       *         queue's ordering.       *         queue's ordering
186       * @throws NullPointerException if the specified collection or an       * @throws NullPointerException if the specified collection or any
187       *         element of the specified collection is <tt>null</tt>.       *         of its elements are null
188       */       */
189      public PriorityQueue(Collection<E> initialElements) {      @SuppressWarnings("unchecked")
190          int sz = initialElements.size();      public PriorityQueue(Collection<? extends E> c) {
191          int initialCapacity = (int)Math.min((sz * 110L) / 100,          if (c instanceof SortedSet<?>) {
192                                              Integer.MAX_VALUE - 1);              SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
193          if (initialCapacity < 1)              this.comparator = (Comparator<? super E>) ss.comparator();
194              initialCapacity = 1;              initElementsFromCollection(ss);
195          queue = new E[initialCapacity + 1];          }
196            else if (c instanceof PriorityQueue<?>) {
197                PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
198                this.comparator = (Comparator<? super E>) pq.comparator();
199                initFromPriorityQueue(pq);
200            }
201            else {
202                this.comparator = null;
203                initFromCollection(c);
204            }
205        }
206    
207        /**
208         * Creates a {@code PriorityQueue} containing the elements in the
209         * specified priority queue.  This priority queue will be
210         * ordered according to the same ordering as the given priority
211         * queue.
212         *
213         * @param  c the priority queue whose elements are to be placed
214         *         into this priority queue
215         * @throws ClassCastException if elements of {@code c} cannot be
216         *         compared to one another according to {@code c}'s
217         *         ordering
218         * @throws NullPointerException if the specified priority queue or any
219         *         of its elements are null
220         */
221        @SuppressWarnings("unchecked")
222        public PriorityQueue(PriorityQueue<? extends E> c) {
223            this.comparator = (Comparator<? super E>) c.comparator();
224            initFromPriorityQueue(c);
225        }
226    
227        /**
228         * Creates a {@code PriorityQueue} containing the elements in the
229         * specified sorted set.   This priority queue will be ordered
230         * according to the same ordering as the given sorted set.
231         *
232         * @param  c the sorted set whose elements are to be placed
233         *         into this priority queue
234         * @throws ClassCastException if elements of the specified sorted
235         *         set cannot be compared to one another according to the
236         *         sorted set's ordering
237         * @throws NullPointerException if the specified sorted set or any
238         *         of its elements are null
239         */
240        @SuppressWarnings("unchecked")
241        public PriorityQueue(SortedSet<? extends E> c) {
242            this.comparator = (Comparator<? super E>) c.comparator();
243            initElementsFromCollection(c);
244        }
245    
246        private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
247            if (c.getClass() == PriorityQueue.class) {
248                this.queue = c.toArray();
249                this.size = c.size();
250            } else {
251                initFromCollection(c);
252            }
253        }
254    
255          /* Commented out to compile with generics compiler      private void initElementsFromCollection(Collection<? extends E> c) {
256            Object[] a = c.toArray();
257            // If c.toArray incorrectly doesn't return Object[], copy it.
258            if (a.getClass() != Object[].class)
259                a = Arrays.copyOf(a, a.length, Object[].class);
260            int len = a.length;
261            if (len == 1 || this.comparator != null)
262                for (Object e : a)
263                    if (e == null)
264                        throw new NullPointerException();
265            this.queue = a;
266            this.size = a.length;
267        }
268    
269          if (initialElements instanceof Sorted) {      /**
270              comparator = ((Sorted)initialElements).comparator();       * Initializes queue array with elements from the given Collection.
271              for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )       *
272                  queue[++size] = i.next();       * @param c the collection
         } else {  
273          */          */
274          {      private void initFromCollection(Collection<? extends E> c) {
275              comparator = null;          initElementsFromCollection(c);
276              for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )          heapify();
                 add(i.next());  
277          }          }
278    
279        /**
280         * The maximum size of array to allocate.
281         * Some VMs reserve some header words in an array.
282         * Attempts to allocate larger arrays may result in
283         * OutOfMemoryError: Requested array size exceeds VM limit
284         */
285        private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
286    
287        /**
288         * Increases the capacity of the array.
289         *
290         * @param minCapacity the desired minimum capacity
291         */
292        private void grow(int minCapacity) {
293            int oldCapacity = queue.length;
294            // Double size if small; else grow by 50%
295            int newCapacity = oldCapacity + ((oldCapacity < 64) ?
296                                             (oldCapacity + 2) :
297                                             (oldCapacity >> 1));
298            // overflow-conscious code
299            if (newCapacity - MAX_ARRAY_SIZE > 0)
300                newCapacity = hugeCapacity(minCapacity);
301            queue = Arrays.copyOf(queue, newCapacity);
302      }      }
303    
304      // Queue Methods      private static int hugeCapacity(int minCapacity) {
305            if (minCapacity < 0) // overflow
306                throw new OutOfMemoryError();
307            return (minCapacity > MAX_ARRAY_SIZE) ?
308                Integer.MAX_VALUE :
309                MAX_ARRAY_SIZE;
310        }
311    
312      /**      /**
313       * Remove and return the minimal element from this priority queue if       * Inserts the specified element into this priority queue.
      * it contains one or more elements, otherwise return <tt>null</tt>.  The term  
      * <i>minimal</i> is defined according to this priority queue's order.  
314       *       *
315       * @return the minimal element from this priority queue if it contains       * @return {@code true} (as specified by {@link Collection#add})
316       *         one or more elements, otherwise <tt>null</tt>.       * @throws ClassCastException if the specified element cannot be
317         *         compared with elements currently in this priority queue
318         *         according to the priority queue's ordering
319         * @throws NullPointerException if the specified element is null
320       */       */
321      public E poll() {      public boolean add(E e) {
322          if (size == 0)          return offer(e);
             return null;  
         return remove(1);  
323      }      }
324    
325      /**      /**
326       * Return, but do not remove, the minimal element from the priority queue,       * Inserts the specified element into this priority queue.
      * or return <tt>null</tt> if the queue is empty.  The term <i>minimal</i> is  
      * defined according to this priority queue's order.  This method returns  
      * the same object reference that would be returned by by the  
      * <tt>poll</tt> method.  The two methods differ in that this method  
      * does not remove the element from the priority queue.  
327       *       *
328       * @return the minimal element from this priority queue if it contains       * @return {@code true} (as specified by {@link Queue#offer})
329       *         one or more elements, otherwise <tt>null</tt>.       * @throws ClassCastException if the specified element cannot be
330         *         compared with elements currently in this priority queue
331         *         according to the priority queue's ordering
332         * @throws NullPointerException if the specified element is null
333       */       */
334        public boolean offer(E e) {
335            if (e == null)
336                throw new NullPointerException();
337            modCount++;
338            int i = size;
339            if (i >= queue.length)
340                grow(i + 1);
341            siftUp(i, e);
342            size = i + 1;
343            return true;
344        }
345    
346        @SuppressWarnings("unchecked")
347      public E peek() {      public E peek() {
348          return queue[1];          return (size == 0) ? null : (E) queue[0];
349      }      }
350    
351      // Collection Methods      private int indexOf(Object o) {
352            if (o != null) {
353                for (int i = 0; i < size; i++)
354                    if (o.equals(queue[i]))
355                        return i;
356            }
357            return -1;
358        }
359    
360      /**      /**
361       * Removes a single instance of the specified element from this priority       * Removes a single instance of the specified element from this queue,
362       * queue, if it is present.  Returns true if this collection contained the       * if it is present.  More formally, removes an element {@code e} such
363       * specified element (or equivalently, if this collection changed as a       * that {@code o.equals(e)}, if this queue contains one or more such
364         * elements.  Returns {@code true} if and only if this queue contained
365         * the specified element (or equivalently, if this queue changed as a
366       * result of the call).       * result of the call).
367       *       *
368       * @param element the element to be removed from this collection, if present.       * @param o element to be removed from this queue, if present
369       * @return <tt>true</tt> if this collection changed as a result of the       * @return {@code true} if this queue changed as a result of the call
      *         call  
      * @throws ClassCastException if the specified element cannot be compared  
      *            with elements currently in the priority queue according  
      *            to the priority queue's ordering.  
      * @throws NullPointerException if the specified element is null.  
370       */       */
371      public boolean remove(Object element) {      public boolean remove(Object o) {
372          if (element == null)          int i = indexOf(o);
373              throw new NullPointerException();          if (i == -1)
374                return false;
375          if (comparator == null) {          else {
376              for (int i = 1; i <= size; i++) {              removeAt(i);
                 if (((Comparable)queue[i]).compareTo(element) == 0) {  
                     remove(i);  
377                      return true;                      return true;
378                  }                  }
379              }              }
380          } else {  
381              for (int i = 1; i <= size; i++) {      /**
382                  if (comparator.compare(queue[i], (E) element) == 0) {       * Version of remove using reference equality, not equals.
383                      remove(i);       * Needed by iterator.remove.
384         *
385         * @param o element to be removed from this queue, if present
386         * @return {@code true} if removed
387         */
388        boolean removeEq(Object o) {
389            for (int i = 0; i < size; i++) {
390                if (o == queue[i]) {
391                    removeAt(i);
392                      return true;                      return true;
393                  }                  }
394              }              }
         }  
395          return false;          return false;
396      }      }
397    
398      /**      /**
399       * Returns an iterator over the elements in this priority queue.  The       * Returns {@code true} if this queue contains the specified element.
400       * elements of the priority queue will be returned by this iterator in the       * More formally, returns {@code true} if and only if this queue contains
401       * order specified by the queue, which is to say the order they would be       * at least one element {@code e} such that {@code o.equals(e)}.
402       * returned by repeated calls to <tt>poll</tt>.       *
403         * @param o object to be checked for containment in this queue
404         * @return {@code true} if this queue contains the specified element
405         */
406        public boolean contains(Object o) {
407            return indexOf(o) >= 0;
408        }
409    
410        /**
411         * Returns an array containing all of the elements in this queue.
412         * The elements are in no particular order.
413         *
414         * <p>The returned array will be "safe" in that no references to it are
415         * maintained by this queue.  (In other words, this method must allocate
416         * a new array).  The caller is thus free to modify the returned array.
417         *
418         * <p>This method acts as bridge between array-based and collection-based
419         * APIs.
420       *       *
421       * @return an <tt>Iterator</tt> over the elements in this priority queue.       * @return an array containing all of the elements in this queue
422         */
423        public Object[] toArray() {
424            return Arrays.copyOf(queue, size);
425        }
426    
427        /**
428         * Returns an array containing all of the elements in this queue; the
429         * runtime type of the returned array is that of the specified array.
430         * The returned array elements are in no particular order.
431         * If the queue fits in the specified array, it is returned therein.
432         * Otherwise, a new array is allocated with the runtime type of the
433         * specified array and the size of this queue.
434         *
435         * <p>If the queue fits in the specified array with room to spare
436         * (i.e., the array has more elements than the queue), the element in
437         * the array immediately following the end of the collection is set to
438         * {@code null}.
439         *
440         * <p>Like the {@link #toArray()} method, this method acts as bridge between
441         * array-based and collection-based APIs.  Further, this method allows
442         * precise control over the runtime type of the output array, and may,
443         * under certain circumstances, be used to save allocation costs.
444         *
445         * <p>Suppose {@code x} is a queue known to contain only strings.
446         * The following code can be used to dump the queue into a newly
447         * allocated array of {@code String}:
448         *
449         * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
450         *
451         * Note that {@code toArray(new Object[0])} is identical in function to
452         * {@code toArray()}.
453         *
454         * @param a the array into which the elements of the queue are to
455         *          be stored, if it is big enough; otherwise, a new array of the
456         *          same runtime type is allocated for this purpose.
457         * @return an array containing all of the elements in this queue
458         * @throws ArrayStoreException if the runtime type of the specified array
459         *         is not a supertype of the runtime type of every element in
460         *         this queue
461         * @throws NullPointerException if the specified array is null
462         */
463        @SuppressWarnings("unchecked")
464        public <T> T[] toArray(T[] a) {
465            final int size = this.size;
466            if (a.length < size)
467                // Make a new array of a's runtime type, but my contents:
468                return (T[]) Arrays.copyOf(queue, size, a.getClass());
469            System.arraycopy(queue, 0, a, 0, size);
470            if (a.length > size)
471                a[size] = null;
472            return a;
473        }
474    
475        /**
476         * Returns an iterator over the elements in this queue. The iterator
477         * does not return the elements in any particular order.
478         *
479         * @return an iterator over the elements in this queue
480       */       */
481      public Iterator<E> iterator() {      public Iterator<E> iterator() {
482          return new Itr();          return new Itr();
483      }      }
484    
485      private class Itr implements Iterator<E> {      private final class Itr implements Iterator<E> {
486          /**          /**
487           * Index (into queue array) of element to be returned by           * Index (into queue array) of element to be returned by
488           * subsequent call to next.           * subsequent call to next.
489           */           */
490          int cursor = 1;          private int cursor;
491    
492          /**          /**
493           * Index of element returned by most recent call to next or           * Index of element returned by most recent call to next,
494           * previous.  Reset to 0 if this element is deleted by a call           * unless that element came from the forgetMeNot list.
495           * to remove.           * Set to -1 if element is deleted by a call to remove.
496             */
497            private int lastRet = -1;
498    
499            /**
500             * A queue of elements that were moved from the unvisited portion of
501             * the heap into the visited portion as a result of "unlucky" element
502             * removals during the iteration.  (Unlucky element removals are those
503             * that require a siftup instead of a siftdown.)  We must visit all of
504             * the elements in this list to complete the iteration.  We do this
505             * after we've completed the "normal" iteration.
506             *
507             * We expect that most iterations, even those involving removals,
508             * will not need to store elements in this field.
509           */           */
510          int lastRet = 0;          private ArrayDeque<E> forgetMeNot;
511    
512            /**
513             * Element returned by the most recent call to next iff that
514             * element was drawn from the forgetMeNot list.
515             */
516            private E lastRetElt;
517    
518          /**          /**
519           * The modCount value that the iterator believes that the backing           * The modCount value that the iterator believes that the backing
520           * List should have.  If this expectation is violated, the iterator           * Queue should have.  If this expectation is violated, the iterator
521           * has detected concurrent modification.           * has detected concurrent modification.
522           */           */
523          int expectedModCount = modCount;          private int expectedModCount = modCount;
524    
525            Itr() {}                        // prevent access constructor creation
526    
527          public boolean hasNext() {          public boolean hasNext() {
528              return cursor <= size;              return cursor < size ||
529                    (forgetMeNot != null && !forgetMeNot.isEmpty());
530          }          }
531    
532            @SuppressWarnings("unchecked")
533          public E next() {          public E next() {
534              checkForComodification();              if (expectedModCount != modCount)
535              if (cursor > size)                  throw new ConcurrentModificationException();
536                if (cursor < size)
537                    return (E) queue[lastRet = cursor++];
538                if (forgetMeNot != null) {
539                    lastRet = -1;
540                    lastRetElt = forgetMeNot.poll();
541                    if (lastRetElt != null)
542                        return lastRetElt;
543                }
544                  throw new NoSuchElementException();                  throw new NoSuchElementException();
             E result = queue[cursor];  
             lastRet = cursor++;  
             return result;  
545          }          }
546    
547          public void remove() {          public void remove() {
548              if (lastRet == 0)              if (expectedModCount != modCount)
549                  throw new IllegalStateException();                  throw new ConcurrentModificationException();
550              checkForComodification();              if (lastRet != -1) {
551                    E moved = PriorityQueue.this.removeAt(lastRet);
552              PriorityQueue.this.remove(lastRet);                  lastRet = -1;
553              if (lastRet < cursor)                  if (moved == null)
554                  cursor--;                  cursor--;
555              lastRet = 0;                  else {
556              expectedModCount = modCount;                      if (forgetMeNot == null)
557                            forgetMeNot = new ArrayDeque<>();
558                        forgetMeNot.add(moved);
559                    }
560                } else if (lastRetElt != null) {
561                    PriorityQueue.this.removeEq(lastRetElt);
562                    lastRetElt = null;
563                } else {
564                    throw new IllegalStateException();
565          }          }
566                expectedModCount = modCount;
         final void checkForComodification() {  
             if (modCount != expectedModCount)  
                 throw new ConcurrentModificationException();  
567          }          }
568      }      }
569    
     /**  
      * Returns the number of elements in this priority queue.  
      *  
      * @return the number of elements in this priority queue.  
      */  
570      public int size() {      public int size() {
571          return size;          return size;
572      }      }
573    
574      /**      /**
575       * Add the specified element to this priority queue.       * Removes all of the elements from this priority queue.
576       *       * The queue will be empty after this call returns.
      * @param element the element to add.  
      * @return true  
      * @throws ClassCastException if the specified element cannot be compared  
      *            with elements currently in the priority queue according  
      *            to the priority queue's ordering.  
      * @throws NullPointerException if the specified element is null.  
      */  
     public boolean offer(E element) {  
         if (element == null)  
             throw new NullPointerException();  
         modCount++;  
   
         // Grow backing store if necessary  
         if (++size == queue.length) {  
             E[] newQueue = new E[2 * queue.length];  
             System.arraycopy(queue, 0, newQueue, 0, size);  
             queue = newQueue;  
         }  
   
         queue[size] = element;  
         fixUp(size);  
         return true;  
     }  
   
     /**  
      * Remove all elements from the priority queue.  
577       */       */
578      public void clear() {      public void clear() {
579          modCount++;          modCount++;
580            for (int i = 0; i < size; i++)
         // Null out element references to prevent memory leak  
         for (int i=1; i<=size; i++)  
581              queue[i] = null;              queue[i] = null;
   
582          size = 0;          size = 0;
583      }      }
584    
585        @SuppressWarnings("unchecked")
586        public E poll() {
587            if (size == 0)
588                return null;
589            int s = --size;
590            modCount++;
591            E result = (E) queue[0];
592            E x = (E) queue[s];
593            queue[s] = null;
594            if (s != 0)
595                siftDown(0, x);
596            return result;
597        }
598    
599      /**      /**
600       * Removes and returns the ith element from queue.  Recall       * Removes the ith element from queue.
      * that queue is one-based, so 1 <= i <= size.  
601       *       *
602       * XXX: Could further special-case i==size, but is it worth it?       * Normally this method leaves the elements at up to i-1,
603       * XXX: Could special-case i==0, but is it worth it?       * inclusive, untouched.  Under these circumstances, it returns
604       */       * null.  Occasionally, in order to maintain the heap invariant,
605      private E remove(int i) {       * it must swap a later element of the list with one earlier than
606          assert i <= size;       * i.  Under these circumstances, this method returns the element
607         * that was previously at the end of the list and is now at some
608         * position before i. This fact is used by iterator.remove so as to
609         * avoid missing traversing elements.
610         */
611        @SuppressWarnings("unchecked")
612        E removeAt(int i) {
613            // assert i >= 0 && i < size;
614          modCount++;          modCount++;
615            int s = --size;
616          E result = queue[i];          if (s == i) // removed last element
617          queue[i] = queue[size];              queue[i] = null;
618          queue[size--] = null;  // Drop extra ref to prevent memory leak          else {
619          if (i <= size)              E moved = (E) queue[s];
620              fixDown(i);              queue[s] = null;
621          return result;              siftDown(i, moved);
622                if (queue[i] == moved) {
623                    siftUp(i, moved);
624                    if (queue[i] != moved)
625                        return moved;
626                }
627            }
628            return null;
629      }      }
630    
631      /**      /**
632       * Establishes the heap invariant (described above) assuming the heap       * Inserts item x at position k, maintaining heap invariant by
633       * satisfies the invariant except possibly for the leaf-node indexed by k       * promoting x up the tree until it is greater than or equal to
634       * (which may have a nextExecutionTime less than its parent's).       * its parent, or is the root.
635       *       *
636       * This method functions by "promoting" queue[k] up the hierarchy       * To simplify and speed up coercions and comparisons, the
637       * (by swapping it with its parent) repeatedly until queue[k]       * Comparable and Comparator versions are separated into different
638       * is greater than or equal to its parent.       * methods that are otherwise identical. (Similarly for siftDown.)
639       */       *
640      private void fixUp(int k) {       * @param k the position to fill
641          if (comparator == null) {       * @param x the item to insert
642              while (k > 1) {       */
643                  int j = k >> 1;      private void siftUp(int k, E x) {
644                  if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)          if (comparator != null)
645                siftUpUsingComparator(k, x);
646            else
647                siftUpComparable(k, x);
648        }
649    
650        @SuppressWarnings("unchecked")
651        private void siftUpComparable(int k, E x) {
652            Comparable<? super E> key = (Comparable<? super E>) x;
653            while (k > 0) {
654                int parent = (k - 1) >>> 1;
655                Object e = queue[parent];
656                if (key.compareTo((E) e) >= 0)
657                      break;                      break;
658                  E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;              queue[k] = e;
659                  k = j;              k = parent;
660              }              }
661          } else {          queue[k] = key;
             while (k > 1) {  
                 int j = k >> 1;  
                 if (comparator.compare(queue[j], queue[k]) <= 0)  
                     break;  
                 E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;  
                 k = j;  
662              }              }
663    
664        @SuppressWarnings("unchecked")
665        private void siftUpUsingComparator(int k, E x) {
666            while (k > 0) {
667                int parent = (k - 1) >>> 1;
668                Object e = queue[parent];
669                if (comparator.compare(x, (E) e) >= 0)
670                    break;
671                queue[k] = e;
672                k = parent;
673          }          }
674            queue[k] = x;
675      }      }
676    
677      /**      /**
678       * Establishes the heap invariant (described above) in the subtree       * Inserts item x at position k, maintaining heap invariant by
679       * rooted at k, which is assumed to satisfy the heap invariant except       * demoting x down the tree repeatedly until it is less than or
680       * possibly for node k itself (which may be greater than its children).       * equal to its children or is a leaf.
681       *       *
682       * This method functions by "demoting" queue[k] down the hierarchy       * @param k the position to fill
683       * (by swapping it with its smaller child) repeatedly until queue[k]       * @param x the item to insert
684       * is less than or equal to its children.       */
685       */      private void siftDown(int k, E x) {
686      private void fixDown(int k) {          if (comparator != null)
687          int j;              siftDownUsingComparator(k, x);
688          if (comparator == null) {          else
689              while ((j = k << 1) <= size) {              siftDownComparable(k, x);
690                  if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)      }
691                      j++; // j indexes smallest kid  
692                  if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)      @SuppressWarnings("unchecked")
693        private void siftDownComparable(int k, E x) {
694            Comparable<? super E> key = (Comparable<? super E>)x;
695            int half = size >>> 1;        // loop while a non-leaf
696            while (k < half) {
697                int child = (k << 1) + 1; // assume left child is least
698                Object c = queue[child];
699                int right = child + 1;
700                if (right < size &&
701                    ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
702                    c = queue[child = right];
703                if (key.compareTo((E) c) <= 0)
704                      break;                      break;
705                  E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;              queue[k] = c;
706                  k = j;              k = child;
707              }              }
708          } else {          queue[k] = key;
709              while ((j = k << 1) <= size) {      }
710                  if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)  
711                      j++; // j indexes smallest kid      @SuppressWarnings("unchecked")
712                  if (comparator.compare(queue[k], queue[j]) <= 0)      private void siftDownUsingComparator(int k, E x) {
713            int half = size >>> 1;
714            while (k < half) {
715                int child = (k << 1) + 1;
716                Object c = queue[child];
717                int right = child + 1;
718                if (right < size &&
719                    comparator.compare((E) c, (E) queue[right]) > 0)
720                    c = queue[child = right];
721                if (comparator.compare(x, (E) c) <= 0)
722                      break;                      break;
723                  E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;              queue[k] = c;
724                  k = j;              k = child;
725              }              }
726            queue[k] = x;
727          }          }
728    
729        /**
730         * Establishes the heap invariant (described above) in the entire tree,
731         * assuming nothing about the order of the elements prior to the call.
732         * This classic algorithm due to Floyd (1964) is known to be O(size).
733         */
734        @SuppressWarnings("unchecked")
735        private void heapify() {
736            final Object[] es = queue;
737            final int half = (size >>> 1) - 1;
738            if (comparator == null)
739                for (int i = half; i >= 0; i--)
740                    siftDownComparable(i, (E) es[i]);
741            else
742                for (int i = half; i >= 0; i--)
743                    siftDownUsingComparator(i, (E) es[i]);
744      }      }
745    
746      /**      /**
747       * Returns the comparator associated with this priority queue, or       * Returns the comparator used to order the elements in this
748       * <tt>null</tt> if it uses its elements' natural ordering.       * queue, or {@code null} if this queue is sorted according to
749         * the {@linkplain Comparable natural ordering} of its elements.
750       *       *
751       * @return the comparator associated with this priority queue, or       * @return the comparator used to order this queue, or
752       *         <tt>null</tt> if it uses its elements' natural ordering.       *         {@code null} if this queue is sorted according to the
753         *         natural ordering of its elements
754       */       */
755      Comparator comparator() {      public Comparator<? super E> comparator() {
756          return comparator;          return comparator;
757      }      }
758    
759      /**      /**
760       * Save the state of the instance to a stream (that       * Saves this queue to a stream (that is, serializes it).
      * is, serialize it).  
761       *       *
762         * @param s the stream
763         * @throws java.io.IOException if an I/O error occurs
764       * @serialData The length of the array backing the instance is       * @serialData The length of the array backing the instance is
765       * emitted (int), followed by all of its elements (each an       *             emitted (int), followed by all of its elements
766       * <tt>Object</tt>) in the proper order.       *             (each an {@code Object}) in the proper order.
767       */       */
768      private synchronized void writeObject(java.io.ObjectOutputStream s)      private void writeObject(java.io.ObjectOutputStream s)
769          throws java.io.IOException{          throws java.io.IOException{
770          // Write out element count, and any hidden stuff          // Write out element count, and any hidden stuff
771          s.defaultWriteObject();          s.defaultWriteObject();
772    
773          // Write out array length          // Write out array length, for compatibility with 1.5 version
774          s.writeInt(queue.length);          s.writeInt(Math.max(2, size + 1));
775    
776          // Write out all elements in the proper order.          // Write out all elements in the "proper order".
777          for (int i=0; i<size; i++)          for (int i=0; i<size; i++)
778              s.writeObject(queue[i]);              s.writeObject(queue[i]);
779      }      }
780    
781      /**      /**
782       * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,       * Reconstitutes the {@code PriorityQueue} instance from a stream
783       * deserialize it).       * (that is, deserializes it).
784         *
785         * @param s the stream
786         * @throws ClassNotFoundException if the class of a serialized object
787         *         could not be found
788         * @throws java.io.IOException if an I/O error occurs
789       */       */
790      private synchronized void readObject(java.io.ObjectInputStream s)      private void readObject(java.io.ObjectInputStream s)
791          throws java.io.IOException, ClassNotFoundException {          throws java.io.IOException, ClassNotFoundException {
792          // Read in size, and any hidden stuff          // Read in size, and any hidden stuff
793          s.defaultReadObject();          s.defaultReadObject();
794    
795          // Read in array length and allocate array          // Read in (and discard) array length
796          int arrayLength = s.readInt();          s.readInt();
797          queue = new E[arrayLength];  
798            queue = new Object[size];
799    
800          // Read in all elements in the proper order.          // Read in all elements.
801          for (int i=0; i<size; i++)          for (int i=0; i<size; i++)
802              queue[i] = (E)s.readObject();              queue[i] = s.readObject();
803    
804            // Elements are guaranteed to be in "proper order", but the
805            // spec has never explained what that might be.
806            heapify();
807        }
808    
809        /**
810         * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
811         * and <em>fail-fast</em> {@link Spliterator} over the elements in this
812         * queue. The spliterator does not traverse elements in any particular order
813         * (the {@link Spliterator#ORDERED ORDERED} characteristic is not reported).
814         *
815         * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
816         * {@link Spliterator#SUBSIZED}, and {@link Spliterator#NONNULL}.
817         * Overriding implementations should document the reporting of additional
818         * characteristic values.
819         *
820         * @return a {@code Spliterator} over the elements in this queue
821         * @since 1.8
822         */
823        public final Spliterator<E> spliterator() {
824            return new PriorityQueueSpliterator(0, -1, 0);
825        }
826    
827        final class PriorityQueueSpliterator implements Spliterator<E> {
828            private int index;            // current index, modified on advance/split
829            private int fence;            // -1 until first use
830            private int expectedModCount; // initialized when fence set
831    
832            /** Creates new spliterator covering the given range. */
833            PriorityQueueSpliterator(int origin, int fence, int expectedModCount) {
834                this.index = origin;
835                this.fence = fence;
836                this.expectedModCount = expectedModCount;
837            }
838    
839            private int getFence() { // initialize fence to size on first use
840                int hi;
841                if ((hi = fence) < 0) {
842                    expectedModCount = modCount;
843                    hi = fence = size;
844                }
845                return hi;
846            }
847    
848            public PriorityQueueSpliterator trySplit() {
849                int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
850                return (lo >= mid) ? null :
851                    new PriorityQueueSpliterator(lo, index = mid, expectedModCount);
852      }      }
853    
854            @SuppressWarnings("unchecked")
855            public void forEachRemaining(Consumer<? super E> action) {
856                if (action == null)
857                    throw new NullPointerException();
858                if (fence < 0) { fence = size; expectedModCount = modCount; }
859                final Object[] a = queue;
860                int i, hi; E e;
861                for (i = index, index = hi = fence; i < hi; i++) {
862                    if ((e = (E) a[i]) == null)
863                        break;      // must be CME
864                    action.accept(e);
865                }
866                if (modCount != expectedModCount)
867                    throw new ConcurrentModificationException();
868            }
869    
870            @SuppressWarnings("unchecked")
871            public boolean tryAdvance(Consumer<? super E> action) {
872                if (action == null)
873                    throw new NullPointerException();
874                if (fence < 0) { fence = size; expectedModCount = modCount; }
875                int i;
876                if ((i = index) < fence) {
877                    index = i + 1;
878                    E e;
879                    if ((e = (E) queue[i]) == null
880                        || modCount != expectedModCount)
881                        throw new ConcurrentModificationException();
882                    action.accept(e);
883                    return true;
884                }
885                return false;
886            }
887    
888            public long estimateSize() {
889                return getFence() - index;
890            }
891    
892            public int characteristics() {
893                return Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.NONNULL;
894            }
895        }
896  }  }

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