java/util/PriorityQueue.java

 package java.util;

/*
 * Todo
 *
 *   1) Make it serializable.
 */

/**
 * An unbounded priority queue based on a priority heap.  This queue orders
 * elements according to the order specified at creation time.  This order is
 * specified as for {@link TreeSet} and {@link TreeMap}: Elements are ordered
 * either according to their <i>natural order</i> (see {@link Comparable}), or
 * according to a {@link Comparator}, depending on which constructor is used.
 * The {@link #peek}, {@link #poll}, and {@link #remove} methods return the
 * minimal element with respect to the specified ordering.  If multiple
 * these elements are tied for least value, no guarantees are made as to
 * which of elements is returned.
 *
 * <p>Each priority queue has a <i>capacity</i>.  The capacity is the size of
 * the array used to store the elements on the queue.  It is always at least
 * as large as the queue size.  As elements are added to a priority list,
 * its capacity grows automatically.  The details of the growth policy are not
 * specified.
 *
 *<p>Implementation note: this implementation provides O(log(n)) time for
 * the <tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt>
 * methods; linear time for the <tt>remove(Object)</tt> and
 * <tt>contains</tt> methods; and constant time for the <tt>peek</tt>,
 * <tt>element</tt>, and <tt>size</tt> methods.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 */
public class PriorityQueue<E> extends AbstractQueue<E>
                              implements Queue<E>
{
    private static final int DEFAULT_INITIAL_CAPACITY = 11;

    /**
     * Priority queue represented as a balanced binary heap: the two children
     * of queue[n] are queue[2*n] and queue[2*n + 1].  The priority queue is
     * ordered by comparator, or by the elements' natural ordering, if
     * comparator is null:  For each node n in the heap, and each descendant
     * of n, d, n <= d.
     *
     * 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.
     */
    private E[] queue;

    /**
     * The number of elements in the priority queue.
     */
    private int size = 0;

    /**
     * The comparator, or null if priority queue uses elements'
     * natural ordering.
     */
    private final Comparator<E> comparator;

    /**
     * The number of times this priority queue has been
     * <i>structurally modified</i>.  See AbstractList for gory details.
     */
    private int modCount = 0;

    /**
     * Create a new priority queue with the default initial capacity (11)
     * that orders its elements according to their natural ordering.
     */
    public PriorityQueue() {
        this(DEFAULT_INITIAL_CAPACITY);
    }

    /**
     * Create a new priority queue with the specified initial capacity
     * that orders its elements according to their natural ordering.
     *
     * @param initialCapacity the initial capacity for this priority queue.
     */
    public PriorityQueue(int initialCapacity) {
        this(initialCapacity, null);
    }

    /**
     * Create a new priority queue with the specified initial capacity (11)
     * that orders its elements according to the specified comparator.
     *
     * @param initialCapacity the initial capacity for this priority queue.
     * @param comparator the comparator used to order this priority queue.
     */
    public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
        if (initialCapacity < 1)
            initialCapacity = 1;
        queue = new E[initialCapacity + 1];
        this.comparator = comparator;
    }

    /**
     * Create a new priority queue containing the elements in the specified
     * collection.  The priority queue has an initial capacity of 110% of the
     * size of the specified collection. If the specified collection
     * implements the {@link Sorted} interface, the priority queue will be
     * sorted according to the same comparator, or according to its elements'
     * natural order if the collection is sorted according to its elements'
     * natural order.  If the specified collection does not implement the
     * <tt>Sorted</tt> interface, the priority queue is ordered according to
     * its elements' natural order.
     *
     * @param initialElements the collection whose elements are to be placed
     *        into this priority queue.
     * @throws ClassCastException if elements of the specified collection
     *         cannot be compared to one another according to the priority
     *         queue's ordering.
     * @throws NullPointerException if the specified collection or an
     *         element of the specified collection is <tt>null</tt>.
     */
    public PriorityQueue(Collection<E> initialElements) {
        int sz = initialElements.size();
        int initialCapacity = (int)Math.min((sz * 110L) / 100,
                                            Integer.MAX_VALUE - 1);
        if (initialCapacity < 1)
            initialCapacity = 1;
        queue = new E[initialCapacity + 1];

        if (initialElements instanceof Sorted) {
            comparator = ((Sorted)initialElements).comparator();
            for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
                queue[++size] = i.next();
        } else {
            comparator = null;
            for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
                add(i.next());
        }
    }

    // Queue Methods

    /**
     * Remove and return the minimal element from this priority queue if
     * it contains one or more elements, otherwise <tt>null</tt>.  The term
     * <i>minimal</i> is defined according to this priority queue's order.
     *
     * @return the minimal element from this priority queue if it contains
     *         one or more elements, otherwise <tt>null</tt>.
     */
    public E poll() {
        if (size == 0)
            return null;
        return remove(1);
    }

    /**
     * Return, but do not remove, the minimal element from the priority queue,
     * or <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.
     *
     * @return the minimal element from this priority queue if it contains
     *         one or more elements, otherwise <tt>null</tt>.
     */
    public E peek() {
        return queue[1];
    }

    // Collection Methods

    /**
     * Removes a single instance of the specified element from this priority
     * queue, if it is present.  Returns true if this collection contained the
     * specified element (or equivalently, if this collection changed as a
     * result of the call).
     *
     * @param o element to be removed from this collection, if present.
     * @return <tt>true</tt> if this collection changed as a result of the
     *         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.
     */
    public boolean remove(Object element) {
        if (element == null)
            throw new NullPointerException();

        if (comparator == null) {
            for (int i = 1; i <= size; i++) {
                if (((Comparable)queue[i]).compareTo(element) == 0) {
                    remove(i);
                    return true;
                }
            }
        } else {
            for (int i = 1; i <= size; i++) {
                if (comparator.compare(queue[i], (E) element) == 0) {
                    remove(i);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Returns an iterator over the elements in this priority queue.  The
     * first element returned by this iterator is the same element that
     * would be returned by a call to <tt>peek</tt>.
     *
     * @return an <tt>Iterator</tt> over the elements in this priority queue.
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    private class Itr implements Iterator<E> {
        /**
         * Index (into queue array) of element to be returned by
         * subsequent call to next.
         */
        int cursor = 1;

        /**
         * Index of element returned by most recent call to next or
         * previous.  Reset to 0 if this element is deleted by a call
         * to remove.
         */
        int lastRet = 0;

        /**
         * The modCount value that the iterator believes that the backing
         * List should have.  If this expectation is violated, the iterator
         * has detected concurrent modification.
         */
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor <= size;
        }

        public E next() {
            checkForComodification();
            if (cursor > size)
                throw new NoSuchElementException();
            E result = queue[cursor];
            lastRet = cursor++;
            return result;
        }

        public void remove() {
            if (lastRet == 0)
                throw new IllegalStateException();
            checkForComodification();

            PriorityQueue.this.remove(lastRet);
            if (lastRet < cursor)
                cursor--;
            lastRet = 0;
            expectedModCount = modCount;
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    /**
     * Returns the number of elements in this priority queue.
     *
     * @return the number of elements in this priority queue.
     */
    public int size() {
        return size;
    }

    /**
     * Add the specified element to this priority queue.
     *
     * @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.
     */
    public void clear() {
        modCount++;

        // Null out element references to prevent memory leak
        for (int i=1; i<=size; i++)
            queue[i] = null;

        size = 0;
    }

    /**
     * Removes and returns the ith element from queue.  Recall
     * that queue is one-based, so 1 <= i <= size.
     *
     * XXX: Could further special-case i==size, but is it worth it?
     * XXX: Could special-case i==0, but is it worth it?
     */
    private E remove(int i) {
        assert i <= size;
        modCount++;

        E result = queue[i];
        queue[i] = queue[size];
        queue[size--] = null;  // Drop extra ref to prevent memory leak
        if (i <= size)
            fixDown(i);
        return result;
    }

    /**
     * Establishes the heap invariant (described above) assuming the heap
     * satisfies the invariant except possibly for the leaf-node indexed by k
     * (which may have a nextExecutionTime less than its parent's).
     *
     * This method functions by "promoting" queue[k] up the hierarchy
     * (by swapping it with its parent) repeatedly until queue[k]
     * is greater than or equal to its parent.
     */
    private void fixUp(int k) {
        if (comparator == null) {
            while (k > 1) {
                int j = k >> 1;
                if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
                    break;
                E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        } else {
            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;
            }
        }
    }

    /**
     * Establishes the heap invariant (described above) in the subtree
     * rooted at k, which is assumed to satisfy the heap invariant except
     * possibly for node k itself (which may be greater than its children).
     *
     * This method functions by "demoting" queue[k] down the hierarchy
     * (by swapping it with its smaller child) repeatedly until queue[k]
     * is less than or equal to its children.
     */
    private void fixDown(int k) {
        int j;
        if (comparator == null) {
            while ((j = k << 1) <= size) {
                if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
                    j++; // j indexes smallest kid
                if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
                    break;
                E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        } else {
            while ((j = k << 1) <= size) {
                if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
                    j++; // j indexes smallest kid
                if (comparator.compare(queue[k], queue[j]) <= 0)
                    break;
                E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        }
    }

    /**
     * Returns the comparator associated with this priority queue, or
     * <tt>null</tt> if it uses its elements' natural ordering.
     *
     * @return the comparator associated with this priority queue, or
     *         <tt>null</tt> if it uses its elements' natural ordering.
     */
    Comparator<E> comparator() {
        return comparator;
    }
}
Revision:	1.4
Committed:	Mon May 19 02:45:07 2003 UTC (20 years, 11 months ago) by tim
Branch:	MAIN
Changes since 1.3:	+0 -4 lines
Log Message:	Use temp version of Sorted to allow a portion of PQ impl to be uncommented.
#	Content
1	package java.util;
2
3	/*
4	* Todo
5	*
6	* 1) Make it serializable.
7	*/
8
9	/**
10	* An unbounded priority queue based on a priority heap. This queue orders
11	* elements according to the order specified at creation time. This order is
12	* specified as for {@link TreeSet} and {@link TreeMap}: Elements are ordered
13	* either according to their <i>natural order</i> (see {@link Comparable}), or
14	* according to a {@link Comparator}, depending on which constructor is used.
15	* The {@link #peek}, {@link #poll}, and {@link #remove} methods return the
16	* minimal element with respect to the specified ordering. If multiple
17	* these elements are tied for least value, no guarantees are made as to
18	* which of elements is returned.
19	*
20	* <p>Each priority queue has a <i>capacity</i>. The capacity is the size of
21	* the array used to store the elements on the queue. It is always at least
22	* as large as the queue size. As elements are added to a priority list,
23	* its capacity grows automatically. The details of the growth policy are not
24	* specified.
25	*
26	*<p>Implementation note: this implementation provides O(log(n)) time for
27	* the <tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt>
28	* methods; linear time for the <tt>remove(Object)</tt> and
29	* <tt>contains</tt> methods; and constant time for the <tt>peek</tt>,
30	* <tt>element</tt>, and <tt>size</tt> methods.
31	*
32	* <p>This class is a member of the
33	* <a href="{@docRoot}/../guide/collections/index.html">
34	* Java Collections Framework</a>.
35	*/
36	public class PriorityQueue<E> extends AbstractQueue<E>
37	implements Queue<E>
38	{
39	private static final int DEFAULT_INITIAL_CAPACITY = 11;
40
41	/**
42	* Priority queue represented as a balanced binary heap: the two children
43	* of queue[n] are queue[2n] and queue[2n + 1]. The priority queue is
44	* ordered by comparator, or by the elements' natural ordering, if
45	* comparator is null: For each node n in the heap, and each descendant
46	* of n, d, n <= d.
47	*
48	* The element with the lowest value is in queue[1] (assuming the queue is
49	* nonempty). A one-based array is used in preference to the traditional
50	* zero-based array to simplify parent and child calculations.
51	*
52	* queue.length must be >= 2, even if size == 0.
53	*/
54	private E[] queue;
55
56	/**
57	* The number of elements in the priority queue.
58	*/
59	private int size = 0;
60
61	/**
62	* The comparator, or null if priority queue uses elements'
63	* natural ordering.
64	*/
65	private final Comparator<E> comparator;
66
67	/**
68	* The number of times this priority queue has been
69	* <i>structurally modified</i>. See AbstractList for gory details.
70	*/
71	private int modCount = 0;
72
73	/**
74	* Create a new priority queue with the default initial capacity (11)
75	* that orders its elements according to their natural ordering.
76	*/
77	public PriorityQueue() {
78	this(DEFAULT_INITIAL_CAPACITY);
79	}
80
81	/**
82	* Create a new priority queue with the specified initial capacity
83	* that orders its elements according to their natural ordering.
84	*
85	* @param initialCapacity the initial capacity for this priority queue.
86	*/
87	public PriorityQueue(int initialCapacity) {
88	this(initialCapacity, null);
89	}
90
91	/**
92	* Create a new priority queue with the specified initial capacity (11)
93	* that orders its elements according to the specified comparator.
94	*
95	* @param initialCapacity the initial capacity for this priority queue.
96	* @param comparator the comparator used to order this priority queue.
97	*/
98	public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
99	if (initialCapacity < 1)
100	initialCapacity = 1;
101	queue = new E[initialCapacity + 1];
102	this.comparator = comparator;
103	}
104
105	/**
106	* Create a new priority queue containing the elements in the specified
107	* collection. The priority queue has an initial capacity of 110% of the
108	* size of the specified collection. If the specified collection
109	* implements the {@link Sorted} interface, the priority queue will be
110	* sorted according to the same comparator, or according to its elements'
111	* natural order if the collection is sorted according to its elements'
112	* natural order. If the specified collection does not implement the
113	* <tt>Sorted</tt> interface, the priority queue is ordered according to
114	* its elements' natural order.
115	*
116	* @param initialElements the collection whose elements are to be placed
117	* into this priority queue.
118	* @throws ClassCastException if elements of the specified collection
119	* cannot be compared to one another according to the priority
120	* queue's ordering.
121	* @throws NullPointerException if the specified collection or an
122	* element of the specified collection is <tt>null</tt>.
123	*/
124	public PriorityQueue(Collection<E> initialElements) {
125	int sz = initialElements.size();
126	int initialCapacity = (int)Math.min((sz * 110L) / 100,
127	Integer.MAX_VALUE - 1);
128	if (initialCapacity < 1)
129	initialCapacity = 1;
130	queue = new E[initialCapacity + 1];
131
132	if (initialElements instanceof Sorted) {
133	comparator = ((Sorted)initialElements).comparator();
134	for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
135	queue[++size] = i.next();
136	} else {
137	comparator = null;
138	for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
139	add(i.next());
140	}
141	}
142
143	// Queue Methods
144
145	/**
146	* Remove and return the minimal element from this priority queue if
147	* it contains one or more elements, otherwise <tt>null</tt>. The term
148	* <i>minimal</i> is defined according to this priority queue's order.
149	*
150	* @return the minimal element from this priority queue if it contains
151	* one or more elements, otherwise <tt>null</tt>.
152	*/
153	public E poll() {
154	if (size == 0)
155	return null;
156	return remove(1);
157	}
158
159	/**
160	* Return, but do not remove, the minimal element from the priority queue,
161	* or <tt>null</tt> if the queue is empty. The term <i>minimal</i> is
162	* defined according to this priority queue's order. This method returns
163	* the same object reference that would be returned by by the
164	* <tt>poll</tt> method. The two methods differ in that this method
165	* does not remove the element from the priority queue.
166	*
167	* @return the minimal element from this priority queue if it contains
168	* one or more elements, otherwise <tt>null</tt>.
169	*/
170	public E peek() {
171	return queue[1];
172	}
173
174	// Collection Methods
175
176	/**
177	* Removes a single instance of the specified element from this priority
178	* queue, if it is present. Returns true if this collection contained the
179	* specified element (or equivalently, if this collection changed as a
180	* result of the call).
181	*
182	* @param o element to be removed from this collection, if present.
183	* @return <tt>true</tt> if this collection changed as a result of the
184	* call
185	* @throws ClassCastException if the specified element cannot be compared
186	* with elements currently in the priority queue according
187	* to the priority queue's ordering.
188	* @throws NullPointerException if the specified element is null.
189	*/
190	public boolean remove(Object element) {
191	if (element == null)
192	throw new NullPointerException();
193
194	if (comparator == null) {
195	for (int i = 1; i <= size; i++) {
196	if (((Comparable)queue[i]).compareTo(element) == 0) {
197	remove(i);
198	return true;
199	}
200	}
201	} else {
202	for (int i = 1; i <= size; i++) {
203	if (comparator.compare(queue[i], (E) element) == 0) {
204	remove(i);
205	return true;
206	}
207	}
208	}
209	return false;
210	}
211
212	/**
213	* Returns an iterator over the elements in this priority queue. The
214	* first element returned by this iterator is the same element that
215	* would be returned by a call to <tt>peek</tt>.
216	*
217	* @return an <tt>Iterator</tt> over the elements in this priority queue.
218	*/
219	public Iterator<E> iterator() {
220	return new Itr();
221	}
222
223	private class Itr implements Iterator<E> {
224	/**
225	* Index (into queue array) of element to be returned by
226	* subsequent call to next.
227	*/
228	int cursor = 1;
229
230	/**
231	* Index of element returned by most recent call to next or
232	* previous. Reset to 0 if this element is deleted by a call
233	* to remove.
234	*/
235	int lastRet = 0;
236
237	/**
238	* The modCount value that the iterator believes that the backing
239	* List should have. If this expectation is violated, the iterator
240	* has detected concurrent modification.
241	*/
242	int expectedModCount = modCount;
243
244	public boolean hasNext() {
245	return cursor <= size;
246	}
247
248	public E next() {
249	checkForComodification();
250	if (cursor > size)
251	throw new NoSuchElementException();
252	E result = queue[cursor];
253	lastRet = cursor++;
254	return result;
255	}
256
257	public void remove() {
258	if (lastRet == 0)
259	throw new IllegalStateException();
260	checkForComodification();
261
262	PriorityQueue.this.remove(lastRet);
263	if (lastRet < cursor)
264	cursor--;
265	lastRet = 0;
266	expectedModCount = modCount;
267	}
268
269	final void checkForComodification() {
270	if (modCount != expectedModCount)
271	throw new ConcurrentModificationException();
272	}
273	}
274
275	/**
276	* Returns the number of elements in this priority queue.
277	*
278	* @return the number of elements in this priority queue.
279	*/
280	public int size() {
281	return size;
282	}
283
284	/**
285	* Add the specified element to this priority queue.
286	*
287	* @param element the element to add.
288	* @return true
289	* @throws ClassCastException if the specified element cannot be compared
290	* with elements currently in the priority queue according
291	* to the priority queue's ordering.
292	* @throws NullPointerException if the specified element is null.
293	*/
294	public boolean offer(E element) {
295	if (element == null)
296	throw new NullPointerException();
297	modCount++;
298
299	// Grow backing store if necessary
300	if (++size == queue.length) {
301	E[] newQueue = new E[2 * queue.length];
302	System.arraycopy(queue, 0, newQueue, 0, size);
303	queue = newQueue;
304	}
305
306	queue[size] = element;
307	fixUp(size);
308	return true;
309	}
310
311	/**
312	* Remove all elements from the priority queue.
313	*/
314	public void clear() {
315	modCount++;
316
317	// Null out element references to prevent memory leak
318	for (int i=1; i<=size; i++)
319	queue[i] = null;
320
321	size = 0;
322	}
323
324	/**
325	* Removes and returns the ith element from queue. Recall
326	* that queue is one-based, so 1 <= i <= size.
327	*
328	* XXX: Could further special-case i==size, but is it worth it?
329	* XXX: Could special-case i==0, but is it worth it?
330	*/
331	private E remove(int i) {
332	assert i <= size;
333	modCount++;
334
335	E result = queue[i];
336	queue[i] = queue[size];
337	queue[size--] = null; // Drop extra ref to prevent memory leak
338	if (i <= size)
339	fixDown(i);
340	return result;
341	}
342
343	/**
344	* Establishes the heap invariant (described above) assuming the heap
345	* satisfies the invariant except possibly for the leaf-node indexed by k
346	* (which may have a nextExecutionTime less than its parent's).
347	*
348	* This method functions by "promoting" queue[k] up the hierarchy
349	* (by swapping it with its parent) repeatedly until queue[k]
350	* is greater than or equal to its parent.
351	*/
352	private void fixUp(int k) {
353	if (comparator == null) {
354	while (k > 1) {
355	int j = k >> 1;
356	if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
357	break;
358	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
359	k = j;
360	}
361	} else {
362	while (k > 1) {
363	int j = k >> 1;
364	if (comparator.compare(queue[j], queue[k]) <= 0)
365	break;
366	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
367	k = j;
368	}
369	}
370	}
371
372	/**
373	* Establishes the heap invariant (described above) in the subtree
374	* rooted at k, which is assumed to satisfy the heap invariant except
375	* possibly for node k itself (which may be greater than its children).
376	*
377	* This method functions by "demoting" queue[k] down the hierarchy
378	* (by swapping it with its smaller child) repeatedly until queue[k]
379	* is less than or equal to its children.
380	*/
381	private void fixDown(int k) {
382	int j;
383	if (comparator == null) {
384	while ((j = k << 1) <= size) {
385	if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
386	j++; // j indexes smallest kid
387	if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
388	break;
389	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
390	k = j;
391	}
392	} else {
393	while ((j = k << 1) <= size) {
394	if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
395	j++; // j indexes smallest kid
396	if (comparator.compare(queue[k], queue[j]) <= 0)
397	break;
398	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
399	k = j;
400	}
401	}
402	}
403
404	/**
405	* Returns the comparator associated with this priority queue, or
406	* <tt>null</tt> if it uses its elements' natural ordering.
407	*
408	* @return the comparator associated with this priority queue, or
409	* <tt>null</tt> if it uses its elements' natural ordering.
410	*/
411	Comparator<E> comparator() {
412	return comparator;
413	}
414	}