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];

        /* Commented out to compile with generics compiler

        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:	1.3
Committed:	Sun May 18 20:36:01 2003 UTC (20 years, 11 months ago) by tim
Branch:	MAIN
Changes since 1.2:	+1 -1 lines
Log Message:	Added another type parameter
#	User	Rev	Content
1	tim	1.2	package java.util;
2	tim	1.1
3	tim	1.2	/*
4			* Todo
5			*
6			* 1) Make it serializable.
7			*/
8	tim	1.1
9			/**
10	tim	1.2	* 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	tim	1.1
41	tim	1.2	/**
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	tim	1.1
56	tim	1.2	/**
57			* The number of elements in the priority queue.
58			*/
59			private int size = 0;
60	tim	1.1
61	tim	1.2	/**
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	tim	1.1	}
80	tim	1.2
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	tim	1.1	}
90	tim	1.2
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	tim	1.1	}
104
105	tim	1.2	/**
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			/* Commented out to compile with generics compiler
133
134			if (initialElements instanceof Sorted) {
135			comparator = ((Sorted)initialElements).comparator();
136			for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
137			queue[++size] = i.next();
138			} else {
139			*/
140			{
141			comparator = null;
142			for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
143			add(i.next());
144			}
145	tim	1.1	}
146
147	tim	1.2	// Queue Methods
148
149			/**
150			* Remove and return the minimal element from this priority queue if
151			* it contains one or more elements, otherwise <tt>null</tt>. The term
152			* <i>minimal</i> is defined according to this priority queue's order.
153			*
154			* @return the minimal element from this priority queue if it contains
155			* one or more elements, otherwise <tt>null</tt>.
156			*/
157	tim	1.1	public E poll() {
158	tim	1.2	if (size == 0)
159			return null;
160			return remove(1);
161	tim	1.1	}
162	tim	1.2
163			/**
164			* Return, but do not remove, the minimal element from the priority queue,
165			* or <tt>null</tt> if the queue is empty. The term <i>minimal</i> is
166			* defined according to this priority queue's order. This method returns
167			* the same object reference that would be returned by by the
168			* <tt>poll</tt> method. The two methods differ in that this method
169			* does not remove the element from the priority queue.
170			*
171			* @return the minimal element from this priority queue if it contains
172			* one or more elements, otherwise <tt>null</tt>.
173			*/
174	tim	1.1	public E peek() {
175	tim	1.2	return queue[1];
176	tim	1.1	}
177
178	tim	1.2	// Collection Methods
179
180			/**
181			* Removes a single instance of the specified element from this priority
182			* queue, if it is present. Returns true if this collection contained the
183			* specified element (or equivalently, if this collection changed as a
184			* result of the call).
185			*
186			* @param o element to be removed from this collection, if present.
187			* @return <tt>true</tt> if this collection changed as a result of the
188			* call
189			* @throws ClassCastException if the specified element cannot be compared
190			* with elements currently in the priority queue according
191			* to the priority queue's ordering.
192			* @throws NullPointerException if the specified element is null.
193			*/
194			public boolean remove(Object element) {
195			if (element == null)
196			throw new NullPointerException();
197
198			if (comparator == null) {
199			for (int i = 1; i <= size; i++) {
200			if (((Comparable)queue[i]).compareTo(element) == 0) {
201			remove(i);
202			return true;
203			}
204			}
205			} else {
206			for (int i = 1; i <= size; i++) {
207			if (comparator.compare(queue[i], (E) element) == 0) {
208			remove(i);
209			return true;
210			}
211			}
212			}
213	tim	1.1	return false;
214			}
215	tim	1.2
216			/**
217			* Returns an iterator over the elements in this priority queue. The
218			* first element returned by this iterator is the same element that
219			* would be returned by a call to <tt>peek</tt>.
220			*
221			* @return an <tt>Iterator</tt> over the elements in this priority queue.
222			*/
223			public Iterator<E> iterator() {
224			return new Itr();
225			}
226
227			private class Itr implements Iterator<E> {
228			/**
229			* Index (into queue array) of element to be returned by
230			* subsequent call to next.
231			*/
232			int cursor = 1;
233
234			/**
235			* Index of element returned by most recent call to next or
236			* previous. Reset to 0 if this element is deleted by a call
237			* to remove.
238			*/
239			int lastRet = 0;
240
241			/**
242			* The modCount value that the iterator believes that the backing
243			* List should have. If this expectation is violated, the iterator
244			* has detected concurrent modification.
245			*/
246			int expectedModCount = modCount;
247
248			public boolean hasNext() {
249			return cursor <= size;
250			}
251
252			public E next() {
253			checkForComodification();
254			if (cursor > size)
255			throw new NoSuchElementException();
256			E result = queue[cursor];
257			lastRet = cursor++;
258			return result;
259			}
260
261			public void remove() {
262			if (lastRet == 0)
263			throw new IllegalStateException();
264			checkForComodification();
265
266			PriorityQueue.this.remove(lastRet);
267			if (lastRet < cursor)
268			cursor--;
269			lastRet = 0;
270			expectedModCount = modCount;
271			}
272
273			final void checkForComodification() {
274			if (modCount != expectedModCount)
275			throw new ConcurrentModificationException();
276			}
277			}
278
279			/**
280			* Returns the number of elements in this priority queue.
281			*
282			* @return the number of elements in this priority queue.
283			*/
284	tim	1.1	public int size() {
285	tim	1.2	return size;
286	tim	1.1	}
287	tim	1.2
288			/**
289			* Add the specified element to this priority queue.
290			*
291			* @param element the element to add.
292			* @return true
293			* @throws ClassCastException if the specified element cannot be compared
294			* with elements currently in the priority queue according
295			* to the priority queue's ordering.
296			* @throws NullPointerException if the specified element is null.
297			*/
298			public boolean offer(E element) {
299			if (element == null)
300			throw new NullPointerException();
301			modCount++;
302
303			// Grow backing store if necessary
304			if (++size == queue.length) {
305			E[] newQueue = new E[2 * queue.length];
306			System.arraycopy(queue, 0, newQueue, 0, size);
307			queue = newQueue;
308			}
309
310			queue[size] = element;
311			fixUp(size);
312			return true;
313	tim	1.1	}
314
315	tim	1.2	/**
316			* Remove all elements from the priority queue.
317			*/
318			public void clear() {
319			modCount++;
320
321			// Null out element references to prevent memory leak
322			for (int i=1; i<=size; i++)
323			queue[i] = null;
324
325			size = 0;
326			}
327
328			/**
329			* Removes and returns the ith element from queue. Recall
330			* that queue is one-based, so 1 <= i <= size.
331			*
332			* XXX: Could further special-case i==size, but is it worth it?
333			* XXX: Could special-case i==0, but is it worth it?
334			*/
335			private E remove(int i) {
336			assert i <= size;
337			modCount++;
338
339			E result = queue[i];
340			queue[i] = queue[size];
341			queue[size--] = null; // Drop extra ref to prevent memory leak
342			if (i <= size)
343			fixDown(i);
344			return result;
345	tim	1.1	}
346
347	tim	1.2	/**
348			* Establishes the heap invariant (described above) assuming the heap
349			* satisfies the invariant except possibly for the leaf-node indexed by k
350			* (which may have a nextExecutionTime less than its parent's).
351			*
352			* This method functions by "promoting" queue[k] up the hierarchy
353			* (by swapping it with its parent) repeatedly until queue[k]
354			* is greater than or equal to its parent.
355			*/
356			private void fixUp(int k) {
357			if (comparator == null) {
358			while (k > 1) {
359			int j = k >> 1;
360			if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
361			break;
362			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
363			k = j;
364			}
365			} else {
366			while (k > 1) {
367			int j = k >> 1;
368			if (comparator.compare(queue[j], queue[k]) <= 0)
369			break;
370			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
371			k = j;
372			}
373			}
374			}
375
376			/**
377			* Establishes the heap invariant (described above) in the subtree
378			* rooted at k, which is assumed to satisfy the heap invariant except
379			* possibly for node k itself (which may be greater than its children).
380			*
381			* This method functions by "demoting" queue[k] down the hierarchy
382			* (by swapping it with its smaller child) repeatedly until queue[k]
383			* is less than or equal to its children.
384			*/
385			private void fixDown(int k) {
386			int j;
387			if (comparator == null) {
388			while ((j = k << 1) <= size) {
389			if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
390			j++; // j indexes smallest kid
391			if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
392			break;
393			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
394			k = j;
395			}
396			} else {
397			while ((j = k << 1) <= size) {
398			if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
399			j++; // j indexes smallest kid
400			if (comparator.compare(queue[k], queue[j]) <= 0)
401			break;
402			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
403			k = j;
404			}
405			}
406			}
407
408			/**
409			* Returns the comparator associated with this priority queue, or
410			* <tt>null</tt> if it uses its elements' natural ordering.
411			*
412			* @return the comparator associated with this priority queue, or
413			* <tt>null</tt> if it uses its elements' natural ordering.
414			*/
415	tim	1.3	Comparator<E> comparator() {
416	tim	1.2	return comparator;
417			}
418	tim	1.1	}