java/util/PriorityQueue.java

 package java.util;

/**
 * An unbounded priority queue based on a priority heap.  This queue orders
 * elements according to an order specified at construction time, which is
 * specified in the same manner as {@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
 * elements are tied for least value, no guarantees are made as to
 * which of these elements is returned.
 *
 * <p>A priority queue has a <i>capacity</i>.  The capacity is the size of
 * the array used internally 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 queue,
 * 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 insertion methods (<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(Object)</tt> methods; and constant time for the retrieval methods (<tt>peek</tt>,
 * <tt>element</tt>, and <tt>size</tt>).
 *
 * <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 d
     * of n, 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 transient 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 transient int modCount = 0;

    /**
     * Create a new priority queue with the default initial capacity (11)
     * that orders its elements according to their natural ordering (using <tt>Comparable</tt>.)
     */
    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 (using <tt>Comparable</tt>.)
     *
     * @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
     * <tt>Sorted</tt>, 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 return <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 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.
     *
     * @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 element the 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
     * elements of the priority queue will be returned by this iterator in the
     * order specified by the queue, which is to say the order they would be
     * returned by repeated calls to <tt>poll</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 comparator() {
        return comparator;
    }

    /**
     * Save the state of the instance to a stream (that
     * is, serialize it).
     *
     * @serialData The length of the array backing the instance is
     * emitted (int), followed by all of its elements (each an
     * <tt>Object</tt>) in the proper order.
     */
    private synchronized void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        s.defaultWriteObject();

        // Write out array length
        s.writeInt(queue.length);

        // Write out all elements in the proper order.
        for (int i=0; i<size; i++)
            s.writeObject(queue[i]);
    }

    /**
     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
     * deserialize it).
     */
    private synchronized void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in array length and allocate array
        int arrayLength = s.readInt();
        queue = new E[arrayLength];

        // Read in all elements in the proper order.
        for (int i=0; i<size; i++)
            queue[i] = (E)s.readObject();
    }

}
Revision:	1.6
Committed:	Mon Jun 23 02:26:15 2003 UTC (20 years, 10 months ago) by brian
Branch:	MAIN
Changes since 1.5:	+25 -24 lines
Log Message:	Partial javadoc pass
#	Content
1	package java.util;
2
3	/**
4	* An unbounded priority queue based on a priority heap. This queue orders
5	* elements according to an order specified at construction time, which is
6	* specified in the same manner as {@link TreeSet} and {@link TreeMap}: elements are ordered
7	* either according to their <i>natural order</i> (see {@link Comparable}), or
8	* according to a {@link Comparator}, depending on which constructor is used.
9	* The {@link #peek}, {@link #poll}, and {@link #remove} methods return the
10	* minimal element with respect to the specified ordering. If multiple
11	* elements are tied for least value, no guarantees are made as to
12	* which of these elements is returned.
13	*
14	* <p>A priority queue has a <i>capacity</i>. The capacity is the size of
15	* the array used internally to store the elements on the queue. It is always at least
16	* as large as the queue size. As elements are added to a priority queue,
17	* its capacity grows automatically. The details of the growth policy are not
18	* specified.
19	*
20	*<p>Implementation note: this implementation provides O(log(n)) time for
21	* the insertion methods (<tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt>)
22	* methods; linear time for the <tt>remove(Object)</tt> and
23	* <tt>contains(Object)</tt> methods; and constant time for the retrieval methods (<tt>peek</tt>,
24	* <tt>element</tt>, and <tt>size</tt>).
25	*
26	* <p>This class is a member of the
27	* <a href="{@docRoot}/../guide/collections/index.html">
28	* Java Collections Framework</a>.
29	*/
30	public class PriorityQueue<E> extends AbstractQueue<E>
31	implements Queue<E>
32	{
33	private static final int DEFAULT_INITIAL_CAPACITY = 11;
34
35	/**
36	* Priority queue represented as a balanced binary heap: the two children
37	* of queue[n] are queue[2n] and queue[2n + 1]. The priority queue is
38	* ordered by comparator, or by the elements' natural ordering, if
39	* comparator is null: For each node n in the heap and each descendant d
40	* of n, n <= d.
41	*
42	* The element with the lowest value is in queue[1], assuming the queue is
43	* nonempty. (A one-based array is used in preference to the traditional
44	* zero-based array to simplify parent and child calculations.)
45	*
46	* queue.length must be >= 2, even if size == 0.
47	*/
48	private transient E[] queue;
49
50	/**
51	* The number of elements in the priority queue.
52	*/
53	private int size = 0;
54
55	/**
56	* The comparator, or null if priority queue uses elements'
57	* natural ordering.
58	*/
59	private final Comparator<E> comparator;
60
61	/**
62	* The number of times this priority queue has been
63	* <i>structurally modified</i>. See AbstractList for gory details.
64	*/
65	private transient int modCount = 0;
66
67	/**
68	* Create a new priority queue with the default initial capacity (11)
69	* that orders its elements according to their natural ordering (using <tt>Comparable</tt>.)
70	*/
71	public PriorityQueue() {
72	this(DEFAULT_INITIAL_CAPACITY);
73	}
74
75	/**
76	* Create a new priority queue with the specified initial capacity
77	* that orders its elements according to their natural ordering (using <tt>Comparable</tt>.)
78	*
79	* @param initialCapacity the initial capacity for this priority queue.
80	*/
81	public PriorityQueue(int initialCapacity) {
82	this(initialCapacity, null);
83	}
84
85	/**
86	* Create a new priority queue with the specified initial capacity (11)
87	* that orders its elements according to the specified comparator.
88	*
89	* @param initialCapacity the initial capacity for this priority queue.
90	* @param comparator the comparator used to order this priority queue.
91	*/
92	public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
93	if (initialCapacity < 1)
94	initialCapacity = 1;
95	queue = new E[initialCapacity + 1];
96	this.comparator = comparator;
97	}
98
99	/**
100	* Create a new priority queue containing the elements in the specified
101	* collection. The priority queue has an initial capacity of 110% of the
102	* size of the specified collection. If the specified collection
103	* implements the {@link Sorted} interface, the priority queue will be
104	* sorted according to the same comparator, or according to its elements'
105	* natural order if the collection is sorted according to its elements'
106	* natural order. If the specified collection does not implement
107	* <tt>Sorted</tt>, the priority queue is ordered according to
108	* its elements' natural order.
109	*
110	* @param initialElements the collection whose elements are to be placed
111	* into this priority queue.
112	* @throws ClassCastException if elements of the specified collection
113	* cannot be compared to one another according to the priority
114	* queue's ordering.
115	* @throws NullPointerException if the specified collection or an
116	* element of the specified collection is <tt>null</tt>.
117	*/
118	public PriorityQueue(Collection<E> initialElements) {
119	int sz = initialElements.size();
120	int initialCapacity = (int)Math.min((sz * 110L) / 100,
121	Integer.MAX_VALUE - 1);
122	if (initialCapacity < 1)
123	initialCapacity = 1;
124	queue = new E[initialCapacity + 1];
125
126	/* Commented out to compile with generics compiler
127
128	if (initialElements instanceof Sorted) {
129	comparator = ((Sorted)initialElements).comparator();
130	for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
131	queue[++size] = i.next();
132	} else {
133	*/
134	{
135	comparator = null;
136	for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
137	add(i.next());
138	}
139	}
140
141	// Queue Methods
142
143	/**
144	* Remove and return the minimal element from this priority queue if
145	* it contains one or more elements, otherwise return <tt>null</tt>. The term
146	* <i>minimal</i> is defined according to this priority queue's order.
147	*
148	* @return the minimal element from this priority queue if it contains
149	* one or more elements, otherwise <tt>null</tt>.
150	*/
151	public E poll() {
152	if (size == 0)
153	return null;
154	return remove(1);
155	}
156
157	/**
158	* Return, but do not remove, the minimal element from the priority queue,
159	* or return <tt>null</tt> if the queue is empty. The term <i>minimal</i> is
160	* defined according to this priority queue's order. This method returns
161	* the same object reference that would be returned by by the
162	* <tt>poll</tt> method. The two methods differ in that this method
163	* does not remove the element from the priority queue.
164	*
165	* @return the minimal element from this priority queue if it contains
166	* one or more elements, otherwise <tt>null</tt>.
167	*/
168	public E peek() {
169	return queue[1];
170	}
171
172	// Collection Methods
173
174	/**
175	* Removes a single instance of the specified element from this priority
176	* queue, if it is present. Returns true if this collection contained the
177	* specified element (or equivalently, if this collection changed as a
178	* result of the call).
179	*
180	* @param element the element to be removed from this collection, if present.
181	* @return <tt>true</tt> if this collection changed as a result of the
182	* call
183	* @throws ClassCastException if the specified element cannot be compared
184	* with elements currently in the priority queue according
185	* to the priority queue's ordering.
186	* @throws NullPointerException if the specified element is null.
187	*/
188	public boolean remove(Object element) {
189	if (element == null)
190	throw new NullPointerException();
191
192	if (comparator == null) {
193	for (int i = 1; i <= size; i++) {
194	if (((Comparable)queue[i]).compareTo(element) == 0) {
195	remove(i);
196	return true;
197	}
198	}
199	} else {
200	for (int i = 1; i <= size; i++) {
201	if (comparator.compare(queue[i], (E) element) == 0) {
202	remove(i);
203	return true;
204	}
205	}
206	}
207	return false;
208	}
209
210	/**
211	* Returns an iterator over the elements in this priority queue. The
212	* elements of the priority queue will be returned by this iterator in the
213	* order specified by the queue, which is to say the order they would be
214	* returned by repeated calls to <tt>poll</tt>.
215	*
216	* @return an <tt>Iterator</tt> over the elements in this priority queue.
217	*/
218	public Iterator<E> iterator() {
219	return new Itr();
220	}
221
222	private class Itr implements Iterator<E> {
223	/**
224	* Index (into queue array) of element to be returned by
225	* subsequent call to next.
226	*/
227	int cursor = 1;
228
229	/**
230	* Index of element returned by most recent call to next or
231	* previous. Reset to 0 if this element is deleted by a call
232	* to remove.
233	*/
234	int lastRet = 0;
235
236	/**
237	* The modCount value that the iterator believes that the backing
238	* List should have. If this expectation is violated, the iterator
239	* has detected concurrent modification.
240	*/
241	int expectedModCount = modCount;
242
243	public boolean hasNext() {
244	return cursor <= size;
245	}
246
247	public E next() {
248	checkForComodification();
249	if (cursor > size)
250	throw new NoSuchElementException();
251	E result = queue[cursor];
252	lastRet = cursor++;
253	return result;
254	}
255
256	public void remove() {
257	if (lastRet == 0)
258	throw new IllegalStateException();
259	checkForComodification();
260
261	PriorityQueue.this.remove(lastRet);
262	if (lastRet < cursor)
263	cursor--;
264	lastRet = 0;
265	expectedModCount = modCount;
266	}
267
268	final void checkForComodification() {
269	if (modCount != expectedModCount)
270	throw new ConcurrentModificationException();
271	}
272	}
273
274	/**
275	* Returns the number of elements in this priority queue.
276	*
277	* @return the number of elements in this priority queue.
278	*/
279	public int size() {
280	return size;
281	}
282
283	/**
284	* Add the specified element to this priority queue.
285	*
286	* @param element the element to add.
287	* @return true
288	* @throws ClassCastException if the specified element cannot be compared
289	* with elements currently in the priority queue according
290	* to the priority queue's ordering.
291	* @throws NullPointerException if the specified element is null.
292	*/
293	public boolean offer(E element) {
294	if (element == null)
295	throw new NullPointerException();
296	modCount++;
297
298	// Grow backing store if necessary
299	if (++size == queue.length) {
300	E[] newQueue = new E[2 * queue.length];
301	System.arraycopy(queue, 0, newQueue, 0, size);
302	queue = newQueue;
303	}
304
305	queue[size] = element;
306	fixUp(size);
307	return true;
308	}
309
310	/**
311	* Remove all elements from the priority queue.
312	*/
313	public void clear() {
314	modCount++;
315
316	// Null out element references to prevent memory leak
317	for (int i=1; i<=size; i++)
318	queue[i] = null;
319
320	size = 0;
321	}
322
323	/**
324	* Removes and returns the ith element from queue. Recall
325	* that queue is one-based, so 1 <= i <= size.
326	*
327	* XXX: Could further special-case i==size, but is it worth it?
328	* XXX: Could special-case i==0, but is it worth it?
329	*/
330	private E remove(int i) {
331	assert i <= size;
332	modCount++;
333
334	E result = queue[i];
335	queue[i] = queue[size];
336	queue[size--] = null; // Drop extra ref to prevent memory leak
337	if (i <= size)
338	fixDown(i);
339	return result;
340	}
341
342	/**
343	* Establishes the heap invariant (described above) assuming the heap
344	* satisfies the invariant except possibly for the leaf-node indexed by k
345	* (which may have a nextExecutionTime less than its parent's).
346	*
347	* This method functions by "promoting" queue[k] up the hierarchy
348	* (by swapping it with its parent) repeatedly until queue[k]
349	* is greater than or equal to its parent.
350	*/
351	private void fixUp(int k) {
352	if (comparator == null) {
353	while (k > 1) {
354	int j = k >> 1;
355	if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
356	break;
357	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
358	k = j;
359	}
360	} else {
361	while (k > 1) {
362	int j = k >> 1;
363	if (comparator.compare(queue[j], queue[k]) <= 0)
364	break;
365	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
366	k = j;
367	}
368	}
369	}
370
371	/**
372	* Establishes the heap invariant (described above) in the subtree
373	* rooted at k, which is assumed to satisfy the heap invariant except
374	* possibly for node k itself (which may be greater than its children).
375	*
376	* This method functions by "demoting" queue[k] down the hierarchy
377	* (by swapping it with its smaller child) repeatedly until queue[k]
378	* is less than or equal to its children.
379	*/
380	private void fixDown(int k) {
381	int j;
382	if (comparator == null) {
383	while ((j = k << 1) <= size) {
384	if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
385	j++; // j indexes smallest kid
386	if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
387	break;
388	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
389	k = j;
390	}
391	} else {
392	while ((j = k << 1) <= size) {
393	if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
394	j++; // j indexes smallest kid
395	if (comparator.compare(queue[k], queue[j]) <= 0)
396	break;
397	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
398	k = j;
399	}
400	}
401	}
402
403	/**
404	* Returns the comparator associated with this priority queue, or
405	* <tt>null</tt> if it uses its elements' natural ordering.
406	*
407	* @return the comparator associated with this priority queue, or
408	* <tt>null</tt> if it uses its elements' natural ordering.
409	*/
410	Comparator comparator() {
411	return comparator;
412	}
413
414	/**
415	* Save the state of the instance to a stream (that
416	* is, serialize it).
417	*
418	* @serialData The length of the array backing the instance is
419	* emitted (int), followed by all of its elements (each an
420	* <tt>Object</tt>) in the proper order.
421	*/
422	private synchronized void writeObject(java.io.ObjectOutputStream s)
423	throws java.io.IOException{
424	// Write out element count, and any hidden stuff
425	s.defaultWriteObject();
426
427	// Write out array length
428	s.writeInt(queue.length);
429
430	// Write out all elements in the proper order.
431	for (int i=0; i<size; i++)
432	s.writeObject(queue[i]);
433	}
434
435	/**
436	* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
437	* deserialize it).
438	*/
439	private synchronized void readObject(java.io.ObjectInputStream s)
440	throws java.io.IOException, ClassNotFoundException {
441	// Read in size, and any hidden stuff
442	s.defaultReadObject();
443
444	// Read in array length and allocate array
445	int arrayLength = s.readInt();
446	queue = new E[arrayLength];
447
448	// Read in all elements in the proper order.
449	for (int i=0; i<size; i++)
450	queue[i] = (E)s.readObject();
451	}
452
453	}