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 <em>head</em> of this queue is the least element with respect to the
 * specified ordering. If multiple elements are tied for least value, the
 * head is one of those elements. A priority queue does not permit
 * <tt>null</tt> elements.
 *
 * <p>The {@link #remove()} and {@link #poll()} methods remove and
 * return the head of the queue.
 *
 * <p>The {@link #element()} and {@link #peek()} methods return, but do
 * not delete, the head of the queue.
 *
 * <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>.
 * @since 1.5
 * @author Josh Bloch
 */
public class PriorityQueue<E> extends AbstractQueue<E>
    implements Queue<E>, java.io.Serializable {

    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 <tt>PriorityQueue</tt> 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, null);
    }

    /**
     * Create a <tt>PriorityQueue</tt> 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 <tt>PriorityQueue</tt> with the specified initial capacity
     * 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.
     * If <tt>null</tt> then the order depends on the elements' natural
     * ordering.
     * @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
     * than 1
     */
    public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
        if (initialCapacity < 1)
            throw new IllegalArgumentException();
        queue = (E[]) new Object[initialCapacity + 1];
        this.comparator = comparator;
    }

    /**
     * Create a <tt>PriorityQueue</tt> containing the elements in the specified
     * collection.  The priority queue has an initial capacity of 110% of the
     * size of the specified collection; or 1 if the collection is empty. 
     * 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 c 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 <tt>c</tt> or any element within it
     * is <tt>null</tt>
     */
    public PriorityQueue(Collection<E> c) {
        int sz = c.size();
        int initialCapacity = (int)Math.min((sz * 110L) / 100,
                                            Integer.MAX_VALUE - 1);
        if (initialCapacity < 1)
            initialCapacity = 1;

        queue = (E[]) new Object[initialCapacity + 1];

        if (c instanceof Sorted) {
            // FIXME: this code assumes too much
            comparator = ((Sorted)c).comparator();
            for (Iterator<E> i = c.iterator(); i.hasNext(); )
                queue[++size] = i.next();
        } else {
            comparator = null;
            for (Iterator<E> i = c.iterator(); i.hasNext(); )
                add(i.next());
        }
    }

    // Queue Methods

    /**
     * Add the specified element to this priority queue.
     *
     * @param element the element to add.
     * @return <tt>true</tt>
     * @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++;
        ++size;

        // Grow backing store if necessary
        while (size >= queue.length) {
            E[] newQueue = (E[]) new Object[2 * queue.length];
            System.arraycopy(queue, 0, newQueue, 0, queue.length);
            queue = newQueue;
        }

        queue[size] = element;
        fixUp(size);
        return true;
    }

    public E poll() {
        if (size == 0)
            return null;
        return remove(1);
    }

    public E peek() {
        return queue[1];
    }

    // Collection Methods

    // these first two override just to get the throws docs

    /**
     * @throws NullPointerException if the specified element is <tt>null</tt>.
     * @throws ClassCastException if the specified element cannot be compared
     * with elements currently in the priority queue according
     * to the priority queue's ordering.
     */
    public boolean add(E element) {
        return super.add(element);
    }

    /**
     * @throws NullPointerException if any element is <tt>null</tt>.
     * @throws ClassCastException if any element cannot be compared
     * with elements currently in the priority queue according
     * to the priority queue's ordering.
     */
    public boolean addAll(Collection<? extends E> c) {
        return super.addAll(c);
    }

    public boolean remove(Object o) {
        if (o == null)
            return false;

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

    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.
         */
        private 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.
         */
        private 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.
         */
        private 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;
    }

    /**
     * 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;
            }
        }
    }

    public 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.
     * @param s the stream
     */
    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).
     * @param s the stream
     */
    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 = (E[]) new Object[arrayLength];

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

}

Revision:	1.15
Committed:	Thu Jul 31 07:18:02 2003 UTC (20 years, 9 months ago) by dholmes
Branch:	MAIN
Changes since 1.14:	+23 -23 lines
Log Message:	Continued updates to explicit and inherited doc comments. Consistency over remove(null) Some inherited doc is still not right.
#	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}:
7	* elements are ordered
8	* either according to their <i>natural order</i> (see {@link Comparable}), or
9	* according to a {@link Comparator}, depending on which constructor is used.
10	* The <em>head</em> of this queue is the least element with respect to the
11	* specified ordering. If multiple elements are tied for least value, the
12	* head is one of those elements. A priority queue does not permit
13	* <tt>null</tt> elements.
14	*
15	* <p>The {@link #remove()} and {@link #poll()} methods remove and
16	* return the head of the queue.
17	*
18	* <p>The {@link #element()} and {@link #peek()} methods return, but do
19	* not delete, the head of the queue.
20	*
21	* <p>A priority queue has a <i>capacity</i>. The capacity is the
22	* size of the array used internally to store the elements on the
23	* queue. It is always at least as large as the queue size. As
24	* elements are added to a priority queue, its capacity grows
25	* automatically. The details of the growth policy are not specified.
26	*
27	* <p>Implementation note: this implementation provides O(log(n)) time
28	* for the insertion methods (<tt>offer</tt>, <tt>poll</tt>,
29	* <tt>remove()</tt> and <tt>add</tt>) methods; linear time for the
30	* <tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and
31	* constant time for the retrieval methods (<tt>peek</tt>,
32	* <tt>element</tt>, and <tt>size</tt>).
33	*
34	* <p>This class is a member of the
35	* <a href="{@docRoot}/../guide/collections/index.html">
36	* Java Collections Framework</a>.
37	* @since 1.5
38	* @author Josh Bloch
39	*/
40	public class PriorityQueue<E> extends AbstractQueue<E>
41	implements Queue<E>, java.io.Serializable {
42
43	private static final int DEFAULT_INITIAL_CAPACITY = 11;
44
45	/**
46	* Priority queue represented as a balanced binary heap: the two children
47	* of queue[n] are queue[2n] and queue[2n + 1]. The priority queue is
48	* ordered by comparator, or by the elements' natural ordering, if
49	* comparator is null: For each node n in the heap and each descendant d
50	* of n, n <= d.
51	*
52	* The element with the lowest value is in queue[1], assuming the queue is
53	* nonempty. (A one-based array is used in preference to the traditional
54	* zero-based array to simplify parent and child calculations.)
55	*
56	* queue.length must be >= 2, even if size == 0.
57	*/
58	private transient E[] queue;
59
60	/**
61	* The number of elements in the priority queue.
62	*/
63	private int size = 0;
64
65	/**
66	* The comparator, or null if priority queue uses elements'
67	* natural ordering.
68	*/
69	private final Comparator<E> comparator;
70
71	/**
72	* The number of times this priority queue has been
73	* <i>structurally modified</i>. See AbstractList for gory details.
74	*/
75	private transient int modCount = 0;
76
77	/**
78	* Create a <tt>PriorityQueue</tt> with the default initial capacity
79	* (11) that orders its elements according to their natural
80	* ordering (using <tt>Comparable</tt>.)
81	*/
82	public PriorityQueue() {
83	this(DEFAULT_INITIAL_CAPACITY, null);
84	}
85
86	/**
87	* Create a <tt>PriorityQueue</tt> with the specified initial capacity
88	* that orders its elements according to their natural ordering
89	* (using <tt>Comparable</tt>.)
90	*
91	* @param initialCapacity the initial capacity for this priority queue.
92	*/
93	public PriorityQueue(int initialCapacity) {
94	this(initialCapacity, null);
95	}
96
97	/**
98	* Create a <tt>PriorityQueue</tt> with the specified initial capacity
99	* that orders its elements according to the specified comparator.
100	*
101	* @param initialCapacity the initial capacity for this priority queue.
102	* @param comparator the comparator used to order this priority queue.
103	* If <tt>null</tt> then the order depends on the elements' natural
104	* ordering.
105	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
106	* than 1
107	*/
108	public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
109	if (initialCapacity < 1)
110	throw new IllegalArgumentException();
111	queue = (E[]) new Object[initialCapacity + 1];
112	this.comparator = comparator;
113	}
114
115	/**
116	* Create a <tt>PriorityQueue</tt> containing the elements in the specified
117	* collection. The priority queue has an initial capacity of 110% of the
118	* size of the specified collection; or 1 if the collection is empty.
119	* If the specified collection
120	* implements the {@link Sorted} interface, the priority queue will be
121	* sorted according to the same comparator, or according to its elements'
122	* natural order if the collection is sorted according to its elements'
123	* natural order. If the specified collection does not implement
124	* <tt>Sorted</tt>, the priority queue is ordered according to
125	* its elements' natural order.
126	*
127	* @param c the collection whose elements are to be placed
128	* into this priority queue.
129	* @throws ClassCastException if elements of the specified collection
130	* cannot be compared to one another according to the priority
131	* queue's ordering.
132	* @throws NullPointerException if <tt>c</tt> or any element within it
133	* is <tt>null</tt>
134	*/
135	public PriorityQueue(Collection<E> c) {
136	int sz = c.size();
137	int initialCapacity = (int)Math.min((sz * 110L) / 100,
138	Integer.MAX_VALUE - 1);
139	if (initialCapacity < 1)
140	initialCapacity = 1;
141
142	queue = (E[]) new Object[initialCapacity + 1];
143
144	if (c instanceof Sorted) {
145	// FIXME: this code assumes too much
146	comparator = ((Sorted)c).comparator();
147	for (Iterator<E> i = c.iterator(); i.hasNext(); )
148	queue[++size] = i.next();
149	} else {
150	comparator = null;
151	for (Iterator<E> i = c.iterator(); i.hasNext(); )
152	add(i.next());
153	}
154	}
155
156	// Queue Methods
157
158	/**
159	* Add the specified element to this priority queue.
160	*
161	* @param element the element to add.
162	* @return <tt>true</tt>
163	* @throws ClassCastException if the specified element cannot be compared
164	* with elements currently in the priority queue according
165	* to the priority queue's ordering.
166	* @throws NullPointerException if the specified element is null.
167	*/
168	public boolean offer(E element) {
169	if (element == null)
170	throw new NullPointerException();
171	modCount++;
172	++size;
173
174	// Grow backing store if necessary
175	while (size >= queue.length) {
176	E[] newQueue = (E[]) new Object[2 * queue.length];
177	System.arraycopy(queue, 0, newQueue, 0, queue.length);
178	queue = newQueue;
179	}
180
181	queue[size] = element;
182	fixUp(size);
183	return true;
184	}
185
186	public E poll() {
187	if (size == 0)
188	return null;
189	return remove(1);
190	}
191
192	public E peek() {
193	return queue[1];
194	}
195
196	// Collection Methods
197
198	// these first two override just to get the throws docs
199
200	/**
201	* @throws NullPointerException if the specified element is <tt>null</tt>.
202	* @throws ClassCastException if the specified element cannot be compared
203	* with elements currently in the priority queue according
204	* to the priority queue's ordering.
205	*/
206	public boolean add(E element) {
207	return super.add(element);
208	}
209
210	/**
211	* @throws NullPointerException if any element is <tt>null</tt>.
212	* @throws ClassCastException if any element cannot be compared
213	* with elements currently in the priority queue according
214	* to the priority queue's ordering.
215	*/
216	public boolean addAll(Collection<? extends E> c) {
217	return super.addAll(c);
218	}
219
220	public boolean remove(Object o) {
221	if (o == null)
222	return false;
223
224	if (comparator == null) {
225	for (int i = 1; i <= size; i++) {
226	if (((Comparable)queue[i]).compareTo(o) == 0) {
227	remove(i);
228	return true;
229	}
230	}
231	} else {
232	for (int i = 1; i <= size; i++) {
233	if (comparator.compare(queue[i], (E)o) == 0) {
234	remove(i);
235	return true;
236	}
237	}
238	}
239	return false;
240	}
241
242	public Iterator<E> iterator() {
243	return new Itr();
244	}
245
246	private class Itr implements Iterator<E> {
247	/**
248	* Index (into queue array) of element to be returned by
249	* subsequent call to next.
250	*/
251	private int cursor = 1;
252
253	/**
254	* Index of element returned by most recent call to next or
255	* previous. Reset to 0 if this element is deleted by a call
256	* to remove.
257	*/
258	private int lastRet = 0;
259
260	/**
261	* The modCount value that the iterator believes that the backing
262	* List should have. If this expectation is violated, the iterator
263	* has detected concurrent modification.
264	*/
265	private int expectedModCount = modCount;
266
267	public boolean hasNext() {
268	return cursor <= size;
269	}
270
271	public E next() {
272	checkForComodification();
273	if (cursor > size)
274	throw new NoSuchElementException();
275	E result = queue[cursor];
276	lastRet = cursor++;
277	return result;
278	}
279
280	public void remove() {
281	if (lastRet == 0)
282	throw new IllegalStateException();
283	checkForComodification();
284
285	PriorityQueue.this.remove(lastRet);
286	if (lastRet < cursor)
287	cursor--;
288	lastRet = 0;
289	expectedModCount = modCount;
290	}
291
292	final void checkForComodification() {
293	if (modCount != expectedModCount)
294	throw new ConcurrentModificationException();
295	}
296	}
297
298	/**
299	* Returns the number of elements in this priority queue.
300	*
301	* @return the number of elements in this priority queue.
302	*/
303	public int size() {
304	return size;
305	}
306
307	/**
308	* Remove all elements from the priority queue.
309	*/
310	public void clear() {
311	modCount++;
312
313	// Null out element references to prevent memory leak
314	for (int i=1; i<=size; i++)
315	queue[i] = null;
316
317	size = 0;
318	}
319
320	/**
321	* Removes and returns the ith element from queue. Recall
322	* that queue is one-based, so 1 <= i <= size.
323	*
324	* XXX: Could further special-case i==size, but is it worth it?
325	* XXX: Could special-case i==0, but is it worth it?
326	*/
327	private E remove(int i) {
328	assert i <= size;
329	modCount++;
330
331	E result = queue[i];
332	queue[i] = queue[size];
333	queue[size--] = null; // Drop extra ref to prevent memory leak
334	if (i <= size)
335	fixDown(i);
336	return result;
337	}
338
339	/**
340	* Establishes the heap invariant (described above) assuming the heap
341	* satisfies the invariant except possibly for the leaf-node indexed by k
342	* (which may have a nextExecutionTime less than its parent's).
343	*
344	* This method functions by "promoting" queue[k] up the hierarchy
345	* (by swapping it with its parent) repeatedly until queue[k]
346	* is greater than or equal to its parent.
347	*/
348	private void fixUp(int k) {
349	if (comparator == null) {
350	while (k > 1) {
351	int j = k >> 1;
352	if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
353	break;
354	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
355	k = j;
356	}
357	} else {
358	while (k > 1) {
359	int j = k >> 1;
360	if (comparator.compare(queue[j], queue[k]) <= 0)
361	break;
362	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
363	k = j;
364	}
365	}
366	}
367
368	/**
369	* Establishes the heap invariant (described above) in the subtree
370	* rooted at k, which is assumed to satisfy the heap invariant except
371	* possibly for node k itself (which may be greater than its children).
372	*
373	* This method functions by "demoting" queue[k] down the hierarchy
374	* (by swapping it with its smaller child) repeatedly until queue[k]
375	* is less than or equal to its children.
376	*/
377	private void fixDown(int k) {
378	int j;
379	if (comparator == null) {
380	while ((j = k << 1) <= size) {
381	if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
382	j++; // j indexes smallest kid
383	if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
384	break;
385	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
386	k = j;
387	}
388	} else {
389	while ((j = k << 1) <= size) {
390	if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
391	j++; // j indexes smallest kid
392	if (comparator.compare(queue[k], queue[j]) <= 0)
393	break;
394	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
395	k = j;
396	}
397	}
398	}
399
400	public Comparator comparator() {
401	return comparator;
402	}
403
404	/**
405	* Save the state of the instance to a stream (that
406	* is, serialize it).
407	*
408	* @serialData The length of the array backing the instance is
409	* emitted (int), followed by all of its elements (each an
410	* <tt>Object</tt>) in the proper order.
411	* @param s the stream
412	*/
413	private synchronized void writeObject(java.io.ObjectOutputStream s)
414	throws java.io.IOException{
415	// Write out element count, and any hidden stuff
416	s.defaultWriteObject();
417
418	// Write out array length
419	s.writeInt(queue.length);
420
421	// Write out all elements in the proper order.
422	for (int i=0; i<size; i++)
423	s.writeObject(queue[i]);
424	}
425
426	/**
427	* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
428	* deserialize it).
429	* @param s the stream
430	*/
431	private synchronized void readObject(java.io.ObjectInputStream s)
432	throws java.io.IOException, ClassNotFoundException {
433	// Read in size, and any hidden stuff
434	s.defaultReadObject();
435
436	// Read in array length and allocate array
437	int arrayLength = s.readInt();
438	queue = (E[]) new Object[arrayLength];
439
440	// Read in all elements in the proper order.
441	for (int i=0; i<size; i++)
442	queue[i] = (E)s.readObject();
443	}
444
445	}
446