java/util/PriorityQueue.java

 package java.util;

/**
 * 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 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.
     */
    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 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:	1.5
Committed:	Tue May 27 18:20:06 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_1, JSR166_PRERELEASE_0_1
Changes since 1.4:	+87 -49 lines
Log Message:	re-checkin initial implementations
#	Content
1	package java.util;
2
3	/**
4	* An unbounded priority queue based on a priority heap. This queue orders
5	* elements according to the order specified at creation time. This order is
6	* specified as for {@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	* these elements are tied for least value, no guarantees are made as to
12	* which of elements is returned.
13	*
14	* <p>Each priority queue has a <i>capacity</i>. The capacity is the size of
15	* the array used 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 list,
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 <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</tt> methods; and constant time for the <tt>peek</tt>,
24	* <tt>element</tt>, and <tt>size</tt> methods.
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
40	* of n, d, 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.
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.
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 the
107	* <tt>Sorted</tt> interface, 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 <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 <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 o 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	* first element returned by this iterator is the same element that
213	* would be returned by a call to <tt>peek</tt>.
214	*
215	* @return an <tt>Iterator</tt> over the elements in this priority queue.
216	*/
217	public Iterator<E> iterator() {
218	return new Itr();
219	}
220
221	private class Itr implements Iterator<E> {
222	/**
223	* Index (into queue array) of element to be returned by
224	* subsequent call to next.
225	*/
226	int cursor = 1;
227
228	/**
229	* Index of element returned by most recent call to next or
230	* previous. Reset to 0 if this element is deleted by a call
231	* to remove.
232	*/
233	int lastRet = 0;
234
235	/**
236	* The modCount value that the iterator believes that the backing
237	* List should have. If this expectation is violated, the iterator
238	* has detected concurrent modification.
239	*/
240	int expectedModCount = modCount;
241
242	public boolean hasNext() {
243	return cursor <= size;
244	}
245
246	public E next() {
247	checkForComodification();
248	if (cursor > size)
249	throw new NoSuchElementException();
250	E result = queue[cursor];
251	lastRet = cursor++;
252	return result;
253	}
254
255	public void remove() {
256	if (lastRet == 0)
257	throw new IllegalStateException();
258	checkForComodification();
259
260	PriorityQueue.this.remove(lastRet);
261	if (lastRet < cursor)
262	cursor--;
263	lastRet = 0;
264	expectedModCount = modCount;
265	}
266
267	final void checkForComodification() {
268	if (modCount != expectedModCount)
269	throw new ConcurrentModificationException();
270	}
271	}
272
273	/**
274	* Returns the number of elements in this priority queue.
275	*
276	* @return the number of elements in this priority queue.
277	*/
278	public int size() {
279	return size;
280	}
281
282	/**
283	* Add the specified element to this priority queue.
284	*
285	* @param element the element to add.
286	* @return true
287	* @throws ClassCastException if the specified element cannot be compared
288	* with elements currently in the priority queue according
289	* to the priority queue's ordering.
290	* @throws NullPointerException if the specified element is null.
291	*/
292	public boolean offer(E element) {
293	if (element == null)
294	throw new NullPointerException();
295	modCount++;
296
297	// Grow backing store if necessary
298	if (++size == queue.length) {
299	E[] newQueue = new E[2 * queue.length];
300	System.arraycopy(queue, 0, newQueue, 0, size);
301	queue = newQueue;
302	}
303
304	queue[size] = element;
305	fixUp(size);
306	return true;
307	}
308
309	/**
310	* Remove all elements from the priority queue.
311	*/
312	public void clear() {
313	modCount++;
314
315	// Null out element references to prevent memory leak
316	for (int i=1; i<=size; i++)
317	queue[i] = null;
318
319	size = 0;
320	}
321
322	/**
323	* Removes and returns the ith element from queue. Recall
324	* that queue is one-based, so 1 <= i <= size.
325	*
326	* XXX: Could further special-case i==size, but is it worth it?
327	* XXX: Could special-case i==0, but is it worth it?
328	*/
329	private E remove(int i) {
330	assert i <= size;
331	modCount++;
332
333	E result = queue[i];
334	queue[i] = queue[size];
335	queue[size--] = null; // Drop extra ref to prevent memory leak
336	if (i <= size)
337	fixDown(i);
338	return result;
339	}
340
341	/**
342	* Establishes the heap invariant (described above) assuming the heap
343	* satisfies the invariant except possibly for the leaf-node indexed by k
344	* (which may have a nextExecutionTime less than its parent's).
345	*
346	* This method functions by "promoting" queue[k] up the hierarchy
347	* (by swapping it with its parent) repeatedly until queue[k]
348	* is greater than or equal to its parent.
349	*/
350	private void fixUp(int k) {
351	if (comparator == null) {
352	while (k > 1) {
353	int j = k >> 1;
354	if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
355	break;
356	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
357	k = j;
358	}
359	} else {
360	while (k > 1) {
361	int j = k >> 1;
362	if (comparator.compare(queue[j], queue[k]) <= 0)
363	break;
364	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
365	k = j;
366	}
367	}
368	}
369
370	/**
371	* Establishes the heap invariant (described above) in the subtree
372	* rooted at k, which is assumed to satisfy the heap invariant except
373	* possibly for node k itself (which may be greater than its children).
374	*
375	* This method functions by "demoting" queue[k] down the hierarchy
376	* (by swapping it with its smaller child) repeatedly until queue[k]
377	* is less than or equal to its children.
378	*/
379	private void fixDown(int k) {
380	int j;
381	if (comparator == null) {
382	while ((j = k << 1) <= size) {
383	if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
384	j++; // j indexes smallest kid
385	if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
386	break;
387	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
388	k = j;
389	}
390	} else {
391	while ((j = k << 1) <= size) {
392	if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
393	j++; // j indexes smallest kid
394	if (comparator.compare(queue[k], queue[j]) <= 0)
395	break;
396	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
397	k = j;
398	}
399	}
400	}
401
402	/**
403	* Returns the comparator associated with this priority queue, or
404	* <tt>null</tt> if it uses its elements' natural ordering.
405	*
406	* @return the comparator associated with this priority queue, or
407	* <tt>null</tt> if it uses its elements' natural ordering.
408	*/
409	Comparator comparator() {
410	return comparator;
411	}
412
413	/**
414	* Save the state of the instance to a stream (that
415	* is, serialize it).
416	*
417	* @serialData The length of the array backing the instance is
418	* emitted (int), followed by all of its elements (each an
419	* <tt>Object</tt>) in the proper order.
420	*/
421	private synchronized void writeObject(java.io.ObjectOutputStream s)
422	throws java.io.IOException{
423	// Write out element count, and any hidden stuff
424	s.defaultWriteObject();
425
426	// Write out array length
427	s.writeInt(queue.length);
428
429	// Write out all elements in the proper order.
430	for (int i=0; i<size; i++)
431	s.writeObject(queue[i]);
432	}
433
434	/**
435	* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
436	* deserialize it).
437	*/
438	private synchronized void readObject(java.io.ObjectInputStream s)
439	throws java.io.IOException, ClassNotFoundException {
440	// Read in size, and any hidden stuff
441	s.defaultReadObject();
442
443	// Read in array length and allocate array
444	int arrayLength = s.readInt();
445	queue = new E[arrayLength];
446
447	// Read in all elements in the proper order.
448	for (int i=0; i<size; i++)
449	queue[i] = (E)s.readObject();
450	}
451
452	}