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>.
 * @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 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 = (E[]) new Object[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 = (E[]) new Object[initialCapacity + 1];


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

    // Queue Methods

    /**
     * Remove and return the minimal element from this priority queue
     * if it contains one or more elements, otherwise 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.
         */
        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;
    }

    /**
     * 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++;
        ++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;
    }

    /**
     * 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.
     */
    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:	1.10
Committed:	Sat Jul 26 13:17:51 2003 UTC (20 years, 9 months ago) by tim
Branch:	MAIN
Changes since 1.9:	+7 -7 lines
Log Message:	Default compiler is now 2.2-ea. Some sources are not compatible with 2.0-ea.
#	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
15	* size of the array used internally to store the elements on the
16	* queue. It is always at least as large as the queue size. As
17	* elements are added to a priority queue, its capacity grows
18	* automatically. The details of the growth policy are not specified.
19	*
20	*<p>Implementation note: this implementation provides O(log(n)) time
21	*for the insertion methods (<tt>offer</tt>, <tt>poll</tt>,
22	*<tt>remove()</tt> and <tt>add</tt>) methods; linear time for the
23	*<tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and
24	*constant time for the retrieval methods (<tt>peek</tt>,
25	*<tt>element</tt>, and <tt>size</tt>).
26	*
27	* <p>This class is a member of the
28	* <a href="{@docRoot}/../guide/collections/index.html">
29	* Java Collections Framework</a>.
30	* @since 1.5
31	* @author Josh Bloch
32	*/
33	public class PriorityQueue<E> extends AbstractQueue<E>
34	implements Queue<E>,
35	java.io.Serializable {
36	private static final int DEFAULT_INITIAL_CAPACITY = 11;
37
38	/**
39	* Priority queue represented as a balanced binary heap: the two children
40	* of queue[n] are queue[2n] and queue[2n + 1]. The priority queue is
41	* ordered by comparator, or by the elements' natural ordering, if
42	* comparator is null: For each node n in the heap and each descendant d
43	* of n, n <= d.
44	*
45	* The element with the lowest value is in queue[1], assuming the queue is
46	* nonempty. (A one-based array is used in preference to the traditional
47	* zero-based array to simplify parent and child calculations.)
48	*
49	* queue.length must be >= 2, even if size == 0.
50	*/
51	private transient E[] queue;
52
53	/**
54	* The number of elements in the priority queue.
55	*/
56	private int size = 0;
57
58	/**
59	* The comparator, or null if priority queue uses elements'
60	* natural ordering.
61	*/
62	private final Comparator<E> comparator;
63
64	/**
65	* The number of times this priority queue has been
66	* <i>structurally modified</i>. See AbstractList for gory details.
67	*/
68	private transient int modCount = 0;
69
70	/**
71	* Create a new priority queue with the default initial capacity
72	* (11) that orders its elements according to their natural
73	* ordering (using <tt>Comparable</tt>.)
74	*/
75	public PriorityQueue() {
76	this(DEFAULT_INITIAL_CAPACITY);
77	}
78
79	/**
80	* Create a new priority queue with the specified initial capacity
81	* that orders its elements according to their natural ordering
82	* (using <tt>Comparable</tt>.)
83	*
84	* @param initialCapacity the initial capacity for this priority queue.
85	*/
86	public PriorityQueue(int initialCapacity) {
87	this(initialCapacity, null);
88	}
89
90	/**
91	* Create a new priority queue with the specified initial capacity (11)
92	* that orders its elements according to the specified comparator.
93	*
94	* @param initialCapacity the initial capacity for this priority queue.
95	* @param comparator the comparator used to order this priority queue.
96	*/
97	public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
98	if (initialCapacity < 1)
99	initialCapacity = 1;
100	queue = (E[]) new Object[initialCapacity + 1];
101	this.comparator = comparator;
102	}
103
104	/**
105	* Create a new priority queue containing the elements in the specified
106	* collection. The priority queue has an initial capacity of 110% of the
107	* size of the specified collection. If the specified collection
108	* implements the {@link Sorted} interface, the priority queue will be
109	* sorted according to the same comparator, or according to its elements'
110	* natural order if the collection is sorted according to its elements'
111	* natural order. If the specified collection does not implement
112	* <tt>Sorted</tt>, the priority queue is ordered according to
113	* its elements' natural order.
114	*
115	* @param initialElements the collection whose elements are to be placed
116	* into this priority queue.
117	* @throws ClassCastException if elements of the specified collection
118	* cannot be compared to one another according to the priority
119	* queue's ordering.
120	* @throws NullPointerException if the specified collection or an
121	* element of the specified collection is <tt>null</tt>.
122	*/
123	public PriorityQueue(Collection<E> initialElements) {
124	int sz = initialElements.size();
125	int initialCapacity = (int)Math.min((sz * 110L) / 100,
126	Integer.MAX_VALUE - 1);
127	if (initialCapacity < 1)
128	initialCapacity = 1;
129	queue = (E[]) new Object[initialCapacity + 1];
130
131
132	if (initialElements instanceof Sorted) {
133	comparator = ((Sorted)initialElements).comparator();
134	for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
135	queue[++size] = i.next();
136	} else {
137	comparator = null;
138	for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
139	add(i.next());
140	}
141	}
142
143	// Queue Methods
144
145	/**
146	* Remove and return the minimal element from this priority queue
147	* if it contains one or more elements, otherwise return
148	* <tt>null</tt>. The term <i>minimal</i> is defined according to
149	* this priority queue's order.
150	*
151	* @return the minimal element from this priority queue if it contains
152	* one or more elements, otherwise <tt>null</tt>.
153	*/
154	public E poll() {
155	if (size == 0)
156	return null;
157	return remove(1);
158	}
159
160	/**
161	* Return, but do not remove, the minimal element from the
162	* priority queue, or return <tt>null</tt> if the queue is empty.
163	* The term <i>minimal</i> is defined according to this priority
164	* queue's order. This method returns the same object reference
165	* that would be returned by by the <tt>poll</tt> method. The two
166	* methods differ in that this method does not remove the element
167	* from the priority queue.
168	*
169	* @return the minimal element from this priority queue if it contains
170	* one or more elements, otherwise <tt>null</tt>.
171	*/
172	public E peek() {
173	return queue[1];
174	}
175
176	// Collection Methods
177
178	/**
179	* Removes a single instance of the specified element from this priority
180	* queue, if it is present. Returns true if this collection contained the
181	* specified element (or equivalently, if this collection changed as a
182	* result of the call).
183	*
184	* @param element the element to be removed from this collection,
185	* if present.
186	* @return <tt>true</tt> if this collection changed as a result of the
187	* call
188	* @throws ClassCastException if the specified element cannot be compared
189	* with elements currently in the priority queue according
190	* to the priority queue's ordering.
191	* @throws NullPointerException if the specified element is null.
192	*/
193	public boolean remove(Object element) {
194	if (element == null)
195	throw new NullPointerException();
196
197	if (comparator == null) {
198	for (int i = 1; i <= size; i++) {
199	if (((Comparable)queue[i]).compareTo(element) == 0) {
200	remove(i);
201	return true;
202	}
203	}
204	} else {
205	for (int i = 1; i <= size; i++) {
206	if (comparator.compare(queue[i], (E) element) == 0) {
207	remove(i);
208	return true;
209	}
210	}
211	}
212	return false;
213	}
214
215	/**
216	* Returns an iterator over the elements in this priority queue. The
217	* elements of the priority queue will be returned by this iterator in the
218	* order specified by the queue, which is to say the order they would be
219	* returned by repeated calls to <tt>poll</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	private 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	private 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	private 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	public int size() {
285	return size;
286	}
287
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	++size;
303
304	// Grow backing store if necessary
305	while (size >= queue.length) {
306	E[] newQueue = (E[]) new Object[2 * queue.length];
307	System.arraycopy(queue, 0, newQueue, 0, queue.length);
308	queue = newQueue;
309	}
310
311	queue[size] = element;
312	fixUp(size);
313	return true;
314	}
315
316	/**
317	* Remove all elements from the priority queue.
318	*/
319	public void clear() {
320	modCount++;
321
322	// Null out element references to prevent memory leak
323	for (int i=1; i<=size; i++)
324	queue[i] = null;
325
326	size = 0;
327	}
328
329	/**
330	* Removes and returns the ith element from queue. Recall
331	* that queue is one-based, so 1 <= i <= size.
332	*
333	* XXX: Could further special-case i==size, but is it worth it?
334	* XXX: Could special-case i==0, but is it worth it?
335	*/
336	private E remove(int i) {
337	assert i <= size;
338	modCount++;
339
340	E result = queue[i];
341	queue[i] = queue[size];
342	queue[size--] = null; // Drop extra ref to prevent memory leak
343	if (i <= size)
344	fixDown(i);
345	return result;
346	}
347
348	/**
349	* Establishes the heap invariant (described above) assuming the heap
350	* satisfies the invariant except possibly for the leaf-node indexed by k
351	* (which may have a nextExecutionTime less than its parent's).
352	*
353	* This method functions by "promoting" queue[k] up the hierarchy
354	* (by swapping it with its parent) repeatedly until queue[k]
355	* is greater than or equal to its parent.
356	*/
357	private void fixUp(int k) {
358	if (comparator == null) {
359	while (k > 1) {
360	int j = k >> 1;
361	if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
362	break;
363	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
364	k = j;
365	}
366	} else {
367	while (k > 1) {
368	int j = k >> 1;
369	if (comparator.compare(queue[j], queue[k]) <= 0)
370	break;
371	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
372	k = j;
373	}
374	}
375	}
376
377	/**
378	* Establishes the heap invariant (described above) in the subtree
379	* rooted at k, which is assumed to satisfy the heap invariant except
380	* possibly for node k itself (which may be greater than its children).
381	*
382	* This method functions by "demoting" queue[k] down the hierarchy
383	* (by swapping it with its smaller child) repeatedly until queue[k]
384	* is less than or equal to its children.
385	*/
386	private void fixDown(int k) {
387	int j;
388	if (comparator == null) {
389	while ((j = k << 1) <= size) {
390	if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
391	j++; // j indexes smallest kid
392	if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
393	break;
394	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
395	k = j;
396	}
397	} else {
398	while ((j = k << 1) <= size) {
399	if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
400	j++; // j indexes smallest kid
401	if (comparator.compare(queue[k], queue[j]) <= 0)
402	break;
403	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
404	k = j;
405	}
406	}
407	}
408
409	/**
410	* Returns the comparator associated with this priority queue, or
411	* <tt>null</tt> if it uses its elements' natural ordering.
412	*
413	* @return the comparator associated with this priority queue, or
414	* <tt>null</tt> if it uses its elements' natural ordering.
415	*/
416	public Comparator comparator() {
417	return comparator;
418	}
419
420	/**
421	* Save the state of the instance to a stream (that
422	* is, serialize it).
423	*
424	* @serialData The length of the array backing the instance is
425	* emitted (int), followed by all of its elements (each an
426	* <tt>Object</tt>) in the proper order.
427	* @param s the stream
428	*/
429	private synchronized void writeObject(java.io.ObjectOutputStream s)
430	throws java.io.IOException{
431	// Write out element count, and any hidden stuff
432	s.defaultWriteObject();
433
434	// Write out array length
435	s.writeInt(queue.length);
436
437	// Write out all elements in the proper order.
438	for (int i=0; i<size; i++)
439	s.writeObject(queue[i]);
440	}
441
442	/**
443	* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
444	* deserialize it).
445	* @param s the stream
446	*/
447	private synchronized void readObject(java.io.ObjectInputStream s)
448	throws java.io.IOException, ClassNotFoundException {
449	// Read in size, and any hidden stuff
450	s.defaultReadObject();
451
452	// Read in array length and allocate array
453	int arrayLength = s.readInt();
454	queue = (E[]) new Object[arrayLength];
455
456	// Read in all elements in the proper order.
457	for (int i=0; i<size; i++)
458	queue[i] = (E)s.readObject();
459	}
460
461	}