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> {
    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.
         */
        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++;

        // 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.
     * @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 = new E[arrayLength];

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

}
Revision:	1.7
Committed:	Tue Jun 24 14:34:30 2003 UTC (20 years, 10 months ago) by dl
Branch:	MAIN
Changes since 1.6:	+78 -69 lines
Log Message:	Added missing javadoc tags; minor reformatting
#	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	private static final int DEFAULT_INITIAL_CAPACITY = 11;
36
37	/**
38	* Priority queue represented as a balanced binary heap: the two children
39	* of queue[n] are queue[2n] and queue[2n + 1]. The priority queue is
40	* ordered by comparator, or by the elements' natural ordering, if
41	* comparator is null: For each node n in the heap and each descendant d
42	* of n, n <= d.
43	*
44	* The element with the lowest value is in queue[1], assuming the queue is
45	* nonempty. (A one-based array is used in preference to the traditional
46	* zero-based array to simplify parent and child calculations.)
47	*
48	* queue.length must be >= 2, even if size == 0.
49	*/
50	private transient E[] queue;
51
52	/**
53	* The number of elements in the priority queue.
54	*/
55	private int size = 0;
56
57	/**
58	* The comparator, or null if priority queue uses elements'
59	* natural ordering.
60	*/
61	private final Comparator<E> comparator;
62
63	/**
64	* The number of times this priority queue has been
65	* <i>structurally modified</i>. See AbstractList for gory details.
66	*/
67	private transient int modCount = 0;
68
69	/**
70	* Create a new priority queue with the default initial capacity
71	* (11) that orders its elements according to their natural
72	* ordering (using <tt>Comparable</tt>.)
73	*/
74	public PriorityQueue() {
75	this(DEFAULT_INITIAL_CAPACITY);
76	}
77
78	/**
79	* Create a new priority queue with the specified initial capacity
80	* that orders its elements according to their natural ordering
81	* (using <tt>Comparable</tt>.)
82	*
83	* @param initialCapacity the initial capacity for this priority queue.
84	*/
85	public PriorityQueue(int initialCapacity) {
86	this(initialCapacity, null);
87	}
88
89	/**
90	* Create a new priority queue with the specified initial capacity (11)
91	* that orders its elements according to the specified comparator.
92	*
93	* @param initialCapacity the initial capacity for this priority queue.
94	* @param comparator the comparator used to order this priority queue.
95	*/
96	public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
97	if (initialCapacity < 1)
98	initialCapacity = 1;
99	queue = new E[initialCapacity + 1];
100	this.comparator = comparator;
101	}
102
103	/**
104	* Create a new priority queue containing the elements in the specified
105	* collection. The priority queue has an initial capacity of 110% of the
106	* size of the specified collection. If the specified collection
107	* implements the {@link Sorted} interface, the priority queue will be
108	* sorted according to the same comparator, or according to its elements'
109	* natural order if the collection is sorted according to its elements'
110	* natural order. If the specified collection does not implement
111	* <tt>Sorted</tt>, the priority queue is ordered according to
112	* its elements' natural order.
113	*
114	* @param initialElements the collection whose elements are to be placed
115	* into this priority queue.
116	* @throws ClassCastException if elements of the specified collection
117	* cannot be compared to one another according to the priority
118	* queue's ordering.
119	* @throws NullPointerException if the specified collection or an
120	* element of the specified collection is <tt>null</tt>.
121	*/
122	public PriorityQueue(Collection<E> initialElements) {
123	int sz = initialElements.size();
124	int initialCapacity = (int)Math.min((sz * 110L) / 100,
125	Integer.MAX_VALUE - 1);
126	if (initialCapacity < 1)
127	initialCapacity = 1;
128	queue = new E[initialCapacity + 1];
129
130	/* Commented out to compile with generics compiler
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	*/
138	{
139	comparator = null;
140	for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
141	add(i.next());
142	}
143	}
144
145	// Queue Methods
146
147	/**
148	* Remove and return the minimal element from this priority queue
149	* if it contains one or more elements, otherwise return
150	* <tt>null</tt>. The term <i>minimal</i> is defined according to
151	* this priority queue's order.
152	*
153	* @return the minimal element from this priority queue if it contains
154	* one or more elements, otherwise <tt>null</tt>.
155	*/
156	public E poll() {
157	if (size == 0)
158	return null;
159	return remove(1);
160	}
161
162	/**
163	* Return, but do not remove, the minimal element from the
164	* priority queue, or return <tt>null</tt> if the queue is empty.
165	* The term <i>minimal</i> is defined according to this priority
166	* queue's order. This method returns the same object reference
167	* that would be returned by by the <tt>poll</tt> method. The two
168	* methods differ in that this method does not remove the element
169	* from the priority queue.
170	*
171	* @return the minimal element from this priority queue if it contains
172	* one or more elements, otherwise <tt>null</tt>.
173	*/
174	public E peek() {
175	return queue[1];
176	}
177
178	// Collection Methods
179
180	/**
181	* Removes a single instance of the specified element from this priority
182	* queue, if it is present. Returns true if this collection contained the
183	* specified element (or equivalently, if this collection changed as a
184	* result of the call).
185	*
186	* @param element the element to be removed from this collection,
187	* if present.
188	* @return <tt>true</tt> if this collection changed as a result of the
189	* call
190	* @throws ClassCastException if the specified element cannot be compared
191	* with elements currently in the priority queue according
192	* to the priority queue's ordering.
193	* @throws NullPointerException if the specified element is null.
194	*/
195	public boolean remove(Object element) {
196	if (element == null)
197	throw new NullPointerException();
198
199	if (comparator == null) {
200	for (int i = 1; i <= size; i++) {
201	if (((Comparable)queue[i]).compareTo(element) == 0) {
202	remove(i);
203	return true;
204	}
205	}
206	} else {
207	for (int i = 1; i <= size; i++) {
208	if (comparator.compare(queue[i], (E) element) == 0) {
209	remove(i);
210	return true;
211	}
212	}
213	}
214	return false;
215	}
216
217	/**
218	* Returns an iterator over the elements in this priority queue. The
219	* elements of the priority queue will be returned by this iterator in the
220	* order specified by the queue, which is to say the order they would be
221	* returned by repeated calls to <tt>poll</tt>.
222	*
223	* @return an <tt>Iterator</tt> over the elements in this priority queue.
224	*/
225	public Iterator<E> iterator() {
226	return new Itr();
227	}
228
229	private class Itr implements Iterator<E> {
230	/**
231	* Index (into queue array) of element to be returned by
232	* subsequent call to next.
233	*/
234	private int cursor = 1;
235
236	/**
237	* Index of element returned by most recent call to next or
238	* previous. Reset to 0 if this element is deleted by a call
239	* to remove.
240	*/
241	private int lastRet = 0;
242
243	/**
244	* The modCount value that the iterator believes that the backing
245	* List should have. If this expectation is violated, the iterator
246	* has detected concurrent modification.
247	*/
248	private int expectedModCount = modCount;
249
250	public boolean hasNext() {
251	return cursor <= size;
252	}
253
254	public E next() {
255	checkForComodification();
256	if (cursor > size)
257	throw new NoSuchElementException();
258	E result = queue[cursor];
259	lastRet = cursor++;
260	return result;
261	}
262
263	public void remove() {
264	if (lastRet == 0)
265	throw new IllegalStateException();
266	checkForComodification();
267
268	PriorityQueue.this.remove(lastRet);
269	if (lastRet < cursor)
270	cursor--;
271	lastRet = 0;
272	expectedModCount = modCount;
273	}
274
275	final void checkForComodification() {
276	if (modCount != expectedModCount)
277	throw new ConcurrentModificationException();
278	}
279	}
280
281	/**
282	* Returns the number of elements in this priority queue.
283	*
284	* @return the number of elements in this priority queue.
285	*/
286	public int size() {
287	return size;
288	}
289
290	/**
291	* Add the specified element to this priority queue.
292	*
293	* @param element the element to add.
294	* @return true
295	* @throws ClassCastException if the specified element cannot be compared
296	* with elements currently in the priority queue according
297	* to the priority queue's ordering.
298	* @throws NullPointerException if the specified element is null.
299	*/
300	public boolean offer(E element) {
301	if (element == null)
302	throw new NullPointerException();
303	modCount++;
304
305	// Grow backing store if necessary
306	if (++size == queue.length) {
307	E[] newQueue = new E[2 * queue.length];
308	System.arraycopy(queue, 0, newQueue, 0, size);
309	queue = newQueue;
310	}
311
312	queue[size] = element;
313	fixUp(size);
314	return true;
315	}
316
317	/**
318	* Remove all elements from the priority queue.
319	*/
320	public void clear() {
321	modCount++;
322
323	// Null out element references to prevent memory leak
324	for (int i=1; i<=size; i++)
325	queue[i] = null;
326
327	size = 0;
328	}
329
330	/**
331	* Removes and returns the ith element from queue. Recall
332	* that queue is one-based, so 1 <= i <= size.
333	*
334	* XXX: Could further special-case i==size, but is it worth it?
335	* XXX: Could special-case i==0, but is it worth it?
336	*/
337	private E remove(int i) {
338	assert i <= size;
339	modCount++;
340
341	E result = queue[i];
342	queue[i] = queue[size];
343	queue[size--] = null; // Drop extra ref to prevent memory leak
344	if (i <= size)
345	fixDown(i);
346	return result;
347	}
348
349	/**
350	* Establishes the heap invariant (described above) assuming the heap
351	* satisfies the invariant except possibly for the leaf-node indexed by k
352	* (which may have a nextExecutionTime less than its parent's).
353	*
354	* This method functions by "promoting" queue[k] up the hierarchy
355	* (by swapping it with its parent) repeatedly until queue[k]
356	* is greater than or equal to its parent.
357	*/
358	private void fixUp(int k) {
359	if (comparator == null) {
360	while (k > 1) {
361	int j = k >> 1;
362	if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
363	break;
364	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
365	k = j;
366	}
367	} else {
368	while (k > 1) {
369	int j = k >> 1;
370	if (comparator.compare(queue[j], queue[k]) <= 0)
371	break;
372	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
373	k = j;
374	}
375	}
376	}
377
378	/**
379	* Establishes the heap invariant (described above) in the subtree
380	* rooted at k, which is assumed to satisfy the heap invariant except
381	* possibly for node k itself (which may be greater than its children).
382	*
383	* This method functions by "demoting" queue[k] down the hierarchy
384	* (by swapping it with its smaller child) repeatedly until queue[k]
385	* is less than or equal to its children.
386	*/
387	private void fixDown(int k) {
388	int j;
389	if (comparator == null) {
390	while ((j = k << 1) <= size) {
391	if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
392	j++; // j indexes smallest kid
393	if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
394	break;
395	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
396	k = j;
397	}
398	} else {
399	while ((j = k << 1) <= size) {
400	if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
401	j++; // j indexes smallest kid
402	if (comparator.compare(queue[k], queue[j]) <= 0)
403	break;
404	E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
405	k = j;
406	}
407	}
408	}
409
410	/**
411	* Returns the comparator associated with this priority queue, or
412	* <tt>null</tt> if it uses its elements' natural ordering.
413	*
414	* @return the comparator associated with this priority queue, or
415	* <tt>null</tt> if it uses its elements' natural ordering.
416	*/
417	Comparator comparator() {
418	return comparator;
419	}
420
421	/**
422	* Save the state of the instance to a stream (that
423	* is, serialize it).
424	*
425	* @serialData The length of the array backing the instance is
426	* emitted (int), followed by all of its elements (each an
427	* <tt>Object</tt>) in the proper order.
428	* @param s the stream
429	*/
430	private synchronized void writeObject(java.io.ObjectOutputStream s)
431	throws java.io.IOException{
432	// Write out element count, and any hidden stuff
433	s.defaultWriteObject();
434
435	// Write out array length
436	s.writeInt(queue.length);
437
438	// Write out all elements in the proper order.
439	for (int i=0; i<size; i++)
440	s.writeObject(queue[i]);
441	}
442
443	/**
444	* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
445	* deserialize it).
446	* @param s the stream
447	*/
448	private synchronized void readObject(java.io.ObjectInputStream s)
449	throws java.io.IOException, ClassNotFoundException {
450	// Read in size, and any hidden stuff
451	s.defaultReadObject();
452
453	// Read in array length and allocate array
454	int arrayLength = s.readInt();
455	queue = new E[arrayLength];
456
457	// Read in all elements in the proper order.
458	for (int i=0; i<size; i++)
459	queue[i] = (E)s.readObject();
460	}
461
462	}