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 = 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];


        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.
     */
    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 = new E[arrayLength];

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

}
Revision:	1.8
Committed:	Tue Jul 1 16:29:45 2003 UTC (20 years, 10 months ago) by dl
Branch:	MAIN
Changes since 1.7:	+3 -5 lines
Log Message:	Misc minor tunings
#	User	Rev	Content
1	tim	1.2	package java.util;
2	tim	1.1
3			/**
4	tim	1.2	* An unbounded priority queue based on a priority heap. This queue orders
5	brian	1.6	* 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	tim	1.2	* 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	brian	1.6	* elements are tied for least value, no guarantees are made as to
12			* which of these elements is returned.
13	tim	1.2	*
14	dl	1.7	* <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	tim	1.2	*
20	dl	1.7	*<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	tim	1.2	*
27			* <p>This class is a member of the
28			* <a href="{@docRoot}/../guide/collections/index.html">
29			* Java Collections Framework</a>.
30	dl	1.7	* @since 1.5
31			* @author Josh Bloch
32	tim	1.2	*/
33			public class PriorityQueue<E> extends AbstractQueue<E>
34	dl	1.8	implements Queue<E>,
35			java.io.Serializable {
36	tim	1.2	private static final int DEFAULT_INITIAL_CAPACITY = 11;
37	tim	1.1
38	tim	1.2	/**
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	brian	1.6	* comparator is null: For each node n in the heap and each descendant d
43			* of n, n <= d.
44	tim	1.2	*
45	brian	1.6	* 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	tim	1.2	*
49			* queue.length must be >= 2, even if size == 0.
50			*/
51	dl	1.5	private transient E[] queue;
52	tim	1.1
53	tim	1.2	/**
54			* The number of elements in the priority queue.
55			*/
56			private int size = 0;
57	tim	1.1
58	tim	1.2	/**
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	dl	1.5	private transient int modCount = 0;
69	tim	1.2
70			/**
71	dl	1.7	* 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	tim	1.2	*/
75			public PriorityQueue() {
76			this(DEFAULT_INITIAL_CAPACITY);
77	tim	1.1	}
78	tim	1.2
79			/**
80			* Create a new priority queue with the specified initial capacity
81	dl	1.7	* that orders its elements according to their natural ordering
82			* (using <tt>Comparable</tt>.)
83	tim	1.2	*
84			* @param initialCapacity the initial capacity for this priority queue.
85			*/
86			public PriorityQueue(int initialCapacity) {
87			this(initialCapacity, null);
88	tim	1.1	}
89	tim	1.2
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 = new E[initialCapacity + 1];
101			this.comparator = comparator;
102	tim	1.1	}
103
104	tim	1.2	/**
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	brian	1.6	* natural order. If the specified collection does not implement
112			* <tt>Sorted</tt>, the priority queue is ordered according to
113	tim	1.2	* 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 = new E[initialCapacity + 1];
130
131	dl	1.5
132	tim	1.2	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	tim	1.1	}
142
143	tim	1.2	// Queue Methods
144
145			/**
146	dl	1.7	* 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	tim	1.2	*
151			* @return the minimal element from this priority queue if it contains
152			* one or more elements, otherwise <tt>null</tt>.
153			*/
154	tim	1.1	public E poll() {
155	tim	1.2	if (size == 0)
156			return null;
157			return remove(1);
158	tim	1.1	}
159	tim	1.2
160			/**
161	dl	1.7	* 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	tim	1.2	*
169			* @return the minimal element from this priority queue if it contains
170			* one or more elements, otherwise <tt>null</tt>.
171			*/
172	tim	1.1	public E peek() {
173	tim	1.2	return queue[1];
174	tim	1.1	}
175
176	tim	1.2	// 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	dl	1.7	* @param element the element to be removed from this collection,
185			* if present.
186	tim	1.2	* @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	dl	1.7	* with elements currently in the priority queue according
190	tim	1.2	* 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	tim	1.1	return false;
213			}
214	tim	1.2
215			/**
216			* Returns an iterator over the elements in this priority queue. The
217	brian	1.6	* 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	dl	1.5	*
221	tim	1.2	* @return an <tt>Iterator</tt> over the elements in this priority queue.
222			*/
223			public Iterator<E> iterator() {
224	dl	1.7	return new Itr();
225	tim	1.2	}
226
227			private class Itr implements Iterator<E> {
228	dl	1.7	/**
229			* Index (into queue array) of element to be returned by
230	tim	1.2	* subsequent call to next.
231	dl	1.7	*/
232			private int cursor = 1;
233	tim	1.2
234	dl	1.7	/**
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	tim	1.2
248	dl	1.7	public boolean hasNext() {
249			return cursor <= size;
250			}
251
252			public E next() {
253	tim	1.2	checkForComodification();
254			if (cursor > size)
255	dl	1.7	throw new NoSuchElementException();
256	tim	1.2	E result = queue[cursor];
257			lastRet = cursor++;
258			return result;
259	dl	1.7	}
260	tim	1.2
261	dl	1.7	public void remove() {
262			if (lastRet == 0)
263			throw new IllegalStateException();
264	tim	1.2	checkForComodification();
265
266			PriorityQueue.this.remove(lastRet);
267			if (lastRet < cursor)
268			cursor--;
269			lastRet = 0;
270			expectedModCount = modCount;
271	dl	1.7	}
272	tim	1.2
273	dl	1.7	final void checkForComodification() {
274			if (modCount != expectedModCount)
275			throw new ConcurrentModificationException();
276			}
277	tim	1.2	}
278
279			/**
280			* Returns the number of elements in this priority queue.
281	dl	1.5	*
282	tim	1.2	* @return the number of elements in this priority queue.
283			*/
284	tim	1.1	public int size() {
285	tim	1.2	return size;
286	tim	1.1	}
287	tim	1.2
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	dl	1.7	* with elements currently in the priority queue according
295	tim	1.2	* 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
303			// Grow backing store if necessary
304			if (++size == queue.length) {
305			E[] newQueue = new E[2 * queue.length];
306			System.arraycopy(queue, 0, newQueue, 0, size);
307			queue = newQueue;
308			}
309
310			queue[size] = element;
311			fixUp(size);
312			return true;
313	tim	1.1	}
314
315	tim	1.2	/**
316			* Remove all elements from the priority queue.
317			*/
318			public void clear() {
319			modCount++;
320
321			// Null out element references to prevent memory leak
322			for (int i=1; i<=size; i++)
323			queue[i] = null;
324
325			size = 0;
326			}
327
328			/**
329			* Removes and returns the ith element from queue. Recall
330			* that queue is one-based, so 1 <= i <= size.
331			*
332			* XXX: Could further special-case i==size, but is it worth it?
333			* XXX: Could special-case i==0, but is it worth it?
334			*/
335			private E remove(int i) {
336			assert i <= size;
337			modCount++;
338
339			E result = queue[i];
340			queue[i] = queue[size];
341			queue[size--] = null; // Drop extra ref to prevent memory leak
342			if (i <= size)
343			fixDown(i);
344			return result;
345	tim	1.1	}
346
347	tim	1.2	/**
348			* Establishes the heap invariant (described above) assuming the heap
349			* satisfies the invariant except possibly for the leaf-node indexed by k
350			* (which may have a nextExecutionTime less than its parent's).
351			*
352			* This method functions by "promoting" queue[k] up the hierarchy
353			* (by swapping it with its parent) repeatedly until queue[k]
354			* is greater than or equal to its parent.
355			*/
356			private void fixUp(int k) {
357			if (comparator == null) {
358			while (k > 1) {
359			int j = k >> 1;
360			if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
361			break;
362			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
363			k = j;
364			}
365			} else {
366			while (k > 1) {
367			int j = k >> 1;
368			if (comparator.compare(queue[j], queue[k]) <= 0)
369			break;
370			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
371			k = j;
372			}
373			}
374			}
375
376			/**
377			* Establishes the heap invariant (described above) in the subtree
378			* rooted at k, which is assumed to satisfy the heap invariant except
379			* possibly for node k itself (which may be greater than its children).
380			*
381			* This method functions by "demoting" queue[k] down the hierarchy
382			* (by swapping it with its smaller child) repeatedly until queue[k]
383			* is less than or equal to its children.
384			*/
385			private void fixDown(int k) {
386			int j;
387			if (comparator == null) {
388			while ((j = k << 1) <= size) {
389			if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
390			j++; // j indexes smallest kid
391			if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
392			break;
393			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
394			k = j;
395			}
396			} else {
397			while ((j = k << 1) <= size) {
398			if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
399			j++; // j indexes smallest kid
400			if (comparator.compare(queue[k], queue[j]) <= 0)
401			break;
402			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
403			k = j;
404			}
405			}
406			}
407
408			/**
409			* Returns the comparator associated with this priority queue, or
410			* <tt>null</tt> if it uses its elements' natural ordering.
411			*
412			* @return the comparator associated with this priority queue, or
413	dl	1.7	* <tt>null</tt> if it uses its elements' natural ordering.
414	tim	1.2	*/
415	dl	1.8	public Comparator comparator() {
416	tim	1.2	return comparator;
417			}
418	dl	1.5
419			/**
420			* Save the state of the instance to a stream (that
421			* is, serialize it).
422			*
423			* @serialData The length of the array backing the instance is
424			* emitted (int), followed by all of its elements (each an
425			* <tt>Object</tt>) in the proper order.
426	dl	1.7	* @param s the stream
427	dl	1.5	*/
428			private synchronized void writeObject(java.io.ObjectOutputStream s)
429			throws java.io.IOException{
430	dl	1.7	// Write out element count, and any hidden stuff
431			s.defaultWriteObject();
432	dl	1.5
433			// Write out array length
434			s.writeInt(queue.length);
435
436	dl	1.7	// Write out all elements in the proper order.
437			for (int i=0; i<size; i++)
438	dl	1.5	s.writeObject(queue[i]);
439			}
440
441			/**
442			* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
443			* deserialize it).
444	dl	1.7	* @param s the stream
445	dl	1.5	*/
446			private synchronized void readObject(java.io.ObjectInputStream s)
447			throws java.io.IOException, ClassNotFoundException {
448	dl	1.7	// Read in size, and any hidden stuff
449			s.defaultReadObject();
450	dl	1.5
451			// Read in array length and allocate array
452			int arrayLength = s.readInt();
453			queue = new E[arrayLength];
454
455	dl	1.7	// Read in all elements in the proper order.
456			for (int i=0; i<size; i++)
457	dl	1.5	queue[i] = (E)s.readObject();
458			}
459
460	tim	1.1	}