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>.
 */
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.
         */
        int cursor = 1;

        /**
         * Index of element returned by most recent call to next or
         * previous.  Reset to 0 if this element is deleted by a call
         * to remove.
         */
        int lastRet = 0;

        /**
         * The modCount value that the iterator believes that the backing
         * List should have.  If this expectation is violated, the iterator
         * has detected concurrent modification.
         */
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor <= size;
        }

        public E next() {
            checkForComodification();
            if (cursor > size)
                throw new NoSuchElementException();
            E result = queue[cursor];
            lastRet = cursor++;
            return result;
        }

        public void remove() {
            if (lastRet == 0)
                throw new IllegalStateException();
            checkForComodification();

            PriorityQueue.this.remove(lastRet);
            if (lastRet < cursor)
                cursor--;
            lastRet = 0;
            expectedModCount = modCount;
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    /**
     * Returns the number of elements in this priority queue.
     * 
     * @return the number of elements in this priority queue.
     */
    public int size() {
        return size;
    }

    /**
     * Add the specified element to this priority queue.
     *
     * @param element the element to add.
     * @return true
     * @throws ClassCastException if the specified element cannot be compared
     *            with elements currently in the priority queue according
     *            to the priority queue's ordering.
     * @throws NullPointerException if the specified element is null.
     */
    public boolean offer(E element) {
        if (element == null)
            throw new NullPointerException();
        modCount++;

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

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

    /**
     * Remove all elements from the priority queue.
     */
    public void clear() {
        modCount++;

        // Null out element references to prevent memory leak
        for (int i=1; i<=size; i++)
            queue[i] = null;

        size = 0;
    }

    /**
     * Removes and returns the ith element from queue.  Recall
     * that queue is one-based, so 1 <= i <= size.
     *
     * XXX: Could further special-case i==size, but is it worth it?
     * XXX: Could special-case i==0, but is it worth it?
     */
    private E remove(int i) {
        assert i <= size;
        modCount++;

        E result = queue[i];
        queue[i] = queue[size];
        queue[size--] = null;  // Drop extra ref to prevent memory leak
        if (i <= size)
            fixDown(i);
        return result;
    }

    /**
     * Establishes the heap invariant (described above) assuming the heap
     * satisfies the invariant except possibly for the leaf-node indexed by k
     * (which may have a nextExecutionTime less than its parent's).
     *
     * This method functions by "promoting" queue[k] up the hierarchy
     * (by swapping it with its parent) repeatedly until queue[k]
     * is greater than or equal to its parent.
     */
    private void fixUp(int k) {
        if (comparator == null) {
            while (k > 1) {
                int j = k >> 1;
                if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
                    break;
                E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        } else {
            while (k > 1) {
                int j = k >> 1;
                if (comparator.compare(queue[j], queue[k]) <= 0)
                    break;
                E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        }
    }

    /**
     * Establishes the heap invariant (described above) in the subtree
     * rooted at k, which is assumed to satisfy the heap invariant except
     * possibly for node k itself (which may be greater than its children).
     *
     * This method functions by "demoting" queue[k] down the hierarchy
     * (by swapping it with its smaller child) repeatedly until queue[k]
     * is less than or equal to its children.
     */
    private void fixDown(int k) {
        int j;
        if (comparator == null) {
            while ((j = k << 1) <= size) {
                if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
                    j++; // j indexes smallest kid
                if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
                    break;
                E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        } else {
            while ((j = k << 1) <= size) {
                if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
                    j++; // j indexes smallest kid
                if (comparator.compare(queue[k], queue[j]) <= 0)
                    break;
                E tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        }
    }

    /**
     * Returns the comparator associated with this priority queue, or
     * <tt>null</tt> if it uses its elements' natural ordering.
     *
     * @return the comparator associated with this priority queue, or
     *         <tt>null</tt> if it uses its elements' natural ordering.
     */
    Comparator comparator() {
        return comparator;
    }

    /**
     * Save the state of the instance to a stream (that
     * is, serialize it).
     *
     * @serialData The length of the array backing the instance is
     * emitted (int), followed by all of its elements (each an
     * <tt>Object</tt>) in the proper order.
     */
    private synchronized void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        s.defaultWriteObject();

        // Write out array length
        s.writeInt(queue.length);

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

    /**
     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
     * deserialize it).
     */
    private synchronized void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in array length and allocate array
        int arrayLength = s.readInt();
        queue = new E[arrayLength];

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

}
Revision:	1.6
Committed:	Mon Jun 23 02:26:15 2003 UTC (20 years, 10 months ago) by brian
Branch:	MAIN
Changes since 1.5:	+25 -24 lines
Log Message:	Partial javadoc pass
#	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	brian	1.6	* <p>A priority queue has a <i>capacity</i>. The capacity is the size of
15			* the array used internally to store the elements on the queue. It is always at least
16			* as large as the queue size. As elements are added to a priority queue,
17	tim	1.2	* its capacity grows automatically. The details of the growth policy are not
18			* specified.
19			*
20			*<p>Implementation note: this implementation provides O(log(n)) time for
21	brian	1.6	* the insertion methods (<tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt>)
22	tim	1.2	* methods; linear time for the <tt>remove(Object)</tt> and
23	brian	1.6	* <tt>contains(Object)</tt> methods; and constant time for the retrieval methods (<tt>peek</tt>,
24			* <tt>element</tt>, and <tt>size</tt>).
25	tim	1.2	*
26			* <p>This class is a member of the
27			* <a href="{@docRoot}/../guide/collections/index.html">
28			* Java Collections Framework</a>.
29			*/
30			public class PriorityQueue<E> extends AbstractQueue<E>
31			implements Queue<E>
32			{
33			private static final int DEFAULT_INITIAL_CAPACITY = 11;
34	tim	1.1
35	tim	1.2	/**
36			* Priority queue represented as a balanced binary heap: the two children
37			* of queue[n] are queue[2n] and queue[2n + 1]. The priority queue is
38			* ordered by comparator, or by the elements' natural ordering, if
39	brian	1.6	* comparator is null: For each node n in the heap and each descendant d
40			* of n, n <= d.
41	tim	1.2	*
42	brian	1.6	* The element with the lowest value is in queue[1], assuming the queue is
43			* nonempty. (A one-based array is used in preference to the traditional
44			* zero-based array to simplify parent and child calculations.)
45	tim	1.2	*
46			* queue.length must be >= 2, even if size == 0.
47			*/
48	dl	1.5	private transient E[] queue;
49	tim	1.1
50	tim	1.2	/**
51			* The number of elements in the priority queue.
52			*/
53			private int size = 0;
54	tim	1.1
55	tim	1.2	/**
56			* The comparator, or null if priority queue uses elements'
57			* natural ordering.
58			*/
59			private final Comparator<E> comparator;
60
61			/**
62			* The number of times this priority queue has been
63			* <i>structurally modified</i>. See AbstractList for gory details.
64			*/
65	dl	1.5	private transient int modCount = 0;
66	tim	1.2
67			/**
68			* Create a new priority queue with the default initial capacity (11)
69	brian	1.6	* that orders its elements according to their natural ordering (using <tt>Comparable</tt>.)
70	tim	1.2	*/
71			public PriorityQueue() {
72			this(DEFAULT_INITIAL_CAPACITY);
73	tim	1.1	}
74	tim	1.2
75			/**
76			* Create a new priority queue with the specified initial capacity
77	brian	1.6	* that orders its elements according to their natural ordering (using <tt>Comparable</tt>.)
78	tim	1.2	*
79			* @param initialCapacity the initial capacity for this priority queue.
80			*/
81			public PriorityQueue(int initialCapacity) {
82			this(initialCapacity, null);
83	tim	1.1	}
84	tim	1.2
85			/**
86			* Create a new priority queue with the specified initial capacity (11)
87			* that orders its elements according to the specified comparator.
88			*
89			* @param initialCapacity the initial capacity for this priority queue.
90			* @param comparator the comparator used to order this priority queue.
91			*/
92			public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
93			if (initialCapacity < 1)
94			initialCapacity = 1;
95			queue = new E[initialCapacity + 1];
96			this.comparator = comparator;
97	tim	1.1	}
98
99	tim	1.2	/**
100			* Create a new priority queue containing the elements in the specified
101			* collection. The priority queue has an initial capacity of 110% of the
102			* size of the specified collection. If the specified collection
103			* implements the {@link Sorted} interface, the priority queue will be
104			* sorted according to the same comparator, or according to its elements'
105			* natural order if the collection is sorted according to its elements'
106	brian	1.6	* natural order. If the specified collection does not implement
107			* <tt>Sorted</tt>, the priority queue is ordered according to
108	tim	1.2	* its elements' natural order.
109			*
110			* @param initialElements the collection whose elements are to be placed
111			* into this priority queue.
112			* @throws ClassCastException if elements of the specified collection
113			* cannot be compared to one another according to the priority
114			* queue's ordering.
115			* @throws NullPointerException if the specified collection or an
116			* element of the specified collection is <tt>null</tt>.
117			*/
118			public PriorityQueue(Collection<E> initialElements) {
119			int sz = initialElements.size();
120			int initialCapacity = (int)Math.min((sz * 110L) / 100,
121			Integer.MAX_VALUE - 1);
122			if (initialCapacity < 1)
123			initialCapacity = 1;
124			queue = new E[initialCapacity + 1];
125
126	dl	1.5	/* Commented out to compile with generics compiler
127
128	tim	1.2	if (initialElements instanceof Sorted) {
129			comparator = ((Sorted)initialElements).comparator();
130			for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
131			queue[++size] = i.next();
132			} else {
133	dl	1.5	*/
134			{
135	tim	1.2	comparator = null;
136			for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
137			add(i.next());
138			}
139	tim	1.1	}
140
141	tim	1.2	// Queue Methods
142
143			/**
144			* Remove and return the minimal element from this priority queue if
145	brian	1.6	* it contains one or more elements, otherwise return <tt>null</tt>. The term
146	tim	1.2	* <i>minimal</i> is defined according to this priority queue's order.
147			*
148			* @return the minimal element from this priority queue if it contains
149			* one or more elements, otherwise <tt>null</tt>.
150			*/
151	tim	1.1	public E poll() {
152	tim	1.2	if (size == 0)
153			return null;
154			return remove(1);
155	tim	1.1	}
156	tim	1.2
157			/**
158			* Return, but do not remove, the minimal element from the priority queue,
159	brian	1.6	* or return <tt>null</tt> if the queue is empty. The term <i>minimal</i> is
160	tim	1.2	* defined according to this priority queue's order. This method returns
161			* the same object reference that would be returned by by the
162	dl	1.5	* <tt>poll</tt> method. The two methods differ in that this method
163	tim	1.2	* does not remove the element from the priority queue.
164			*
165			* @return the minimal element from this priority queue if it contains
166			* one or more elements, otherwise <tt>null</tt>.
167			*/
168	tim	1.1	public E peek() {
169	tim	1.2	return queue[1];
170	tim	1.1	}
171
172	tim	1.2	// Collection Methods
173
174			/**
175			* Removes a single instance of the specified element from this priority
176			* queue, if it is present. Returns true if this collection contained the
177			* specified element (or equivalently, if this collection changed as a
178			* result of the call).
179			*
180	brian	1.6	* @param element the element to be removed from this collection, if present.
181	tim	1.2	* @return <tt>true</tt> if this collection changed as a result of the
182			* call
183			* @throws ClassCastException if the specified element cannot be compared
184	dl	1.5	* with elements currently in the priority queue according
185	tim	1.2	* to the priority queue's ordering.
186			* @throws NullPointerException if the specified element is null.
187			*/
188			public boolean remove(Object element) {
189			if (element == null)
190			throw new NullPointerException();
191
192			if (comparator == null) {
193			for (int i = 1; i <= size; i++) {
194			if (((Comparable)queue[i]).compareTo(element) == 0) {
195			remove(i);
196			return true;
197			}
198			}
199			} else {
200			for (int i = 1; i <= size; i++) {
201			if (comparator.compare(queue[i], (E) element) == 0) {
202			remove(i);
203			return true;
204			}
205			}
206			}
207	tim	1.1	return false;
208			}
209	tim	1.2
210			/**
211			* Returns an iterator over the elements in this priority queue. The
212	brian	1.6	* elements of the priority queue will be returned by this iterator in the
213			* order specified by the queue, which is to say the order they would be
214			* returned by repeated calls to <tt>poll</tt>.
215	dl	1.5	*
216	tim	1.2	* @return an <tt>Iterator</tt> over the elements in this priority queue.
217			*/
218			public Iterator<E> iterator() {
219	dl	1.5	return new Itr();
220	tim	1.2	}
221
222			private class Itr implements Iterator<E> {
223	dl	1.5	/**
224			* Index (into queue array) of element to be returned by
225	tim	1.2	* subsequent call to next.
226	dl	1.5	*/
227			int cursor = 1;
228	tim	1.2
229	dl	1.5	/**
230			* Index of element returned by most recent call to next or
231			* previous. Reset to 0 if this element is deleted by a call
232			* to remove.
233			*/
234			int lastRet = 0;
235
236			/**
237			* The modCount value that the iterator believes that the backing
238			* List should have. If this expectation is violated, the iterator
239			* has detected concurrent modification.
240			*/
241			int expectedModCount = modCount;
242
243			public boolean hasNext() {
244			return cursor <= size;
245			}
246	tim	1.2
247	dl	1.5	public E next() {
248	tim	1.2	checkForComodification();
249			if (cursor > size)
250	dl	1.5	throw new NoSuchElementException();
251	tim	1.2	E result = queue[cursor];
252			lastRet = cursor++;
253			return result;
254	dl	1.5	}
255	tim	1.2
256	dl	1.5	public void remove() {
257			if (lastRet == 0)
258			throw new IllegalStateException();
259	tim	1.2	checkForComodification();
260
261			PriorityQueue.this.remove(lastRet);
262			if (lastRet < cursor)
263			cursor--;
264			lastRet = 0;
265			expectedModCount = modCount;
266	dl	1.5	}
267	tim	1.2
268	dl	1.5	final void checkForComodification() {
269			if (modCount != expectedModCount)
270			throw new ConcurrentModificationException();
271			}
272	tim	1.2	}
273
274			/**
275			* Returns the number of elements in this priority queue.
276	dl	1.5	*
277	tim	1.2	* @return the number of elements in this priority queue.
278			*/
279	tim	1.1	public int size() {
280	tim	1.2	return size;
281	tim	1.1	}
282	tim	1.2
283			/**
284			* Add the specified element to this priority queue.
285			*
286			* @param element the element to add.
287			* @return true
288			* @throws ClassCastException if the specified element cannot be compared
289	dl	1.5	* with elements currently in the priority queue according
290	tim	1.2	* to the priority queue's ordering.
291			* @throws NullPointerException if the specified element is null.
292			*/
293			public boolean offer(E element) {
294			if (element == null)
295			throw new NullPointerException();
296			modCount++;
297
298			// Grow backing store if necessary
299			if (++size == queue.length) {
300			E[] newQueue = new E[2 * queue.length];
301			System.arraycopy(queue, 0, newQueue, 0, size);
302			queue = newQueue;
303			}
304
305			queue[size] = element;
306			fixUp(size);
307			return true;
308	tim	1.1	}
309
310	tim	1.2	/**
311			* Remove all elements from the priority queue.
312			*/
313			public void clear() {
314			modCount++;
315
316			// Null out element references to prevent memory leak
317			for (int i=1; i<=size; i++)
318			queue[i] = null;
319
320			size = 0;
321			}
322
323			/**
324			* Removes and returns the ith element from queue. Recall
325			* that queue is one-based, so 1 <= i <= size.
326			*
327			* XXX: Could further special-case i==size, but is it worth it?
328			* XXX: Could special-case i==0, but is it worth it?
329			*/
330			private E remove(int i) {
331			assert i <= size;
332			modCount++;
333
334			E result = queue[i];
335			queue[i] = queue[size];
336			queue[size--] = null; // Drop extra ref to prevent memory leak
337			if (i <= size)
338			fixDown(i);
339			return result;
340	tim	1.1	}
341
342	tim	1.2	/**
343			* Establishes the heap invariant (described above) assuming the heap
344			* satisfies the invariant except possibly for the leaf-node indexed by k
345			* (which may have a nextExecutionTime less than its parent's).
346			*
347			* This method functions by "promoting" queue[k] up the hierarchy
348			* (by swapping it with its parent) repeatedly until queue[k]
349			* is greater than or equal to its parent.
350			*/
351			private void fixUp(int k) {
352			if (comparator == null) {
353			while (k > 1) {
354			int j = k >> 1;
355			if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
356			break;
357			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
358			k = j;
359			}
360			} else {
361			while (k > 1) {
362			int j = k >> 1;
363			if (comparator.compare(queue[j], queue[k]) <= 0)
364			break;
365			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
366			k = j;
367			}
368			}
369			}
370
371			/**
372			* Establishes the heap invariant (described above) in the subtree
373			* rooted at k, which is assumed to satisfy the heap invariant except
374			* possibly for node k itself (which may be greater than its children).
375			*
376			* This method functions by "demoting" queue[k] down the hierarchy
377			* (by swapping it with its smaller child) repeatedly until queue[k]
378			* is less than or equal to its children.
379			*/
380			private void fixDown(int k) {
381			int j;
382			if (comparator == null) {
383			while ((j = k << 1) <= size) {
384			if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
385			j++; // j indexes smallest kid
386			if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
387			break;
388			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
389			k = j;
390			}
391			} else {
392			while ((j = k << 1) <= size) {
393			if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
394			j++; // j indexes smallest kid
395			if (comparator.compare(queue[k], queue[j]) <= 0)
396			break;
397			E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
398			k = j;
399			}
400			}
401			}
402
403			/**
404			* Returns the comparator associated with this priority queue, or
405			* <tt>null</tt> if it uses its elements' natural ordering.
406			*
407			* @return the comparator associated with this priority queue, or
408	dl	1.5	* <tt>null</tt> if it uses its elements' natural ordering.
409	tim	1.2	*/
410	dl	1.5	Comparator comparator() {
411	tim	1.2	return comparator;
412			}
413	dl	1.5
414			/**
415			* Save the state of the instance to a stream (that
416			* is, serialize it).
417			*
418			* @serialData The length of the array backing the instance is
419			* emitted (int), followed by all of its elements (each an
420			* <tt>Object</tt>) in the proper order.
421			*/
422			private synchronized void writeObject(java.io.ObjectOutputStream s)
423			throws java.io.IOException{
424			// Write out element count, and any hidden stuff
425			s.defaultWriteObject();
426
427			// Write out array length
428			s.writeInt(queue.length);
429
430			// Write out all elements in the proper order.
431			for (int i=0; i<size; i++)
432			s.writeObject(queue[i]);
433			}
434
435			/**
436			* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
437			* deserialize it).
438			*/
439			private synchronized void readObject(java.io.ObjectInputStream s)
440			throws java.io.IOException, ClassNotFoundException {
441			// Read in size, and any hidden stuff
442			s.defaultReadObject();
443
444			// Read in array length and allocate array
445			int arrayLength = s.readInt();
446			queue = new E[arrayLength];
447
448			// Read in all elements in the proper order.
449			for (int i=0; i<size; i++)
450			queue[i] = (E)s.readObject();
451			}
452
453	tim	1.1	}