java/util/PriorityQueue.java

 package java.util;

/**
 * An unbounded priority {@linkplain Queue 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 java.util.TreeSet} and
 * {@link java.util.TreeMap}: elements are ordered
 * either according to their <i>natural order</i> (see {@link Comparable}), or
 * according to a {@link java.util.Comparator}, depending on which
 * constructor is used.
 * <p>The <em>head</em> of this queue is the <em>least</em> element with
 * respect to the specified ordering.
 * If multiple elements are tied for least value, the
 * head is one of those elements. A priority queue does not permit
 * <tt>null</tt> elements.
 *
 * <p>The {@link #remove()} and {@link #poll()} methods remove and
 * return the head of the queue.
 *
 * <p>The {@link #element()} and {@link #peek()} methods return, but do
 * not delete, the head of the queue.
 *
 * <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>The Iterator provided in method {@link #iterator()} is <em>not</em>
 * guaranteed to traverse the elements of the PriorityQueue in any
 * particular order. If you need ordered traversal, consider using
 * <tt>Arrays.sort(pq.toArray())</tt>.
 *
 * <p> <strong>Note that this implementation is not synchronized.</strong>
 * Multiple threads should not access a <tt>PriorityQueue</tt>
 * instance concurrently if any of the threads modifies the list
 * structurally. Instead, use the thread-safe {@link
 * java.util.concurrent.PriorityBlockingQueue} class.
 *
 * 
 * <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 long serialVersionUID = -7720805057305804111L;

    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 Object[] 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<? super 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;

    /**
     * Creates a <tt>PriorityQueue</tt> 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, null);
    }

    /**
     * Creates a <tt>PriorityQueue</tt> 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.
     * @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
     * than 1
     */
    public PriorityQueue(int initialCapacity) {
        this(initialCapacity, null);
    }

    /**
     * Creates a <tt>PriorityQueue</tt> with the specified initial capacity
     * 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.
     * If <tt>null</tt> then the order depends on the elements' natural
     * ordering.
     * @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
     * than 1
     */
    public PriorityQueue(int initialCapacity, 
                         Comparator<? super E> comparator) {
        if (initialCapacity < 1)
            throw new IllegalArgumentException();
        this.queue = new Object[initialCapacity + 1];
        this.comparator = comparator;
    }

    /**
     * Common code to initialize underlying queue array across
     * constructors below.
     */
    private void initializeArray(Collection<? extends E> c) {
        int sz = c.size();
        int initialCapacity = (int)Math.min((sz * 110L) / 100,
                                            Integer.MAX_VALUE - 1);
        if (initialCapacity < 1)
            initialCapacity = 1;

        this.queue = new Object[initialCapacity + 1];
    }

    /**
     * Initially fill elements of the queue array under the 
     * knowledge that it is sorted or is another PQ, in which
     * case we can just place the elements without fixups.
     */
    private void fillFromSorted(Collection<? extends E> c) {
        for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
            queue[++size] = i.next();
    }


    /**
     * Initially fill elements of the queue array that is
     * not to our knowledge sorted, so we must add them
     * one by one.
     */
    private void fillFromUnsorted(Collection<? extends E> c) {
        for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
            add(i.next());
    }

    /**
     * Creates a <tt>PriorityQueue</tt> containing the elements in the
     * specified collection.  The priority queue has an initial
     * capacity of 110% of the size of the specified collection or 1
     * if the collection is empty.  If the specified collection is an
     * instance of a {@link java.util.SortedSet} or is another
     * <tt>PriorityQueue</tt>, 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.  Otherwise, the priority queue is
     * ordered according to its elements' natural order.
     *
     * @param c 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 <tt>c</tt> or any element within it
     * is <tt>null</tt>
     */
    public PriorityQueue(Collection<? extends E> c) {
        initializeArray(c);
        if (c instanceof SortedSet) {
            // @fixme double-cast workaround for compiler
            SortedSet<? extends E> s = (SortedSet<? extends E>) (SortedSet)c;
            comparator = (Comparator<? super E>)s.comparator();
            fillFromSorted(s);
        } else if (c instanceof PriorityQueue) {
            PriorityQueue<? extends E> s = (PriorityQueue<? extends E>) c;
            comparator = (Comparator<? super E>)s.comparator();
            fillFromSorted(s);
        } else {
            comparator = null;
            fillFromUnsorted(c);
        }
    }

    /**
     * Creates a <tt>PriorityQueue</tt> containing the elements in the
     * specified collection.  The priority queue has an initial
     * capacity of 110% of the size of the specified collection or 1
     * if the collection is empty.  This priority queue will be sorted
     * according to the same comparator as the given collection, or
     * according to its elements' natural order if the collection is
     * sorted according to its elements' natural order.
     *
     * @param c 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 <tt>c</tt> or any element within it
     * is <tt>null</tt>
     */
    public PriorityQueue(PriorityQueue<? extends E> c) {
        initializeArray(c);
        comparator = (Comparator<? super E>)c.comparator();
        fillFromSorted(c);
    }

    /**
     * Creates a <tt>PriorityQueue</tt> containing the elements in the
     * specified collection.  The priority queue has an initial
     * capacity of 110% of the size of the specified collection or 1
     * if the collection is empty.  This priority queue will be sorted
     * according to the same comparator as the given collection, or
     * according to its elements' natural order if the collection is
     * sorted according to its elements' natural order.
     *
     * @param c 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 <tt>c</tt> or any element within it
     * is <tt>null</tt>
     */
    public PriorityQueue(SortedSet<? extends E> c) {
        initializeArray(c);
        comparator = (Comparator<? super E>)c.comparator();
        fillFromSorted(c);
    }

    /**
     * Resize array, if necessary, to be able to hold given index
     */
    private void grow(int index) {
        int newlen = queue.length;
        if (index < newlen) // don't need to grow
            return;
        if (index == Integer.MAX_VALUE)
            throw new OutOfMemoryError();
        while (newlen <= index) {
            if (newlen >= Integer.MAX_VALUE / 2)  // avoid overflow
                newlen = Integer.MAX_VALUE;
            else
                newlen <<= 2;
        }
        Object[] newQueue = new Object[newlen];
        System.arraycopy(queue, 0, newQueue, 0, queue.length);
        queue = newQueue;
    }
            
    // Queue Methods


    /**
     * Add the specified element to this priority queue.
     *
     * @return <tt>true</tt>
     * @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 <tt>null</tt>.
     */
    public boolean offer(E o) {
        if (o == null)
            throw new NullPointerException();
        modCount++;
        ++size;

        // Grow backing store if necessary
        if (size >= queue.length) 
            grow(size);

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

    public E poll() {
        if (size == 0)
            return null;
        return (E) remove(1);
    }

    public E peek() {
        return (E) queue[1];
    }

    // Collection Methods - the first two override to update docs

    /**
     * Adds the specified element to this queue.
     * @return <tt>true</tt> (as per the general contract of
     * <tt>Collection.add</tt>).
     *
     * @throws NullPointerException {@inheritDoc}
     * @throws ClassCastException if the specified element cannot be compared
     * with elements currently in the priority queue according
     * to the priority queue's ordering.
     */
    public boolean add(E o) {
        return super.add(o);
    }

   
    /**
     * Adds all of the elements in the specified collection to this queue.
     * The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (This implies that the behavior of this call is undefined if
     * the specified collection is this queue, and this queue is nonempty.)
     * <p>
     * This implementation iterates over the specified collection, and adds
     * each object returned by the iterator to this collection, in turn.
     * @throws NullPointerException {@inheritDoc}
     * @throws ClassCastException if any element cannot be compared
     * with elements currently in the priority queue according
     * to the priority queue's ordering.
     */
    public boolean addAll(Collection<? extends E> c) {
        return super.addAll(c);
    }


 /**
     * Removes a single instance of the specified element from this
     * queue, if it is present.  More formally,
     * removes an element <tt>e</tt> such that <tt>(o==null ? e==null :
     * o.equals(e))</tt>, if the queue contains one or more such
     * elements.  Returns <tt>true</tt> if the queue contained the
     * specified element (or equivalently, if the queue changed as a
     * result of the call).
     *
     * <p>This implementation iterates over the queue looking for the
     * specified element.  If it finds the element, it removes the element
     * from the queue using the iterator's remove method.<p>
     *
     */
    public boolean remove(Object o) {
        if (o == null)
            return false;

        if (comparator == null) {
            for (int i = 1; i <= size; i++) {
                if (((Comparable<E>)queue[i]).compareTo((E)o) == 0) {
                    remove(i);
                    return true;
                }
            }
        } else {
            for (int i = 1; i <= size; i++) {
                if (comparator.compare((E)queue[i], (E)o) == 0) {
                    remove(i);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Returns an iterator over the elements in this queue. The iterator
     * does not return the elements in any particular order.
     *
     * @return an iterator over the elements in this 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;

        // Workarounds until version that better handles remove() installed.
        // These are used to copy-on-write the array upon first remove
        private Object[] q = queue;
        private int qsize = size;

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

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

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

            // Copy on first remove
            if (q == queue) {
                q = new Object[queue.length];
                System.arraycopy(queue, 0, q, 0, queue.length);
            }
            PriorityQueue.this.remove(q[lastRet]);

            lastRet = 0;
            expectedModCount = modCount;
        }

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

    public int size() {
        return size;
    }

    /**
     * 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.
     *
     */
    private E remove(int i) {
        assert i <= size;
        modCount++;

        E result = (E) queue[i];
        queue[i] = queue[size];
        queue[size--] = null;  // Drop extra ref to prevent memory leak
        if (i <= size) {
            fixDown(i);
            fixUp(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<E>)queue[j]).compareTo((E)queue[k]) <= 0)
                    break;
                Object tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        } else {
            while (k > 1) {
                int j = k >>> 1;
                if (comparator.compare((E)queue[j], (E)queue[k]) <= 0)
                    break;
                Object 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 && (j > 0)) {
                if (j<size && 
                    ((Comparable<E>)queue[j]).compareTo((E)queue[j+1]) > 0)
                    j++; // j indexes smallest kid

                if (((Comparable<E>)queue[k]).compareTo((E)queue[j]) <= 0)
                    break;
                Object tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        } else {
            while ((j = k << 1) <= size && (j > 0)) {
                if (j<size && 
                    comparator.compare((E)queue[j], (E)queue[j+1]) > 0)
                    j++; // j indexes smallest kid
                if (comparator.compare((E)queue[k], (E)queue[j]) <= 0)
                    break;
                Object tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
                k = j;
            }
        }

    }


    /**
     * Returns the comparator used to order this collection, or <tt>null</tt>
     * if this collection is sorted according to its elements natural ordering
     * (using <tt>Comparable</tt>).
     *
     * @return the comparator used to order this collection, or <tt>null</tt>
     * if this collection is sorted according to its elements natural ordering.
     */
    public Comparator<? super E> 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 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 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 Object[arrayLength];

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

}

Revision:	1.35
Committed:	Wed Aug 27 10:27:07 2003 UTC (20 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.34:	+27 -9 lines
Log Message:	Fix remove(i); COW workaround for iter.remove()
#	Content
1	package java.util;
2
3	/**
4	* An unbounded priority {@linkplain Queue queue} based on a priority heap.
5	* This queue orders
6	* elements according to an order specified at construction time, which is
7	* specified in the same manner as {@link java.util.TreeSet} and
8	* {@link java.util.TreeMap}: elements are ordered
9	* either according to their <i>natural order</i> (see {@link Comparable}), or
10	* according to a {@link java.util.Comparator}, depending on which
11	* constructor is used.
12	* <p>The <em>head</em> of this queue is the <em>least</em> element with
13	* respect to the specified ordering.
14	* If multiple elements are tied for least value, the
15	* head is one of those elements. A priority queue does not permit
16	* <tt>null</tt> elements.
17	*
18	* <p>The {@link #remove()} and {@link #poll()} methods remove and
19	* return the head of the queue.
20	*
21	* <p>The {@link #element()} and {@link #peek()} methods return, but do
22	* not delete, the head of the queue.
23	*
24	* <p>A priority queue has a <i>capacity</i>. The capacity is the
25	* size of the array used internally to store the elements on the
26	* queue.
27	* It is always at least as large as the queue size. As
28	* elements are added to a priority queue, its capacity grows
29	* automatically. The details of the growth policy are not specified.
30	*
31	* <p>The Iterator provided in method {@link #iterator()} is <em>not</em>
32	* guaranteed to traverse the elements of the PriorityQueue in any
33	* particular order. If you need ordered traversal, consider using
34	* <tt>Arrays.sort(pq.toArray())</tt>.
35	*
36	* <p> <strong>Note that this implementation is not synchronized.</strong>
37	* Multiple threads should not access a <tt>PriorityQueue</tt>
38	* instance concurrently if any of the threads modifies the list
39	* structurally. Instead, use the thread-safe {@link
40	* java.util.concurrent.PriorityBlockingQueue} class.
41	*
42	*
43	* <p>Implementation note: this implementation provides O(log(n)) time
44	* for the insertion methods (<tt>offer</tt>, <tt>poll</tt>,
45	* <tt>remove()</tt> and <tt>add</tt>) methods; linear time for the
46	* <tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and
47	* constant time for the retrieval methods (<tt>peek</tt>,
48	* <tt>element</tt>, and <tt>size</tt>).
49	*
50	* <p>This class is a member of the
51	* <a href="{@docRoot}/../guide/collections/index.html">
52	* Java Collections Framework</a>.
53	* @since 1.5
54	* @author Josh Bloch
55	*/
56	public class PriorityQueue<E> extends AbstractQueue<E>
57	implements Queue<E>, java.io.Serializable {
58
59	private static final long serialVersionUID = -7720805057305804111L;
60
61	private static final int DEFAULT_INITIAL_CAPACITY = 11;
62
63	/**
64	* Priority queue represented as a balanced binary heap: the two children
65	* of queue[n] are queue[2n] and queue[2n + 1]. The priority queue is
66	* ordered by comparator, or by the elements' natural ordering, if
67	* comparator is null: For each node n in the heap and each descendant d
68	* of n, n <= d.
69	*
70	* The element with the lowest value is in queue[1], assuming the queue is
71	* nonempty. (A one-based array is used in preference to the traditional
72	* zero-based array to simplify parent and child calculations.)
73	*
74	* queue.length must be >= 2, even if size == 0.
75	*/
76	private transient Object[] queue;
77
78	/**
79	* The number of elements in the priority queue.
80	*/
81	private int size = 0;
82
83	/**
84	* The comparator, or null if priority queue uses elements'
85	* natural ordering.
86	*/
87	private final Comparator<? super E> comparator;
88
89	/**
90	* The number of times this priority queue has been
91	* <i>structurally modified</i>. See AbstractList for gory details.
92	*/
93	private transient int modCount = 0;
94
95	/**
96	* Creates a <tt>PriorityQueue</tt> with the default initial capacity
97	* (11) that orders its elements according to their natural
98	* ordering (using <tt>Comparable</tt>).
99	*/
100	public PriorityQueue() {
101	this(DEFAULT_INITIAL_CAPACITY, null);
102	}
103
104	/**
105	* Creates a <tt>PriorityQueue</tt> with the specified initial capacity
106	* that orders its elements according to their natural ordering
107	* (using <tt>Comparable</tt>).
108	*
109	* @param initialCapacity the initial capacity for this priority queue.
110	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
111	* than 1
112	*/
113	public PriorityQueue(int initialCapacity) {
114	this(initialCapacity, null);
115	}
116
117	/**
118	* Creates a <tt>PriorityQueue</tt> with the specified initial capacity
119	* that orders its elements according to the specified comparator.
120	*
121	* @param initialCapacity the initial capacity for this priority queue.
122	* @param comparator the comparator used to order this priority queue.
123	* If <tt>null</tt> then the order depends on the elements' natural
124	* ordering.
125	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
126	* than 1
127	*/
128	public PriorityQueue(int initialCapacity,
129	Comparator<? super E> comparator) {
130	if (initialCapacity < 1)
131	throw new IllegalArgumentException();
132	this.queue = new Object[initialCapacity + 1];
133	this.comparator = comparator;
134	}
135
136	/**
137	* Common code to initialize underlying queue array across
138	* constructors below.
139	*/
140	private void initializeArray(Collection<? extends E> c) {
141	int sz = c.size();
142	int initialCapacity = (int)Math.min((sz * 110L) / 100,
143	Integer.MAX_VALUE - 1);
144	if (initialCapacity < 1)
145	initialCapacity = 1;
146
147	this.queue = new Object[initialCapacity + 1];
148	}
149
150	/**
151	* Initially fill elements of the queue array under the
152	* knowledge that it is sorted or is another PQ, in which
153	* case we can just place the elements without fixups.
154	*/
155	private void fillFromSorted(Collection<? extends E> c) {
156	for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
157	queue[++size] = i.next();
158	}
159
160
161	/**
162	* Initially fill elements of the queue array that is
163	* not to our knowledge sorted, so we must add them
164	* one by one.
165	*/
166	private void fillFromUnsorted(Collection<? extends E> c) {
167	for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
168	add(i.next());
169	}
170
171	/**
172	* Creates a <tt>PriorityQueue</tt> containing the elements in the
173	* specified collection. The priority queue has an initial
174	* capacity of 110% of the size of the specified collection or 1
175	* if the collection is empty. If the specified collection is an
176	* instance of a {@link java.util.SortedSet} or is another
177	* <tt>PriorityQueue</tt>, the priority queue will be sorted
178	* according to the same comparator, or according to its elements'
179	* natural order if the collection is sorted according to its
180	* elements' natural order. Otherwise, the priority queue is
181	* ordered according to its elements' natural order.
182	*
183	* @param c the collection whose elements are to be placed
184	* into this priority queue.
185	* @throws ClassCastException if elements of the specified collection
186	* cannot be compared to one another according to the priority
187	* queue's ordering.
188	* @throws NullPointerException if <tt>c</tt> or any element within it
189	* is <tt>null</tt>
190	*/
191	public PriorityQueue(Collection<? extends E> c) {
192	initializeArray(c);
193	if (c instanceof SortedSet) {
194	// @fixme double-cast workaround for compiler
195	SortedSet<? extends E> s = (SortedSet<? extends E>) (SortedSet)c;
196	comparator = (Comparator<? super E>)s.comparator();
197	fillFromSorted(s);
198	} else if (c instanceof PriorityQueue) {
199	PriorityQueue<? extends E> s = (PriorityQueue<? extends E>) c;
200	comparator = (Comparator<? super E>)s.comparator();
201	fillFromSorted(s);
202	} else {
203	comparator = null;
204	fillFromUnsorted(c);
205	}
206	}
207
208	/**
209	* Creates a <tt>PriorityQueue</tt> containing the elements in the
210	* specified collection. The priority queue has an initial
211	* capacity of 110% of the size of the specified collection or 1
212	* if the collection is empty. This priority queue will be sorted
213	* according to the same comparator as the given collection, or
214	* according to its elements' natural order if the collection is
215	* sorted according to its elements' natural order.
216	*
217	* @param c the collection whose elements are to be placed
218	* into this priority queue.
219	* @throws ClassCastException if elements of the specified collection
220	* cannot be compared to one another according to the priority
221	* queue's ordering.
222	* @throws NullPointerException if <tt>c</tt> or any element within it
223	* is <tt>null</tt>
224	*/
225	public PriorityQueue(PriorityQueue<? extends E> c) {
226	initializeArray(c);
227	comparator = (Comparator<? super E>)c.comparator();
228	fillFromSorted(c);
229	}
230
231	/**
232	* Creates a <tt>PriorityQueue</tt> containing the elements in the
233	* specified collection. The priority queue has an initial
234	* capacity of 110% of the size of the specified collection or 1
235	* if the collection is empty. This priority queue will be sorted
236	* according to the same comparator as the given collection, or
237	* according to its elements' natural order if the collection is
238	* sorted according to its elements' natural order.
239	*
240	* @param c the collection whose elements are to be placed
241	* into this priority queue.
242	* @throws ClassCastException if elements of the specified collection
243	* cannot be compared to one another according to the priority
244	* queue's ordering.
245	* @throws NullPointerException if <tt>c</tt> or any element within it
246	* is <tt>null</tt>
247	*/
248	public PriorityQueue(SortedSet<? extends E> c) {
249	initializeArray(c);
250	comparator = (Comparator<? super E>)c.comparator();
251	fillFromSorted(c);
252	}
253
254	/**
255	* Resize array, if necessary, to be able to hold given index
256	*/
257	private void grow(int index) {
258	int newlen = queue.length;
259	if (index < newlen) // don't need to grow
260	return;
261	if (index == Integer.MAX_VALUE)
262	throw new OutOfMemoryError();
263	while (newlen <= index) {
264	if (newlen >= Integer.MAX_VALUE / 2) // avoid overflow
265	newlen = Integer.MAX_VALUE;
266	else
267	newlen <<= 2;
268	}
269	Object[] newQueue = new Object[newlen];
270	System.arraycopy(queue, 0, newQueue, 0, queue.length);
271	queue = newQueue;
272	}
273
274	// Queue Methods
275
276
277
278	/**
279	* Add the specified element to this priority queue.
280	*
281	* @return <tt>true</tt>
282	* @throws ClassCastException if the specified element cannot be compared
283	* with elements currently in the priority queue according
284	* to the priority queue's ordering.
285	* @throws NullPointerException if the specified element is <tt>null</tt>.
286	*/
287	public boolean offer(E o) {
288	if (o == null)
289	throw new NullPointerException();
290	modCount++;
291	++size;
292
293	// Grow backing store if necessary
294	if (size >= queue.length)
295	grow(size);
296
297	queue[size] = o;
298	fixUp(size);
299	return true;
300	}
301
302	public E poll() {
303	if (size == 0)
304	return null;
305	return (E) remove(1);
306	}
307
308	public E peek() {
309	return (E) queue[1];
310	}
311
312	// Collection Methods - the first two override to update docs
313
314	/**
315	* Adds the specified element to this queue.
316	* @return <tt>true</tt> (as per the general contract of
317	* <tt>Collection.add</tt>).
318	*
319	* @throws NullPointerException {@inheritDoc}
320	* @throws ClassCastException if the specified element cannot be compared
321	* with elements currently in the priority queue according
322	* to the priority queue's ordering.
323	*/
324	public boolean add(E o) {
325	return super.add(o);
326	}
327
328
329	/**
330	* Adds all of the elements in the specified collection to this queue.
331	* The behavior of this operation is undefined if
332	* the specified collection is modified while the operation is in
333	* progress. (This implies that the behavior of this call is undefined if
334	* the specified collection is this queue, and this queue is nonempty.)
335	* <p>
336	* This implementation iterates over the specified collection, and adds
337	* each object returned by the iterator to this collection, in turn.
338	* @throws NullPointerException {@inheritDoc}
339	* @throws ClassCastException if any element cannot be compared
340	* with elements currently in the priority queue according
341	* to the priority queue's ordering.
342	*/
343	public boolean addAll(Collection<? extends E> c) {
344	return super.addAll(c);
345	}
346
347
348	/**
349	* Removes a single instance of the specified element from this
350	* queue, if it is present. More formally,
351	* removes an element <tt>e</tt> such that <tt>(o==null ? e==null :
352	* o.equals(e))</tt>, if the queue contains one or more such
353	* elements. Returns <tt>true</tt> if the queue contained the
354	* specified element (or equivalently, if the queue changed as a
355	* result of the call).
356	*
357	* <p>This implementation iterates over the queue looking for the
358	* specified element. If it finds the element, it removes the element
359	* from the queue using the iterator's remove method.<p>
360	*
361	*/
362	public boolean remove(Object o) {
363	if (o == null)
364	return false;
365
366	if (comparator == null) {
367	for (int i = 1; i <= size; i++) {
368	if (((Comparable<E>)queue[i]).compareTo((E)o) == 0) {
369	remove(i);
370	return true;
371	}
372	}
373	} else {
374	for (int i = 1; i <= size; i++) {
375	if (comparator.compare((E)queue[i], (E)o) == 0) {
376	remove(i);
377	return true;
378	}
379	}
380	}
381	return false;
382	}
383
384	/**
385	* Returns an iterator over the elements in this queue. The iterator
386	* does not return the elements in any particular order.
387	*
388	* @return an iterator over the elements in this queue.
389	*/
390	public Iterator<E> iterator() {
391	return new Itr();
392	}
393
394	private class Itr implements Iterator<E> {
395
396	/**
397	* Index (into queue array) of element to be returned by
398	* subsequent call to next.
399	*/
400	private int cursor = 1;
401
402	/**
403	* Index of element returned by most recent call to next or
404	* previous. Reset to 0 if this element is deleted by a call
405	* to remove.
406	*/
407	private int lastRet = 0;
408
409	/**
410	* The modCount value that the iterator believes that the backing
411	* List should have. If this expectation is violated, the iterator
412	* has detected concurrent modification.
413	*/
414	private int expectedModCount = modCount;
415
416	// Workarounds until version that better handles remove() installed.
417	// These are used to copy-on-write the array upon first remove
418	private Object[] q = queue;
419	private int qsize = size;
420
421	public boolean hasNext() {
422	return cursor <= qsize;
423	}
424
425	public E next() {
426	checkForComodification();
427	if (cursor > qsize)
428	throw new NoSuchElementException();
429	E result = (E) q[cursor];
430	lastRet = cursor++;
431	return result;
432	}
433
434	public void remove() {
435	if (lastRet == 0)
436	throw new IllegalStateException();
437	checkForComodification();
438
439	// Copy on first remove
440	if (q == queue) {
441	q = new Object[queue.length];
442	System.arraycopy(queue, 0, q, 0, queue.length);
443	}
444	PriorityQueue.this.remove(q[lastRet]);
445
446	lastRet = 0;
447	expectedModCount = modCount;
448	}
449
450	final void checkForComodification() {
451	if (modCount != expectedModCount)
452	throw new ConcurrentModificationException();
453	}
454	}
455
456	public int size() {
457	return size;
458	}
459
460	/**
461	* Remove all elements from the priority queue.
462	*/
463	public void clear() {
464	modCount++;
465
466	// Null out element references to prevent memory leak
467	for (int i=1; i<=size; i++)
468	queue[i] = null;
469
470	size = 0;
471	}
472
473	/**
474	* Removes and returns the ith element from queue. Recall
475	* that queue is one-based, so 1 <= i <= size.
476	*
477	*/
478	private E remove(int i) {
479	assert i <= size;
480	modCount++;
481
482	E result = (E) queue[i];
483	queue[i] = queue[size];
484	queue[size--] = null; // Drop extra ref to prevent memory leak
485	if (i <= size) {
486	fixDown(i);
487	fixUp(i);
488	}
489
490	return result;
491	}
492
493	/**
494	* Establishes the heap invariant (described above) assuming the heap
495	* satisfies the invariant except possibly for the leaf-node indexed by k
496	* (which may have a nextExecutionTime less than its parent's).
497	*
498	* This method functions by "promoting" queue[k] up the hierarchy
499	* (by swapping it with its parent) repeatedly until queue[k]
500	* is greater than or equal to its parent.
501	*/
502	private void fixUp(int k) {
503	if (comparator == null) {
504	while (k > 1) {
505	int j = k >> 1;
506	if (((Comparable<E>)queue[j]).compareTo((E)queue[k]) <= 0)
507	break;
508	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
509	k = j;
510	}
511	} else {
512	while (k > 1) {
513	int j = k >>> 1;
514	if (comparator.compare((E)queue[j], (E)queue[k]) <= 0)
515	break;
516	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
517	k = j;
518	}
519	}
520	}
521
522	/**
523	* Establishes the heap invariant (described above) in the subtree
524	* rooted at k, which is assumed to satisfy the heap invariant except
525	* possibly for node k itself (which may be greater than its children).
526	*
527	* This method functions by "demoting" queue[k] down the hierarchy
528	* (by swapping it with its smaller child) repeatedly until queue[k]
529	* is less than or equal to its children.
530	*/
531	private void fixDown(int k) {
532	int j;
533	if (comparator == null) {
534	while ((j = k << 1) <= size && (j > 0)) {
535	if (j<size &&
536	((Comparable<E>)queue[j]).compareTo((E)queue[j+1]) > 0)
537	j++; // j indexes smallest kid
538
539	if (((Comparable<E>)queue[k]).compareTo((E)queue[j]) <= 0)
540	break;
541	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
542	k = j;
543	}
544	} else {
545	while ((j = k << 1) <= size && (j > 0)) {
546	if (j<size &&
547	comparator.compare((E)queue[j], (E)queue[j+1]) > 0)
548	j++; // j indexes smallest kid
549	if (comparator.compare((E)queue[k], (E)queue[j]) <= 0)
550	break;
551	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
552	k = j;
553	}
554	}
555
556	}
557
558
559	/**
560	* Returns the comparator used to order this collection, or <tt>null</tt>
561	* if this collection is sorted according to its elements natural ordering
562	* (using <tt>Comparable</tt>).
563	*
564	* @return the comparator used to order this collection, or <tt>null</tt>
565	* if this collection is sorted according to its elements natural ordering.
566	*/
567	public Comparator<? super E> comparator() {
568	return comparator;
569	}
570
571	/**
572	* Save the state of the instance to a stream (that
573	* is, serialize it).
574	*
575	* @serialData The length of the array backing the instance is
576	* emitted (int), followed by all of its elements (each an
577	* <tt>Object</tt>) in the proper order.
578	* @param s the stream
579	*/
580	private void writeObject(java.io.ObjectOutputStream s)
581	throws java.io.IOException{
582	// Write out element count, and any hidden stuff
583	s.defaultWriteObject();
584
585	// Write out array length
586	s.writeInt(queue.length);
587
588	// Write out all elements in the proper order.
589	for (int i=0; i<size; i++)
590	s.writeObject(queue[i]);
591	}
592
593	/**
594	* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
595	* deserialize it).
596	* @param s the stream
597	*/
598	private void readObject(java.io.ObjectInputStream s)
599	throws java.io.IOException, ClassNotFoundException {
600	// Read in size, and any hidden stuff
601	s.defaultReadObject();
602
603	// Read in array length and allocate array
604	int arrayLength = s.readInt();
605	queue = new Object[arrayLength];
606
607	// Read in all elements in the proper order.
608	for (int i=0; i<size; i++)
609	queue[i] = s.readObject();
610	}
611
612	}
613