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>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 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;

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

        public E next() {
            checkForComodification();
            if (cursor > size)
                throw new NoSuchElementException();
            E result = (E) 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();
        }
    }

    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.
     *
     * 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 = (E) 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<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) {
                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) {
                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:	1.28
Committed:	Wed Aug 13 14:11:59 2003 UTC (20 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.27:	+2 -1 lines
Log Message:	Added raw cast as workaround for compiler bug
#	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>Implementation note: this implementation provides O(log(n)) time
32	* for the insertion methods (<tt>offer</tt>, <tt>poll</tt>,
33	* <tt>remove()</tt> and <tt>add</tt>) methods; linear time for the
34	* <tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and
35	* constant time for the retrieval methods (<tt>peek</tt>,
36	* <tt>element</tt>, and <tt>size</tt>).
37	*
38	* <p>This class is a member of the
39	* <a href="{@docRoot}/../guide/collections/index.html">
40	* Java Collections Framework</a>.
41	* @since 1.5
42	* @author Josh Bloch
43	*/
44	public class PriorityQueue<E> extends AbstractQueue<E>
45	implements Queue<E>, java.io.Serializable {
46
47	private static final int DEFAULT_INITIAL_CAPACITY = 11;
48
49	/**
50	* Priority queue represented as a balanced binary heap: the two children
51	* of queue[n] are queue[2n] and queue[2n + 1]. The priority queue is
52	* ordered by comparator, or by the elements' natural ordering, if
53	* comparator is null: For each node n in the heap and each descendant d
54	* of n, n <= d.
55	*
56	* The element with the lowest value is in queue[1], assuming the queue is
57	* nonempty. (A one-based array is used in preference to the traditional
58	* zero-based array to simplify parent and child calculations.)
59	*
60	* queue.length must be >= 2, even if size == 0.
61	*/
62	private transient Object[] queue;
63
64	/**
65	* The number of elements in the priority queue.
66	*/
67	private int size = 0;
68
69	/**
70	* The comparator, or null if priority queue uses elements'
71	* natural ordering.
72	*/
73	private final Comparator<? super E> comparator;
74
75	/**
76	* The number of times this priority queue has been
77	* <i>structurally modified</i>. See AbstractList for gory details.
78	*/
79	private transient int modCount = 0;
80
81	/**
82	* Creates a <tt>PriorityQueue</tt> with the default initial capacity
83	* (11) that orders its elements according to their natural
84	* ordering (using <tt>Comparable</tt>).
85	*/
86	public PriorityQueue() {
87	this(DEFAULT_INITIAL_CAPACITY, null);
88	}
89
90	/**
91	* Creates a <tt>PriorityQueue</tt> with the specified initial capacity
92	* that orders its elements according to their natural ordering
93	* (using <tt>Comparable</tt>).
94	*
95	* @param initialCapacity the initial capacity for this priority queue.
96	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
97	* than 1
98	*/
99	public PriorityQueue(int initialCapacity) {
100	this(initialCapacity, null);
101	}
102
103	/**
104	* Creates a <tt>PriorityQueue</tt> with the specified initial capacity
105	* that orders its elements according to the specified comparator.
106	*
107	* @param initialCapacity the initial capacity for this priority queue.
108	* @param comparator the comparator used to order this priority queue.
109	* If <tt>null</tt> then the order depends on the elements' natural
110	* ordering.
111	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
112	* than 1
113	*/
114	public PriorityQueue(int initialCapacity,
115	Comparator<? super E> comparator) {
116	if (initialCapacity < 1)
117	throw new IllegalArgumentException();
118	this.queue = new Object[initialCapacity + 1];
119	this.comparator = comparator;
120	}
121
122	/**
123	* Common code to initialize underlying queue array across
124	* constructors below.
125	*/
126	private void initializeArray(Collection<? extends E> c) {
127	int sz = c.size();
128	int initialCapacity = (int)Math.min((sz * 110L) / 100,
129	Integer.MAX_VALUE - 1);
130	if (initialCapacity < 1)
131	initialCapacity = 1;
132
133	this.queue = new Object[initialCapacity + 1];
134	}
135
136	/**
137	* Initially fill elements of the queue array under the
138	* knowledge that it is sorted or is another PQ, in which
139	* case we can just place the elements without fixups.
140	*/
141	private void fillFromSorted(Collection<? extends E> c) {
142	for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
143	queue[++size] = i.next();
144	}
145
146
147	/**
148	* Initially fill elements of the queue array that is
149	* not to our knowledge sorted, so we must add them
150	* one by one.
151	*/
152	private void fillFromUnsorted(Collection<? extends E> c) {
153	for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
154	add(i.next());
155	}
156
157	/**
158	* Creates a <tt>PriorityQueue</tt> containing the elements in the
159	* specified collection. The priority queue has an initial
160	* capacity of 110% of the size of the specified collection or 1
161	* if the collection is empty. If the specified collection is an
162	* instance of a {@link java.util.SortedSet} or is another
163	* <tt>PriorityQueue</tt>, the priority queue will be sorted
164	* according to the same comparator, or according to its elements'
165	* natural order if the collection is sorted according to its
166	* elements' natural order. Otherwise, the priority queue is
167	* ordered according to its elements' natural order.
168	*
169	* @param c the collection whose elements are to be placed
170	* into this priority queue.
171	* @throws ClassCastException if elements of the specified collection
172	* cannot be compared to one another according to the priority
173	* queue's ordering.
174	* @throws NullPointerException if <tt>c</tt> or any element within it
175	* is <tt>null</tt>
176	*/
177	public PriorityQueue(Collection<? extends E> c) {
178	initializeArray(c);
179	if (c instanceof SortedSet) {
180	// @fixme double-cast workaround for compiler
181	SortedSet<? extends E> s = (SortedSet<? extends E>) (SortedSet)c;
182	comparator = (Comparator<? super E>)s.comparator();
183	fillFromSorted(s);
184	} else if (c instanceof PriorityQueue) {
185	PriorityQueue<? extends E> s = (PriorityQueue<? extends E>) c;
186	comparator = (Comparator<? super E>)s.comparator();
187	fillFromSorted(s);
188	} else {
189	comparator = null;
190	fillFromUnsorted(c);
191	}
192	}
193
194	/**
195	* Creates a <tt>PriorityQueue</tt> containing the elements in the
196	* specified collection. The priority queue has an initial
197	* capacity of 110% of the size of the specified collection or 1
198	* if the collection is empty. This priority queue will be sorted
199	* according to the same comparator as the given collection, or
200	* according to its elements' natural order if the collection is
201	* sorted according to its elements' natural order.
202	*
203	* @param c the collection whose elements are to be placed
204	* into this priority queue.
205	* @throws ClassCastException if elements of the specified collection
206	* cannot be compared to one another according to the priority
207	* queue's ordering.
208	* @throws NullPointerException if <tt>c</tt> or any element within it
209	* is <tt>null</tt>
210	*/
211	public PriorityQueue(PriorityQueue<? extends E> c) {
212	initializeArray(c);
213	comparator = (Comparator<? super E>)c.comparator();
214	fillFromSorted(c);
215	}
216
217	/**
218	* Creates a <tt>PriorityQueue</tt> containing the elements in the
219	* specified collection. The priority queue has an initial
220	* capacity of 110% of the size of the specified collection or 1
221	* if the collection is empty. This priority queue will be sorted
222	* according to the same comparator as the given collection, or
223	* according to its elements' natural order if the collection is
224	* sorted according to its elements' natural order.
225	*
226	* @param c the collection whose elements are to be placed
227	* into this priority queue.
228	* @throws ClassCastException if elements of the specified collection
229	* cannot be compared to one another according to the priority
230	* queue's ordering.
231	* @throws NullPointerException if <tt>c</tt> or any element within it
232	* is <tt>null</tt>
233	*/
234	public PriorityQueue(SortedSet<? extends E> c) {
235	initializeArray(c);
236	comparator = (Comparator<? super E>)c.comparator();
237	fillFromSorted(c);
238	}
239
240	/**
241	* Resize array, if necessary, to be able to hold given index
242	*/
243	private void grow(int index) {
244	int newlen = queue.length;
245	if (index < newlen) // don't need to grow
246	return;
247	if (index == Integer.MAX_VALUE)
248	throw new OutOfMemoryError();
249	while (newlen <= index) {
250	if (newlen >= Integer.MAX_VALUE / 2) // avoid overflow
251	newlen = Integer.MAX_VALUE;
252	else
253	newlen <<= 2;
254	}
255	Object[] newQueue = new Object[newlen];
256	System.arraycopy(queue, 0, newQueue, 0, queue.length);
257	queue = newQueue;
258	}
259
260	// Queue Methods
261
262
263
264	/**
265	* Add the specified element to this priority queue.
266	*
267	* @return <tt>true</tt>
268	* @throws ClassCastException if the specified element cannot be compared
269	* with elements currently in the priority queue according
270	* to the priority queue's ordering.
271	* @throws NullPointerException if the specified element is <tt>null</tt>.
272	*/
273	public boolean offer(E o) {
274	if (o == null)
275	throw new NullPointerException();
276	modCount++;
277	++size;
278
279	// Grow backing store if necessary
280	if (size >= queue.length)
281	grow(size);
282
283	queue[size] = o;
284	fixUp(size);
285	return true;
286	}
287
288	public E poll() {
289	if (size == 0)
290	return null;
291	return (E) remove(1);
292	}
293
294	public E peek() {
295	return (E) queue[1];
296	}
297
298	// Collection Methods - the first two override to update docs
299
300	/**
301	* Adds the specified element to this queue.
302	* @return <tt>true</tt> (as per the general contract of
303	* <tt>Collection.add</tt>).
304	*
305	* @throws NullPointerException {@inheritDoc}
306	* @throws ClassCastException if the specified element cannot be compared
307	* with elements currently in the priority queue according
308	* to the priority queue's ordering.
309	*/
310	public boolean add(E o) {
311	return super.add(o);
312	}
313
314
315	/**
316	* Adds all of the elements in the specified collection to this queue.
317	* The behavior of this operation is undefined if
318	* the specified collection is modified while the operation is in
319	* progress. (This implies that the behavior of this call is undefined if
320	* the specified collection is this queue, and this queue is nonempty.)
321	* <p>
322	* This implementation iterates over the specified collection, and adds
323	* each object returned by the iterator to this collection, in turn.
324	* @throws NullPointerException {@inheritDoc}
325	* @throws ClassCastException if any element cannot be compared
326	* with elements currently in the priority queue according
327	* to the priority queue's ordering.
328	*/
329	public boolean addAll(Collection<? extends E> c) {
330	return super.addAll(c);
331	}
332
333
334	/**
335	* Removes a single instance of the specified element from this
336	* queue, if it is present. More formally,
337	* removes an element <tt>e</tt> such that <tt>(o==null ? e==null :
338	* o.equals(e))</tt>, if the queue contains one or more such
339	* elements. Returns <tt>true</tt> if the queue contained the
340	* specified element (or equivalently, if the queue changed as a
341	* result of the call).
342	*
343	* <p>This implementation iterates over the queue looking for the
344	* specified element. If it finds the element, it removes the element
345	* from the queue using the iterator's remove method.<p>
346	*
347	*/
348	public boolean remove(Object o) {
349	if (o == null)
350	return false;
351
352	if (comparator == null) {
353	for (int i = 1; i <= size; i++) {
354	if (((Comparable<E>)queue[i]).compareTo((E)o) == 0) {
355	remove(i);
356	return true;
357	}
358	}
359	} else {
360	for (int i = 1; i <= size; i++) {
361	if (comparator.compare((E)queue[i], (E)o) == 0) {
362	remove(i);
363	return true;
364	}
365	}
366	}
367	return false;
368	}
369
370	/**
371	* Returns an iterator over the elements in this queue. The iterator
372	* does not return the elements in any particular order.
373	*
374	* @return an iterator over the elements in this queue.
375	*/
376	public Iterator<E> iterator() {
377	return new Itr();
378	}
379
380	private class Itr implements Iterator<E> {
381	/**
382	* Index (into queue array) of element to be returned by
383	* subsequent call to next.
384	*/
385	private int cursor = 1;
386
387	/**
388	* Index of element returned by most recent call to next or
389	* previous. Reset to 0 if this element is deleted by a call
390	* to remove.
391	*/
392	private int lastRet = 0;
393
394	/**
395	* The modCount value that the iterator believes that the backing
396	* List should have. If this expectation is violated, the iterator
397	* has detected concurrent modification.
398	*/
399	private int expectedModCount = modCount;
400
401	public boolean hasNext() {
402	return cursor <= size;
403	}
404
405	public E next() {
406	checkForComodification();
407	if (cursor > size)
408	throw new NoSuchElementException();
409	E result = (E) queue[cursor];
410	lastRet = cursor++;
411	return result;
412	}
413
414	public void remove() {
415	if (lastRet == 0)
416	throw new IllegalStateException();
417	checkForComodification();
418
419	PriorityQueue.this.remove(lastRet);
420	if (lastRet < cursor)
421	cursor--;
422	lastRet = 0;
423	expectedModCount = modCount;
424	}
425
426	final void checkForComodification() {
427	if (modCount != expectedModCount)
428	throw new ConcurrentModificationException();
429	}
430	}
431
432	public int size() {
433	return size;
434	}
435
436	/**
437	* Remove all elements from the priority queue.
438	*/
439	public void clear() {
440	modCount++;
441
442	// Null out element references to prevent memory leak
443	for (int i=1; i<=size; i++)
444	queue[i] = null;
445
446	size = 0;
447	}
448
449	/**
450	* Removes and returns the ith element from queue. Recall
451	* that queue is one-based, so 1 <= i <= size.
452	*
453	* XXX: Could further special-case i==size, but is it worth it?
454	* XXX: Could special-case i==0, but is it worth it?
455	*/
456	private E remove(int i) {
457	assert i <= size;
458	modCount++;
459
460	E result = (E) queue[i];
461	queue[i] = queue[size];
462	queue[size--] = null; // Drop extra ref to prevent memory leak
463	if (i <= size)
464	fixDown(i);
465	return result;
466	}
467
468	/**
469	* Establishes the heap invariant (described above) assuming the heap
470	* satisfies the invariant except possibly for the leaf-node indexed by k
471	* (which may have a nextExecutionTime less than its parent's).
472	*
473	* This method functions by "promoting" queue[k] up the hierarchy
474	* (by swapping it with its parent) repeatedly until queue[k]
475	* is greater than or equal to its parent.
476	*/
477	private void fixUp(int k) {
478	if (comparator == null) {
479	while (k > 1) {
480	int j = k >> 1;
481	if (((Comparable<E>)queue[j]).compareTo((E)queue[k]) <= 0)
482	break;
483	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
484	k = j;
485	}
486	} else {
487	while (k > 1) {
488	int j = k >> 1;
489	if (comparator.compare((E)queue[j], (E)queue[k]) <= 0)
490	break;
491	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
492	k = j;
493	}
494	}
495	}
496
497	/**
498	* Establishes the heap invariant (described above) in the subtree
499	* rooted at k, which is assumed to satisfy the heap invariant except
500	* possibly for node k itself (which may be greater than its children).
501	*
502	* This method functions by "demoting" queue[k] down the hierarchy
503	* (by swapping it with its smaller child) repeatedly until queue[k]
504	* is less than or equal to its children.
505	*/
506	private void fixDown(int k) {
507	int j;
508	if (comparator == null) {
509	while ((j = k << 1) <= size) {
510	if (j<size && ((Comparable<E>)queue[j]).compareTo((E)queue[j+1]) > 0)
511	j++; // j indexes smallest kid
512	if (((Comparable<E>)queue[k]).compareTo((E)queue[j]) <= 0)
513	break;
514	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
515	k = j;
516	}
517	} else {
518	while ((j = k << 1) <= size) {
519	if (j < size && comparator.compare((E)queue[j], (E)queue[j+1]) > 0)
520	j++; // j indexes smallest kid
521	if (comparator.compare((E)queue[k], (E)queue[j]) <= 0)
522	break;
523	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
524	k = j;
525	}
526	}
527	}
528
529
530	/**
531	* Returns the comparator used to order this collection, or <tt>null</tt>
532	* if this collection is sorted according to its elements natural ordering
533	* (using <tt>Comparable</tt>).
534	*
535	* @return the comparator used to order this collection, or <tt>null</tt>
536	* if this collection is sorted according to its elements natural ordering.
537	*/
538	public Comparator<? super E> comparator() {
539	return comparator;
540	}
541
542	/**
543	* Save the state of the instance to a stream (that
544	* is, serialize it).
545	*
546	* @serialData The length of the array backing the instance is
547	* emitted (int), followed by all of its elements (each an
548	* <tt>Object</tt>) in the proper order.
549	* @param s the stream
550	*/
551	private void writeObject(java.io.ObjectOutputStream s)
552	throws java.io.IOException{
553	// Write out element count, and any hidden stuff
554	s.defaultWriteObject();
555
556	// Write out array length
557	s.writeInt(queue.length);
558
559	// Write out all elements in the proper order.
560	for (int i=0; i<size; i++)
561	s.writeObject(queue[i]);
562	}
563
564	/**
565	* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
566	* deserialize it).
567	* @param s the stream
568	*/
569	private void readObject(java.io.ObjectInputStream s)
570	throws java.io.IOException, ClassNotFoundException {
571	// Read in size, and any hidden stuff
572	s.defaultReadObject();
573
574	// Read in array length and allocate array
575	int arrayLength = s.readInt();
576	queue = new Object[arrayLength];
577
578	// Read in all elements in the proper order.
579	for (int i=0; i<size; i++)
580	queue[i] = s.readObject();
581	}
582
583	}
584