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 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.
     */
    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 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<? extends E>) {
            SortedSet<? extends E> s = (SortedSet<? extends E>) c;
            comparator = (Comparator<? super E>)s.comparator();
            fillFromSorted(s);
        }
        else if (c instanceof PriorityQueue<? extends E>) {
            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

    // these first two override just to get the throws docs

    /**
     * @throws NullPointerException if the specified element is <tt>null</tt>.
     * @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);
    }

    /**
     * @throws ClassCastException if any element cannot be compared
     * with elements currently in the priority queue according
     * to the priority queue's ordering.
     * @throws NullPointerException if <tt>c</tt> or any element in <tt>c</tt>
     * is <tt>null</tt>
     */
    public boolean addAll(Collection<? extends E> c) {
        return super.addAll(c);
    }

    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;
    }

    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();
        }
    }

    /**
     * Returns the number of elements in this priority queue.
     *
     * @return the number of elements in this priority queue.
     */
    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;
            }
        }
    }

    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:	1.22
Committed:	Tue Aug 5 12:11:08 2003 UTC (20 years, 9 months ago) by dl
Branch:	MAIN
Changes since 1.21:	+128 -32 lines
Log Message:	Remove Sorted interface, adjust PQ and PBQ
#	Content
1	package java.util;
2
3	/**
4	* An unbounded priority queue based on a priority heap. This queue orders
5	* elements according to an order specified at construction time, which is
6	* specified in the same manner as {@link java.util.TreeSet} and
7	* {@link java.util.TreeMap}: elements are ordered
8	* either according to their <i>natural order</i> (see {@link Comparable}), or
9	* according to a {@link java.util.Comparator}, depending on which
10	* constructor is used.
11	* <p>The <em>head</em> of this queue is the <em>least</em> element with
12	* respect to the specified ordering.
13	* If multiple elements are tied for least value, the
14	* head is one of those elements. A priority queue does not permit
15	* <tt>null</tt> elements.
16	*
17	* <p>The {@link #remove()} and {@link #poll()} methods remove and
18	* return the head of the queue.
19	*
20	* <p>The {@link #element()} and {@link #peek()} methods return, but do
21	* not delete, the head of the queue.
22	*
23	* <p>A priority queue has a <i>capacity</i>. The capacity is the
24	* size of the array used internally to store the elements on the
25	* queue.
26	* It is always at least as large as the queue size. As
27	* elements are added to a priority queue, its capacity grows
28	* automatically. The details of the growth policy are not specified.
29	*
30	* <p>Implementation note: this implementation provides O(log(n)) time
31	* for the insertion methods (<tt>offer</tt>, <tt>poll</tt>,
32	* <tt>remove()</tt> and <tt>add</tt>) methods; linear time for the
33	* <tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and
34	* constant time for the retrieval methods (<tt>peek</tt>,
35	* <tt>element</tt>, and <tt>size</tt>).
36	*
37	* <p>This class is a member of the
38	* <a href="{@docRoot}/../guide/collections/index.html">
39	* Java Collections Framework</a>.
40	* @since 1.5
41	* @author Josh Bloch
42	*/
43	public class PriorityQueue<E> extends AbstractQueue<E>
44	implements Queue<E>, java.io.Serializable {
45
46	private static final int DEFAULT_INITIAL_CAPACITY = 11;
47
48	/**
49	* Priority queue represented as a balanced binary heap: the two children
50	* of queue[n] are queue[2n] and queue[2n + 1]. The priority queue is
51	* ordered by comparator, or by the elements' natural ordering, if
52	* comparator is null: For each node n in the heap and each descendant d
53	* of n, n <= d.
54	*
55	* The element with the lowest value is in queue[1], assuming the queue is
56	* nonempty. (A one-based array is used in preference to the traditional
57	* zero-based array to simplify parent and child calculations.)
58	*
59	* queue.length must be >= 2, even if size == 0.
60	*/
61	private transient Object[] queue;
62
63	/**
64	* The number of elements in the priority queue.
65	*/
66	private int size = 0;
67
68	/**
69	* The comparator, or null if priority queue uses elements'
70	* natural ordering.
71	*/
72	private final Comparator<? super E> comparator;
73
74	/**
75	* The number of times this priority queue has been
76	* <i>structurally modified</i>. See AbstractList for gory details.
77	*/
78	private transient int modCount = 0;
79
80	/**
81	* Creates a <tt>PriorityQueue</tt> with the default initial capacity
82	* (11) that orders its elements according to their natural
83	* ordering (using <tt>Comparable</tt>.)
84	*/
85	public PriorityQueue() {
86	this(DEFAULT_INITIAL_CAPACITY, null);
87	}
88
89	/**
90	* Creates a <tt>PriorityQueue</tt> with the specified initial capacity
91	* that orders its elements according to their natural ordering
92	* (using <tt>Comparable</tt>.)
93	*
94	* @param initialCapacity the initial capacity for this priority queue.
95	*/
96	public PriorityQueue(int initialCapacity) {
97	this(initialCapacity, null);
98	}
99
100	/**
101	* Creates a <tt>PriorityQueue</tt> with the specified initial capacity
102	* that orders its elements according to the specified comparator.
103	*
104	* @param initialCapacity the initial capacity for this priority queue.
105	* @param comparator the comparator used to order this priority queue.
106	* If <tt>null</tt> then the order depends on the elements' natural
107	* ordering.
108	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
109	* than 1
110	*/
111	public PriorityQueue(int initialCapacity, Comparator<? super E> comparator) {
112	if (initialCapacity < 1)
113	throw new IllegalArgumentException();
114	this.queue = new Object[initialCapacity + 1];
115	this.comparator = comparator;
116	}
117
118	/**
119	* Common code to initialize underlying queue array across
120	* constructors below.
121	*/
122	private void initializeArray(Collection<? extends E> c) {
123	int sz = c.size();
124	int initialCapacity = (int)Math.min((sz * 110L) / 100,
125	Integer.MAX_VALUE - 1);
126	if (initialCapacity < 1)
127	initialCapacity = 1;
128
129	this.queue = new Object[initialCapacity + 1];
130	}
131
132	/**
133	* Initially fill elements of the queue array under the
134	* knowledge that it is sorted or is another PQ, in which
135	* case we can just place the elements without fixups.
136	*/
137	private void fillFromSorted(Collection<? extends E> c) {
138	for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
139	queue[++size] = i.next();
140	}
141
142
143	/**
144	* Initially fill elements of the queue array that is
145	* not to our knowledge sorted, so we must add them
146	* one by one.
147	*/
148	private void fillFromUnsorted(Collection<? extends E> c) {
149	for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
150	add(i.next());
151	}
152
153	/**
154	* Creates a <tt>PriorityQueue</tt> containing the elements in the
155	* specified collection. The priority queue has an initial
156	* capacity of 110% of the size of the specified collection or 1
157	* if the collection is empty. If the specified collection is an
158	* instance of a {@link SortedSet} or is another
159	* <tt>PriorityQueue</tt>, the priority queue will be sorted
160	* according to the same comparator, or according to its elements'
161	* natural order if the collection is sorted according to its
162	* elements' natural order. Otherwise, the priority queue is
163	* ordered according to its elements' natural order.
164	*
165	* @param c the collection whose elements are to be placed
166	* into this priority queue.
167	* @throws ClassCastException if elements of the specified collection
168	* cannot be compared to one another according to the priority
169	* queue's ordering.
170	* @throws NullPointerException if <tt>c</tt> or any element within it
171	* is <tt>null</tt>
172	*/
173	public PriorityQueue(Collection<? extends E> c) {
174	initializeArray(c);
175	if (c instanceof SortedSet<? extends E>) {
176	SortedSet<? extends E> s = (SortedSet<? extends E>) c;
177	comparator = (Comparator<? super E>)s.comparator();
178	fillFromSorted(s);
179	}
180	else if (c instanceof PriorityQueue<? extends E>) {
181	PriorityQueue<? extends E> s = (PriorityQueue<? extends E>) c;
182	comparator = (Comparator<? super E>)s.comparator();
183	fillFromSorted(s);
184	}
185	else {
186	comparator = null;
187	fillFromUnsorted(c);
188	}
189	}
190
191	/**
192	* Creates a <tt>PriorityQueue</tt> containing the elements in the
193	* specified collection. The priority queue has an initial
194	* capacity of 110% of the size of the specified collection or 1
195	* if the collection is empty. This priority queue will be sorted
196	* according to the same comparator as the given collection, or
197	* according to its elements' natural order if the collection is
198	* sorted according to its elements' natural order.
199	*
200	* @param c the collection whose elements are to be placed
201	* into this priority queue.
202	* @throws ClassCastException if elements of the specified collection
203	* cannot be compared to one another according to the priority
204	* queue's ordering.
205	* @throws NullPointerException if <tt>c</tt> or any element within it
206	* is <tt>null</tt>
207	*/
208	public PriorityQueue(PriorityQueue<? extends E> c) {
209	initializeArray(c);
210	comparator = (Comparator<? super E>)c.comparator();
211	fillFromSorted(c);
212	}
213
214	/**
215	* Creates a <tt>PriorityQueue</tt> containing the elements in the
216	* specified collection. The priority queue has an initial
217	* capacity of 110% of the size of the specified collection or 1
218	* if the collection is empty. This priority queue will be sorted
219	* according to the same comparator as the given collection, or
220	* according to its elements' natural order if the collection is
221	* sorted according to its elements' natural order.
222	*
223	* @param c the collection whose elements are to be placed
224	* into this priority queue.
225	* @throws ClassCastException if elements of the specified collection
226	* cannot be compared to one another according to the priority
227	* queue's ordering.
228	* @throws NullPointerException if <tt>c</tt> or any element within it
229	* is <tt>null</tt>
230	*/
231	public PriorityQueue(SortedSet<? extends E> c) {
232	initializeArray(c);
233	comparator = (Comparator<? super E>)c.comparator();
234	fillFromSorted(c);
235	}
236
237	/**
238	* Resize array, if necessary, to be able to hold given index
239	*/
240	private void grow(int index) {
241	int newlen = queue.length;
242	if (index < newlen) // don't need to grow
243	return;
244	if (index == Integer.MAX_VALUE)
245	throw new OutOfMemoryError();
246	while (newlen <= index) {
247	if (newlen >= Integer.MAX_VALUE / 2) // avoid overflow
248	newlen = Integer.MAX_VALUE;
249	else
250	newlen <<= 2;
251	}
252	Object[] newQueue = new Object[newlen];
253	System.arraycopy(queue, 0, newQueue, 0, queue.length);
254	queue = newQueue;
255	}
256
257	// Queue Methods
258
259	/**
260	* Add the specified element to this priority queue.
261	*
262	* @return <tt>true</tt>
263	* @throws ClassCastException if the specified element cannot be compared
264	* with elements currently in the priority queue according
265	* to the priority queue's ordering.
266	* @throws NullPointerException if the specified element is <tt>null</tt>.
267	*/
268	public boolean offer(E o) {
269	if (o == null)
270	throw new NullPointerException();
271	modCount++;
272	++size;
273
274	// Grow backing store if necessary
275	if (size >= queue.length)
276	grow(size);
277
278	queue[size] = o;
279	fixUp(size);
280	return true;
281	}
282
283	public E poll() {
284	if (size == 0)
285	return null;
286	return (E) remove(1);
287	}
288
289	public E peek() {
290	return (E) queue[1];
291	}
292
293	// Collection Methods
294
295	// these first two override just to get the throws docs
296
297	/**
298	* @throws NullPointerException if the specified element is <tt>null</tt>.
299	* @throws ClassCastException if the specified element cannot be compared
300	* with elements currently in the priority queue according
301	* to the priority queue's ordering.
302	*/
303	public boolean add(E o) {
304	return super.add(o);
305	}
306
307	/**
308	* @throws ClassCastException if any element cannot be compared
309	* with elements currently in the priority queue according
310	* to the priority queue's ordering.
311	* @throws NullPointerException if <tt>c</tt> or any element in <tt>c</tt>
312	* is <tt>null</tt>
313	*/
314	public boolean addAll(Collection<? extends E> c) {
315	return super.addAll(c);
316	}
317
318	public boolean remove(Object o) {
319	if (o == null)
320	return false;
321
322	if (comparator == null) {
323	for (int i = 1; i <= size; i++) {
324	if (((Comparable<E>)queue[i]).compareTo((E)o) == 0) {
325	remove(i);
326	return true;
327	}
328	}
329	} else {
330	for (int i = 1; i <= size; i++) {
331	if (comparator.compare((E)queue[i], (E)o) == 0) {
332	remove(i);
333	return true;
334	}
335	}
336	}
337	return false;
338	}
339
340	public Iterator<E> iterator() {
341	return new Itr();
342	}
343
344	private class Itr implements Iterator<E> {
345	/**
346	* Index (into queue array) of element to be returned by
347	* subsequent call to next.
348	*/
349	private int cursor = 1;
350
351	/**
352	* Index of element returned by most recent call to next or
353	* previous. Reset to 0 if this element is deleted by a call
354	* to remove.
355	*/
356	private int lastRet = 0;
357
358	/**
359	* The modCount value that the iterator believes that the backing
360	* List should have. If this expectation is violated, the iterator
361	* has detected concurrent modification.
362	*/
363	private int expectedModCount = modCount;
364
365	public boolean hasNext() {
366	return cursor <= size;
367	}
368
369	public E next() {
370	checkForComodification();
371	if (cursor > size)
372	throw new NoSuchElementException();
373	E result = (E) queue[cursor];
374	lastRet = cursor++;
375	return result;
376	}
377
378	public void remove() {
379	if (lastRet == 0)
380	throw new IllegalStateException();
381	checkForComodification();
382
383	PriorityQueue.this.remove(lastRet);
384	if (lastRet < cursor)
385	cursor--;
386	lastRet = 0;
387	expectedModCount = modCount;
388	}
389
390	final void checkForComodification() {
391	if (modCount != expectedModCount)
392	throw new ConcurrentModificationException();
393	}
394	}
395
396	/**
397	* Returns the number of elements in this priority queue.
398	*
399	* @return the number of elements in this priority queue.
400	*/
401	public int size() {
402	return size;
403	}
404
405	/**
406	* Remove all elements from the priority queue.
407	*/
408	public void clear() {
409	modCount++;
410
411	// Null out element references to prevent memory leak
412	for (int i=1; i<=size; i++)
413	queue[i] = null;
414
415	size = 0;
416	}
417
418	/**
419	* Removes and returns the ith element from queue. Recall
420	* that queue is one-based, so 1 <= i <= size.
421	*
422	* XXX: Could further special-case i==size, but is it worth it?
423	* XXX: Could special-case i==0, but is it worth it?
424	*/
425	private E remove(int i) {
426	assert i <= size;
427	modCount++;
428
429	E result = (E) queue[i];
430	queue[i] = queue[size];
431	queue[size--] = null; // Drop extra ref to prevent memory leak
432	if (i <= size)
433	fixDown(i);
434	return result;
435	}
436
437	/**
438	* Establishes the heap invariant (described above) assuming the heap
439	* satisfies the invariant except possibly for the leaf-node indexed by k
440	* (which may have a nextExecutionTime less than its parent's).
441	*
442	* This method functions by "promoting" queue[k] up the hierarchy
443	* (by swapping it with its parent) repeatedly until queue[k]
444	* is greater than or equal to its parent.
445	*/
446	private void fixUp(int k) {
447	if (comparator == null) {
448	while (k > 1) {
449	int j = k >> 1;
450	if (((Comparable<E>)queue[j]).compareTo((E)queue[k]) <= 0)
451	break;
452	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
453	k = j;
454	}
455	} else {
456	while (k > 1) {
457	int j = k >> 1;
458	if (comparator.compare((E)queue[j], (E)queue[k]) <= 0)
459	break;
460	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
461	k = j;
462	}
463	}
464	}
465
466	/**
467	* Establishes the heap invariant (described above) in the subtree
468	* rooted at k, which is assumed to satisfy the heap invariant except
469	* possibly for node k itself (which may be greater than its children).
470	*
471	* This method functions by "demoting" queue[k] down the hierarchy
472	* (by swapping it with its smaller child) repeatedly until queue[k]
473	* is less than or equal to its children.
474	*/
475	private void fixDown(int k) {
476	int j;
477	if (comparator == null) {
478	while ((j = k << 1) <= size) {
479	if (j<size && ((Comparable<E>)queue[j]).compareTo((E)queue[j+1]) > 0)
480	j++; // j indexes smallest kid
481	if (((Comparable<E>)queue[k]).compareTo((E)queue[j]) <= 0)
482	break;
483	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
484	k = j;
485	}
486	} else {
487	while ((j = k << 1) <= size) {
488	if (j < size && comparator.compare((E)queue[j], (E)queue[j+1]) > 0)
489	j++; // j indexes smallest kid
490	if (comparator.compare((E)queue[k], (E)queue[j]) <= 0)
491	break;
492	Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
493	k = j;
494	}
495	}
496	}
497
498	public Comparator<? super E> comparator() {
499	return comparator;
500	}
501
502	/**
503	* Save the state of the instance to a stream (that
504	* is, serialize it).
505	*
506	* @serialData The length of the array backing the instance is
507	* emitted (int), followed by all of its elements (each an
508	* <tt>Object</tt>) in the proper order.
509	* @param s the stream
510	*/
511	private void writeObject(java.io.ObjectOutputStream s)
512	throws java.io.IOException{
513	// Write out element count, and any hidden stuff
514	s.defaultWriteObject();
515
516	// Write out array length
517	s.writeInt(queue.length);
518
519	// Write out all elements in the proper order.
520	for (int i=0; i<size; i++)
521	s.writeObject(queue[i]);
522	}
523
524	/**
525	* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
526	* deserialize it).
527	* @param s the stream
528	*/
529	private void readObject(java.io.ObjectInputStream s)
530	throws java.io.IOException, ClassNotFoundException {
531	// Read in size, and any hidden stuff
532	s.defaultReadObject();
533
534	// Read in array length and allocate array
535	int arrayLength = s.readInt();
536	queue = new Object[arrayLength];
537
538	// Read in all elements in the proper order.
539	for (int i=0; i<size; i++)
540	queue[i] = s.readObject();
541	}
542
543	}
544