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*Tue Jun 24 14:34:30 2003 UTC*
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package java.util; /** * An unbounded priority queue based on a priority heap. This queue orders * elements according to an order specified at construction time, which is * specified in the same manner as {@link TreeSet} and {@link TreeMap}: elements are ordered * either according to their <i>natural order</i> (see {@link Comparable}), or * according to a {@link Comparator}, depending on which constructor is used. * The {@link #peek}, {@link #poll}, and {@link #remove} methods return the * minimal element with respect to the specified ordering. If multiple * elements are tied for least value, no guarantees are made as to * which of these elements is returned. * * <p>A priority queue has a <i>capacity</i>. The capacity is the * size of the array used internally to store the elements on the * queue. It is always at least as large as the queue size. As * elements are added to a priority queue, its capacity grows * automatically. The details of the growth policy are not specified. * *<p>Implementation note: this implementation provides O(log(n)) time *for the insertion methods (<tt>offer</tt>, <tt>poll</tt>, *<tt>remove()</tt> and <tt>add</tt>) methods; linear time for the *<tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and *constant time for the retrieval methods (<tt>peek</tt>, *<tt>element</tt>, and <tt>size</tt>). * * <p>This class is a member of the * <a href="{@docRoot}/../guide/collections/index.html"> * Java Collections Framework</a>. * @since 1.5 * @author Josh Bloch */ public class PriorityQueue<E> extends AbstractQueue<E> implements Queue<E> { private static final int DEFAULT_INITIAL_CAPACITY = 11; /** * Priority queue represented as a balanced binary heap: the two children * of queue[n] are queue[2*n] and queue[2*n + 1]. The priority queue is * ordered by comparator, or by the elements' natural ordering, if * comparator is null: For each node n in the heap and each descendant d * of n, n <= d. * * The element with the lowest value is in queue[1], assuming the queue is * nonempty. (A one-based array is used in preference to the traditional * zero-based array to simplify parent and child calculations.) * * queue.length must be >= 2, even if size == 0. */ private transient E[] queue; /** * The number of elements in the priority queue. */ private int size = 0; /** * The comparator, or null if priority queue uses elements' * natural ordering. */ private final Comparator<E> comparator; /** * The number of times this priority queue has been * <i>structurally modified</i>. See AbstractList for gory details. */ private transient int modCount = 0; /** * Create a new priority queue with the default initial capacity * (11) that orders its elements according to their natural * ordering (using <tt>Comparable</tt>.) */ public PriorityQueue() { this(DEFAULT_INITIAL_CAPACITY); } /** * Create a new priority queue with the specified initial capacity * that orders its elements according to their natural ordering * (using <tt>Comparable</tt>.) * * @param initialCapacity the initial capacity for this priority queue. */ public PriorityQueue(int initialCapacity) { this(initialCapacity, null); } /** * Create a new priority queue with the specified initial capacity (11) * that orders its elements according to the specified comparator. * * @param initialCapacity the initial capacity for this priority queue. * @param comparator the comparator used to order this priority queue. */ public PriorityQueue(int initialCapacity, Comparator<E> comparator) { if (initialCapacity < 1) initialCapacity = 1; queue = new E[initialCapacity + 1]; this.comparator = comparator; } /** * Create a new priority queue containing the elements in the specified * collection. The priority queue has an initial capacity of 110% of the * size of the specified collection. If the specified collection * implements the {@link Sorted} interface, the priority queue will be * sorted according to the same comparator, or according to its elements' * natural order if the collection is sorted according to its elements' * natural order. If the specified collection does not implement * <tt>Sorted</tt>, the priority queue is ordered according to * its elements' natural order. * * @param initialElements the collection whose elements are to be placed * into this priority queue. * @throws ClassCastException if elements of the specified collection * cannot be compared to one another according to the priority * queue's ordering. * @throws NullPointerException if the specified collection or an * element of the specified collection is <tt>null</tt>. */ public PriorityQueue(Collection<E> initialElements) { int sz = initialElements.size(); int initialCapacity = (int)Math.min((sz * 110L) / 100, Integer.MAX_VALUE - 1); if (initialCapacity < 1) initialCapacity = 1; queue = new E[initialCapacity + 1]; /* Commented out to compile with generics compiler if (initialElements instanceof Sorted) { comparator = ((Sorted)initialElements).comparator(); for (Iterator<E> i = initialElements.iterator(); i.hasNext(); ) queue[++size] = i.next(); } else { */ { comparator = null; for (Iterator<E> i = initialElements.iterator(); i.hasNext(); ) add(i.next()); } } // Queue Methods /** * Remove and return the minimal element from this priority queue * if it contains one or more elements, otherwise return * <tt>null</tt>. The term <i>minimal</i> is defined according to * this priority queue's order. * * @return the minimal element from this priority queue if it contains * one or more elements, otherwise <tt>null</tt>. */ public E poll() { if (size == 0) return null; return remove(1); } /** * Return, but do not remove, the minimal element from the * priority queue, or return <tt>null</tt> if the queue is empty. * The term <i>minimal</i> is defined according to this priority * queue's order. This method returns the same object reference * that would be returned by by the <tt>poll</tt> method. The two * methods differ in that this method does not remove the element * from the priority queue. * * @return the minimal element from this priority queue if it contains * one or more elements, otherwise <tt>null</tt>. */ public E peek() { return queue[1]; } // Collection Methods /** * Removes a single instance of the specified element from this priority * queue, if it is present. Returns true if this collection contained the * specified element (or equivalently, if this collection changed as a * result of the call). * * @param element the element to be removed from this collection, * if present. * @return <tt>true</tt> if this collection changed as a result of the * call * @throws ClassCastException if the specified element cannot be compared * with elements currently in the priority queue according * to the priority queue's ordering. * @throws NullPointerException if the specified element is null. */ public boolean remove(Object element) { if (element == null) throw new NullPointerException(); if (comparator == null) { for (int i = 1; i <= size; i++) { if (((Comparable)queue[i]).compareTo(element) == 0) { remove(i); return true; } } } else { for (int i = 1; i <= size; i++) { if (comparator.compare(queue[i], (E) element) == 0) { remove(i); return true; } } } return false; } /** * Returns an iterator over the elements in this priority queue. The * elements of the priority queue will be returned by this iterator in the * order specified by the queue, which is to say the order they would be * returned by repeated calls to <tt>poll</tt>. * * @return an <tt>Iterator</tt> over the elements in this priority queue. */ public Iterator<E> iterator() { return new Itr(); } private class Itr implements Iterator<E> { /** * Index (into queue array) of element to be returned by * subsequent call to next. */ private int cursor = 1; /** * Index of element returned by most recent call to next or * previous. Reset to 0 if this element is deleted by a call * to remove. */ private int lastRet = 0; /** * The modCount value that the iterator believes that the backing * List should have. If this expectation is violated, the iterator * has detected concurrent modification. */ private int expectedModCount = modCount; public boolean hasNext() { return cursor <= size; } public E next() { checkForComodification(); if (cursor > size) throw new NoSuchElementException(); E result = queue[cursor]; lastRet = cursor++; return result; } public void remove() { if (lastRet == 0) throw new IllegalStateException(); checkForComodification(); PriorityQueue.this.remove(lastRet); if (lastRet < cursor) cursor--; lastRet = 0; expectedModCount = modCount; } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } } /** * Returns the number of elements in this priority queue. * * @return the number of elements in this priority queue. */ public int size() { return size; } /** * Add the specified element to this priority queue. * * @param element the element to add. * @return true * @throws ClassCastException if the specified element cannot be compared * with elements currently in the priority queue according * to the priority queue's ordering. * @throws NullPointerException if the specified element is null. */ public boolean offer(E element) { if (element == null) throw new NullPointerException(); modCount++; // Grow backing store if necessary if (++size == queue.length) { E[] newQueue = new E[2 * queue.length]; System.arraycopy(queue, 0, newQueue, 0, size); queue = newQueue; } queue[size] = element; fixUp(size); return true; } /** * Remove all elements from the priority queue. */ public void clear() { modCount++; // Null out element references to prevent memory leak for (int i=1; i<=size; i++) queue[i] = null; size = 0; } /** * Removes and returns the ith element from queue. Recall * that queue is one-based, so 1 <= i <= size. * * XXX: Could further special-case i==size, but is it worth it? * XXX: Could special-case i==0, but is it worth it? */ private E remove(int i) { assert i <= size; modCount++; E result = queue[i]; queue[i] = queue[size]; queue[size--] = null; // Drop extra ref to prevent memory leak if (i <= size) fixDown(i); return result; } /** * Establishes the heap invariant (described above) assuming the heap * satisfies the invariant except possibly for the leaf-node indexed by k * (which may have a nextExecutionTime less than its parent's). * * This method functions by "promoting" queue[k] up the hierarchy * (by swapping it with its parent) repeatedly until queue[k] * is greater than or equal to its parent. */ private void fixUp(int k) { if (comparator == null) { while (k > 1) { int j = k >> 1; if (((Comparable)queue[j]).compareTo(queue[k]) <= 0) break; E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; k = j; } } else { while (k > 1) { int j = k >> 1; if (comparator.compare(queue[j], queue[k]) <= 0) break; E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; k = j; } } } /** * Establishes the heap invariant (described above) in the subtree * rooted at k, which is assumed to satisfy the heap invariant except * possibly for node k itself (which may be greater than its children). * * This method functions by "demoting" queue[k] down the hierarchy * (by swapping it with its smaller child) repeatedly until queue[k] * is less than or equal to its children. */ private void fixDown(int k) { int j; if (comparator == null) { while ((j = k << 1) <= size) { if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0) j++; // j indexes smallest kid if (((Comparable)queue[k]).compareTo(queue[j]) <= 0) break; E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; k = j; } } else { while ((j = k << 1) <= size) { if (j < size && comparator.compare(queue[j], queue[j+1]) > 0) j++; // j indexes smallest kid if (comparator.compare(queue[k], queue[j]) <= 0) break; E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; k = j; } } } /** * Returns the comparator associated with this priority queue, or * <tt>null</tt> if it uses its elements' natural ordering. * * @return the comparator associated with this priority queue, or * <tt>null</tt> if it uses its elements' natural ordering. */ Comparator comparator() { return comparator; } /** * Save the state of the instance to a stream (that * is, serialize it). * * @serialData The length of the array backing the instance is * emitted (int), followed by all of its elements (each an * <tt>Object</tt>) in the proper order. * @param s the stream */ private synchronized void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{ // Write out element count, and any hidden stuff s.defaultWriteObject(); // Write out array length s.writeInt(queue.length); // Write out all elements in the proper order. for (int i=0; i<size; i++) s.writeObject(queue[i]); } /** * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, * deserialize it). * @param s the stream */ private synchronized void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { // Read in size, and any hidden stuff s.defaultReadObject(); // Read in array length and allocate array int arrayLength = s.readInt(); queue = new E[arrayLength]; // Read in all elements in the proper order. for (int i=0; i<size; i++) queue[i] = (E)s.readObject(); } }

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