699 lines
24 KiB
Java
699 lines
24 KiB
Java
package java.util;
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public class PriorityQueue<E> extends AbstractQueue<E>
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implements java.io.Serializable {
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private static final long serialVersionUID = -7720805057305804111L;
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private static final int DEFAULT_INITIAL_CAPACITY = 11;
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/**
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* Priority queue represented as a balanced binary heap: the two
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* children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
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* priority queue is ordered by comparator, or by the elements'
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* natural ordering, if comparator is null: For each node n in the
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* heap and each descendant d of n, n <= d. The element with the
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* lowest value is in queue[0], assuming the queue is nonempty.
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*/
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private transient Object[] queue;
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/**
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* The number of elements in the priority queue.
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*/
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private int size = 0;
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/**
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* The comparator, or null if priority queue uses elements'
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* natural ordering.
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*/
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private final Comparator<? super E> comparator;
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/**
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* The number of times this priority queue has been
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* <i>structurally modified</i>. See AbstractList for gory details.
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*/
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private transient int modCount = 0;
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/**
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* Creates a {@code PriorityQueue} with the default initial
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* capacity (11) that orders its elements according to their
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* {@linkplain Comparable natural ordering}.
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*/
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public PriorityQueue() {
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this(DEFAULT_INITIAL_CAPACITY, null);
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}
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/**
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* Creates a {@code PriorityQueue} with the specified initial
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* capacity that orders its elements according to their
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* {@linkplain Comparable natural ordering}.
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*
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* @param initialCapacity the initial capacity for this priority queue
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* @throws IllegalArgumentException if {@code initialCapacity} is less
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* than 1
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*/
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public PriorityQueue(int initialCapacity) {
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this(initialCapacity, null);
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}
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/**
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* Creates a {@code PriorityQueue} with the specified initial capacity
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* that orders its elements according to the specified comparator.
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*
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* @param initialCapacity the initial capacity for this priority queue
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* @param comparator the comparator that will be used to order this
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* priority queue. If {@code null}, the {@linkplain Comparable
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* natural ordering} of the elements will be used.
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* @throws IllegalArgumentException if {@code initialCapacity} is
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* less than 1
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*/
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public PriorityQueue(int initialCapacity,
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Comparator<? super E> comparator) {
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// Note: This restriction of at least one is not actually needed,
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// but continues for 1.5 compatibility
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if (initialCapacity < 1)
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throw new IllegalArgumentException();
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this.queue = new Object[initialCapacity];
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this.comparator = comparator;
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}
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/**
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* Creates a {@code PriorityQueue} containing the elements in the
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* specified collection. If the specified collection is an instance of
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* a {@link SortedSet} or is another {@code PriorityQueue}, this
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* priority queue will be ordered according to the same ordering.
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* Otherwise, this priority queue will be ordered according to the
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* {@linkplain Comparable natural ordering} of its elements.
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*
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* @param c the collection whose elements are to be placed
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* into this priority queue
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* @throws ClassCastException if elements of the specified collection
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* cannot be compared to one another according to the priority
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* queue's ordering
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* @throws NullPointerException if the specified collection or any
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* of its elements are null
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*/
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@SuppressWarnings("unchecked")
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public PriorityQueue(Collection<? extends E> c) {
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if (c instanceof SortedSet<?>) {
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SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
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this.comparator = (Comparator<? super E>) ss.comparator();
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initElementsFromCollection(ss);
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}
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else if (c instanceof PriorityQueue<?>) {
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PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
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this.comparator = (Comparator<? super E>) pq.comparator();
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initFromPriorityQueue(pq);
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}
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else {
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this.comparator = null;
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initFromCollection(c);
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}
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}
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/**
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* Creates a {@code PriorityQueue} containing the elements in the
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* specified priority queue. This priority queue will be
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* ordered according to the same ordering as the given priority
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* queue.
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*
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* @param c the priority queue whose elements are to be placed
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* into this priority queue
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* @throws ClassCastException if elements of {@code c} cannot be
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* compared to one another according to {@code c}'s
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* ordering
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* @throws NullPointerException if the specified priority queue or any
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* of its elements are null
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*/
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@SuppressWarnings("unchecked")
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public PriorityQueue(PriorityQueue<? extends E> c) {
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this.comparator = (Comparator<? super E>) c.comparator();
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initFromPriorityQueue(c);
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}
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/**
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* Creates a {@code PriorityQueue} containing the elements in the
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* specified sorted set. This priority queue will be ordered
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* according to the same ordering as the given sorted set.
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*
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* @param c the sorted set whose elements are to be placed
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* into this priority queue
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* @throws ClassCastException if elements of the specified sorted
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* set cannot be compared to one another according to the
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* sorted set's ordering
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* @throws NullPointerException if the specified sorted set or any
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* of its elements are null
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*/
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@SuppressWarnings("unchecked")
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public PriorityQueue(SortedSet<? extends E> c) {
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this.comparator = (Comparator<? super E>) c.comparator();
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initElementsFromCollection(c);
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}
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private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
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if (c.getClass() == PriorityQueue.class) {
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this.queue = c.toArray();
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this.size = c.size();
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} else {
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initFromCollection(c);
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}
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}
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private void initElementsFromCollection(Collection<? extends E> c) {
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Object[] a = c.toArray();
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// If c.toArray incorrectly doesn't return Object[], copy it.
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if (a.getClass() != Object[].class)
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a = Arrays.copyOf(a, a.length, Object[].class);
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int len = a.length;
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if (len == 1 || this.comparator != null)
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for (int i = 0; i < len; i++)
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if (a[i] == null)
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throw new NullPointerException();
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this.queue = a;
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this.size = a.length;
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}
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/**
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* Initializes queue array with elements from the given Collection.
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*
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* @param c the collection
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*/
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private void initFromCollection(Collection<? extends E> c) {
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initElementsFromCollection(c);
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heapify();
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}
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/**
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* The maximum size of array to allocate.
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* Some VMs reserve some header words in an array.
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* Attempts to allocate larger arrays may result in
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* OutOfMemoryError: Requested array size exceeds VM limit
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*/
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private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
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/**
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* Increases the capacity of the array.
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*
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* @param minCapacity the desired minimum capacity
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*/
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private void grow(int minCapacity) {
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int oldCapacity = queue.length;
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// Double size if small; else grow by 50%
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int newCapacity = oldCapacity + ((oldCapacity < 64) ?
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(oldCapacity + 2) :
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(oldCapacity >> 1));
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// overflow-conscious code
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if (newCapacity - MAX_ARRAY_SIZE > 0)
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newCapacity = hugeCapacity(minCapacity);
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queue = Arrays.copyOf(queue, newCapacity);
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}
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private static int hugeCapacity(int minCapacity) {
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if (minCapacity < 0) // overflow
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throw new OutOfMemoryError();
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return (minCapacity > MAX_ARRAY_SIZE) ?
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Integer.MAX_VALUE :
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MAX_ARRAY_SIZE;
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}
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/**
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* Inserts the specified element into this priority queue.
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*
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* @return {@code true} (as specified by {@link Collection#add})
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* @throws ClassCastException if the specified element cannot be
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* compared with elements currently in this priority queue
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* according to the priority queue's ordering
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* @throws NullPointerException if the specified element is null
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*/
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public boolean add(E e) {
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return offer(e);
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}
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/**
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* Inserts the specified element into this priority queue.
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*
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* @return {@code true} (as specified by {@link Queue#offer})
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* @throws ClassCastException if the specified element cannot be
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* compared with elements currently in this priority queue
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* according to the priority queue's ordering
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* @throws NullPointerException if the specified element is null
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*/
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public boolean offer(E e) {
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if (e == null)
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throw new NullPointerException();
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modCount++;
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int i = size;
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if (i >= queue.length)
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grow(i + 1);
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size = i + 1;
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if (i == 0)
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queue[0] = e;
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else
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siftUp(i, e);
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return true;
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}
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public E peek() {
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if (size == 0)
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return null;
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return (E) queue[0];
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}
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private int indexOf(Object o) {
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if (o != null) {
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for (int i = 0; i < size; i++)
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if (o.equals(queue[i]))
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return i;
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}
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return -1;
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}
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/**
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* Removes a single instance of the specified element from this queue,
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* if it is present. More formally, removes an element {@code e} such
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* that {@code o.equals(e)}, if this queue contains one or more such
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* elements. Returns {@code true} if and only if this queue contained
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* the specified element (or equivalently, if this queue changed as a
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* result of the call).
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*
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* @param o element to be removed from this queue, if present
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* @return {@code true} if this queue changed as a result of the call
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*/
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public boolean remove(Object o) {
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int i = indexOf(o);
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if (i == -1)
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return false;
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else {
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removeAt(i);
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return true;
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}
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}
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/**
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* Version of remove using reference equality, not equals.
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* Needed by iterator.remove.
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*
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* @param o element to be removed from this queue, if present
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* @return {@code true} if removed
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*/
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boolean removeEq(Object o) {
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for (int i = 0; i < size; i++) {
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if (o == queue[i]) {
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removeAt(i);
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return true;
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}
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}
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return false;
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}
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/**
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* Returns {@code true} if this queue contains the specified element.
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* More formally, returns {@code true} if and only if this queue contains
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* at least one element {@code e} such that {@code o.equals(e)}.
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*
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* @param o object to be checked for containment in this queue
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* @return {@code true} if this queue contains the specified element
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*/
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public boolean contains(Object o) {
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return indexOf(o) != -1;
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}
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/**
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* Returns an array containing all of the elements in this queue.
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* The elements are in no particular order.
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*
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* <p>The returned array will be "safe" in that no references to it are
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* maintained by this queue. (In other words, this method must allocate
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* a new array). The caller is thus free to modify the returned array.
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*
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* <p>This method acts as bridge between array-based and collection-based
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* APIs.
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*
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* @return an array containing all of the elements in this queue
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*/
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public Object[] toArray() {
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return Arrays.copyOf(queue, size);
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}
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/**
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* Returns an array containing all of the elements in this queue; the
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* runtime type of the returned array is that of the specified array.
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* The returned array elements are in no particular order.
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* If the queue fits in the specified array, it is returned therein.
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* Otherwise, a new array is allocated with the runtime type of the
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* specified array and the size of this queue.
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*
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* <p>If the queue fits in the specified array with room to spare
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* (i.e., the array has more elements than the queue), the element in
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* the array immediately following the end of the collection is set to
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* {@code null}.
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*
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* <p>Like the {@link #toArray()} method, this method acts as bridge between
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* array-based and collection-based APIs. Further, this method allows
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* precise control over the runtime type of the output array, and may,
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* under certain circumstances, be used to save allocation costs.
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*
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* <p>Suppose <tt>x</tt> is a queue known to contain only strings.
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* The following code can be used to dump the queue into a newly
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* allocated array of <tt>String</tt>:
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*
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* <pre>
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* String[] y = x.toArray(new String[0]);</pre>
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*
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* Note that <tt>toArray(new Object[0])</tt> is identical in function to
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* <tt>toArray()</tt>.
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*
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* @param a the array into which the elements of the queue are to
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* be stored, if it is big enough; otherwise, a new array of the
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* same runtime type is allocated for this purpose.
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* @return an array containing all of the elements in this queue
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* @throws ArrayStoreException if the runtime type of the specified array
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* is not a supertype of the runtime type of every element in
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* this queue
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* @throws NullPointerException if the specified array is null
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*/
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public <T> T[] toArray(T[] a) {
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if (a.length < size)
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// Make a new array of a's runtime type, but my contents:
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return (T[]) Arrays.copyOf(queue, size, a.getClass());
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System.arraycopy(queue, 0, a, 0, size);
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if (a.length > size)
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a[size] = null;
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return a;
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}
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/**
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* Returns an iterator over the elements in this queue. The iterator
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* does not return the elements in any particular order.
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*
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* @return an iterator over the elements in this queue
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*/
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public Iterator<E> iterator() {
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return new Itr();
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}
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private final class Itr implements Iterator<E> {
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/**
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* Index (into queue array) of element to be returned by
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* subsequent call to next.
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*/
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private int cursor = 0;
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/**
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* Index of element returned by most recent call to next,
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* unless that element came from the forgetMeNot list.
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* Set to -1 if element is deleted by a call to remove.
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*/
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private int lastRet = -1;
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/**
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* A queue of elements that were moved from the unvisited portion of
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* the heap into the visited portion as a result of "unlucky" element
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* removals during the iteration. (Unlucky element removals are those
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* that require a siftup instead of a siftdown.) We must visit all of
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* the elements in this list to complete the iteration. We do this
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* after we've completed the "normal" iteration.
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*
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* We expect that most iterations, even those involving removals,
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* will not need to store elements in this field.
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*/
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private ArrayDeque<E> forgetMeNot = null;
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/**
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* Element returned by the most recent call to next iff that
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* element was drawn from the forgetMeNot list.
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*/
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private E lastRetElt = null;
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/**
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* The modCount value that the iterator believes that the backing
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* Queue should have. If this expectation is violated, the iterator
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* has detected concurrent modification.
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*/
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private int expectedModCount = modCount;
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public boolean hasNext() {
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return cursor < size ||
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(forgetMeNot != null && !forgetMeNot.isEmpty());
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}
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public E next() {
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if (expectedModCount != modCount)
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throw new ConcurrentModificationException();
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if (cursor < size)
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return (E) queue[lastRet = cursor++];
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if (forgetMeNot != null) {
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lastRet = -1;
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lastRetElt = forgetMeNot.poll();
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if (lastRetElt != null)
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return lastRetElt;
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}
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throw new NoSuchElementException();
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}
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public void remove() {
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if (expectedModCount != modCount)
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throw new ConcurrentModificationException();
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if (lastRet != -1) {
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E moved = PriorityQueue.this.removeAt(lastRet);
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lastRet = -1;
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if (moved == null)
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cursor--;
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else {
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if (forgetMeNot == null)
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forgetMeNot = new ArrayDeque<>();
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forgetMeNot.add(moved);
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}
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} else if (lastRetElt != null) {
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PriorityQueue.this.removeEq(lastRetElt);
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lastRetElt = null;
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} else {
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throw new IllegalStateException();
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}
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expectedModCount = modCount;
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}
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}
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public int size() {
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return size;
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}
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/**
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* Removes all of the elements from this priority queue.
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* The queue will be empty after this call returns.
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*/
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public void clear() {
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modCount++;
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for (int i = 0; i < size; i++)
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queue[i] = null;
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size = 0;
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}
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public E poll() {
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if (size == 0)
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return null;
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int s = --size;
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modCount++;
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E result = (E) queue[0];
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E x = (E) queue[s];
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queue[s] = null;
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if (s != 0)
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siftDown(0, x);
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return result;
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}
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/**
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* Removes the ith element from queue.
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*
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* Normally this method leaves the elements at up to i-1,
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* inclusive, untouched. Under these circumstances, it returns
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* null. Occasionally, in order to maintain the heap invariant,
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* it must swap a later element of the list with one earlier than
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* i. Under these circumstances, this method returns the element
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* that was previously at the end of the list and is now at some
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* position before i. This fact is used by iterator.remove so as to
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* avoid missing traversing elements.
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*/
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private E removeAt(int i) {
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assert i >= 0 && i < size;
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modCount++;
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int s = --size;
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if (s == i) // removed last element
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queue[i] = null;
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else {
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E moved = (E) queue[s];
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queue[s] = null;
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siftDown(i, moved);
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if (queue[i] == moved) {
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siftUp(i, moved);
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if (queue[i] != moved)
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return moved;
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}
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}
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return null;
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}
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/**
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* Inserts item x at position k, maintaining heap invariant by
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* promoting x up the tree until it is greater than or equal to
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* its parent, or is the root.
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*
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* To simplify and speed up coercions and comparisons. the
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* Comparable and Comparator versions are separated into different
|
|
* methods that are otherwise identical. (Similarly for siftDown.)
|
|
*
|
|
* @param k the position to fill
|
|
* @param x the item to insert
|
|
*/
|
|
private void siftUp(int k, E x) {
|
|
if (comparator != null)
|
|
siftUpUsingComparator(k, x);
|
|
else
|
|
siftUpComparable(k, x);
|
|
}
|
|
|
|
private void siftUpComparable(int k, E x) {
|
|
Comparable<? super E> key = (Comparable<? super E>) x;
|
|
while (k > 0) {
|
|
int parent = (k - 1) >>> 1;
|
|
Object e = queue[parent];
|
|
if (key.compareTo((E) e) >= 0)
|
|
break;
|
|
queue[k] = e;
|
|
k = parent;
|
|
}
|
|
queue[k] = key;
|
|
}
|
|
|
|
private void siftUpUsingComparator(int k, E x) {
|
|
while (k > 0) {
|
|
int parent = (k - 1) >>> 1;
|
|
Object e = queue[parent];
|
|
if (comparator.compare(x, (E) e) >= 0)
|
|
break;
|
|
queue[k] = e;
|
|
k = parent;
|
|
}
|
|
queue[k] = x;
|
|
}
|
|
|
|
/**
|
|
* Inserts item x at position k, maintaining heap invariant by
|
|
* demoting x down the tree repeatedly until it is less than or
|
|
* equal to its children or is a leaf.
|
|
*
|
|
* @param k the position to fill
|
|
* @param x the item to insert
|
|
*/
|
|
private void siftDown(int k, E x) {
|
|
if (comparator != null)
|
|
siftDownUsingComparator(k, x);
|
|
else
|
|
siftDownComparable(k, x);
|
|
}
|
|
|
|
private void siftDownComparable(int k, E x) {
|
|
Comparable<? super E> key = (Comparable<? super E>)x;
|
|
int half = size >>> 1; // loop while a non-leaf
|
|
while (k < half) {
|
|
int child = (k << 1) + 1; // assume left child is least
|
|
Object c = queue[child];
|
|
int right = child + 1;
|
|
if (right < size &&
|
|
((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
|
|
c = queue[child = right];
|
|
if (key.compareTo((E) c) <= 0)
|
|
break;
|
|
queue[k] = c;
|
|
k = child;
|
|
}
|
|
queue[k] = key;
|
|
}
|
|
|
|
private void siftDownUsingComparator(int k, E x) {
|
|
int half = size >>> 1;
|
|
while (k < half) {
|
|
int child = (k << 1) + 1;
|
|
Object c = queue[child];
|
|
int right = child + 1;
|
|
if (right < size &&
|
|
comparator.compare((E) c, (E) queue[right]) > 0)
|
|
c = queue[child = right];
|
|
if (comparator.compare(x, (E) c) <= 0)
|
|
break;
|
|
queue[k] = c;
|
|
k = child;
|
|
}
|
|
queue[k] = x;
|
|
}
|
|
|
|
/**
|
|
* Establishes the heap invariant (described above) in the entire tree,
|
|
* assuming nothing about the order of the elements prior to the call.
|
|
*/
|
|
private void heapify() {
|
|
for (int i = (size >>> 1) - 1; i >= 0; i--)
|
|
siftDown(i, (E) queue[i]);
|
|
}
|
|
|
|
/**
|
|
* Returns the comparator used to order the elements in this
|
|
* queue, or {@code null} if this queue is sorted according to
|
|
* the {@linkplain Comparable natural ordering} of its elements.
|
|
*
|
|
* @return the comparator used to order this queue, or
|
|
* {@code null} if this queue is sorted according to the
|
|
* natural ordering of its elements
|
|
*/
|
|
public Comparator<? super E> comparator() {
|
|
return comparator;
|
|
}
|
|
|
|
/**
|
|
* Saves the state of the instance to a stream (that
|
|
* is, serializes it).
|
|
*
|
|
* @serialData The length of the array backing the instance is
|
|
* emitted (int), followed by all of its elements
|
|
* (each an {@code Object}) 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, for compatibility with 1.5 version
|
|
s.writeInt(Math.max(2, size + 1));
|
|
|
|
// Write out all elements in the "proper order".
|
|
for (int i = 0; i < size; i++)
|
|
s.writeObject(queue[i]);
|
|
}
|
|
|
|
/**
|
|
* Reconstitutes the {@code PriorityQueue} instance from a stream
|
|
* (that is, deserializes 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 (and discard) array length
|
|
s.readInt();
|
|
|
|
queue = new Object[size];
|
|
|
|
// Read in all elements.
|
|
for (int i = 0; i < size; i++)
|
|
queue[i] = s.readObject();
|
|
|
|
// Elements are guaranteed to be in "proper order", but the
|
|
// spec has never explained what that might be.
|
|
heapify();
|
|
}
|
|
}
|