PriorityQueue 源代码

1、优先队列初始化、扩容机制

        //默认初始化容量    private static final int DEFAULT_INITIAL_CAPACITY = 11;    /**     * Increases the capacity of the array.     *     * @param minCapacity the desired minimum capacity     */    //扩容机制代码    private void grow(int minCapacity) {        int oldCapacity = queue.length;        // Double size if small; else grow by 50%        //如果容量小于64,则为 2倍+2 ，如果容量大于等于64，则为 1.5倍        int newCapacity = oldCapacity + ((oldCapacity < 64) ?                                         (oldCapacity + 2) :                                         (oldCapacity >> 1));        // overflow-conscious code        if (newCapacity - MAX_ARRAY_SIZE > 0)            newCapacity = hugeCapacity(minCapacity);        queue = Arrays.copyOf(queue, newCapacity);    }

2、优先队列底层是根据堆排序来实现的【最小堆】

具体代码：

    /**     * Inserts item x at position k, maintaining heap invariant by     * promoting x up the tree until it is greater than or equal to     * its parent, or is the root.     *     * To simplify and speed up coercions and comparisons. the     * 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     */    //在k位置插入元素x，调整堆使x大于或等于它的父节点【从下到上比较】    private void siftUp(int k, E x) {        //区分是否有自己的比较器，该方法实现最小堆调整        if (comparator != null)            siftUpUsingComparator(k, x);        else            siftUpComparable(k, x);    }    @SuppressWarnings("unchecked")    private void siftUpComparable(int k, E x) {        Comparable<? super E> key = (Comparable<? super E>) x;        while (k > 0) {            //计算得到parent的下标            int parent = (k - 1) >>> 1;            Object e = queue[parent];            if (key.compareTo((E) e) >= 0)                break;            //如果比父结点小，则跟父节点替换            queue[k] = e;            k = parent;        }        queue[k] = key;    }    @SuppressWarnings("unchecked")    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;            //如果比父结点小，则跟父节点替换            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     */    //在k位置中插入元素x，调整堆使元素x小于或等于它儿子【从上到下比较】    private void siftDown(int k, E x) {        if (comparator != null)            siftDownUsingComparator(k, x);        else            siftDownComparable(k, x);    }    @SuppressWarnings("unchecked")    private void siftDownComparable(int k, E x) {        Comparable<? super E> key = (Comparable<? super E>)x;        int half = size >>> 1;        // loop while a non-leaf        //只要k < size/2, 说明k有孩子        while (k < half) {            //计算出左孩子的下标            int child = (k << 1) + 1; // assume left child is least            Object c = queue[child];            //右孩子下标            int right = child + 1;            //如果有右孩子，左孩子跟右孩子比较，将小的结点赋到c            if (right < size &&                ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)                c = queue[child = right];            if (key.compareTo((E) c) <= 0)                //key比c小，说明比孩子结点小，直接退出                break;            //比孩子大，则跟孩子替换            queue[k] = c;            k = child;        }        queue[k] = key;    }    @SuppressWarnings("unchecked")    private void siftDownUsingComparator(int k, E x) {        int half = size >>> 1;        //只要k < size/2, 说明k有孩子        while (k < half) {            //计算出左孩子的下标            int child = (k << 1) + 1;            Object c = queue[child];            //右孩子下标            int right = child + 1;            //如果有右孩子，左孩子跟右孩子比较，将小的结点赋到c            if (right < size &&                comparator.compare((E) c, (E) queue[right]) > 0)                c = queue[child = right];            if (comparator.compare(x, (E) c) <= 0)                //key比c小，说明比孩子结点小，直接退出                break;            //比孩子大，则跟孩子替换            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.     */    @SuppressWarnings("unchecked")    private void heapify() {        //建立堆结构        for (int i = (size >>> 1) - 1; i >= 0; i--)            siftDown(i, (E) queue[i]);    }

3、PriorityQueue源代码

/* * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */package java.util;import java.util.function.Consumer;/** * An unbounded priority {@linkplain Queue queue} based on a priority heap. * The elements of the priority queue are ordered according to their * {@linkplain Comparable natural ordering}, or by a {@link Comparator} * provided at queue construction time, depending on which constructor is * used.  A priority queue does not permit {@code null} elements. * A priority queue relying on natural ordering also does not permit * insertion of non-comparable objects (doing so may result in * {@code ClassCastException}). * * <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 -- ties are * broken arbitrarily.  The queue retrieval operations {@code poll}, * {@code remove}, {@code peek}, and {@code element} access the * element at the head of the queue. * * <p>A priority queue is unbounded, but has an internal * <i>capacity</i> governing the size of an array used 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>This class and its iterator implement all of the * <em>optional</em> methods of the {@link Collection} and {@link * Iterator} interfaces.  The Iterator provided in method {@link * #iterator()} is <em>not</em> guaranteed to traverse the elements of * the priority queue in any particular order. If you need ordered * traversal, consider using {@code Arrays.sort(pq.toArray())}. * * <p><strong>Note that this implementation is not synchronized.</strong> * Multiple threads should not access a {@code PriorityQueue} * instance concurrently if any of the threads modifies the queue. * Instead, use the thread-safe {@link * java.util.concurrent.PriorityBlockingQueue} class. * * <p>Implementation note: this implementation provides * O(log(n)) time for the enqueuing and dequeuing methods * ({@code offer}, {@code poll}, {@code remove()} and {@code add}); * linear time for the {@code remove(Object)} and {@code contains(Object)} * methods; and constant time for the retrieval methods * ({@code peek}, {@code element}, and {@code size}). * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @since 1.5 * @author Josh Bloch, Doug Lea * @param <E> the type of elements held in this collection *//*一个基于优先级堆的无界优先级队列。优先级队列的元素按照其自然顺序进行排序，或者根据构造队列时提供的 Comparator进行排序，具体取决于所使用的构造方法。优先级队列不允许使用 null 元素。依靠自然顺序的优先级队列还不允许插入不可比较的对象（这样做可能导致 ClassCastException）。此队列的头 是按指定排序方式确定的最小 元素。如果多个元素都是最小值，则头是其中一个元素——选择方法是任意的。队列获取操作 poll、remove、peek 和 element 访问处于队列头的元素。优先级队列是无界的，但是有一个内部容量，控制着用于存储队列元素的数组大小。它通常至少等于队列的大小。随着不断向优先级队列添加元素，其容量会自动增加。无需指定容量增加策略的细节。此类及其迭代器实现了 Collection 和 Iterator 接口的所有可选 方法。方法 iterator() 中提供的迭代器不保证以任何特定的顺序遍历优先级队列中的元素。如果需要按顺序遍历，请考虑使用 Arrays.sort(pq.toArray())。注意，此实现不是同步的。如果多个线程中的任意线程修改了队列，则这些线程不应同时访问 PriorityQueue 实例。相反，请使用线程安全的 PriorityBlockingQueue 类。实现注意事项：此实现为排队和出队方法（offer、poll、remove() 和 add）提供 O(log(n)) 时间；为 remove(Object) 和 contains(Object) 方法提供线性时间；为获取方法（peek、element 和 size）提供固定时间。此类是 Java Collections Framework 的成员 */public class PriorityQueue<E> extends AbstractQueue<E>    implements java.io.Serializable {    private static final long serialVersionUID = -7720805057305804111L;    //默认初始化容量    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+1] 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[0], assuming the queue is nonempty.     */    //优先级队列可以当做平衡二叉堆，queue[n]的儿子为queue[2*n+1]、queue[2*(n+1)]    //通过比较器来实现优先级，如果比较器为空，则以递增方式，元素不能为null    transient Object[] queue; // non-private to simplify nested class access    /**     * The number of elements in the priority queue.     */    //优先队列中的元素个数    private int size = 0;    /**     * The comparator, or null if priority queue uses elements'     * natural ordering.     */    //比较器    private final Comparator<? super E> comparator;    /**     * The number of times this priority queue has been     * <i>structurally modified</i>.  See AbstractList for gory details.     */    //结构修改时就modCount++    transient int modCount = 0; // non-private to simplify nested class access    /**     * Creates a {@code PriorityQueue} with the default initial     * capacity (11) that orders its elements according to their     * {@linkplain Comparable natural ordering}.     */    //构造函数，默认初始化为11个元素    public PriorityQueue() {        this(DEFAULT_INITIAL_CAPACITY, null);    }    /**     * Creates a {@code PriorityQueue} with the specified initial     * capacity that orders its elements according to their     * {@linkplain Comparable natural ordering}.     *     * @param initialCapacity the initial capacity for this priority queue     * @throws IllegalArgumentException if {@code initialCapacity} is less     *         than 1     */    //传入初始化容量    public PriorityQueue(int initialCapacity) {        this(initialCapacity, null);    }    /**     * Creates a {@code PriorityQueue} with the default initial capacity and     * whose elements are ordered according to the specified comparator.     *     * @param  comparator the comparator that will be used to order this     *         priority queue.  If {@code null}, the {@linkplain Comparable     *         natural ordering} of the elements will be used.     * @since 1.8     */    //传入比较器    public PriorityQueue(Comparator<? super E> comparator) {        this(DEFAULT_INITIAL_CAPACITY, comparator);    }    /**     * Creates a {@code PriorityQueue} 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 that will be used to order this     *         priority queue.  If {@code null}, the {@linkplain Comparable     *         natural ordering} of the elements will be used.     * @throws IllegalArgumentException if {@code initialCapacity} is     *         less than 1     */    //传入初始化容量和比较器    public PriorityQueue(int initialCapacity,                         Comparator<? super E> comparator) {        // Note: This restriction of at least one is not actually needed,        // but continues for 1.5 compatibility        if (initialCapacity < 1)            throw new IllegalArgumentException();        this.queue = new Object[initialCapacity];        this.comparator = comparator;    }    /**     * Creates a {@code PriorityQueue} containing the elements in the     * specified collection.  If the specified collection is an instance of     * a {@link SortedSet} or is another {@code PriorityQueue}, this     * priority queue will be ordered according to the same ordering.     * Otherwise, this priority queue will be ordered according to the     * {@linkplain Comparable natural ordering} of its elements.     *     * @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 the specified collection or any     *         of its elements are null     */    //传入容器    @SuppressWarnings("unchecked")    public PriorityQueue(Collection<? extends E> c) {        if (c instanceof SortedSet<?>) {            SortedSet<? extends E> ss = (SortedSet<? extends E>) c;            this.comparator = (Comparator<? super E>) ss.comparator();            initElementsFromCollection(ss);        }        else if (c instanceof PriorityQueue<?>) {            PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;            this.comparator = (Comparator<? super E>) pq.comparator();            initFromPriorityQueue(pq);        }        else {            this.comparator = null;            initFromCollection(c);        }    }    /**     * Creates a {@code PriorityQueue} containing the elements in the     * specified priority queue.  This priority queue will be     * ordered according to the same ordering as the given priority     * queue.     *     * @param  c the priority queue whose elements are to be placed     *         into this priority queue     * @throws ClassCastException if elements of {@code c} cannot be     *         compared to one another according to {@code c}'s     *         ordering     * @throws NullPointerException if the specified priority queue or any     *         of its elements are null     */    @SuppressWarnings("unchecked")    public PriorityQueue(PriorityQueue<? extends E> c) {        this.comparator = (Comparator<? super E>) c.comparator();        initFromPriorityQueue(c);    }    /**     * Creates a {@code PriorityQueue} containing the elements in the     * specified sorted set.   This priority queue will be ordered     * according to the same ordering as the given sorted set.     *     * @param  c the sorted set whose elements are to be placed     *         into this priority queue     * @throws ClassCastException if elements of the specified sorted     *         set cannot be compared to one another according to the     *         sorted set's ordering     * @throws NullPointerException if the specified sorted set or any     *         of its elements are null     */    @SuppressWarnings("unchecked")    public PriorityQueue(SortedSet<? extends E> c) {        this.comparator = (Comparator<? super E>) c.comparator();        initElementsFromCollection(c);    }    //将队列c的元素初始化为本队列元素    private void initFromPriorityQueue(PriorityQueue<? extends E> c) {        if (c.getClass() == PriorityQueue.class) {            this.queue = c.toArray();            this.size = c.size();        } else {            initFromCollection(c);        }    }    //初始化元素，将容器c的元素赋到本队列中    private void initElementsFromCollection(Collection<? extends E> c) {        Object[] a = c.toArray();        // If c.toArray incorrectly doesn't return Object[], copy it.        if (a.getClass() != Object[].class)            a = Arrays.copyOf(a, a.length, Object[].class);        int len = a.length;        //有比较器则元素不能为空        if (len == 1 || this.comparator != null)            for (int i = 0; i < len; i++)                if (a[i] == null)                    throw new NullPointerException();        this.queue = a;        this.size = a.length;    }    /**     * Initializes queue array with elements from the given Collection.     *     * @param c the collection     */    //初始化队列    private void initFromCollection(Collection<? extends E> c) {        initElementsFromCollection(c);        //建立堆结构        heapify();    }    /**     * The maximum size of array to allocate.     * Some VMs reserve some header words in an array.     * Attempts to allocate larger arrays may result in     * OutOfMemoryError: Requested array size exceeds VM limit     */    //队列中最大的容量    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;    /**     * Increases the capacity of the array.     *     * @param minCapacity the desired minimum capacity     */    private void grow(int minCapacity) {        int oldCapacity = queue.length;        // Double size if small; else grow by 50%        //如果容量小于64,则为 2倍+2 ，如果容量大于等于64，则为 1.5倍        int newCapacity = oldCapacity + ((oldCapacity < 64) ?                                         (oldCapacity + 2) :                                         (oldCapacity >> 1));        // overflow-conscious code        if (newCapacity - MAX_ARRAY_SIZE > 0)            newCapacity = hugeCapacity(minCapacity);        queue = Arrays.copyOf(queue, newCapacity);    }    private static int hugeCapacity(int minCapacity) {        if (minCapacity < 0) // overflow            throw new OutOfMemoryError();        //最大为Integer.MAX_VALUE        return (minCapacity > MAX_ARRAY_SIZE) ?            Integer.MAX_VALUE :            MAX_ARRAY_SIZE;    }    /**     * Inserts the specified element into this priority queue.     *     * @return {@code true} (as specified by {@link Collection#add})     * @throws ClassCastException if the specified element cannot be     *         compared with elements currently in this priority queue     *         according to the priority queue's ordering     * @throws NullPointerException if the specified element is null     */    public boolean add(E e) {        return offer(e);    }    /**     * Inserts the specified element into this priority queue.     *     * @return {@code true} (as specified by {@link Queue#offer})     * @throws ClassCastException if the specified element cannot be     *         compared with elements currently in this priority queue     *         according to the priority queue's ordering     * @throws NullPointerException if the specified element is null     */    public boolean offer(E e) {        if (e == null)            throw new NullPointerException();        modCount++;        int i = size;        //扩容        if (i >= queue.length)            grow(i + 1);        size = i + 1;        if (i == 0)            //堆顶            queue[0] = e;        else            //插入元素，修改堆结构，从下到上扫描            siftUp(i, e);        return true;    }    @SuppressWarnings("unchecked")    public E peek() {        return (size == 0) ? null : (E) queue[0];    }    //返回对象o第一次出现的位置    private int indexOf(Object o) {        if (o != null) {            for (int i = 0; i < size; i++)                if (o.equals(queue[i]))                    return i;        }        return -1;    }    /**     * Removes a single instance of the specified element from this queue,     * if it is present.  More formally, removes an element {@code e} such     * that {@code o.equals(e)}, if this queue contains one or more such     * elements.  Returns {@code true} if and only if this queue contained     * the specified element (or equivalently, if this queue changed as a     * result of the call).     *     * @param o element to be removed from this queue, if present     * @return {@code true} if this queue changed as a result of the call     */    //移除掉第一次出现的o    public boolean remove(Object o) {        int i = indexOf(o);        if (i == -1)            return false;        else {            //移除的真正实现方法            removeAt(i);            return true;        }    }    /**     * Version of remove using reference equality, not equals.     * Needed by iterator.remove.http://weidian.com/i/1614441547?wfr=wx     *     * @param o element to be removed from this queue, if present     * @return {@code true} if removed     */    //使用 == 来对比对象的引用，不是用equals    boolean removeEq(Object o) {        for (int i = 0; i < size; i++) {            if (o == queue[i]) {                removeAt(i);                return true;            }        }        return false;    }    /**     * Returns {@code true} if this queue contains the specified element.     * More formally, returns {@code true} if and only if this queue contains     * at least one element {@code e} such that {@code o.equals(e)}.     *     * @param o object to be checked for containment in this queue     * @return {@code true} if this queue contains the specified element     */    public boolean contains(Object o) {        return indexOf(o) != -1;    }    /**     * Returns an array containing all of the elements in this queue.     * The elements are in no particular order.     *     * <p>The returned array will be "safe" in that no references to it are     * maintained by this queue.  (In other words, this method must allocate     * a new array).  The caller is thus free to modify the returned array.     *     * <p>This method acts as bridge between array-based and collection-based     * APIs.     *     * @return an array containing all of the elements in this queue     */    public Object[] toArray() {        return Arrays.copyOf(queue, size);    }    /**     * Returns an array containing all of the elements in this queue; the     * runtime type of the returned array is that of the specified array.     * The returned array elements are in no particular order.     * If the queue fits in the specified array, it is returned therein.     * Otherwise, a new array is allocated with the runtime type of the     * specified array and the size of this queue.     *     * <p>If the queue fits in the specified array with room to spare     * (i.e., the array has more elements than the queue), the element in     * the array immediately following the end of the collection is set to     * {@code null}.     *     * <p>Like the {@link #toArray()} method, this method acts as bridge between     * array-based and collection-based APIs.  Further, this method allows     * precise control over the runtime type of the output array, and may,     * under certain circumstances, be used to save allocation costs.     *     * <p>Suppose {@code x} is a queue known to contain only strings.     * The following code can be used to dump the queue into a newly     * allocated array of {@code String}:     *     *  <pre> {@code String[] y = x.toArray(new String[0]);}</pre>     *     * Note that {@code toArray(new Object[0])} is identical in function to     * {@code toArray()}.     *     * @param a the array into which the elements of the queue are to     *          be stored, if it is big enough; otherwise, a new array of the     *          same runtime type is allocated for this purpose.     * @return an array containing all of the elements in this queue     * @throws ArrayStoreException if the runtime type of the specified array     *         is not a supertype of the runtime type of every element in     *         this queue     * @throws NullPointerException if the specified array is null     */    @SuppressWarnings("unchecked")    public <T> T[] toArray(T[] a) {        final int size = this.size;        if (a.length < size)            //传进来的数组的容量不够            // Make a new array of a's runtime type, but my contents:            return (T[]) Arrays.copyOf(queue, size, a.getClass());        System.arraycopy(queue, 0, a, 0, size);        if (a.length > size)            a[size] = null;        return a;    }    /**     * Returns an iterator over the elements in this queue. The iterator     * does not return the elements in any particular order.     *     * @return an iterator over the elements in this queue     */    public Iterator<E> iterator() {        return new Itr();    }    private final class Itr implements Iterator<E> {        /**         * Index (into queue array) of element to be returned by         * subsequent call to next.         */        //下次调用next时返回值的下标        private int cursor = 0;        /**         * Index of element returned by most recent call to next,         * unless that element came from the forgetMeNot list.         * Set to -1 if element is deleted by a call to remove.         */        //最近调用next返回值的下标        private int lastRet = -1;        /**         * A queue of elements that were moved from the unvisited portion of         * the heap into the visited portion as a result of "unlucky" element         * removals during the iteration.  (Unlucky element removals are those         * that require a siftup instead of a siftdown.)  We must visit all of         * the elements in this list to complete the iteration.  We do this         * after we've completed the "normal" iteration.         *         * We expect that most iterations, even those involving removals,         * will not need to store elements in this field.         */        //记录那些没有被调用的元素        private ArrayDeque<E> forgetMeNot = null;        /**         * Element returned by the most recent call to next iff that         * element was drawn from the forgetMeNot list.         */        //上一次调用next的元素        private E lastRetElt = null;        /**         * The modCount value that the iterator believes that the backing         * Queue should have.  If this expectation is violated, the iterator         * has detected concurrent modification.         */        private int expectedModCount = modCount;        public boolean hasNext() {            return cursor < size ||                (forgetMeNot != null && !forgetMeNot.isEmpty());        }        @SuppressWarnings("unchecked")        public E next() {            if (expectedModCount != modCount)                throw new ConcurrentModificationException();            if (cursor < size)                return (E) queue[lastRet = cursor++];            if (forgetMeNot != null) {                lastRet = -1;                lastRetElt = forgetMeNot.poll();                if (lastRetElt != null)                    return lastRetElt;            }            throw new NoSuchElementException();        }        public void remove() {            if (expectedModCount != modCount)                throw new ConcurrentModificationException();            if (lastRet != -1) {                E moved = PriorityQueue.this.removeAt(lastRet);                lastRet = -1;                if (moved == null)                    cursor--;                else {                    if (forgetMeNot == null)                        forgetMeNot = new ArrayDeque<>();                    forgetMeNot.add(moved);                }            } else if (lastRetElt != null) {                PriorityQueue.this.removeEq(lastRetElt);                lastRetElt = null;            } else {                throw new IllegalStateException();            }            expectedModCount = modCount;        }    }    public int size() {        return size;    }    /**     * Removes all of the elements from this priority queue.     * The queue will be empty after this call returns.     */    public void clear() {        modCount++;        for (int i = 0; i < size; i++)            queue[i] = null;        size = 0;    }    @SuppressWarnings("unchecked")    public E poll() {        if (size == 0)            return null;        int s = --size;        modCount++;        E result = (E) queue[0];        E x = (E) queue[s];        queue[s] = null;        //需要重新调整堆结构        if (s != 0)            siftDown(0, x);        return result;    }    /**     * Removes the ith element from queue.     *     * Normally this method leaves the elements at up to i-1,     * inclusive, untouched.  Under these circumstances, it returns     * null.  Occasionally, in order to maintain the heap invariant,     * it must swap a later element of the list with one earlier than     * i.  Under these circumstances, this method returns the element     * that was previously at the end of the list and is now at some     * position before i. This fact is used by iterator.remove so as to     * avoid missing traversing elements.     */    //移除掉第i个元素    @SuppressWarnings("unchecked")    private E removeAt(int i) {        // assert i >= 0 && i < size;        modCount++;        int s = --size;        if (s == i) // removed last element            //移除掉最后一个元素，不需要调整堆结构            queue[i] = null;        else {            E moved = (E) queue[s];            queue[s] = null;            //将最后一个元素和第i个元素替换            //从i往下比较，比较i下面的孩子们            siftDown(i, moved);            if (queue[i] == moved) {                //比较堆中i以上的元素                siftUp(i, moved);                if (queue[i] != moved)                    return moved;            }        }        return null;    }    /**     * Inserts item x at position k, maintaining heap invariant by     * promoting x up the tree until it is greater than or equal to     * its parent, or is the root.     *     * To simplify and speed up coercions and comparisons. the     * 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     */    //在k位置插入元素x，调整堆使x大于或等于它的父节点【从下到上比较】    private void siftUp(int k, E x) {        //区分是否有自己的比较器，该方法实现最小堆调整        if (comparator != null)            siftUpUsingComparator(k, x);        else            siftUpComparable(k, x);    }    @SuppressWarnings("unchecked")    private void siftUpComparable(int k, E x) {        Comparable<? super E> key = (Comparable<? super E>) x;        while (k > 0) {            //计算得到parent的下标            int parent = (k - 1) >>> 1;            Object e = queue[parent];            if (key.compareTo((E) e) >= 0)                break;            //如果比父结点小，则跟父节点替换            queue[k] = e;            k = parent;        }        queue[k] = key;    }    @SuppressWarnings("unchecked")    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;            //如果比父结点小，则跟父节点替换            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     */    //在k位置中插入元素x，调整堆使元素x小于或等于它儿子【从上到下比较】    private void siftDown(int k, E x) {        if (comparator != null)            siftDownUsingComparator(k, x);        else            siftDownComparable(k, x);    }    @SuppressWarnings("unchecked")    private void siftDownComparable(int k, E x) {        Comparable<? super E> key = (Comparable<? super E>)x;        int half = size >>> 1;        // loop while a non-leaf        //只要k < size/2, 说明k有孩子        while (k < half) {            //计算出左孩子的下标            int child = (k << 1) + 1; // assume left child is least            Object c = queue[child];            //右孩子下标            int right = child + 1;            //如果有右孩子，左孩子跟右孩子比较，将小的结点赋到c            if (right < size &&                ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)                c = queue[child = right];            if (key.compareTo((E) c) <= 0)                //key比c小，说明比孩子结点小，直接退出                break;            //比孩子大，则跟孩子替换            queue[k] = c;            k = child;        }        queue[k] = key;    }    @SuppressWarnings("unchecked")    private void siftDownUsingComparator(int k, E x) {        int half = size >>> 1;        //只要k < size/2, 说明k有孩子        while (k < half) {            //计算出左孩子的下标            int child = (k << 1) + 1;            Object c = queue[child];            //右孩子下标            int right = child + 1;            //如果有右孩子，左孩子跟右孩子比较，将小的结点赋到c            if (right < size &&                comparator.compare((E) c, (E) queue[right]) > 0)                c = queue[child = right];            if (comparator.compare(x, (E) c) <= 0)                //key比c小，说明比孩子结点小，直接退出                break;            //比孩子大，则跟孩子替换            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.     */    @SuppressWarnings("unchecked")    private void heapify() {        //建立堆结构        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 this queue 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();    }    /**     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>     * and <em>fail-fast</em> {@link Spliterator} over the elements in this     * queue.     *     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},     * {@link Spliterator#SUBSIZED}, and {@link Spliterator#NONNULL}.     * Overriding implementations should document the reporting of additional     * characteristic values.     *     * @return a {@code Spliterator} over the elements in this queue     * @since 1.8     */    public final Spliterator<E> spliterator() {        return new PriorityQueueSpliterator<E>(this, 0, -1, 0);    }    static final class PriorityQueueSpliterator<E> implements Spliterator<E> {        /*         * This is very similar to ArrayList Spliterator, except for         * extra null checks.         */        private final PriorityQueue<E> pq;        private int index;            // current index, modified on advance/split        private int fence;            // -1 until first use        private int expectedModCount; // initialized when fence set        /** Creates new spliterator covering the given range */        PriorityQueueSpliterator(PriorityQueue<E> pq, int origin, int fence,                             int expectedModCount) {            this.pq = pq;            this.index = origin;            this.fence = fence;            this.expectedModCount = expectedModCount;        }        private int getFence() { // initialize fence to size on first use            int hi;            if ((hi = fence) < 0) {                expectedModCount = pq.modCount;                hi = fence = pq.size;            }            return hi;        }        public PriorityQueueSpliterator<E> trySplit() {            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;            return (lo >= mid) ? null :                new PriorityQueueSpliterator<E>(pq, lo, index = mid,                                                expectedModCount);        }        @SuppressWarnings("unchecked")        public void forEachRemaining(Consumer<? super E> action) {            int i, hi, mc; // hoist accesses and checks from loop            PriorityQueue<E> q; Object[] a;            if (action == null)                throw new NullPointerException();            if ((q = pq) != null && (a = q.queue) != null) {                if ((hi = fence) < 0) {                    mc = q.modCount;                    hi = q.size;                }                else                    mc = expectedModCount;                if ((i = index) >= 0 && (index = hi) <= a.length) {                    for (E e;; ++i) {                        if (i < hi) {                            if ((e = (E) a[i]) == null) // must be CME                                break;                            action.accept(e);                        }                        else if (q.modCount != mc)                            break;                        else                            return;                    }                }            }            throw new ConcurrentModificationException();        }        public boolean tryAdvance(Consumer<? super E> action) {            if (action == null)                throw new NullPointerException();            int hi = getFence(), lo = index;            if (lo >= 0 && lo < hi) {                index = lo + 1;                @SuppressWarnings("unchecked") E e = (E)pq.queue[lo];                if (e == null)                    throw new ConcurrentModificationException();                action.accept(e);                if (pq.modCount != expectedModCount)                    throw new ConcurrentModificationException();                return true;            }            return false;        }        public long estimateSize() {            return (long) (getFence() - index);        }        public int characteristics() {            return Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.NONNULL;        }    }}