PriorityQueue 源代码

//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);
}

/**
* 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]);
}

/*
* Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
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*
*
*
*
*
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*
*/

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
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
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;
}
}
}