PriorityQueue源码浅谈
/*
* @(#)PriorityQueue.java 1.16 06/04/21
*
* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util;
/**
* 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 enqueing and dequeing 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
* @version 1.16, 04/21/06
* @author Josh Bloch, Doug Lea
* @param <E> the type of elements held in this collection
*/
public class PriorityQueue<E> extends AbstractQueue<E>
implements java.io.Serializable {
private static final long serialVersionUID = -7720805057305804111L;
// 初始化时数组大小默认为 11
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.
*/
// 具有二叉堆性质的数组 queue[n]的左右孩子分别为queue[2*n+1]、queue[2*(n+1)]
// 优先队列通过比较器comparator来对元素进行排序
// 如果没有比较器，元素的顺序就是他们插入时的顺序，
// 第一个插入的元素在堆顶queue[0](我个人的理解)
private transient Object[] queue;
/**
* The number of elements in the priority queue.
*/
private int size = 0;
/**
* The comparator, or null if priority queue uses elements'
* natural ordering.
*/
private final Comparator<? super E> comparator;
/**
* The number of times this priority queue has been
* <i>structurally modified</i>. See AbstractList for gory details.
*/
private transient int modCount = 0;
/**
* Creates a {@code PriorityQueue} with the default initial
* capacity (11) that orders its elements according to their
* {@linkplain Comparable natural ordering}.
*/
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
*/
public PriorityQueue(int initialCapacity) {
this(initialCapacity, null);
}
/**
* 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
*/
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
*/
public PriorityQueue(Collection<? extends E> c) {
// 初始化数组queue，也就是调用Arrays.copyOf进行数组的复制
initFromCollection(c);
if (c instanceof SortedSet)
comparator = (Comparator<? super E>)
((SortedSet<? extends E>)c).comparator();
else if (c instanceof PriorityQueue)
comparator = (Comparator<? super E>)
((PriorityQueue<? extends E>)c).comparator();
else {
// 如果C不是SortedSet或者PriorityQueue的子类
// 则要进行堆调整，以保证堆的有序性/
// 通过这里我们可以猜测SortedSet的实现也是具有与二叉堆相似的算法实现
comparator = null;
heapify();
}
}
/**
* 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
*/
public PriorityQueue(PriorityQueue<? extends E> c) {
comparator = (Comparator<? super E>)c.comparator();
initFromCollection(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
*/
public PriorityQueue(SortedSet<? extends E> c) {
comparator = (Comparator<? super E>)c.comparator();
initFromCollection(c);
}
/**
* Initializes queue array with elements from the given Collection.
*
* @param c the collection
*/
private void initFromCollection(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);
queue = a;
size = a.length;
}
/**
* Increases the capacity of the array.
*
* @param minCapacity the desired minimum capacity
*/
private void grow(int minCapacity) {
// 扩容操作
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
int oldCapacity = queue.length;
// Double size if small; else grow by 50%
// 如果当前的元素个数小于 64，则扩大为原来的2倍
// 否则只扩大为原来的1.5倍
int newCapacity = ((oldCapacity < 64)?
((oldCapacity + 1) * 2):
((oldCapacity / 2) * 3));
// 如果最后的计算结果溢出了，则扩容为Integer.MAX_VALUE
// 可见优先队列的元素是有个数限制的
if (newCapacity < 0) // overflow
newCapacity = Integer.MAX_VALUE;
if (newCapacity < minCapacity)
// 用户请求的扩容大于计算的应该扩容值是，使用用户的值
newCapacity = minCapacity;
queue = Arrays.copyOf(queue, newCapacity);
}
/**
* 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) {
// add(E e)调用了 offer(E e)，不知道为什么会提供这样的一个add(E e)
if (e == null)
throw new NullPointerException();
modCount++;
int i = size;
if (i >= queue.length)
// 如果当前数组queue已经装满元素了，则进行扩容，看似只新增一个容量
// 其实不然，可能扩大为原来的2倍或者1.5倍，具体看grow的算法
grow(i + 1);
size = i + 1;
if (i == 0)
// 如果当前数组没有值，则直接保存在queue[0]
queue[0] = e;
else
// 调整堆的实现
siftUp(i, e);
return true;
}
public E peek() {
if (size == 0)
return null;
return (E) queue[0];
}
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
*/
public boolean remove(Object o) {
int i = indexOf(o);
if (i == -1)
return false;
else {
removeAt(i);
return true;
}
}
/**
* Version of remove using reference equality, not equals.
* Needed by iterator.remove.
*
* @param o element to be removed from this queue, if present
* @return {@code true} if removed
*/
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 <tt>x</tt> is a queue known to contain only strings.
* The following code can be used to dump the queue into a newly
* allocated array of <tt>String</tt>:
*
* <pre>
* String[] y = x.toArray(new String[0]);</pre>
*
* Note that <tt>toArray(new Object[0])</tt> is identical in function to
* <tt>toArray()</tt>.
*
* @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
*/
public <T> T[] toArray(T[] a) {
if (a.length < size)
// 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.
*/
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.
*/
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.
*/
private ArrayDeque<E> forgetMeNot = null;
/**
* Element returned by the most recent call to next iff that
* element was drawn from the forgetMeNot list.
*/
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());
}
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<E>();
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;
}
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;
if (s != 0)
// 调整堆
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.
*/
private E removeAt(int i) {
// 删除指定下标的数组
// 判断是否越界，使用了assert关键字，如果表达式为false这终止执行后续代码
// 前提是开启断言开关，否则无效，感觉这是个不太好的地方，
// 一般assert只用在开发当中进行调试使用
assert i >= 0 && i < size;
modCount++;
int s = --size;
if (s == i) // removed last element
// 删除的是最后一个元素
queue[i] = null;
else {
// 获取最后一个元素
E moved = (E) queue[s];
// 将最后一个元素所在的地址置空
queue[s] = null;
// 将元素moved（也就是数组的最后一个元素）插入到 i 下标的位置
// 然后和它的孩子比较，如果大于（假设优先队列使用的是最小堆算法）它的孩子，
// 则和它的孩子交换位置,依次递推执行，直到moved小于等于它的孩子，或者它已经没有孩子了
siftDown(i, moved);
if (queue[i] == moved) {
// 说明moved小于等于queue[i]
// 这时就要判断，moved是不是小于queue[i]的父元素，不小于交换位置
siftUp(i, moved);
// 这个看作者给的注释说明，好像是为了给迭代器使用
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
*/
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) {
// queue[k]的父节点下标
int parent = (k - 1) >>> 1;
// queue[k]的父节点
Object e = queue[parent];
if (key.compareTo((E) e) >= 0)
// key大于queue[k]的父节点结束循环
// queue[k]就是key要插入的位置
break;
// 把父节点移到其孩子queue[k]的位置
queue[k] = e;
// k变为父节点的下标，下次比较的就是父节点和父节点的父节点的比较
// 听着有点绕，其实就是依次网速比较（脑海中要有树的轮廓）
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
// 因为数组queue的特点（二叉堆），数组后半部分的元素都没有左右孩子（叶子节点）
while (k < half) {
int child = (k << 1) + 1; // assume left child is least
// 左孩子
Object c = queue[child];
int right = child + 1;
if (right < size &&
((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
// 有右孩子，且左孩子大于右孩子
// 把右孩子赋给变量C
c = queue[child = right];
if (key.compareTo((E) c) <= 0)
// 如果key小于C(此时c是要删除元素的左右孩子中最小的那个值)，结束循环，堆调整结束
break;
// 把最小值移到要删除元素下标的位置
queue[k] = c;
// 然后k变为右孩子的下标，递归遍历，
// 直到key小于queue[child]和queue[right],或者queue[k]已没有孩子
// 此时k就是key要插入的位置
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();
}
}
小结：PriorityQueue就是使用数组实现了推排序的算法，至于是最大堆（树根的值最大array[0]）还事最小堆，可以由我们提供的comparator来实现。PriorityQueue可以使用迭代器来顺序访问......