队列总结（二）PriorityQueue_priorityqueue offer poll-优快云博客

队列的实现

首先尝试动手写一个类实现Queue接口，发现要实现的方法太多了，麻烦。所幸，JDK为我们提供了一个骨架实现类AbstractQueue，最大限度地减少了实现Queue接口所需的工作。

AbstractQueue

/**
 * 该骨架对于允许null元素的实现来讲是不合适的，其方法add 、 remove和element分别基于offer 、 poll和peek来实现。
 *  所以继承该类的队列实现必须提供一个不允许插入null元素的offer方法
 */
public abstract class AbstractQueue<E>
    extends AbstractCollection<E>
    implements Queue<E> {

    protected AbstractQueue() {
    }
    
    public boolean add(E e) {
        if (offer(e))
            return true;
        else
            throw new IllegalStateException("Queue full");
    }

    public E remove() {
        E x = poll();
        if (x != null)
            return x;
        else
            throw new NoSuchElementException();
    }

    public E element() {
        E x = peek();
        if (x != null)
            return x;
        else
            throw new NoSuchElementException();
    }

    public void clear() {
        while (poll() != null)
            ;
    }

    public boolean addAll(Collection<? extends E> c) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        boolean modified = false;
        for (E e : c)
            if (add(e))
                modified = true;
        return modified;
    }

再找一个基于AbstractQueue的实现PriorityQueue来学习一波。

PriorityQueue

二叉堆实现，理解了二叉堆，这个就实在没什么好讲的了，本质是个最小堆，参考文章https://www.jianshu.com/p/6d3a12fe2d04

但是有几点需要拎出来单独分析一下

	/**
	 * 从队列中移除第 i 个元素。 与常规数组的移除不同，堆的移除操作，首先将最后一个元素e替换到位置i上，同时还要进行下沉或上浮操作以维持堆结构。
	 * 如果在执行上浮或下沉时，元素e恰好都未移位，即最后一个元素替换到i位置上，刚好完美契合堆结构，即此队列的前i-1（包括 i-1）位的元素皆不受影响。在这些情况下，它返回 null。 
	 * 假如在上浮或下沉过程中，元素e与早于 i 的元素（前i-1位）发生了交换。即元素e不在预料的i位置上，而是前i-1的某个位置上。在这些情况下，它返回元素e。 
	 *  这个事实被 iterator.remove 使用，以避免丢失遍历元素。（介绍迭代时会解释）
	 */
    private E removeAt(int i) {
        // assert i >= 0 && i < size;
        modCount++;
        int s = --size;
        if (s == i) // removed last element
            queue[i] = null;
        else {
            E moved = (E) queue[s];
            queue[s] = null;
            siftDown(i, moved);
            // 说明并未下沉移位
            if (queue[i] == moved) {
                siftUp(i, moved);
                  // 说明发生了上浮移位
                if (queue[i] != moved)
                    return moved;
            }
        }
        return null;
    }

我这里提供一个简单例子体会一下：
一个最大堆[8,4,6,2,3,5,1]移除下标1的元素4，这里会发生下沉
一个最大堆[8,4,6,2,5,1,5]移除下标4的元素3，这里会发生上浮
（注意PriorityQueue是个最小堆，如果要测试，注意提供额外的比较器）

package test;

import org.junit.Test;

import java.util.Iterator;
import java.util.PriorityQueue;

public class Client {

    @Test
    public void testUp() {
        PriorityQueue<Integer> queue = new PriorityQueue<>((a, b) -> -a.compareTo(b));

        queue.add(8);
        queue.add(4);
        queue.add(6);
        queue.add(2);
        queue.add(3);
        queue.add(5);
        queue.add(1);

        queue.remove(4);
        StringBuilder sb = new StringBuilder();
        for (int i : queue) {
            sb.append(i).append(",");
        }
        System.out.println(sb);
    }

    @Test
    public void testDown() {
        PriorityQueue<Integer> queue = new PriorityQueue<>((a, b) -> -a.compareTo(b));

        queue.add(8);
        queue.add(4);
        queue.add(6);
        queue.add(2);
        queue.add(3);
        queue.add(1);
        queue.add(5);

        queue.remove(3);
        StringBuilder sb = new StringBuilder();
        for (int i : queue) {
            sb.append(i).append(",");
        }
        System.out.println(sb);
    }

    @Test
    public void testItr() {
        PriorityQueue<Integer> queue = new PriorityQueue<>((a, b) -> -a.compareTo(b));

        queue.add(8);
        queue.add(4);
        queue.add(6);
        queue.add(2);
        queue.add(3);
        queue.add(1);
        queue.add(5);

        StringBuilder sb = new StringBuilder();
        Iterator<Integer> iterator = queue.iterator();
        while (iterator.hasNext()) {
            Integer next = iterator.next();
            if (next == 3) {
                iterator.remove();
            }
            sb.append(next).append(",");
        }
        System.out.println(sb);
    }

}

迭代器的实现（偷个懒，懒得写分析了）

    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());
        }

        @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;
        }
    }