Prioprity源码分析--priority如何实现优先级排序

目录

本文围绕的核心问题：priority是怎么实现优先级排序？

Demo

源码分析：

offer方法

siftUp方法

siftUpUsingComparator方法

接口Comparator中的compare方法（默认比较器）

自定义的compare方法

siftUpComparable 方法

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本文围绕的核心问题：priority是怎么实现优先级排序？

Demo

public class Demo {
    public static void main(String[] args) {
        //需求：priority中age越大优先级越高
        PriorityQueue<Student> priorityQueue = new PriorityQueue<>(2, new StudentComparator());
        Student s1 = new Student("jack", 2200);
        Student s2 = new Student("Tom", 223);
        priorityQueue.offer(s1);
        priorityQueue.offer(s2);
        Student poll = priorityQueue.poll();
        System.out.println(poll.name+":"+poll.age);
    }
}


public class StudentComparator implements Comparator<Student> {
    @Override
    public int compare(Student current, Student middle) {
        if (current.age>middle.age)
            return -1;
        else if (current.age<middle.age)
            return 1;
        else
            return 0;
    }
}
  

public class Student {
    String name;
    int age;
    public Student(String name,int age){
        this.name=name;
        this.age=age;
    }
}

源码分析：

• 底层的队列依靠数组实现
• 内部有比较器，可以在构造函数中指定自定义的比较器

public class PriorityQueue<E> extends AbstractQueue<E>
    implements java.io.Serializable {
    ...    
    /**
     * 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.
     */
    transient Object[] queue; // non-private to simplify nested class access

    /**
     * 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;

    ...

    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;
    }
}

 

offer方法

    public boolean offer(E e) {
        if (e == null)
            throw new NullPointerException();
        modCount++;
        //i记录当前数组长度
        int i = size;
        //越界扩容处理
        if (i >= queue.length)
            grow(i + 1);
        //加入新元素e，长度+1
        size = i + 1;
        //第一个元素直接放置
        if (i == 0)
            queue[0] = e;
        //之后加入队列的元素进行筛选
        else
            siftUp(i, e);
        return true;
    }

 

siftUp方法

private void siftUp(int k, E x) {
    //判断是否有比较器
    if (comparator != null)
        //使用自定义的比较器进行插入
        siftUpUsingComparator(k, x);
    else
        //使用默认比较器
        siftUpComparable(k, x);
}

 

siftUpUsingComparator方法

private void siftUpUsingComparator(int k, E x) {
    //从当前位置，不断的往前遍历，通过比较器修改每个位置
    while (k > 0) {
        //parent是中间位置的角标
        int parent = (k - 1) >>> 1;
        Object e = queue[parent];
        //调用比较器的compare方法，比较当前要插入的元素和中间位置的大小关系
        if (comparator.compare(x, (E) e) >= 0)
            //默认情况下构建的是一个完全二叉树实现的小顶堆。
            //我们这里通过自定义的比较器构建一个大顶堆，在比较器中需要设置当前元素小于中间元素返回1，才能跳出循环
            break;
        //否则把当前元素放置到中间位置，    
        queue[k] = e;
        //k的值变成中间位置，方便后续不断的调整优先队列中前半部分保持优先级的降序
        k = parent;
    }
    //确定好优先级位置后放置x
    queue[k] = x;
}

 

接口Comparator中的compare方法（默认比较器）

public interface Comparator<T> {
    /**
     * Compares its two arguments for order.  Returns a negative integer,
     * zero, or a positive integer as the first argument is less than, equal
     * to, or greater than the second.<p>
     * 比较两个参数进行排序，返回负数，零，正整数，
     * 分别代表第一个参数小于，等于，大于第二个参数
     *...
     */
    int compare(T o1, T o2);
}

 

自定义的compare方法

目标：（让队列最后是降序排序）

// 给age降序排序
public class StudentComparator implements Comparator<Student> {
    @Override
    public int compare(Student current, Student middle) {
        if (current.age>middle.age)
            return -1;
        else if (current.age<middle.age)
            return 1;
        else
            return 0;
    }
}

 

siftUpComparable 方法

代码类似siftUpUsingComparator，使用的是默认比较器

    private void siftUpComparable(int k, E x) {
        Comparable<? super E> key = (Comparable<? super E>) x;
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = queue[parent];
            if (key.compareTo((E) e) >= 0)
                break;
            queue[k] = e;
            k = parent;
        }
        queue[k] = key;
    }

 

 

 

本文地址：https://blog.csdn.net/qq_38783664/article/details/110471807