java - Java 中的 PriorityQueue 说明

Question

在下面的代码中，我想知道创建 PrioriyQueue 时 10 的意义。我知道它的初始容量，但它会影响性能吗？

import java.util.*;

class Test {
    static class PQsort implements Comparator<Integer> { // inverse sort
        public int compare(Integer one, Integer two) {
            return two - one; // unboxing
        }
    }

    public static void main(String[] args) {
        int[] ia = { 1, 5, 3, 7, 6, 9, 8 }; // unordered data
        PriorityQueue<Integer> pq1 = new PriorityQueue<Integer>(); // use
                                                                    // natural
                                                                    // order
        for (int x : ia)
            pq1.offer(x);
        for (int x : ia)
            // review queue
            System.out.print(pq1.poll() + " ");
        System.out.println("");
        PQsort pqs = new PQsort(); // get a Comparator
        PriorityQueue<Integer> pq2 = new PriorityQueue<Integer>(10, pqs); // use
                                                                            // Comparator
        for (int x : ia)
            // load queue
            pq2.offer(x);
        System.out.println("size " + pq2.size());
        System.out.println("peek " + pq2.peek());
        System.out.println("size " + pq2.size());
        System.out.println("poll " + pq2.poll());
        System.out.println("size " + pq2.size());
        for (int x : ia)
            // review queue
            System.out.print(pq2.poll() + " ");
    }
}

Answer 1

Javadoc解释:

A priority queue is unbounded, but has an internal capacity 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.

换句话说，如果发现队列花费太多时间增长其内部数组，则能够指定初始容量是优化性能的一种方法。