java PriorityQueue优先级队列使用

package com.example.base.concurrent;

import java.util.Comparator;
import java.util.PriorityQueue;

public class MyPriorityQueue {

    public static void main(String[] args) {
        foo();
    }
    
    private static void foo() {
        PriorityQueue<User> queue = generateQueue(null);
        System.out.println("******read queue without order******");
        for (User user : queue) {
            System.out.println(user);
        }
        //queue = generateQueue();
        System.out.println("******read queue with order******");
        while(!queue.isEmpty()) {
            System.out.println(queue.poll());
        }
        
        //产生队列时使用comparator，不使用类的Comparable接口
        queue = generateQueue((x,y) ->{
            int delta = y.getAge() - x.getAge();
            return delta < 0 ? -1 : (delta == 0 ? 0 : 1);
        });
        System.out.println("******read queue with order comparator******");
        while(!queue.isEmpty()) {
            System.out.println(queue.poll());
        }
    }

    /**
     * @param comparator 优先级队列中进行元素比较的比较器
     *    如果不为null，则使用comparator进行元素比价，优先使用comparator
     *    如果为null，则使用元素的compareTo方法（需要继承Comparable接口）
     * @return
     */
    private static PriorityQueue<User> generateQueue(Comparator<User> comparator) {
        PriorityQueue<User> queue = new PriorityQueue<>(comparator);
        for (int i = 30; i > 20; i--) {
            queue.add(new User(i, "gc"+i));
        }
        return queue;
    }
    
    private static class User implements Comparable<User> {
        private int age;
        private String name;
        
        public User(int age, String name) {
            this.age = age;
            this.name = name;
        }
        public int getAge() {
            return age;
        }
        @Override
        public int compareTo(User o) {
            int delta = this.getAge() - o.getAge();
            return delta < 0 ? -1 : (delta == 0 ? 0 : 1);
        }
        @Override
        public String toString() {
            return "User [age=" + age + ", name=" + name + "]";
        }
    }
}

******read queue without order******
User [age=21, name=gc21]
User [age=22, name=gc22]
User [age=25, name=gc25]
User [age=24, name=gc24]
User [age=23, name=gc23]
User [age=29, name=gc29]
User [age=26, name=gc26]
User [age=30, name=gc30]
User [age=27, name=gc27]
User [age=28, name=gc28]
******read queue with order******
User [age=21, name=gc21]
User [age=22, name=gc22]
User [age=23, name=gc23]
User [age=24, name=gc24]
User [age=25, name=gc25]
User [age=26, name=gc26]
User [age=27, name=gc27]
User [age=28, name=gc28]
User [age=29, name=gc29]
User [age=30, name=gc30]
******read queue with order2******
User [age=30, name=gc30]
User [age=29, name=gc29]
User [age=28, name=gc28]
User [age=27, name=gc27]
User [age=26, name=gc26]
User [age=25, name=gc25]
User [age=24, name=gc24]
User [age=23, name=gc23]
User [age=22, name=gc22]
User [age=21, name=gc21]