CountDownLatch模拟田径赛跑

群里得一面试题，正好用来练手

现有一场田径比赛，共有8个选手参赛。他们编号分别为： A,B,C,D,E,F,G,H；赛事观察员分别在开始后 20s和30s拍下两张照片，各个选手的排名情况如下， 20s：F,A,H,G,C,B,E,D；30s：G,H,A,C,F,D,B,E；请你写出一个程序，模拟出这些参赛选手在20-30s这10s的内排名变化，每1s显示一次排名。

主要是考察countDownLatch。
计算的时候clone出快照。避免计算的过程中瞬间值又有变化，导致算出的值不准确。

拍照也就是存一下快照而已。
ps:可能是对题理解有误，下面的效果不追求固定名次。
固定名次的写法思路是：写死各个选手在20s的时候的距离（a），根据a减去各个选手当前的距离（0），然后再随机到每一秒，就是选手每秒的跑的距离。 30s 的时候重新定义一下目标距离(b)，b减去选手的20s时候的距离a,随机到每一秒。

效果

代码

package com.gkwind.concurrent;import java.time.Instant;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.IntStream;/*** 现有一场田径比赛，共有8个选手参赛。他们编号分别为： A,B,C,D,E,F,G,H；* 赛事观察员分别在开始后   20s和30s拍下两张照片，各个选手的排名情况如下，* 20s：F,A,H,G,C,B,E,D；30s：G,H,A,C,F,D,B,E；请你写出一个程序，* 模拟出这些参赛选手在20-30s这10s的内排名变化，每1s显示一次排名。** @Author thewindkee* @Date 2019/7/18 0018  22:36*/
public class Athletics {static final CountDownLatch START_GUN = new CountDownLatch(1);static final LocalDateTime DATE_TIME = LocalDateTime.now();static volatile boolean over = false;static long startTime;static final int PICTURE_TIME = 20;static final int PICTURE_TIME2 = 30;public static void main(String[] args) throws InterruptedException {int num = 8;CountDownLatch latch = new CountDownLatch(num);List<Athlete> athletes = initAthletes(num, latch);athletes.stream().forEach(athlete -> athlete.start());System.out.println("裁判等待选手就位");Computer computer = new Computer(athletes);computer.start();latch.await();System.out.println(DATE_TIME + "时裁判开枪");startTime = Instant.now().toEpochMilli();START_GUN.countDown();computer.join();computer.showPhotos();}private static List<Athlete> initAthletes(int num, CountDownLatch latch) {return IntStream.range(0, num).mapToObj(i -> new Athlete(String.valueOf((char) ('A' + i)), latch)).collect(Collectors.toList());}static class Computer extends Thread {List<Picture<Athlete>> pictures = new ArrayList<>();static class Picture<E> {final List<E> pictureItems;final long timestamp;public Picture(List<E> items) {this.pictureItems = Collections.unmodifiableList(items);;this.timestamp = Instant.now().toEpochMilli();}public void show() {pictureItems.stream().forEach(item -> System.out.println(item));}@Overridepublic String toString() {return "照片拍摄于" + LocalDateTime.ofInstant(Instant.ofEpochMilli(timestamp), ZoneId.systemDefault()) +":" + pictureItems +'}';}}final List<Athlete> athletes;public Computer(List<Athlete> athletes) {this.athletes = Collections.unmodifiableList(athletes);}public List<Athlete> compute() {return athletes.stream().map(new Function<Athlete, Athlete>() {@Overridepublic Athlete apply(Athlete athlete) {try {//拍照，返回运动员的成绩快照return (Athlete) athlete.clone();} catch (CloneNotSupportedException e) {e.printStackTrace();}throw new RuntimeException("计算机故障");}}).sorted(Comparator.reverseOrder()).collect(Collectors.toList());}@Overridepublic void run() {try {System.out.println("电脑已启动");START_GUN.await();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("\n电脑开始工作");sleepThenTakeFirstPhoto();while (!over && !isInterrupted()) {System.out.println();long currentSecond = (Instant.now().toEpochMilli() - startTime) / 1000 + 1;try {TimeUnit.SECONDS.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}List<Athlete> sortedAthletes = compute();System.out.print(currentSecond + "秒: ");sortedAthletes.stream().forEach(athlete -> System.out.print(athlete.getName() + " " + athlete.curLength + " "));if (currentSecond >= PICTURE_TIME2) {//第二次拍照takePhoto(sortedAthletes);over = true;break;}}}private void sleepThenTakeFirstPhoto() {while (true) {long currentSecond = (Instant.now().toEpochMilli() - startTime) / 1000 + 1;try {TimeUnit.SECONDS.sleep(1);System.out.print(currentSecond + "秒  ");} catch (InterruptedException e) {e.printStackTrace();}if (PICTURE_TIME <= currentSecond) {System.out.println(currentSecond + "秒");//第一次拍照List<Athlete> sortedAthletes = compute();System.out.print(currentSecond + "秒: ");sortedAthletes.stream().forEach(athlete -> System.out.print(athlete.getName() + " " + athlete.curLength + " "));takePhoto(sortedAthletes);break;}}}private <E> void takePhoto(List<E> datas) {System.out.print("\ttakePhoto");pictures.add(new Picture(datas));}public void showPhotos() {System.out.println("\n打印照片");pictures.stream().forEach(e -> System.out.println(e));}}static class Athlete extends Thread implements Comparable<Athlete>, Cloneable {final CountDownLatch latch;final String name;private volatile int curLength = 0;public Athlete(String name, CountDownLatch latch) {super(name);this.name = name;this.latch = latch;}public void ready() {System.out.println(this.name + " 进场");try {TimeUnit.MILLISECONDS.sleep(ThreadLocalRandom.current().nextInt(100));} catch (InterruptedException e) {e.printStackTrace();}System.out.println("选手" + this.name + " 准备好了,等待发令枪");latch.countDown();}@Overridepublic void run() {ready();try {START_GUN.await();} catch (InterruptedException e) {e.printStackTrace();}System.out.print("选手" + this.name + " 起跑\t");doRun();}private void doRun() {while (!over && !isInterrupted()) {curLength += ThreadLocalRandom.current().nextInt(8)+1;//模拟秒速try {TimeUnit.SECONDS.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}}}@Overridepublic int compareTo(Athlete o) {return Integer.compare(this.curLength, o.curLength);}@Overrideprotected Object clone() throws CloneNotSupportedException {Athlete athlete = new Athlete(this.name, null);athlete.curLength = this.curLength;return athlete;}@Overridepublic String toString() {return "Athlete{" +"name='" + name + '\'' +", curLength=" + curLength +'}';}}
}

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CountDownLatch模拟田径赛跑

效果

代码

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