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 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 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> pictures = new ArrayList<>();
static class Picture {
final List pictureItems;
final long timestamp;
public Picture(List items) {
this.pictureItems = Collections.unmodifiableList(items);
;
this.timestamp = Instant.now().toEpochMilli();
}
public void show() {
pictureItems.stream().forEach(item -> System.out.println(item));
}
@Override
public String toString() {
return "照片拍摄于" + LocalDateTime.ofInstant(Instant.ofEpochMilli(timestamp), ZoneId.systemDefault()) +
":" + pictureItems +
'}';
}
}
final List athletes;
public Computer(List athletes) {
this.athletes = Collections.unmodifiableList(athletes);
}
public List compute() {
return athletes.stream().map(new Function() {
@Override
public 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());
}
@Override
public 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 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 sortedAthletes = compute();
System.out.print(currentSecond + "秒: ");
sortedAthletes.stream().forEach(athlete -> System.out.print(athlete.getName() + " " + athlete.curLength + " "));
takePhoto(sortedAthletes);
break;
}
}
}
private void takePhoto(List 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, 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();
}
@Override
public 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();
}
}
}
@Override
public int compareTo(Athlete o) {
return Integer.compare(this.curLength, o.curLength);
}
@Override
protected Object clone() throws CloneNotSupportedException {
Athlete athlete = new Athlete(this.name, null);
athlete.curLength = this.curLength;
return athlete;
}
@Override
public String toString() {
return "Athlete{" +
"name='" + name + '\'' +
", curLength=" + curLength +
'}';
}
}
}