在循环中创建线程，并等待所有线程在Java中完成

Question

我是新来Java的，最近在学校学习多线程，我尝试创建一个小程序，可以将任务分割成一个小的部分，使用循环在多个线程中运行。问题是，在循环之后，我需要将研究结果求和并打印出来，在线程完成之前，循环下的打印运行。而我的同学所做的只是在打印结果之前添加睡眠，但是当线程花费太长时它就不起作用了。在运行其他代码之前，是否需要等待所有的线程在循环中首先完成？

import java.util.concurrent.ExecutorService;

import java.util.concurrent.TimeUnit;

public class threatGenerator {
    static int count = 0;
    public static void main(String[] args) throws InterruptedException {
        Scanner sc = new Scanner(System.in);
        System.out.print("Input start: ");
        int start = Integer.parseInt(sc.nextLine());
        System.out.print("Input end: ");
        int end = Integer.parseInt(sc.nextLine());
        
        int threadNumber = (int)Math.ceil((end - start)/100.0) ;

        System.out.println("Running "+threadNumber+" threads.");
       
        for(int i = 0 ;i < threadNumber;i++){
            int temp = start+ 99;
            
            if(temp>end) temp = end;

            String name = String.valueOf(i);
            ThreadDemo thread = new ThreadDemo(name,start,temp);
            thread.start();
            
            start = temp+1;
            
        }
        Thread.sleep(10);
        System.out.println("\nSum of primes = "+count);
        sc.close();
    }

    public static void awaitTerminationAfterShutdown(ExecutorService threadPool) {
        threadPool.shutdown();
        try {
            if (!threadPool.awaitTermination(60, TimeUnit.SECONDS)) {
                threadPool.shutdownNow();
            }
        } catch (InterruptedException ex) {
            threadPool.shutdownNow();
            Thread.currentThread().interrupt();
        }
    }
}

class ThreadDemo extends Thread {
    private Thread t;
    private String threadName;
    private int start;
    private int end;
    private int num = 0;

    ThreadDemo( String name,int start,int end) {
        threadName= name;
        this.start = start;
        this.end = end;
    }
    public void run() {
        Prime p = new Prime();

        for(int i = start ; i<=end;i++){
            if(p.checkPrime(i)!=true){
                System.out.print("t"+threadName+"-"+i+" ");
                ++num;
            }
        }
        threatGenerator.count = threatGenerator.count + num;
    }
    
    public void setCount(int count){
        this.num=count;
    }

    public int getCount() {
        return this.num;
    }

    public void start () {
        // System.out.println("Starting " + threadName);
        if (t == null) {
            t = new Thread (this, threadName);
            t.start();
        }
        
    }
    
}

Answer 1

是不是要等待所有的线程在循环中完成...？

如果更改程序以保留所有thread引用（例如，通过将它们添加到arraylist )中，则可以编写第二个循环，使join()s所有线程。

https://docs.oracle.com/en/Java/javase/12/docs/api/Java.base/Java/lang/thread.html#join()

t.join()等待（即，不返回），直到线程t终止。