How can I print something after all the threads have finished?

Question

I am trying to print something after two threads have finished running. I have been reading answers to similar questions and all of them were about trying join() method. This is a problem for me as I try not to ruin the way the two threads are alternatively running. If I write use the method for the first thread, the second one won't get a chance to participate in the action I want them to do. And the other way around.

How can I print something right after both threads have finished running alternatively?

I will attach the code here. Files f1 and f2 each contain ten random numbers on separate lines.

import java.io.FileReader;
import java.io.IOException;
import java.util.Scanner;

public class JavaTip3Thread extends Thread
{
    public Thread thread;
    static int a[] = new int[10];
    static int b[] = new int[10];
    static int c[] = new int[10];
    static int index = 0;
    static boolean fin = false;
    static int ok;

    public JavaTip3Thread()
    {
        thread = new Thread(this);
    }

    public static int[] read(FileReader in)
    {
        Scanner s = new Scanner(in);
        int[] x = new int[10];;

        while(s.hasNextLine())
        {
            for(int i = 0; i < 10; i++)
            {
                x[i] = s.nextInt();
            }
        }

        s.close();
        return x;
    }

    public void sum()
    {
        while(fin != true)
        {
            int sum = 0;
            sum += a[index] + b[index];
            c[index] = sum;

            System.out.println(a[index] + " + " + b[index] + " = " + c[index]);
            index++;

            if(index == a.length)
            {
                fin = true;
            }
        }
    }

    public void run()
    {
        sum();
    } 

    public static void main(String args[]) throws IOException
    {
        FileReader in = new FileReader("D:\\IESC\\Java\\JavaTip3Thread\\src\\f1.txt");
        FileReader in2 = new FileReader("D:\\IESC\\Java\\JavaTip3Thread\\src\\f2.txt");

        a = read(in);
        b = read(in2);

        JavaTip3Thread t1 = new JavaTip3Thread();
        JavaTip3Thread t2 = new JavaTip3Thread();

        t1.start();
        t2.start();

        for(int i = 0; i < 10; i++)
        {
            System.out.println("c[" + i + "]= " + c[i] + "  ");
        }

        in.close();
        in2.close();
    }
}

Answer 1

t1.start();
t2.start();

t1.join();
t2.join();

This will trigger both threads and only then will wait for the first thread to finish and then wait for the second thread.

HTH.