How is the order of text output determined in a multithreaded program?

Question

Java

Hello!
I'm currently learning multithreading in Java and I have a small question.
Let's say we have a program like this:

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class Task implements Runnable {
    private int countdown = 5;
    @Override
    public void run() {
        while (countdown-- > 0)
            System.out.println(Thread.currentThread().getName() + "- (" + countdown + ")");
    }
}

public class Program {
    public static void main(String[] args) {
        ExecutorService exc = Executors.newFixedThreadPool(5);
        for (int i = 0; i < 5; i++) {
            exc.execute(new Task());
        }
        exc.shutdown();
    }
}

And the output looks like this:

pool-1-thread-1- (4)
pool-1-thread-1- (3)
pool-1-thread-1- (2)
pool-1-thread-1- (1)
pool-1-thread-1- (0)
pool-1-thread-2- (4)
pool-1-thread-2- (3)
pool-1-thread-2- (2)
pool-1-thread-2- (1)
pool-1-thread-2- (0)
pool-1-thread-3- (4)
pool-1-thread-3- (3)
pool-1-thread-3- (2)
pool-1-thread-3- (1)
pool-1-thread-3- (0)
pool-1-thread-5- (4)
pool-1-thread-5- (3)
pool-1-thread-5- (2)
pool-1-thread-5- (1)
pool-1-thread-5- (0)
pool-1-thread-4- (4)
pool-1-thread-4- (3)
pool-1-thread-4- (2)
pool-1-thread-4- (1)
pool-1-thread-4- (0)

As I correctly understood, all 5 created threads perform 5 of these tasks in parallel, but how is the output determined and carried out? This, it turns out, depends on how the scheduler switches between processes?

Answer 1

This is "Chinese random". The same program will output data in a different order each time it is run.