import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Main {

    public static void main(String[] args) {
        int sleepSec = 10;

        final SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");

        final ScheduledThreadPoolExecutor exec = new ScheduledThreadPoolExecutor(1);

        exec.scheduleAtFixedRate(new Runnable() {
            public void run() {
                try {
                    Calendar cal = Calendar.getInstance();
                    System.out.println(sdf.format(cal.getTime()));

                } catch (Exception e) {
                    e.printStackTrace();
                    exec.shutdown();
                }
            }
        }, 0, sleepSec, TimeUnit.SECONDS);
    }
}