Почему поток не прерывается при вызове clockthread.interrupt()?

package com.javarush.task.task16.task1617;

/*
Отсчет на гонках
*/

import java.sql.SQLOutput;

public class Solution {
    public static int countSeconds = 4;

    public static void main(String[] args) throws InterruptedException {
        RacingClock clock = new RacingClock();
        Thread clockthread = new Thread(clock);
        //clockthread.start();
        Thread.sleep(3500);
        clockthread.interrupt();

        //System.out.println();
    }

    public static class RacingClock extends Thread {
        public RacingClock() {
            start();
        }

        public void run() {

            try {
                Thread current = Thread.currentThread();
                while (!current.isInterrupted()) {
                    System.out.print(countSeconds + " ");
                    current.sleep(1000);
                    countSeconds--;
                    if (countSeconds == 0) {System.out.print(" Mарш!"); break;}
                }
            } catch (InterruptedException e)
            {
                System.out.print(" Прервано!");
            }
        }
    }
}