Объясните плз, почему я не могу в мейне вызвать showwarning???пробовал и через extend Thread и через implement Runnable

package com.javarush.task.task16.task1632;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

/*
Клубок
*/

public class Solution {
    public static List<Thread> threads = new ArrayList<>(5);

    static {
        threads.add(new Thread(new FirstThread()));

        threads.add(new Thread(new SecondThread()));

        threads.add(new Thread(new ThirdThread()));

        threads.add(new ForthThread());
        threads.get(3).showWarning(); // какого фига тут ошибка?
        threads.add(new Thread(new FithfThread()));




    }

    public static void main(String[] args) {
    }

    static class FirstThread implements Runnable {
        public void run() {
            System.out.println("Нить 1 запустилась");
            while (true) {
            }
        }
    }

    static class SecondThread implements Runnable {
        public void run() {
            System.out.println("Нить 2 запустилась");
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                System.out.println("InterruptedException");
            }
        }
    }

    static class ThirdThread implements Runnable {
        public void run() {
            while (true) {
                try {
                    System.out.println("Ура");
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                }

            }
        }
    }
    static class ForthThread  extends Thread implements Runnable, Message {
        @Override

        public void run() {
            while(!isInterrupted()) {
            }
        }

        @Override
        public void showWarning() {
            this.interrupt();
        }
    }

    static class FithfThread implements Runnable{
        @Override
        public void run() {
            BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
            int sum = 0;
            while (true) {
                try {
                    String str = reader.readLine();
                    if (str.equals("N"))
                        break;
                    sum += Integer.parseInt(str);
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            System.out.print(sum);
        }
    }
    }