Почему ругается на создание и запуск нитей для каждого файла? 
package com.javarush.task.task18.task1823;

import java.io.*;
import java.util.HashMap;
import java.util.Map;

/*
Нити и байты
*/

public class Solution {
    public static Map<String, Integer> resultMap = new HashMap<String, Integer>();

    public static void main(String[] args) {
        String fileName;
        try (BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in))) {
            while (true) {
                fileName = bufferedReader.readLine();
                new ReadThread(fileName).start();
                if (fileName.equals("exit")) {
                    break;
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static class ReadThread extends Thread {
        private String fileName;

        public ReadThread(String fileName) {
            this.fileName = fileName;
        }

        @Override
        public void run() {
            try (BufferedReader bufferedReader = new BufferedReader(new FileReader(fileName))) {

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}