А так то все работает :)
package com.javarush.task.task18.task1827;
/*
Прайсы
*/
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;
public class Solution {
public static void main(String[] args) throws Exception {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String name = reader.readLine();
List<String> strings = Files.readAllLines(Paths.get(name));
switch (args[0]) {
case "-c": {
int idMax = strings
.stream()
.map(s -> s.substring(0, 8).trim())
.mapToInt(Integer::parseInt)
.max()
.getAsInt();
strings.add(String.format("%-8s%-30s%-8s%-4s", ++idMax, args[1], args[2], args[3]));
}
}
Files.write(Paths.get(name), strings);
}
}