По факту работает, но валидатор не принимает.
package com.javarush.task.task18.task1822;
/*
Поиск данных внутри файла
*/
import java.io.FileInputStream;
import java.io.BufferedReader;
import java.io.InputStreamReader;
public class Solution {
public static void main(String[] args) throws Exception {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String fileName = reader.readLine();
reader.close();
FileInputStream fromFile = new FileInputStream(fileName);
StringBuilder str = new StringBuilder();
while(fromFile.available() > 0) {
str.append((char)fromFile.read());
}
fromFile.close();
for(String elem : str.toString().split("\n")) {
if(elem.startsWith(args[0])) {
System.out.println(elem);
break;
}
}
}
}