Тестил, вроде всё ок.
package com.javarush.task.task18.task1822;
/*
Поиск данных внутри файла
*/
import java.io.*;
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
BufferedInputStream file = new BufferedInputStream(new FileInputStream(reader.readLine()));
reader.close();
StringBuilder s = new StringBuilder();
while (file.available() > 0) {
s.append((char) file.read());
}
file.close();
String[] arr = s.toString().split("\n");
for (String h:arr
) {
if(h.startsWith(args[0])){
System.out.println(h);
}
}
}
}