Почему? В идеа работает корректно.
package com.javarush.task.task18.task1809;
/*
Реверс файла
*/
import java.io.*;
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
FileInputStream firstFile = new FileInputStream(reader.readLine());
FileOutputStream secondFile = new FileOutputStream(reader.readLine());
byte[] buffer = new byte[20];
int count = 0;
while (firstFile.available() > 0) {
count = firstFile.read(buffer);
}
byte tempCount = 0;
for (int i = 0; i < count / 2; i++) {
tempCount = buffer[i];
buffer[i] = buffer[count - 1 - i];
buffer[count - 1 - i] = tempCount;
}
secondFile.write(buffer, 0, count);
secondFile.close();
firstFile.close();
reader.close();
}
}