а почему таким образом нельзя?
package com.javarush.task.task18.task1808;
/*
Разделение файла
*/
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
public class Solution {
public static void main(String[] args) throws Exception {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String file1 = reader.readLine();
String file2 = reader.readLine();
String file3 = reader.readLine();
FileInputStream inputStream = new FileInputStream(file1);
FileOutputStream outputStream1 = new FileOutputStream(file2);
FileOutputStream outputStream2 = new FileOutputStream(file3);
while (inputStream.available()>0){
int x = inputStream.read();
if (inputStream.available()>=inputStream.available()/2){
outputStream1.write(x);
}
else outputStream2.write(x);
}
inputStream.close();
outputStream1.close();
outputStream2.close();
}
}