Очень странно....
package com.javarush.task.task18.task1819;
import java.io.*;
import java.util.Arrays;
/*
Объединение файлов
*/
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String fileName1 = reader.readLine();
String fileName2 = reader.readLine();
reader.close();
FileInputStream inputStream1 = new FileInputStream(fileName1);
byte[] buffer = new byte[inputStream1.available()];
while (inputStream1.available() > 0) {
int i = inputStream1.read(buffer);
}
inputStream1.close();
FileInputStream inputStream2 = new FileInputStream(fileName2);
byte[] buffer2 = new byte[inputStream2.available()];
while (inputStream2.available() > 0) {
int i = inputStream2.read(buffer);
}
inputStream2.close();
byte[] bigbuff = new byte[inputStream1.available() + inputStream2.available()];
System.arraycopy(buffer2, 0, bigbuff, 0, buffer2.length);
System.arraycopy(buffer, 0, bigbuff, buffer2.length, buffer.length);
FileOutputStream output = new FileOutputStream(fileName1);
output.write(bigbuff);
output.close();
}
}