Тестил программу на файлах - все работает. Тем не менее, валидатор заворачивает по 5 и 6 пунтам.
package com.javarush.task.task18.task1819;
/*
Объединение файлов
*/
import java.io.*;
import java.util.ArrayList;
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();
ArrayList<Integer> Buffer1 = new ArrayList<>();
ArrayList<Integer> Buffer2 = new ArrayList<>();
reader.close();
BufferedInputStream inputStream = new BufferedInputStream(new FileInputStream(filename1));
while (inputStream.available() > 1) {
Buffer1.add(inputStream.read());
}
inputStream.close();
BufferedOutputStream outputStream = new BufferedOutputStream(new FileOutputStream(filename1));
inputStream = new BufferedInputStream(new FileInputStream(filename2));
while (inputStream.available() > 1) {
Buffer2.add(inputStream.read());
}
inputStream.close();
Buffer2.addAll(Buffer1);
for (int i = 0; i < Buffer2.size(); i++) {
outputStream.write(Buffer2.get(i));
}
outputStream.close();
}
}