public static void main(String[] args) throws IOException { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); String fileName1 = reader.readLine(); String fileName2 = reader.readLine(); try (FileInputStream fileReader1 = new FileInputStream(fileName1); FileOutputStream fileWriter = new FileOutputStream(fileName1); FileInputStream fileReader2 = new FileInputStream(fileName2)) { byte [] buffer = new byte[fileReader1.available()]; int read = fileReader1.read(buffer); while (fileReader2.available() > 0) { fileWriter.write(fileReader2.read()); } fileWriter.write(buffer); } } } Этот код не работает как нужно, но проходит