Считал первый файл в конец второго, затем все содержимое второго файла сохранил в первом файле с замещением данных первого файла.
package com.javarush.task.task18.task1819;
/*
Объединение файлов
*/
import java.io.*;
public class Solution {
public Solution() throws FileNotFoundException {
}
public static void main(String[] args) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
//FileInputStream inputStream = new FileInputStream("C:/Users/alexa/OneDrive/Рабочий стол/a.txt");
FileInputStream inputStream = new FileInputStream(reader.readLine());
//FileOutputStream outputStream = new FileOutputStream("C:/Users/alexa/OneDrive/Рабочий стол/b.txt", true);
FileOutputStream outputStream = new FileOutputStream(reader.readLine(), true);
if (inputStream.available() > 0) {
byte[] buffer = new byte[inputStream.available()];
int count = inputStream.read(buffer);
outputStream.write(buffer, 0, count);
}
inputStream.close();
outputStream.close();
//FileInputStream inputStream2 = new FileInputStream("C:/Users/alexa/OneDrive/Рабочий стол/b.txt");
FileInputStream inputStream2 = new FileInputStream(reader.readLine());
//FileOutputStream outputStream2 = new FileOutputStream("C:/Users/alexa/OneDrive/Рабочий стол/a.txt");
FileOutputStream outputStream2 = new FileOutputStream(reader.readLine());
if (inputStream2.available() > 0) {
byte[] buffer = new byte[inputStream2.available()];
int count = inputStream2.read(buffer);
outputStream2.write(buffer, 0, count);
}
inputStream2.close();
outputStream2.close();
}
}