Но все отрабатывает как нужно.
package com.javarush.task.task18.task1819;
import java.io.*;
import java.util.ArrayList;
/*
Объединение файлов
*/
public class Solution {
public static void main(String[] args)throws IOException {
BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
String pathFile1 = input.readLine();
String pathFile2 = input.readLine();
BufferedReader fileReader1 = new BufferedReader(new FileReader(pathFile1));
BufferedReader fileReader2 = new BufferedReader(new FileReader(pathFile2));
ArrayList<String> listDataFile = new ArrayList<>();
while (fileReader2.ready()){
listDataFile.add(fileReader2.readLine());
}
fileReader2.close();
while (fileReader1.ready()){
listDataFile.add(fileReader1.readLine());
}
fileReader1.close();
BufferedWriter fileWriter1 = new BufferedWriter(new FileWriter(pathFile1));
for (String cell : listDataFile){
fileWriter1.write(cell+"\n");
}
fileWriter1.close();
}
}