Не понимаю в чем ошибка вообще, запускал выводилось все правильно
package com.javarush.task.task18.task1825;
import java.io.*;
import java.nio.file.Path;
import java.util.ArrayList;
import java.util.Collections;
public class Solution {
public static void main(String[] args) throws IOException {
ArrayList<String> list = new ArrayList<>();
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
while (true) {
String r = reader.readLine();
if (r.equals("end")) {
break;
}
list.add(r);
}
reader.close();
Collections.sort(list);
String fileName = "";
fileName = list.get(0).split(".part[0-9]+")[0];
File file = new File(fileName);
FileOutputStream fileOutputStream = new FileOutputStream(file,true);
for (String iterator : list) {
String fileNameReader = iterator;
int fileReaderIndex = fileNameReader.indexOf(".part");
fileNameReader = fileNameReader.substring(0, fileReaderIndex);
FileInputStream fileInputStream = new FileInputStream(fileNameReader);
byte[] buffer = new byte[fileInputStream.available()];
fileInputStream.read(buffer);
fileOutputStream.write(buffer);
fileOutputStream.flush();
fileInputStream.close();
}
fileOutputStream.close();
}
}