Мои тесты решение проходит.
package com.javarush.task.task18.task1825;
import java.io.*;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.Set;
import java.util.TreeSet;
/*
Собираем файл
*/
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
String filePath;
TreeSet<String> pathFile = new TreeSet();
while (!(filePath = bufferedReader.readLine()).equals("end")) {
pathFile.add(filePath);
}
bufferedReader.close();
String pathFileReslult = pathFile.first().replaceFirst("part.+", "txt");
BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(new FileOutputStream(pathFileReslult));
ArrayList<Integer> buffer = new ArrayList<>();
BufferedInputStream bufferedInputStream = null;
for (String s : pathFile){
bufferedInputStream = new BufferedInputStream(new FileInputStream(s));
while (bufferedInputStream.available() > 0){
buffer.add(bufferedInputStream.read());
}
bufferedInputStream.close();
}
bufferedInputStream.close();
for (Integer i : buffer){
bufferedOutputStream.write(i);
}
bufferedOutputStream.close();
}
}
//C:\Users\User\Desktop\test\Lion.avi.part0003.txt