пАмАгИтИ тИтИ

package com.javarush.task.task18.task1825;

import java.io.*;
import java.util.*;

/*
Собираем файл
*/

public class Solution {
    public static void main(String[] args) throws IOException {
        String readedName = null;
        List<String> partsOfNewFile = new ArrayList<>();
        String absolutePathnameOfNewFile = null;
        TreeMap<Integer, String> sortedParts = null;

        Scanner scan = new Scanner(System.in);
        while (!(readedName = scan.nextLine()).equalsIgnoreCase("end"))
            partsOfNewFile.add(readedName);

        absolutePathnameOfNewFile =
                determinateAbsolutePathname(partsOfNewFile);

        sortedParts = sortNamesOfFiles(partsOfNewFile);

        try (BufferedOutputStream bos =
                     new BufferedOutputStream(new FileOutputStream(absolutePathnameOfNewFile))) {
            for (Map.Entry<Integer, String> entry : sortedParts.entrySet()) {
                readAndWriteAllFiles(bos, entry.getValue());
            }
        }
    }
}

private static String determinateAbsolutePathname(List<String> partsOfNewFile) {
        String abstractPathname = null;

        if (partsOfNewFile.size() > 0) {
            String nameOfPartFile = partsOfNewFile.get(0);
            int nameOfFilenameStart = nameOfPartFile.lastIndexOf('\\');
            int nameOfFilenameEnds = nameOfPartFile.lastIndexOf('.');

            String directoryOfNewFile = nameOfPartFile.substring(0,
                    nameOfFilenameStart + 1);
            String nameOfNewFile =
                    nameOfPartFile.substring(nameOfFilenameStart + 1,
                            nameOfFilenameEnds);
            abstractPathname = directoryOfNewFile + nameOfNewFile;
        }

        return abstractPathname;
    }

private static TreeMap<Integer, String> sortNamesOfFiles(List<String> partsOfNewFile) {
       TreeMap<Integer, String> sortedNames = new TreeMap<>();

       for (String name : partsOfNewFile) {
           int indexStart = name.lastIndexOf('.') + 1;
           int numberOfFile = Integer.parseInt(name.substring(indexStart).replaceAll(
                   "\\D", ""));
           sortedNames.put(numberOfFile, name);
       }

       return sortedNames;
   }

private static void readAndWriteAllFiles(BufferedOutputStream bos,
                                             String nameOfPart) throws IOException {
        try (BufferedInputStream bis =
                     new BufferedInputStream(
                             new FileInputStream(nameOfPart))) {
            while (bis.available() > 0)
                bos.write(bis.read());
        }
    }

while (!(readedName = scan.nextLine()).equalsIgnoreCase("end"))
            partsOfNewFile.add(readedName);

void shouldProperlyParceAndStore_ifValidDataProvided() {}
void shouldReturnNotFound_if....() // ну и так далее