Собираем файл

task1825.java

package com.javarush.task.task18.task1825;

import java.io.*;
import java.util.Map;
import java.util.TreeMap;
import java.util.regex.Matcher;
import java.util.regex.Pattern;


/* 
Собираем файл
*/

public class Solution {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        TreeMap<Integer, BufferedInputStream> streams = new TreeMap<>();
        BufferedOutputStream target = null;

        String line;
        while (!(line = reader.readLine()).equals("end")) {
            Matcher matcher = Pattern.compile("(^.+)\\.part(\\d+)$").matcher(line);
            if (matcher.find()) {
                streams.put(Integer.parseInt(matcher.group(2)), new BufferedInputStream(new FileInputStream(line)));
                if (target == null) target = new BufferedOutputStream(new FileOutputStream(matcher.group(1)));
            }
        }
        reader.close();

        for (Map.Entry<Integer, BufferedInputStream> entry : streams.entrySet()) {
            BufferedInputStream input = entry.getValue();
            byte[] buffer = new byte[input.available()];
            int count = input.read(buffer, 0, buffer.length);
            input.close();
            target.write(buffer, 0, count);
        }
        assert target != null;
        target.close();
    }
}