Чего хочет, шайтан?
public static void main(String[] args) throws IOException {
TreeMap<Integer, String> treeMap = new TreeMap<>();
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String file;
while(!(file = reader.readLine()).equals("end")) {
treeMap.put(Integer.parseInt(file.substring(file.indexOf(".part") + 5)), file);
}
reader.close();
file = treeMap.firstEntry().getValue();
File f = new File(file.substring(0, file.lastIndexOf(".")));
FileOutputStream outputStream = new FileOutputStream(f);
FileInputStream inputStream = null;
for (Map.Entry<Integer, String> g : treeMap.entrySet()) {
inputStream = new FileInputStream(g.getValue());
byte[] buffer = new byte[inputStream.available()];
inputStream.read(buffer);
outputStream.write(buffer);
inputStream.close();
}
outputStream.close();
}
}