LG

  DatagramSocket socket = new DatagramSocket (PORT);
  byte[] buffer = new byte[1024];
  DatagramPacket paquet = new DatagramPacket (buffer, buffer.length);
  socket.receive (paquet);
  String data = new String (paquet.getData (), 0, paquet.getLength (), Charset.forName ("UTF-8"));

  String messageRenvoye = new String (paquet.getData (), 0, paquet.getLength (), Charset.forName ("UTF-8"));
  DatagramPacket envoie = new DatagramPacket (messageRenvoye.getBytes (Charset.forName ("UTF-8")),
      messageRenvoye.getBytes (Charset.forName ("UTF-8")).length, paquet.getAddress (), paquet.getPort ());
  socket.send (envoie);

OutputStream outputStream;
...
PrintWriter pstream = new PrintWriter (new OutputStreamWriter (outputStream, StandardCharsets.UTF_8), true);

BufferedReader br = new BufferedReader (new InputStreamReader (new FileInputStream (fileName), Charset.forName ("UTF-8")));