ServerSocket sv_sock = new ServerSocket(ポート番号);
Socket sock = sv_sock.accept();

BufferedReader br = new BufferedReader(new InputStreamReader(sock.getInputStream()));
String msg = br.readLine();
br.close();

sock.close();
sv_sock.close();

Socket sock = new Socket(InetAddress.getByName("ホスト名"),ポート番号) ;

PrintWriter osw = new PrintWriter(new OutputStreamWriter(sock.getOutputStream())) ;
osw.println("送信メッセージ") ;
osw.close() ;

sock.close() ;