class PlainOioServer {
    public void serve(int port) throws IOException {<FILL_FUNCTION_BODY>}
}

        final ServerSocket socket = new ServerSocket(port);
        try {
            for(;;) {
                final Socket clientSocket = socket.accept();
                System.out.println(
                        "Accepted connection from " + clientSocket);
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        OutputStream out;
                        try {
                            out = clientSocket.getOutputStream();
                            out.write("Hi!\r\n".getBytes(
                                    Charset.forName("UTF-8")));
                            out.flush();
                            clientSocket.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        } finally {
                            try {
                                clientSocket.close();
                            } catch (IOException ex) {
                                // ignore on close
                            }
                        }
                    }
                }).start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    
<no_super_class>