Erro na postagem do json

Meu código esta assim.

public String post(String json) {
    try {
        URL url = new URL("https://www.caelum.com.br/mobile");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestProperty("Content-type", "application/json");
        connection.setRequestProperty("Accept", "application/json");
        connection.setDoOutput(true);
        connection.setDoInput(true);
        PrintStream output = new PrintStream(connection.getOutputStream());
        output.println(json);
        connection.connect();
        Scanner scanner = new Scanner(connection.getInputStream());
        String resposta = scanner.next();
        return resposta;
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return null;
}

porem sempre estoura uma exceção na linh a catch (IOException e) { e.printStackTrace(); } a dando o seguinte erro. "java.io.FileNotFoundException: https://www.caelum.com.br/mobile"

sera que é o servidor caelum ???