Proses Registrasi Android Studio

public class proses_registrasi extends AsyncTask<String,Void, String> {

    private String url_proses_daftar = "http://ip kalian/proses_android/proses_daftar.php"; // link proses daftar di php

    Context context;
    ProgressDialog progressDialog;
    Activity activity;
    AlertDialog.Builder builder;

    public proses_registrasi(Context context) {
        this.context = context;
        activity = (Activity) context;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        builder = new AlertDialog.Builder(activity);
        progressDialog = new ProgressDialog(context);
        progressDialog.setMessage("Koneksi ke server...");
        progressDialog.setIndeterminate(true);
        progressDialog.setCancelable(false);
        progressDialog.show();
    }

    @Override
    protected String doInBackground(String... params) {
        String method = params[0];
        if (method.equals("Daftar")){
            try {
                URL url = new URL(url_proses_daftar);
                HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                connection.setRequestMethod("POST");
                connection.setDoOutput(true);
                connection.setDoInput(true);
                OutputStream outputStream = connection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                String username, password;
                username = params[1];
                password = params[2];

                String data_daftar = URLEncoder.encode("username", "UTF-8") + "=" + URLEncoder.encode(username, "UTF-8") + "&" +
                        URLEncoder.encode("password", "UTF-8") + "=" + URLEncoder.encode(password, "UTF-8");

                bufferedWriter.write(data_daftar);
                bufferedWriter.flush();
                bufferedWriter.close();
                outputStream.close();

                InputStream inputStream = connection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
                StringBuilder stringBuilder = new StringBuilder();
                String line = "";

                while ((line = bufferedReader.readLine()) != null){
                    stringBuilder.append(line + "\n");
                }

                connection.disconnect();
                Thread.sleep(10000);
                return stringBuilder.toString().trim();

            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    @Override
    protected void onPostExecute(String s) {
        super.onPostExecute(s);
        try {
            JSONObject jsonObject = new JSONObject(s);
            JSONArray jsonArray = jsonObject.getJSONArray("Server");
            JSONObject JO = jsonArray.getJSONObject(0);
            String code = JO.getString("code");
            String message = JO.getString("pesan");
            if (code.equals("daftar_true")){
                builder.setMessage(message);
                builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        progressDialog.dismiss();
                    }
                });
                AlertDialog alertDialog = builder.create();
                alertDialog.show();
            }else if (code.equals("daftar_false")){
                builder.setMessage(message);
                builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        progressDialog.dismiss();
                    }
                });
                AlertDialog alertDialog = builder.create();
                alertDialog.show();
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }

    }
}