class AuthenticatorTask extends AsyncTask<Void, Void, Integer> {
        String username;
        String password;
        Exception ex = null;

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            EditText usernameField = (EditText) findViewById(R.id.edit_text_username);
            EditText passwordField = (EditText) findViewById(R.id.edit_text_password);
            username = usernameField.getText().toString();
            password = passwordField.getText().toString();
        }

        @Override
        protected Integer doInBackground(Void... voids) {
            try {
                Authenticator auth = new Authenticator();
                return auth.getUserId(username, password);
            } catch (IOException e1) {
                ex = e1;
            }
        }

        @Override
        protected void onPostExecute(Integer userId) {
            super.onPostExecute(userId);

            if (ex != null) {
                Toast.makeText(SignInActivity.this, "Error: Could not access server!", Toast.LENGTH_SHORT).show();
                return;
            }

            if (userId == -1) {
                Toast.makeText(SignInActivity.this, "Invalid username or password", Toast.LENGTH_SHORT).show();
            } else {
                startActivity(new Intent(SignInActivity.this, DashboardActivity.class));
            }

        }
    }