Android : can't update member variable

Somehow line mResponseText = response.body().string(); isn't writing member variable. Instead it appears to be creating and logging it locally.

Any ideas why? The more I look at it the more clueless I'm getting :(

public class Gateway extends Activity  {
    private static final String TAG = Gateway.class.getSimpleName();
    public static final MediaType JSON
            = MediaType.parse("application/json; charset=utf-8");
    private String mResponseText = "[{'comment' : 'fake' , 'nickname' : 'fake', 'created':'now', 'parent_class':''}]";



    public Gateway (String url, String json,  Context context) {

        if(isNetworkAvailable(context)) {
            //if network is available build request

            OkHttpClient client = new OkHttpClient();
           // RequestBody body = RequestBody.create(JSON, json);
            Request request = new Request.Builder()
                    .url(url)
                    //.post(body)
                    .build();

            Call call = client.newCall(request);
            call.enqueue(new Callback() {
                //execute call
                @Override
                public void onFailure(Request request, IOException e) {
                    // if request failed
                    Toast.makeText(Gateway.this, "request failed", Toast.LENGTH_LONG).show();
                }

                @Override
                public void onResponse(Response response) throws IOException {
                    // if succeeded
                    if(response.isSuccessful()){
                        mResponseText = response.body().string();
                        Log.v(TAG, "RESPONSE IS SUCCESSFUL");
                        Log.v(TAG, mResponseText);
                    } else {
                        alertUserAboutError();
                    }
                }
            });
        } else {
            Toast.makeText(this, "No network", Toast.LENGTH_LONG).show();
        }

    }

    public String getResponse () { return mResponseText; }


    // check if network is available
    private boolean isNetworkAvailable(Context c) {
        ConnectivityManager manager = (ConnectivityManager) c.getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = manager.getActiveNetworkInfo();
        boolean isAvailable = false;
        if (networkInfo != null && networkInfo.isConnected()) {
            isAvailable = true;
        }

        return isAvailable;
    }

    private void alertUserAboutError() {
        AlertDialogFragment dialog = new AlertDialogFragment();
        dialog.show(getFragmentManager(), "error_dialog");
    }
}