Android : Android get result of processFinish as an Interface method with AsyncTask

in this below simple code i must be return value from call method before get value from AsyncResponse interface class. my code is not correct and return NULL and i can not get result from processFinish to return that with call

Interface:

public interface AsyncResponse {
    void processFinish(String output);

}

public class WSDLHelper implements AsyncResponse{ public SoapObject request;

private String MainResult;

public String call(SoapObject rq){

    new ProcessTask(rq, this).execute();
    return MainResult;

}

@Override
public void processFinish(String output) {
    MainResult = output;
}

}

class ProcessTask extends AsyncTask { public AsyncResponse delegate;

SoapObject req1;

public ProcessTask(SoapObject rq, AsyncResponse delegate) {
    req1 = rq;
    this.delegate = delegate;
}

@Override
protected void onPreExecute() {
    super.onPreExecute();
}

@Override
protected String doInBackground(Void... params) {

    SoapSerializationEnvelope envelope = new SoapSerializationEnvelope(SoapEnvelope.VER11);
    envelope.setOutputSoapObject(this.req1);

    AndroidHttpTransport transport = new AndroidHttpTransport(Strings.URL_TSMS);
    transport.debug = true;

    String result = null;
    try {
        transport.call(Strings.URL_TSMS + this.req1.getName(), envelope);
        result = envelope.getResponse().toString();
    } catch (IOException ex) {
        Log.e("" , ex.getMessage());
    } catch (XmlPullParserException ex) {
        Log.e("" , ex.getMessage());
    }

    if (result.equals(String.valueOf(Integers.CODE_USER_PASS_FALSE))) {
        try {
            throw new TException(PublicErrorList.USERNAME_PASSWORD_ERROR);
        } catch (TException e) {
            e.printStackTrace();
        }
    }

    return result;
}

protected void onPostExecute(String result) {
    delegate.processFinish(result);
}

}