我怎样才能让应用程序等到字符串不是 null？

Question

我使用谷歌 api 来获取坐标，我用 asynTask 来做。 在我获得坐标后，我将其发送到另一个需要借助我的坐标从互联网获取信息的 asyncTask。 我无法添加回调，因为我需要检查字符串是否不是 null。 我试图添加回调，但它似乎没有帮助。 我可能做错了。 帮助

如果他不是 NULL 我需要的字符串是“尝试”内的 MyDownloadTask 类中的字符串“坐标”

我获取坐标的代码：

public class MyCoordinateGiver{

public final String API_URL = "https://maps.googleapis.com/maps/api/geocode/json?address=";
private String cityName;
private final String andKey = "&key=";
private final String apiKey = "ENTER_YOU_API_KEY";
private String fullUrl;
private String rawData;
JSONObject jsonObject;

private double lat;
private double lng;

public MyCoordinateGiver(String cityName) {
    this.cityName = cityName;
    this.fullUrl = this.API_URL + cityName + this.andKey + this.apiKey;
}

private JSONObject readFromJsonUrl() throws IOException, JSONException {
    InputStream is = new URL(this.fullUrl).openStream();
    BufferedReader br = new BufferedReader(new InputStreamReader(is));
    String jsonText = readAll(br);
    jsonObject = new JSONObject(jsonText);
    return jsonObject;
}

private String readAll(BufferedReader br) throws IOException {
    StringBuilder sb = new StringBuilder();
    String cp;
    while((cp = br.readLine()) != null){
        sb.append(cp);
    }
    rawData = sb.toString();
    return sb.toString();
}

private String getRaw(){
    return rawData;
}

private double getLat(JSONObject location) throws IOException, JSONException {
    double lat = location.getDouble("lat");
    return lat;

}

private double getLng(JSONObject location) throws IOException, JSONException {
    double lng = location.getDouble("lng");
    return lng;
}


public String makeCoorString() throws IOException, JSONException {
    JSONObject myData = readFromJsonUrl(); // reads the data
    JSONArray results = myData.getJSONArray("results"); // gets to results in json array
    JSONObject geometry = results.getJSONObject(0).getJSONObject("geometry"); // gets to jsonobject inside the json array
    JSONObject location = geometry.getJSONObject("location"); // gets to jsonobjects inside the json array

    lat = getLat(location);
    lng = getLng(location);
    String coor = lat + "," + lng;
    return coor;
}

需要获取坐标的 asyncTask：

public class MyDownloadTask extends AsyncTask<String, String, String> {

Activity secondActivity;

public MyDownloadTask(Activity secondActivity) {
    this.secondActivity = secondActivity;
}

boolean tof;

@Override
protected String doInBackground(String... urls) {

    MyBoiler myData = null;
    try {
        String coordinates = MyCoorTask.cityCoordinates;
        String cityName = MainActivity.CITY;

        myData = new MyBoiler("Enter_darksky_api_key", coordinates, cityName);
        tof = myData.openBoiler(30, 20);

    } catch (IOException e) {
        e.printStackTrace();
    } catch (JSONException e) {
        e.printStackTrace();
    }
    return tof ? "true" : "false";
}

@Override
protected void onPostExecute(String s) {

    try {
        printToTV(tof, secondActivity, secondActivity);
    } catch (IOException e) {
        e.printStackTrace();
    } catch (JSONException e) {
        e.printStackTrace();
    }
}