Java Stack 由于某种原因变空

Question

我正在开发一个 Android 应用程序，它将调用 DarkSky 天气 API(出于明显的原因，我已在此处编辑了我的 API key )。当我解析 JSON 数据并将其推送到我命名为 dataStack 的堆栈时，我的问题就出现了。在推送堆栈时，我记录了它的大小并且它显示正确。但是，当我的代码到达 buildGraph() 方法时，堆栈现在为空，并且所有数据都消失了。是什么原因导致堆栈为空？

编辑:发布后 30 分钟，我找到了解决方法。我现在返回字符串并在我的 MainActivity Android 类中解析它。但是，我仍然不知道为什么堆栈被删除。我很想知道:)

public class MainActivity extends AppCompatActivity {

    Button button;
    TextView progressLabel;

    GraphView graph;

    JSONObject jsonObject;

    static Stack<DataPoint> dataStack = new Stack<>(); // stack for graph data points

    static final String API_URL = "https://api.darksky.net/forecast/API_KEY/42.3611,-71.0570,"; // #TODO: delete API key before comitting to GitHub
    static final String URL_TAIL = "?exclude=currently,flags,hourly"; // end of URL
    static final long currTime = System.currentTimeMillis() / 1000L; // current UNIX time
    static long unixTime = currTime;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button = findViewById(R.id.button);
        progressLabel = findViewById(R.id.progressLabel);

        graph =  findViewById(R.id.graph);
    }

    public void loadResults(View view) {
        for (int x = 0; x < 2; x++) { // 7 API calls for each of 7 days
            new APICall().execute();
            unixTime -= 86400; // subtract 24 hours in UNIX time
            dataStack.size();
        }
        buildGraph(); // after all data is gathered, build a graph using it
    }

    private void buildGraph() {
// #TODO: Method to build graph
        Log.i("STACK pop", String.valueOf(dataStack.size())); 
    }


    class APICall extends AsyncTask<Void, Void, String> { // Extend AsyncTask so we don't hijack the main UI thread

        protected void onPreExecute() {
            // Do stuff before executing the AsyncTask
            progressLabel.setText("Fetching Data");
        }

        protected String doInBackground(Void... urls) {
            // Execute background task here


            try {
                final String FULL_URL = API_URL + unixTime + URL_TAIL; // build the full URL with latest time
                URL url = new URL(FULL_URL); // URL for the API call
                HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection(); // connection to URL
                try {
                    // tools for reading API results
                    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(urlConnection.getInputStream()));
                    StringBuilder stringBuilder = new StringBuilder();
                    String line;
                    // accumulate results
                    while ((line = bufferedReader.readLine()) != null) {
                        stringBuilder.append(line).append("\n");
                    }

                    bufferedReader.close(); // always close buffered reader

                     return stringBuilder.toString(); // return results
                }
                finally {
                    // inside a finally block so that no matter what we always end a connection that has been started
                    urlConnection.disconnect(); // end the connection

                }
            }
            catch(Exception ex) {
                Log.e("ERROR", ex.getMessage(), ex);
                return null;
            }
        }

        protected void onPostExecute(String response) {
            // Do stuff after we're finished executing

            if (response == null) {
                response = "AN ERROR HAS OCCURRED";
            }
            else {
                try {
                    jsonObject = new JSONObject(response); // create object from our response
                    JSONArray arr = jsonObject.getJSONObject("daily").getJSONArray("data"); // get data Array
                    String arrString = arr.getString(0); // full String
                    String[] splitString = arrString.split(","); // split String into array by comma


                    String time = splitString[0].substring(8); // time is the first index of the array, use substring to cutout unecessary info
                    String temp = splitString[11].substring(18);

                    dataStack.push(new DataPoint(Integer.valueOf(time), Float.valueOf(temp))); // push our data onto the stack as a DataPoint

                    Log.i("STACK push", String.valueOf(dataStack.toString()));

                    response = "Data received"; // display this to user
                }
                catch(Exception ex) {
                    response = "ERROR DURING JSON PARSING";
                }

            }

            progressLabel.setText(response);
            // parse data here
            Log.i("INFO", response);

        }
    }

}

Answer 1

堆栈为空，因为结果尚未输入。问题在于您的 loadResults()。

    public void loadResults(View view) {
        for (int x = 0; x < 2; x++) { // 7 API calls for each of 7 days
            new APICall().execute();
            unixTime -= 86400; // subtract 24 hours in UNIX time
            dataStack.size();
        }
        buildGraph(); // after all data is gathered, build a graph using it
    }

您发出 new APICall().execute(); 来请求数据并更新 dataStack，并且您希望获得 dataStack在同一个函数 loadResults() 中“立即”得到结果？这是不可能的。

一种解决方案是将 loadResults() 中的 buildGraph() 删除到 onPostExecute() 内部。