java - 对非静态字段和JSON对象感到困惑

我正在通过制作使用yahoo weather来显示今天天气的应用程序来学习使用REST API。我创建了一个AsyncTask，一个自定义适配器和一个在MainActivity中调用的WeatherData类。我在使用这个WeatherData类和自定义适配器时遇到了一些麻烦。

在适配器中，尝试从天气数据类调用JSON对象时收到错误消息。它说不能从静态上下文中引用非静态字段“天气”和“日期”。我尝试阅读静态上下文，但确实使我感到困惑，它是什么以及它的作用。

第二个问题是，当我尝试将日期和天气添加到我的天气数据ArrayList时，我的AsyncTask现在也给出了错误。它说WeatherData中的WeatherData（JSONobject）不能应用于JSONObject，java.lang.String。我认为这与以下事实有关：我将天气和日期用作WeatherData类中的对象，然后尝试将它们用作数组列表中的字符串。我试图通过简单地将它们更改为WeatherData类中的字符串来修复它，但是随后出现各种新错误。有人可以帮助我解决我的问题吗？

因此，这是我的MainActivity的一部分，我在其中调用数据：

public void getData(View view) {
    ClassAsyncTask task = new ClassAsyncTask(this);
    task.execute(chosenloc);
}

public void setData(ArrayList<WeatherData> weatherdata) {
    // Construct the data source
    ArrayList<WeatherData> arrayOfWeather = new ArrayList<WeatherData>();
    // Create the adapter to convert the array to views
    CustomArrayAdapter adapter = new CustomArrayAdapter(this, arrayOfWeather);
    // Attach the adapter to a Listview
    ListView listView = (ListView) findViewById(R.id.weatherListView);
    listView.setAdapter(adapter);
}

这是我的自定义适配器：

public class CustomArrayAdapter extends ArrayAdapter<WeatherData> {
public CustomArrayAdapter(Context context, ArrayList<WeatherData> weatherdata){
    super(context, 0, weatherdata);
}

@Override
public View getView(int position, View convertView, ViewGroup parent){
    // Get the data item for this position
    WeatherData weather = getItem(position);
    // Check if an exising view is being reused, otherwise inflate the view
    if (convertView == null){
        convertView = LayoutInflater.from(getContext()).inflate(R.layout.custom_array_layout, parent, false);
    }
    // Lookup view for data population
    TextView date = (TextView) convertView.findViewById(R.id.date);
    TextView weather = (TextView) convertView.findViewById(R.id.weather);

    // Populate the data into the template view using the data object
    date.setText(WeatherData.date);
    weather.setText(WeatherData.weather);

    // Return the completed view to render on screen
    return convertView;
}
}

我的WeatherData类：

public class WeatherData {
// Fields
public JSONObject date;
public String weather;

// Constructor
public WeatherData(JSONObject object) {
    try {
        JSONArray dates = object.getJSONArray("date");
        JSONArray weather = object.getJSONArray("text");
    } catch (JSONException e) {
        e.printStackTrace();
    }
}
    // Method to convert an array of JSON objects into a list of objects
    public static ArrayList<WeatherData> fromJson(JSONArray jsonObjects){
        ArrayList<WeatherData> weather = new ArrayList<WeatherData>();
        for (int i = 0; i < jsonObjects.length(); i++) {
            try {
                weather.add(new WeatherData(jsonObjects.getJSONObject(i)));
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
        return weather;
    }
}

最后，我的AsyncTask解析JSON的部分：

protected void onPostExecute(String result){
    super.onPostExecute(result);

    // Alert user if nothing was found
    if(result.length() == 0){
        Toast.makeText(context, "Nothing was found", Toast.LENGTH_SHORT).show();
    }
    else {
        // Parse JSON
        ArrayList<WeatherData> weatherdata = new ArrayList<>();
        try {
            JSONObject respObj = new JSONObject(result);
            JSONObject forecastObj = respObj.getJSONObject("forecast");
            JSONArray dates = forecastObj.getJSONArray("date");
            JSONArray weatherArray = forecastObj.getJSONArray("text");

            for (int i = 0; i<dates.length(); i++){
                JSONObject date = dates.getJSONObject(i);
                String weather = date.getString("text");
                weatherdata.add(new WeatherData(date, weather));
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }
        // Update MainActivity
        this.activity.setData(weatherdata);
    }
}

最佳答案

您有很多语义错误，理解静态和非静态的差异和细微差别非常重要，但是要修复您的代码，只需注意在何处使用的变量即可。我不会为您阅读Yahoo weather API，因此我将以一种通用的方式在此处显示一些内容。

固定代码如下：

这应该是您的getView

// Get the data item for this position
WeatherData weather = getItem(position);
// Check if an exising view is being reused, otherwise inflate the view
if (convertView == null){
    convertView = LayoutInflater.from(getContext()).inflate(R.layout.custom_array_layout, parent, false);
}
// CHANGED HERE the textView "weather` had the same name of the "WeatherData" defined a few lines above
// Lookup view for data population
TextView textDate = (TextView) convertView.findViewById(R.id.date);
TextView textWeather = (TextView) convertView.findViewById(R.id.weather);

// CHANGED HERE to properly call the textView methods and to read the fields from the instance of "WeatherData"
// Populate the data into the template view using the data object
textDate.setText(weather.date);
textWeather.setText(weather.weather);

// Return the completed view to render on screen
return convertView;

您的“ WeatherData”构造函数也不合逻辑，将导致NPE。我不知道您要在其中实现什么，但是在编写JSONArray dates = object...时，您提取的信息将被添加到该JsonArray中，然后被丢弃。字段public JSONObject date或public String weather从未初始化，并且不包含任何有用的信息。您可能想做的是这样的：

// Fields
public String val1;
public String val2;
public String val3;
  ... any value necessary

// Constructor
public WeatherData(JSONObject object) {
    try {
         val1 = object.optString("val1");
         val2 = object.optString("val2");
         val3 = object.optString("val3");
        ... parse here EVERYTHING from the object
    } catch (JSONException e) {
        e.printStackTrace();
    }
}

另外，在您的postExecute上，您试图创建新的WeatherData，将两个值传递给它，例如new WeatherData(date, weather)。同样，读取WeatherData构造函数，它仅使用1个值，即JsonObject object。

最后是一种性能，您不应将string返回到onPostExecute。您应该执行一个返回ArrayList<WeatherData> result的AsyncTask。然后，所有解析代码都应在background执行中。

编辑：示例asyncTask

   public class MyAsyncTask extends AsyncTask<String, Integer, List<WeatherObject>>{

      @Override protected List<WeatherObject> doInBackground(String... params) {
         // here you call the API and parse the results
         // return ArrayList<WeatherObject> if successful, or `null` if failed
         return null;
      }

      @Override protected void onPostExecute(List<WeatherObject> weatherObjectList) {
           // here you simply check for null and use the List
           if(weatherObjectList == null){
               // fail to load data
           } else {
               // set data to adapter
           }
      }
   }

关于java - 对非静态字段和JSON对象感到困惑，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/36101895/