获取 JSON 数组到 ArrayList 到 ListView

Question

我有一个 php 页面 (urldisplay) 返回这个问题对象数组：

[{
"suggestid": "1",
"question": "What does Aotearoa mean?",
"answer1": "Maui and the Whale",
"answer2": "2 Islands",
"answer3": "Land of the Long White Cloud",
"answer4": "Isle of Greenstone",
"correctanswer": "Land of the Long White Cloud",
"userID": "1"
 }, {
"suggestid": "2",
"question": "What does Haere Ra mean?",
"answer1": "Hello",
"answer2": "Goodbye",
"answer3": "Thanks",
"answer4": "Sorry",
"correctanswer": "Goodbye",
"userID": "1"
}, {
"suggestid": "3",
"question": "Where is Zealand?",
"answer1": "France",
"answer2": "Germany",
"answer3": "Denamrk",
"answer4": "Finland",
"correctanswer": "Denmark",
"userID": "1"
}, {
"suggestid": "4",
"question": "What does Aotearoa mean?",
"answer1": "Maui and the Whale",
"answer2": "2 Islands",
"answer3": "Land of the Long White Cloud",
"answer4": "Isle of Greenstone",
"correctanswer": "Land of the Long White Cloud",
"userID": "2"
  }, {
"suggestid": "5",
"question": "What does Aotearoa mean?",
"answer1": "Maui and the Whale",
"answer2": "2 Islands",
"answer3": "Land of the Long White Cloud",
"answer4": "Isle of Greenstone",
"correctanswer": "Land of the Long White Cloud",
"userID": "3"
}]

我正在尝试通过此类（在我的 DAO 类中）读取对象，它将结果保存到问题数组列表中

ArrayList<Question> quest;

    public ArrayList<Question> ListSugg()
{
    ListSuggestions sugg = (ListSuggestions)new ListSuggestions().execute();
    return quest;
}

public class ListSuggestions extends AsyncTask<Void, Void, Void> {

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

        try {
            URL url = new URL(urldisplay);
            HttpURLConnection con = (HttpURLConnection) url.openConnection();
            JSONArray jsonArray = new JSONArray(con);
            quest = new ArrayList<Question>();
            for (int i = 0; i < jsonArray.length(); i++) {
                JSONObject jsonObject = jsonArray.getJSONObject(i);
                String question = jsonObject.getString("question");
                String answer1 = jsonObject.getString("answer1");
                String answer2 = jsonObject.getString("answer2");
                String answer3 = jsonObject.getString("answer3");
                String answer4 = jsonObject.getString("answer4");
                String correctanswer = jsonObject.getString("correctanswer");
                Question ques = new Question(0, question, answer1, answer2, answer3, answer4, correctanswer);
                quest.add(ques);
            }
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e1) {
            e1.printStackTrace();
        }
        return null;
    }
}

任务仍然是空的。然后我想将问题对象添加到 ListView。

public class ListActivity extends AppCompatActivity {
ListView listv;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list);
    listv = (ListView)findViewById(R.id.ListDisplay);
    DAO dao = new DAO();
    ArrayList list = dao.ListSugg();
    ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(
            this,
            android.R.layout.simple_list_item_1,
            list );

    listv.setAdapter(arrayAdapter);
}

我已经尝试了很多其他 SO 帖子，但我似乎在追赶我的尾巴。谁能发现我的错误/我有更好的方法吗？

干杯，

Answer 1

问题是 AsyncTask 执行代码 Asunchrounsly，这意味着当你调用 ListSugg() 时，它会在另一个线程上运行后台任务，并在后台任务完成之前返回任务对象，这意味着你会有一个静止的空列表。 read here for more info

您可以做的是覆盖onPostExecute 方法，并在那里设置适配器，如下所示：

public class ListSuggestions extends AsyncTask<Void, Void, Void> {

    @Override
    protected Boolean doInBackground(String... params) {
        // do something
        return true;
    }

    @Override
    protected void onPostExecute(String result) {
        // The results of the above method
        // Processing the results here

        ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(
             this,
             android.R.layout.simple_list_item_1,
             list);

        listv.setAdapter(arrayAdapter);

    }

}

但要使其正常工作，您需要将 AsyncTask 包含在 MainActivity 中。

这应该可以解决您的问题，希望对您有所帮助。

但是，还有更好、更简单的解决方案，例如使用 okhttp 或 Retrofit 等 HTTP 网络库，以及使用 Gson 等 JSON 解析器，如果您愿意，可以查看它们。

Answer 2

您在触发后台任务后立即返回ques。后台任务需要一些时间才能完成，在此之前您的 ques 列表将保持为空，因此您将看不到数据。

更新 AsyncTask 的 postExecute 方法中的列表。

Answer 3

试试这个

        quest = new ArrayList<Question>();
        for (int i = 0; i < jsonArray.length(); i++) {

            JSONObject jsonObject = jsonArray.getJSONObject(i);

            Question ques = new Question(); //inti Question
            //Enter Values to Question Object
            ques.setQuestion( jsonObject.getString("question"));
            ques.setAnswer1( jsonObject.getString("answer1"));
            ques.setAnswer2( jsonObject.getString("answer2"));
            ques.setAnswer3( jsonObject.getString("answer3"));
            ques.setAnswer4( jsonObject.getString("answer4"));
            ques.setCorrectAnswer( jsonObject.getString("correctanswer"));
            // Add Question Object to ArrayList
            quest.add(ques);
        }

Answer 4

它是空的，因为您从 doInBackground() 返回 null。理想的方法是将结果传递给 onPostExecute() 并在那里设置适配器。像这样：-

public class ListSuggestions extends AsyncTask<Void, Void, ArrayList<Question>> {

    @Override
    protected ArrayList<Question> doInBackground(Void... params) {

        try {
            URL url = new URL(urldisplay);
            HttpURLConnection con = (HttpURLConnection) url.openConnection();
            //JSONArray jsonArray = new JSONArray(con);
            quest = new ArrayList<Question>();
            for (int i = 0; i < jsonArray.length(); i++) {
                JSONObject jsonObject = jsonArray.getJSONObject(i);
                String question = jsonObject.getString("question");
                String answer1 = jsonObject.getString("answer1");
                String answer2 = jsonObject.getString("answer2");
                String answer3 = jsonObject.getString("answer3");
                String answer4 = jsonObject.getString("answer4");
                String correctanswer = jsonObject.getString("correctanswer");
                Question ques = new Question(0, question, answer1, answer2, answer3, answer4, correctanswer);
                quest.add(ques);
            }
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e1) {
            e1.printStackTrace();
        }
        return quest;
    }

    @Override
    protected void onPostExecute(ArrayList<Question> questions) {
        super.onPostExecute(questions);

        ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(
                this,
                android.R.layout.simple_list_item_1,
                list );

        listv.setAdapter(arrayAdapter);
    }
}