Question

我正在尝试在首次加载onCreate时在列表视图中选择第一个索引。我尝试了listView.setSelection（0），但是会发生一些错误。这是我的代码...

public class QandAPractice extends AppCompatActivity {
String qsid;
ListView listView;
ArrayList<String> myqid;
RadioButton r1,r2,r3,r4;
String txtcontent, txtTimer,txtAnskey,txtImg, txtA, txtB, txtC, txtD, txtID;
private static final String TAG_QID = "id";
private static final String TAG_TITLE = "content";
private static final String TAG_TIMER = "timer";
private static final String TAG_IMAGE = "images";
private static final String TAG_QSID = "qsid";
private static final String TAG_KEY = "key";
private static final String TAG_A = "A";
private static final String TAG_B = "B";
private static final String TAG_C = "C";
private static final String TAG_D = "D";
ArrayList<HashMap<String, String>> questions;
Dialog quizDialog;
public int i = 60;
public int loadindex = 0;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_qand_apractice);
    questions = new ArrayList<>();
    myqid = new ArrayList<>();
    Bundle b = getIntent().getExtras();
    setTitle(b.getString("subject"));
    qsid = b.getString("qsid");
    quizDialog = new Dialog(this,android.R.style.Theme_Black_NoTitleBar_Fullscreen);
    listView = findViewById(R.id.lvquestions);
    getJSON(Constants.ROOT_URL+"mobile_question_index.php?qsid="+qsid);

    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

            txtID = ((TextView) view.findViewById(R.id.quiz_id)).getText()
                    .toString();
            txtcontent = ((TextView) view.findViewById(R.id.quiz_content)).getText()
                    .toString();
            txtTimer = ((TextView) view.findViewById(R.id.quiz_timer)).getText()
                    .toString();
            txtAnskey = ((TextView) view.findViewById(R.id.quiz_key)).getText()
                    .toString();
            txtA = ((TextView) view.findViewById(R.id.quiz_A)).getText()
                    .toString();
            txtB = ((TextView) view.findViewById(R.id.quiz_B)).getText()
                    .toString();
            txtC = ((TextView) view.findViewById(R.id.quiz_C)).getText()
                    .toString();
            txtD = ((TextView) view.findViewById(R.id.quiz_D)).getText()
                    .toString();
            txtImg = ((TextView) view.findViewById(R.id.quiz_image)).getText()
                    .toString();
            showQuiz();

        }
    });
    listView.setSelection(0);
    listView.getSelectedView().setSelected(true);
}
private void getJSON(final String urlWebService) {

    class GetJSON extends AsyncTask<Void, Void, String> {

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


        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            try {
                loadIntoListView(s);
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }

        @Override
        protected String doInBackground(Void... voids) {
            try {
                URL url = new URL(urlWebService);
                HttpURLConnection con = (HttpURLConnection) url.openConnection();
                StringBuilder sb = new StringBuilder();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(con.getInputStream()));
                String json;
                while ((json = bufferedReader.readLine()) != null) {
                    sb.append(json + "\n");
                }
                return sb.toString().trim();
            } catch (Exception e) {
                return null;
            }
        }
    }
    GetJSON getJSON = new GetJSON();
    getJSON.execute();
}

private void loadIntoListView(String json) throws JSONException {
    JSONArray jsonArray = new JSONArray(json);
    //String[] question = new String[jsonArray.length()];

    for (int i = 0; i < jsonArray.length(); i++) {
        JSONObject obj = jsonArray.getJSONObject(i);
        String id = obj.getString(TAG_QID);
        String content = obj.getString(TAG_TITLE);
        String image = obj.getString(TAG_IMAGE);
        String a = obj.getString(TAG_A);
        String b = obj.getString(TAG_B);
        String c = obj.getString(TAG_C);
        String d = obj.getString(TAG_D);
        String key = obj.getString(TAG_KEY);
        String timer = obj.getString(TAG_TIMER);
        HashMap<String, String> map = new HashMap<String, String>();

        map.put(TAG_QID,id);
        map.put(TAG_TITLE,content);
        map.put(TAG_IMAGE,image);
        map.put(TAG_A,a);
        map.put(TAG_B,b);
        map.put(TAG_C,c);
        map.put(TAG_D,d);
        map.put(TAG_KEY,key);
        map.put(TAG_TIMER,timer);
        myqid.add(id);
        questions.add(map);
    }
    // ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, question);
    SimpleAdapter adapter = new SimpleAdapter(
            QandAPractice.this, questions,
            R.layout.quizlayout, new String[]{TAG_QID,
            TAG_TITLE, TAG_IMAGE,TAG_A,TAG_B,TAG_C,TAG_D,TAG_KEY,TAG_TIMER},
            new int[]{R.id.quiz_id, R.id.quiz_content, R.id.quiz_image,R.id.quiz_A,R.id.quiz_B,
                    R.id.quiz_C,R.id.quiz_D,R.id.quiz_key,R.id.quiz_timer});
/*    AtomicReference<ListAdapter> la =
            new AtomicReference<>(new ListAdapter(getApplicationContext(), questions));*/

    listView.setAdapter(adapter);
}

这是在调试logcat中提取错误。

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setSelected(boolean)' on a null object reference
    at com.example.jhan08.engineeringexclusivereviewer.QandAPractice.onCreate(QandAPractice.java:90)
    at android.app.Activity.performCreate(Activity.java:6351)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1114)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2470)

我读过一些与我的问题相同的线程。他们使用适配器来解决此问题，但就我而言，仍然会发生此错误。非常感谢您的帮助。

Answer 1

在将适配器设置为列表视图之后进行选择

private void loadIntoListView(String json) throws JSONException {
JSONArray jsonArray = new JSONArray(json);
//String[] question = new String[jsonArray.length()];

for (int i = 0; i < jsonArray.length(); i++) {
    JSONObject obj = jsonArray.getJSONObject(i);
    String id = obj.getString(TAG_QID);
    String content = obj.getString(TAG_TITLE);
    String image = obj.getString(TAG_IMAGE);
    String a = obj.getString(TAG_A);
    String b = obj.getString(TAG_B);
    String c = obj.getString(TAG_C);
    String d = obj.getString(TAG_D);
    String key = obj.getString(TAG_KEY);
    String timer = obj.getString(TAG_TIMER);
    HashMap<String, String> map = new HashMap<String, String>();

    map.put(TAG_QID,id);
    map.put(TAG_TITLE,content);
    map.put(TAG_IMAGE,image);
    map.put(TAG_A,a);
    map.put(TAG_B,b);
    map.put(TAG_C,c);
    map.put(TAG_D,d);
    map.put(TAG_KEY,key);
    map.put(TAG_TIMER,timer);
    myqid.add(id);
    questions.add(map);
}
// ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, question);
SimpleAdapter adapter = new SimpleAdapter(
        QandAPractice.this, questions,
        R.layout.quizlayout, new String[]{TAG_QID,
        TAG_TITLE, TAG_IMAGE,TAG_A,TAG_B,TAG_C,TAG_D,TAG_KEY,TAG_TIMER},
        new int[]{R.id.quiz_id, R.id.quiz_content, R.id.quiz_image,R.id.quiz_A,R.id.quiz_B,
                R.id.quiz_C,R.id.quiz_D,R.id.quiz_key,R.id.quiz_timer});
/*    AtomicReference<ListAdapter> la =
        new AtomicReference<>(new ListAdapter(getApplicationContext(), questions));*/

  listView.setAdapter(adapter);
  listView.setSelection(0);
  listView.getSelectedView().setSelected(true);

 }

Answer 2

我想我知道您遇到的主要问题。您尝试在填充列表视图之前选择一个项目。您需要在将适配器添加到列表视图之后进行设置。添加适配器后添加以下行。

listView.setAdapter(adapter);
listView.setSelection(0);
listView.getSelectedView().setSelected(true);

或使用

listView.setAdapter(adapter);
listView.setItemChecked(0, true)

这将显示一个吐司消息，第一次打开。这应该在适配器设置之后添加。

Iterator myVeryOwnIterator = questions.get(0).keySet().iterator();
while(myVeryOwnIterator.hasNext()) {
    String key=(String)myVeryOwnIterator.next();
    String value=(String)meMap.get(key);
    Toast.makeText(ctx, "Key: "+key+" Value: "+value, Toast.LENGTH_LONG).show();
}

如何在listview onCreate（）中选择第一个索引？

2 个答案: