我试图在第一次加载 onCreate 时选择列表视图中的第一个索引。我尝试了 listView.setSelection(0) 但发生了一些错误。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);
}
这是调试 logcat 中的错误获取..
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)
我读过一些与我有相同问题的线程。他们使用适配器来解决问题，但在我的情况下，这个错误仍然存在。任何帮助深表感谢。