我是android的新手，我正在尝试将以下asnyctask代码转换为排球，但是如何完全成功地获得了很多错误建议
private class AsyncJsonObject extends AsyncTask<String, Void, String> {
private ProgressDialog progressDialog;
@Override
protected String doInBackground(String... params) {
HttpClient httpClient = new DefaultHttpClient(new BasicHttpParams());
HttpPost httpPost = new HttpPost("http://192.168.0.2/testquiz/index.php");
String jsonResult = "";
try {
HttpResponse response = httpClient.execute(httpPost);
jsonResult = inputStreamToString(response.getEntity().getContent()).toString();
System.out.println("Returned Json object " + jsonResult.toString());
} catch (ClientProtocolException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return jsonResult;
}
@Override
protected void onPreExecute() {
// TODO Auto-generated method stub
super.onPreExecute();
progressDialog = ProgressDialog.show(QuizActivity.this, "Downloading Quiz","Wait....", true);
}
@Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
progressDialog.dismiss();
System.out.println("Resulted Value: " + result);
parsedObject = returnParsedJsonObject(result);
if(parsedObject == null){
return;
}
quizCount = parsedObject.size();
firstQuestion = parsedObject.get(0);
quizQuestion.setText(firstQuestion.getQuestion());
String[] possibleAnswers = firstQuestion.getAnswers().split(",");
optionOne.setText(possibleAnswers[0]);
optionTwo.setText(possibleAnswers[1]);
optionThree.setText(possibleAnswers[2]);
optionFour.setText(possibleAnswers[3]);
}
private StringBuilder inputStreamToString(InputStream is) {
String rLine = "";
StringBuilder answer = new StringBuilder();
BufferedReader br = new BufferedReader(new InputStreamReader(is));
try {
while ((rLine = br.readLine()) != null) {
answer.append(rLine);
}
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return answer;
}
}
*/
private List<QuizWrapper> returnParsedJsonObject(String result) {
List<QuizWrapper> jsonObject = new ArrayList<QuizWrapper>();
JSONObject resultObject = null;
JSONArray jsonArray = null;
QuizWrapper newItemObject = null;
try {
resultObject = new JSONObject(result);
System.out.println("Testing the water " + resultObject.toString());
jsonArray = resultObject.optJSONArray("quiz_questions");
}
catch (JSONException e) {
e.printStackTrace();
}
if (jsonArray != null) { // check jsonArray is null?
for (int i = 0; i < jsonArray.length(); i++) {
JSONObject jsonChildNode = null;
try {
jsonChildNode = jsonArray.getJSONObject(i);
int id = jsonChildNode.getInt("id");
String question = jsonChildNode.getString("question");
String answerOptions = jsonChildNode.getString("possible_answers");
int correctAnswer = jsonChildNode.getInt("correct_answer");
newItemObject = new QuizWrapper(id, question, answerOptions, correctAnswer);
jsonObject.add(newItemObject);
} catch (JSONException e) {
e.printStackTrace();
}
}
}
return jsonObject;
}
这是我迄今取得的成就，任何帮助将不胜感激
public Object getQuestion() {
JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(Request.Method.POST, showUrl, null, new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
try {
JSONArray questions = response.getJSONArray("questions");
for(int i=0; i<questions.length(); i++){
JSONObject quiz_questions = questions.getJSONObject(i);
String firstQuestion = quiz_questions.getString("firstQuestion");
String optionOne = quiz_questions.getString("optionOne");
String optionTwo = quiz_questions.getString("optionTwo");
String optionThree = quiz_questions.getString("optionThree");
String optionFour = quiz_questions.getString("optionFour");
quizQuestion.append(firstQuestion+ "" +optionOne+"" +optionTwo+""+optionThree+""+optionFour+ "");
}
} catch (JSONException e) {
e.printStackTrace();
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
}
});
requestQueue.add(jsonObjectRequest);
return question;
}
发生错误
FATAL EXCEPTION: main
Process: test.com.okcupid, PID: 31544
java.lang.RuntimeException: Unable to start activity ComponentInfo{test.com.okcupid/test.com.okcupid.QuizActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.android.volley.Request com.android.volley.RequestQueue.add(com.android.volley.Request)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.android.volley.Request com.android.volley.RequestQueue.add(com.android.volley.Request)' on a null object reference
at test.com.okcupid.QuizActivity.getQuestion(QuizActivity.java:175)
at test.com.okcupid.QuizActivity.onCreate(QuizActivity.java:118)
at android.app.Activity.performCreate(Activity.java:6679)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
答案 0 :(得分：0)
问题出在这一行：
requestQueue.add(jsonObjectRequest);
到达行requestQueue的点由于某种原因为空。在使用变量之前，您需要确保变量已初始化。如果你无法确定，那么总是用
if (requestQueue != null)
您需要快速了解多线程的工作原理。因为在getQuestion中你启动了一个异步请求（JsonObjectRequest），它之后的行（requestQueue.add（jsonObjectRequest）和return语句）将在 onResponse
之前执行