android asynctask问题

Question

我的应用程序存在另一个问题，但这次我的堆栈跟踪说了不同的东西，所以我只是希望有人能够帮助解决这个问题。我只需要有人告诉我什么是错的，我能做些什么来解决它。

07-26 13:40:26.983: E/AndroidRuntime(837): FATAL EXCEPTION: main 
07-26 13:40:26.983: E/AndroidRuntime(837): java.lang.NullPointerException 
07-26 13:40:26.983: E/AndroidRuntime(837): at com.theproblemsolver.asynctask.onPostExecute(MainActivity.java:72) 
07-26 13:40:26.983: E/AndroidRuntime(837): at com.theproblemsolver.asynctask.onPostExecute(MainActivity.java:1) 
07-26 13:40:26.983: E/AndroidRuntime(837): at android.os.AsyncTask.finish(AsyncTask.java:631) 
07-26 13:40:26.983: E/AndroidRuntime(837): at android.os.AsyncTask.access$600(AsyncTask.java:177) 
07-26 13:40:26.983: E/AndroidRuntime(837): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644) 
07-26 13:40:26.983: E/AndroidRuntime(837): at android.os.Handler.dispatchMessage(Handler.java:99) 
07-26 13:40:26.983: E/AndroidRuntime(837): at android.os.Looper.loop(Looper.java:137) 
07-26 13:40:26.983: E/AndroidRuntime(837): at android.app.ActivityThread.main(ActivityThread.java:5041) 
07-26 13:40:26.983: E/AndroidRuntime(837): at java.lang.reflect.Method.invokeNative(Native Method) 
07-26 13:40:26.983: E/AndroidRuntime(837): at java.lang.reflect.Method.invoke(Method.java:511) 
07-26 13:40:26.983: E/AndroidRuntime(837): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
07-26 13:40:26.983: E/AndroidRuntime(837): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
07-26 13:40:26.983: E/AndroidRuntime(837): at dalvik.system.NativeStart.main(Native Method) 
07-26 13:40:31.994: E/Trace(864): error opening trace file: No such file or directory (2) 

MainActivity

public class MainActivity extends Activity { 

    EditText et; 
    Button getanswer; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     Button getanswer = (Button) findViewById(R.id.button1); 
     getanswer.setOnClickListener(new OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      new asynctask().execute(); 
      Intent i = new Intent(MainActivity.this, ListView.class); 
      startActivity(i); 
     } 

     //protected void launchNewActivity(){ 
     //startActivity(new Intent(MainActivity.this, ListView.class)); 


     }); 
    } 
} 

     class asynctask extends AsyncTask<String, Void, JSONObject> { 

      private static final String TAG_RESULTS = "ResultsSet"; 
      private static final String TAG_SUBJECT = "Subject"; 
      private static final String TAG_NUMANSWERS = "NumAnswers"; 
      private static final String TAG_QUESTION = "Question"; 
      private static final String TAG_QUESTION_CONTENT = "Content"; 
      private static final String TAG_QUESTION_CHOSENANSWER = "ChosenAnswer"; 
      private static final String TAG_QUESTION_ANSWERS = "Answers"; 
      public JSONArray json; 

     @Override 
     protected JSONObject doInBackground(String... params) { 

      JSONParsser jParser = new JSONParsser(); 

      String URI = "http://example.com/json"; 

      JSONObject json = jParser.getJSONFromURI(URI); 

      return json ; 

     }{ } 
      @Override 
      protected void onPostExecute(JSONObject json) { 

       JSONArray ResultsSet = null ; { 
       try { 
        ResultsSet = json.getJSONArray(TAG_RESULTS); 
       } catch (JSONException e) { 
        e.printStackTrace(); 
       } 
      return; 
      }} 
     } 

的ListView活动

public class ListView extends ListActivity { 


@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    //setContentView(R.layout.listview); 



    //protected void doInBackground(String... params) { 
    JSONParsser jParser = new JSONParsser(); 

    String URI = "http://example.com/json";; 

    JSONObject json = jParser.getJSONFromURI(URI); 

    //protected void onPostExecute(JSONObject json) { 

    final String TAG_RESULTS = "ResultsSet"; 
    final String TAG_SUBJECT = "Subject"; 
    final String TAG_NUMANSWERS = "NumAnswers"; 
    final String TAG_QUESTION = "Question"; 
    final String TAG_QUESTION_CONTENT = "Content"; 
    final String TAG_QUESTION_CHOSENANSWER = "ChosenAnswer"; 
    final String TAG_QUESTION_ANSWERS = "Answers"; 

      JSONArray ResultsSet = null; 

      ArrayList<HashMap<String, String>> questionList = new ArrayList<HashMap<String, String>>(); 
    try { 
     ResultsSet = json.getJSONArray(TAG_RESULTS); 

     for(int i = 0; i < ResultsSet.length(); i++){ 
      JSONObject r = ResultsSet.getJSONObject(i); 

      String Subject = r.getString(TAG_SUBJECT); 
      String NumAnswers = r.getString(TAG_NUMANSWERS); 

      JSONObject Question = r.getJSONObject(TAG_QUESTION); 
      String Content = Question.getString(TAG_QUESTION_CONTENT); 
      String ChosenAnswer = Question.getString(TAG_QUESTION_CHOSENANSWER); 
      String Answers = Question.getString(TAG_QUESTION_ANSWERS); 

      HashMap<String, String> map = new HashMap<String, String>(); 

      map.put(TAG_SUBJECT, Subject); 
      map.put(TAG_NUMANSWERS, NumAnswers); 

      questionList.add(map); 

     } 
    } catch (JSONException e) { 
     e.printStackTrace(); 
     } 

    ListAdapter adapter = new SimpleAdapter(this, questionList, 
      R.layout.row, 
      new String[] { TAG_SUBJECT, TAG_NUMANSWERS }, new int[] { 
      R.id.Subject, R.id.NumAnswers }); 

    setListAdapter(adapter); 


    android.widget.ListView lv = getListView(); 

    lv.setOnItemClickListener(new OnItemClickListener() { 

     @Override 
     public void onItemClick(AdapterView<?> parent, View view, 
       int position, long id) { 
      String Subject = view.findViewById(R.id.Subject).getContext().toString(); 

      Intent i = new Intent(getApplicationContext(), SingleListItem.class); 
      i.putExtra(TAG_SUBJECT, Subject); 
      startActivity(i); 

     } 

    }); 

} 

} 

SingleListItem

public class SingleListItem extends Activity { 

    TextView subject; 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     this.setContentView(R.layout.singlelistitem); 

    subject = (TextView) findViewById(R.id.Subject2); 

    Intent i = getIntent(); 
    String Subject = i.getStringExtra("TAG_SUBJECT"); 
    subject.setText(Subject); 

    } 
} 

Answer 1

试试这个：

您的列表视图活动应该是这样的：

public class ListView extends ListActivity { 

private ProgressDialog pDialog; 

JSONParser jParser = new JSONParser(); 
ArrayList<HashMap<String, String>> questionList; 

private static String url = "http://example.com/json";; 

final String TAG_RESULTS = "ResultsSet"; 
final String TAG_SUBJECT = "Subject"; 
final String TAG_NUMANSWERS = "NumAnswers"; 
final String TAG_QUESTION = "Question"; 
final String TAG_QUESTION_CONTENT = "Content"; 
final String TAG_QUESTION_CHOSENANSWER = "ChosenAnswer"; 
final String TAG_QUESTION_ANSWERS = "Answers"; 


JSONArray ResultsSet = null; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.listview); 

    questionList = new ArrayList<HashMap<String, String>>(); 

    new LoadAllData().execute(); 


    ListView lv = getListView(); 

    lv.setOnItemClickListener(new OnItemClickListener() { 

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


     } 
    }); 

} 

@Override 
protected void onActivityResult(int requestCode, int resultCode, Intent data) { 
    super.onActivityResult(requestCode, resultCode, data); 

    if (resultCode == 100) { 
     Intent intent = getIntent(); 
     finish(); 
     startActivity(intent); 
    } 
} 


class LoadAllData extends AsyncTask<String, String, String> { 

    @Override 
    protected void onPreExecute() { 
     super.onPreExecute(); 
     pDialog = new ProgressDialog(ListView.this); 
     pDialog.setMessage("Loading Data. Please wait..."); 
     pDialog.setIndeterminate(false); 
     pDialog.setCancelable(false); 
     pDialog.show(); 
    } 


    protected String doInBackground(String... args) { 

     try { 
       JSONObject json = jParser.getJSONFromUrl(url); 
       ResultsSet = json.getJSONArray(TAG_RESULTS); 

       for(int i = 0; i < ResultsSet.length(); i++) { 
        JSONObject r = ResultsSet.getJSONObject(i); 
        String Subject = r.getString(TAG_SUBJECT); 
        String NumAnswers = r.getString(TAG_NUMANSWERS); 

        JSONObject Question = r.getJSONObject(TAG_QUESTION); 
        String Content = Question.getString(TAG_QUESTION_CONTENT); 
        String ChosenAnswer = Question.getString(TAG_QUESTION_CHOSENANSWER); 
        String Answers = Question.getString(TAG_QUESTION_ANSWERS); 

        HashMap<String, String> map = new HashMap<String, String>(); 

        map.put(TAG_SUBJECT, Subject); 
        map.put(TAG_NUMANSWERS, NumAnswers); 
       } 



     } catch (JSONException e) { 
      e.printStackTrace(); 
     } 
     return null; 


    } 

    protected void onPostExecute(String file_url) { 

     pDialog.dismiss(); 
     runOnUiThread(new Runnable() { 
     public void run() { 
      ListAdapter adapter = new SimpleAdapter(this, questionList, 
        R.layout.row, 
        new String[] { TAG_SUBJECT, TAG_NUMANSWERS }, new int[] { 
        R.id.Subject, R.id.NumAnswers }); 

      setListAdapter(adapter); 

      } 
     }); 

    } 
} 
} 

，并在MainActivity

public class MainActivity extends Activity { 

EditText et; 
Button getanswer; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    Button getanswer = (Button) findViewById(R.id.button1); 
    getanswer.setOnClickListener(new OnClickListener() { 

    @Override 
    public void onClick(View v) { 

     Intent i = new Intent(MainActivity.this, ListView.class); 
     startActivity(i); 
    } 


    }); 
    } 
} 

只是固定所有结束大括号:)