AsyncTask错误Android App崩溃

Question

My oncreate code:

 protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.board);
    ArrayList<String> passing = new ArrayList<String>();
  new AsyncTaskParseJson().execute();


}

Here is my code:

public class AsyncTaskParseJson extends AsyncTask<String, String, String> {
    final String TAG = "AsyncTaskParseJson.java";
    String yourJsonStringUrl = "my url here";
    JSONArray dataJsonArr = null;

    @Override
    protected void onPreExecute() {}

    @SuppressWarnings("null")
    @Override
     public String doInBackground(String... arg0) {

        final String[] values = null;
         try {

             // instantiate our json parser
             JsonParser jParser = new JsonParser();

             // get json string from url
             JSONObject json = jParser.getJSONFromUrl(yourJsonStringUrl);

             // get the array of users
             dataJsonArr = json.getJSONArray("response");

             // loop through all users
             for (int i = 1; i < dataJsonArr.length(); i++) {

                 JSONObject c = dataJsonArr.getJSONObject(i);
                 String text = c.getString("text");
                values[i]=text;
             }
          //   ArrayAdapter<String> adapter = new ArrayAdapter<String>(Board.this,R.layout.item, R.id.label, values);
          //   list1.setAdapter(adapter);


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


         return null;
     }

     @Override
     protected void onPostExecute(String strFromDoInBg) {}



 }

So, it's starts activity when I comment the section values[i] like this:

      //values[i]=text;

BUT when it's not commented, app just crashes. I want to doInBackground to return String[] array, how I do that?

Answer 1

you have re declare it again before the for loop

values = new String[dataJsonArr.length()];
for (int i = 1; i < dataJsonArr.length(); i++) {

             JSONObject c = dataJsonArr.getJSONObject(i);
             String text = c.getString("text");
            values[i]=text;
         }

you need to reserve its size in the memory by declaring new

Answer 2

You need to have

 values = new String[dataJsonArr.length()]; 

Also note the commented lines are in doInbackground . But you can't update ui from a background thread. So the below should go to onPostExecute .

 //   ArrayAdapter<String> adapter = new ArrayAdapter<String>(Board.this,R.layout.item, R.id.label, values);
 //   list1.setAdapter(adapter);

Instead of using a String array it is better to use a ArrayList<String>

You need to change the args type also

public class AsyncTaskParseJson extends AsyncTask<String, String, ArrayList<String>> {

Then

 @Override
 public  ArrayList<String> doInBackground(String... arg0) {

    ArrayList<String> values = new ArrayList<String>();
     try {

         // instantiate our json parser
         JsonParser jParser = new JsonParser();

         // get json string from url
         JSONObject json = jParser.getJSONFromUrl(yourJsonStringUrl);

         // get the array of users
         dataJsonArr = json.getJSONArray("response");

         // loop through all users
         for (int i = 1; i < dataJsonArr.length(); i++) {

             JSONObject c = dataJsonArr.getJSONObject(i);
             String text = c.getString("text");
             values.add(text); 
         }
     }  
     catch (JSONException e) {
         e.printStackTrace();
     }

     return values;
 }

Then

 @Override
 protected void onPostExecute(ArrayList<String> result) {
  super.onPostExecute(result);
   ArrayAdapter<String> adapter = new ArrayAdapter<String>(Board.this,R.layout.item, R.id.label, result);
   list1.setAdapter(adapter);
 }

Also make sure list1 is initialized before you setAdapter to it coz i do not see the code where list1 is initialized.

Answer 3

Just declare String[] array at class level like below,

private static String[] array = null;


public class AsyncTaskParseJson extends AsyncTask<String, String, String> {
    final String TAG = "AsyncTaskParseJson.java";
    String yourJsonStringUrl = "my url here";
    JSONArray dataJsonArr = null;

    @Override
    protected void onPreExecute() {}

    @SuppressWarnings("null")
    @Override
     public String doInBackground(String... arg0) {

        //  final String[] values = null;          
         try {

             // instantiate our json parser
             JsonParser jParser = new JsonParser();

             // get json string from url
             JSONObject json = jParser.getJSONFromUrl(yourJsonStringUrl);

             // get the array of users
             dataJsonArr = json.getJSONArray("response");

             // loop through all users
             for (int i = 1; i < dataJsonArr.length(); i++) {

                 JSONObject c = dataJsonArr.getJSONObject(i);
                 String text = c.getString("text");
                values[i]=text;
             }
          //   ArrayAdapter<String> adapter = new ArrayAdapter<String>(Board.this,R.layout.item, R.id.label, values);
          //   list1.setAdapter(adapter);


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


         return null;
     }

     @Override
     protected void onPostExecute(String strFromDoInBg) {}
}

Answer 4

Here, you are creating an Array object with to major problem...

1. creating Array object without defining it's size.
2. defining as final which will not allow you to add new element to that object.

You declaring an Array which is null and you defining it as final . When you declare an Array as final then you can't change that Array . That's why you can't insert new value to that Array , as a result, when you are going to access that Array its returning null value. It's the cause to crash your app.

So, what you have to do is, define the values array with a fixed size and don't make it final . These will solve your all problem.

Now, replace the following line...

final String[] values = null;

with these...

String[] values;
values = new String[dataJsonArr.length()];