Display JSONArray to Custom ListView Exception Error Android AsyncTask Thread

Question

I am working on getting a JSONArray from a server, and then inserting it into a custom ListView. My code executes, but I have no idea why the program crashes before anything is displayed. I put some System.out.println(x) to see how far the console gets before it crashes, but it runs through all of them that I place?

Thanks in advance for any help!

public class MainActivity extends Activity
{
    private ListView list_view;
    private TrendingAdapter ta = null;

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

        //Create a ListView, defined in the activity_main.xml file
        this.list_view = (ListView) findViewById(R.id.list_view);
        System.out.println("1");

        //Display trending results
        try
        {
            get_trending();
            System.out.println("16");
        }
        catch (Exception e) 
        {
            e.printStackTrace();
        }       
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu)
    {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    //Async class...
    private class AsyncOp extends AsyncTask<String, Void, String>
    {
        @Override
        protected String doInBackground(String... strings)
        {
            System.out.println("5");
            JSONObject response_json = new JSONObject();
            JSONObject request_json = new JSONObject();

            //Take passed String[0] and turn into JSON
            try
            {
                request_json = new JSONObject(strings[0]);
            }
            catch (Exception e) 
            {
                e.printStackTrace();
            }

            //Send the JSON to the server and grab response
            try
            {
                System.out.println("6");
                HttpClient client = new DefaultHttpClient();
                String request_url = "http://SOME_IP/api/api.php?package=" + Uri.encode(request_json.toString(), null);
                HttpGet get_request = new HttpGet(request_url);
                HttpResponse get_response = client.execute(get_request);
                HttpEntity get_entity = get_response.getEntity();
                System.out.println("7");

                //Ensure a response before doing anything foolish
                if (get_entity != null)
                {
                    System.out.println("8");
                    String response = EntityUtils.toString(get_entity);
                    response_json = new JSONObject(response);
                    return response_json.toString();
                }               
            }
            catch (Exception e) 
            {
                e.printStackTrace();
            }

            //If we're here, the above response didn't work for some reason...
            return "crazy shit happened";
        }

        //What to do after the request is done...
        @Override
        protected void onPostExecute(String result)
        {
            System.out.println("9");
            //Call the resppnse parser function...
            try
            {
                server_response(result);
                System.out.println("14");
            }
            catch (Exception e) 
            {
                e.printStackTrace();
            }
        }       
    }

    private class TrendingAdapter extends ArrayAdapter<TrendingTopic>
    {
        private Context context;
        private int resource_id;
        private ArrayList<TrendingTopic> trending_list = new ArrayList<TrendingTopic>();

        public TrendingAdapter(Context context, int resource_id, ArrayList<TrendingTopic> trending_list)
        {
            super(context, resource_id, trending_list);
            this.resource_id = resource_id;
            this.context = context;
            this.trending_list = trending_list;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent)
        {
            ViewHolder holder = new ViewHolder(convertView);
            holder = (ViewHolder) convertView.getTag();
            holder.populateFrom(trending_list.get(position));

            return (convertView);
        }       
    }

    private class ViewHolder
    {
        public TextView topic_rank = null;
        public TextView topic_title = null;
        public TextView num_comments = null;

        public ViewHolder(View row)
        {
            this.topic_rank = (TextView) row.findViewById(R.id.topic_rank);
            this.topic_title = (TextView) row.findViewById(R.id.topic_title);
            this.num_comments = (TextView) row.findViewById(R.id.num_comments);
        }

        public void populateFrom(TrendingTopic topic)
        {
            this.topic_rank.setText(topic.rank);
            this.topic_title.setText(topic.body.substring(0, Math.min(topic.body.length(), 30)));
            this.num_comments.setText(topic.comments);
        }
    }

    public void get_trending() throws JSONException
    {
        System.out.println("2");
        JSONObject type = new JSONObject();
        JSONObject request = new JSONObject();

        type.put("type", "trending");
        request.put("key", "SOME_KEY");
        request.put("request", "info");
        request.put("info", type);

        System.out.println("3");
        AsyncOp operation = new AsyncOp();
        System.out.println("4");
        operation.execute(new String[] {request.toString()});
        System.out.println("15");
    }

    public void server_response(String json_string) throws JSONException
    {
        System.out.println("10");
        //Get JSON info into ArrayList
        JSONObject json = new JSONObject(json_string);
        JSONArray trending = json.getJSONArray("trending");
        ArrayList<TrendingTopic> trending_list = new ArrayList<TrendingTopic>();

        int x = 0;
        int end_of_list = trending.length();
        TrendingTopic topic = new TrendingTopic();
        while (x < end_of_list)
        {
            topic = new TrendingTopic(trending.getJSONObject(x));
            trending_list.add(topic);
            x++;
        }       

        System.out.println("11");
        //Display response into main ListView
        this.ta = new TrendingAdapter(this, android.R.layout.simple_list_item_1, trending_list);
        System.out.println("12");
        this.list_view.setAdapter(ta);
        System.out.println("13");
    }
}

And this is what is produced in the LogCat

05-14 03:11:16.410: I/System.out(1706): 1
05-14 03:11:16.410: I/System.out(1706): 2
05-14 03:11:16.410: I/System.out(1706): 3
05-14 03:11:16.410: I/System.out(1706): 4
05-14 03:11:16.420: I/System.out(1706): 15
05-14 03:11:16.420: I/System.out(1706): 16
05-14 03:11:16.430: I/System.out(1706): 5
05-14 03:11:16.430: I/System.out(1706): 6
05-14 03:11:16.561: D/gralloc_goldfish(1706): Emulator without GPU emulation detected.
05-14 03:11:17.771: I/System.out(1706): 7
05-14 03:11:17.771: I/System.out(1706): 8
05-14 03:11:17.840: D/dalvikvm(1706): GC_CONCURRENT freed 844K, 24% free 3177K/4164K, paused 5ms+9ms, total 36ms
05-14 03:11:18.320: I/System.out(1706): 9
05-14 03:11:18.320: I/System.out(1706): 10
05-14 03:11:18.470: D/dalvikvm(1706): GC_CONCURRENT freed 370K, 22% free 3278K/4164K, paused 12ms+21ms, total 101ms
05-14 03:11:18.491: I/System.out(1706): 11
05-14 03:11:18.500: I/System.out(1706): 12
05-14 03:11:18.500: I/System.out(1706): 13
05-14 03:11:18.500: I/System.out(1706): 14
05-14 03:11:18.510: D/AndroidRuntime(1706): Shutting down VM
05-14 03:11:18.510: W/dalvikvm(1706): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
05-14 03:11:18.542: E/AndroidRuntime(1706): FATAL EXCEPTION: main
05-14 03:11:18.542: E/AndroidRuntime(1706): java.lang.NullPointerException
05-14 03:11:18.542: E/AndroidRuntime(1706):     at com.example.actionbartutorial.MainActivity$ViewHolder.<init>(MainActivity.java:160)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at com.example.actionbartutorial.MainActivity$TrendingAdapter.getView(MainActivity.java:144)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.widget.AbsListView.obtainView(AbsListView.java:2159)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.widget.ListView.measureHeightOfChildren(ListView.java:1246)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.widget.ListView.onMeasure(ListView.java:1158)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.View.measure(View.java:15518)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.widget.RelativeLayout.measureChild(RelativeLayout.java:666)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:477)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.View.measure(View.java:15518)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.View.measure(View.java:15518)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:847)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.View.measure(View.java:15518)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2176)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.View.measure(View.java:15518)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1874)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1089)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1265)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:989)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4351)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.Choreographer.doCallbacks(Choreographer.java:562)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.Choreographer.doFrame(Choreographer.java:532)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.os.Handler.handleCallback(Handler.java:725)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.os.Handler.dispatchMessage(Handler.java:92)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.os.Looper.loop(Looper.java:137)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at android.app.ActivityThread.main(ActivityThread.java:5041)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at java.lang.reflect.Method.invokeNative(Native Method)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at java.lang.reflect.Method.invoke(Method.java:511)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
05-14 03:11:18.542: E/AndroidRuntime(1706):     at dalvik.system.NativeStart.main(Native Method)

Devunwired · Accepted Answer · 2013-05-14 03:55:56Z

You haven't accounted for the fact that convertView is only a valid object once views start getting recycled. Initially, when the first views are created for the list, convertView is null, and your ViewHolder constructor doesn't handle a null parameter very gracefully.

Your ViewHolder pattern is a bit broken. When convertView is null, you have to inflate and return a new instance of the view to be used, and only in that instance create a new holder. You should have something more like:

ViewHolder holder;
if (convertView == null) {
    convertView = getLayoutInflater().inflate(resource_id, parent, false);
    holder = new ViewHolder(convertView);
    convertView.setTag(holder);
} else {
    holder = (ViewHolder) convertView.getTag();
}

holder.populateFrom(trending_list.get(position));

return convertView;

Thanks for the point in the right direction, but after this update to the code, I still get the same errors in the LogCat...? — nathansizemore, May 15 '13 at 1:35

nathansizemore · Answer 2 · 2013-05-15 03:00:00Z

The error was in using the android.R.layout.simple_list_item_1 instead of using the custom XML row that I had made. Although, I doubt it would of went much further without @Devunwired responding with the help, so I chose his for the answer...

asked	11 months ago
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