asynctask无法更新主线程集合内容

public class MainActivity extends Activity {
private ImageView iv;
public List data = new ArrayList();
private Bitmap[] res;
private int index;
static String URL="http://zhbj.qianlong.com/static/api/news/10007/list_1.json";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new AsyncUtil().execute(URL);
stratImage();
}

private List<TestData> getJson(String url){
    List<TestData> data  = new ArrayList<TestData>();
    try {
        InputStream is = new URL(url).openStream();
        String json = readStream(is);
        JSONObject obj;
        TestData td;
        obj = new JSONObject(json);
        obj = obj.getJSONObject("data");
        JSONArray array = obj.getJSONArray("topnews");
        for(int i = 0; i < array.length(); i++){
            obj = array.getJSONObject(i);
            td = new TestData();
            td.icon = obj.getString("topimage");
            td.dataTitle = obj.getString("title");
            td.dataText = obj.getString("url");
            data.add(td);
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return data;

}

private String readStream(InputStream is){
    InputStreamReader isr = null;
    String result ="";
    try {
        String line = "";
        isr = new InputStreamReader(is,"utf-8");
        BufferedReader reader = new BufferedReader(isr);
        while((line = reader.readLine()) != null){
            result += line; 
        }
    } catch (IOException e) {
        e.printStackTrace();
    }

    return result;
}

//TODO  
public void addBitmapArray(List<TestData> list){
    Bitmap bit = null;
    InputStream is = null;
    for (int i = 0; i < list.size(); i++) {
        try {
            URL u = new URL(list.get(i).icon);
            HttpsURLConnection conn = (HttpsURLConnection) u.openConnection();
            is = conn.getInputStream();
            bit = BitmapFactory.decodeStream(is);
            conn.disconnect();
            res[i] = bit;
        } catch (Exception e) {
            e.printStackTrace();
        }finally{
            try {
                is.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

    }
}


public Handler handler = new Handler(){
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        iv.setImageBitmap(res[(Integer)msg.obj]);
    }
};

public void stratImage(){
    new Thread(new Runnable(){
        @Override
        public void run() {
            while(true){
                Message msg = new Message();
                msg.obj = index;
                handler.sendMessage(msg);
                index++;
                if(index >= data.size()){
                    index = 0;
                }
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }).start();
}




//TODO 
class AsyncUtil extends AsyncTask<String, Void, List<TestData>>{

    @Override
    protected List<TestData> doInBackground(String... params) {
        return getJson(params[0]);
    }

    @Override
    protected void onPostExecute(List<TestData> result) {
        data = result;
        addBitmapArray(result);
        super.onPostExecute(result);    
    }
}

}

AsyncTask不能更新UI，必须用handler，looper，ruanable才行