Why does it say The method setListAdapter(ArrayAdapter<String>) is undefined for the type MainList?

protected String[] mBlogPostTitles;
public static final int NUMBER_OF_POSTS = 20;
public static final String TAG = MainListActivity.class.getSimpleName();
protected JSONObject mblogData;



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

    if(isNetWorkAvailable()){
    GetBlogPostTask getBlogPostTask = new GetBlogPostTask();
    getBlogPostTask.execute();
    }
    else{
        Toast.makeText(this, "The network is down",  Toast.LENGTH_LONG).show();
    }

}

private boolean isNetWorkAvailable() {
    ConnectivityManager manager = (ConnectivityManager) getSystemService(CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = manager.getActiveNetworkInfo();

    boolean isAvailable = false;
    if(networkInfo != null && networkInfo.isConnected()){
        isAvailable = true;
    }
    return isAvailable;
}

@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_list, menu);
    return true;
}


public void UpdateList() {
    if(mblogData == null){
        //TODO: Handle error
    }
    else{
        try {
            JSONArray jsonPosts = mblogData.getJSONArray("posts");
            mBlogPostTitles = new String[jsonPosts.length()];
            for(int i = 0; i < jsonPosts.length(); i++ ){
                JSONObject posts = jsonPosts.getJSONObject(i);
                String title = posts.getString("title");
                title = Html.fromHtml(title).toString();
                mBlogPostTitles[i] = title;
            }
            ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, mBlogPostTitles);
            setListAdapter(adapter);


        } catch (JSONException e) {
            Log.e(TAG, "Json Exception Found: " , e);
        }
    }

}

private class GetBlogPostTask extends AsyncTask<Object, Void, JSONObject>{

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

        int responseCode = -1;
        JSONObject jsonResponse = null;

        try {
            URL blogFeedUrl = new URL("http://blog.teamtreehouse.com/api/get_recent_summary/?count=" + NUMBER_OF_POSTS);
            HttpURLConnection connection = (HttpURLConnection) blogFeedUrl.openConnection();
            connection.connect();

            responseCode = connection.getResponseCode();
            if(responseCode == HttpURLConnection.HTTP_OK);
            InputStream inputStream = connection.getInputStream();
            Reader reader = new InputStreamReader(inputStream);
            int content = connection.getContentLength();
            char[] charArray = new char [content];
            reader.read(charArray);
            String responseData = new String(charArray);

            jsonResponse = new JSONObject(responseData);



        } catch (MalformedURLException e) {
            Log.e(TAG, "Exception Found: " , e );
        } catch (IOException e) {
            Log.e(TAG, "Exception Found: " , e );
        }catch (Exception e){
            Log.e(TAG, "Exception Found: " , e );
        }
        return jsonResponse;
    }
    protected void onPostExecute (JSONObject result){
        mblogData = result;
        UpdateList();
    }

}





@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}