Listview items with checkbox checked at start

I have a fragment with a listView.

The view is populated from a remote received JSON array as follows:

private void callVolley(){



        SharedPreferences prefs3 =
                getActivity().getSharedPreferences(MIEXAMEN, Context.MODE_PRIVATE);

        final String id_materia= "42";
        final String num_examen= "787878";
        pDialog = new ProgressDialog(getActivity());
        pDialog.setMessage("Cargando temas de la materia seleccionada...");
        showDialog();


        JsonArrayRequest jArr = new JsonArrayRequest(url+"?id="+id_materia, new Response.Listener<JSONArray>() {
            @Override
            public void onResponse(JSONArray response) {
                Log.d(TAG, response.toString());
                hideDialog();




                // Parsing json
                for (int i = 0; i < response.length(); i++) {
                    try {
                        JSONObject obj = response.getJSONObject(i);

                        Data item = new Data();

                        item.setMenu(obj.getString(TAG_NOMBRE));
                        item.setId(obj.getString(TAG_ID));


                        itemList.add(item);



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

                // list.invalidateViews();
                adapter.notifyDataSetChanged();



            }
        }, new Response.ErrorListener() {

            @Override
            public void onErrorResponse(VolleyError error) {
                VolleyLog.d(TAG, "Error: " + error.getMessage());
                hideDialog();
            }
        });


        AppController.getInstance().addToRequestQueue(jArr);
    }

Then I add programmatically a checkbox to each list item. This is the adapter:

 public class Adapter extends BaseAdapter {

    private Context activity;
    private ArrayList<Data> data;
    private static LayoutInflater inflater = null;
    private View vi;
    private ViewHolder viewHolder;

    public Adapter(Context context, ArrayList<Data> items) {
        this.activity = context;
        this.data = items;
        inflater = (LayoutInflater)activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    @Override
    public int getCount() {
        return data.size();
    }

    @Override
    public Object getItem(int i) {
        return i;
    }

    @Override
    public long getItemId(int i) {
        return i;
    }

    @Override
    public View getView(int position, View view, ViewGroup viewGroup) {
        vi = view;
        final int pos = position;
        Data items = data.get(pos);

        if(view == null) {
            vi = inflater.inflate(R.layout.list_row, null);
            viewHolder = new ViewHolder();
            viewHolder.checkBox = (CheckBox) vi.findViewById(R.id.cb);
            viewHolder.menu = (TextView) vi.findViewById(R.id.nama_menu);
            vi.setTag(viewHolder);
        }else {
            viewHolder = (ViewHolder) view.getTag();

        }
        viewHolder.menu.setText(items.getMenu());
        if(items.isCheckbox()){
            viewHolder.checkBox.setChecked(true);
        } else {
            viewHolder.checkBox.setChecked(false);
        }

        return vi;
    }

    public ArrayList<Data> getAllData(){
        return data;
    }

    public void setCheckBox(int position){
        Data items = data.get(position);
        items.setCheckbox(!items.isCheckbox());
        notifyDataSetChanged();
    }

    public class ViewHolder{
        TextView menu;
        CheckBox checkBox;
    }
}

At start I need all checkboxes to be checked.

Then the user can check/uncheck the desired items.

On the fragment there is a button that reads all items checkbox states.

What should I implement to put all items in status checked so that on button clicked all items are recognized as checked?

This is a screenshot at start with all items unchecked: