Android : ListView Mode is Multiple But does not select items

I have set my ListView choice mode as below:

 listView.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);

but after selecting some items and trying to Toast selected items it show me empty string .

how I Toast Selected Items:

 String selected = "";
    int cntChoice = listView.getCount();

    SparseBooleanArray sparseBooleanArray = listView.getCheckedItemPositions();
    for (int i = 0; i < cntChoice; i++) {
        if (sparseBooleanArray.get(i)) {
            selected +=listView.getItemAtPosition(i).toString() + "\n";
        }
    }
    Toast.makeText(getApplicationContext(),selected,Toast.LENGTH_SHORT).show();

what is the problem ? and how can i solve it?