If your radio group is defined in a layout xml file, each button can be assigned an id. Then you just check a button like this
radioGroup.check(R.id.myButtonId);
If you created your radio group programmatically (I’m not even sure how you do this…), you might want to consider creating a special layout xml file just for the radio group so that you can assign R.id.* ids to the buttons.
Please see the answer below if you are, in fact, looking to set the radio button group by index, see the answer below.
((RadioButton)radioGroup.getChildAt(index)).setChecked(true);