In the XML file set the android:checkedButton
field in your RadioGroup
, with the id of your default RadioButton
:
<RadioGroup
....
android:checkedButton="@+id/button_1">
<RadioButton
android:id="@+id/button_1"
...../>
<RadioButton
android:id="@+id/button_2"
...../>
<RadioButton
android:id="@+id/button_3"
...../>
</RadioGroup>