Should be able to use setStepSize() and use a 1.0 to allow increments of 1 star instead of 0.5 for half stars
setStepSize()
1.0
0.5
Here is the docs on it.
It can be set in the xml too, with
android:stepSize="1"
Try to set the Step size like this
ratingBar.setStepSize(1.0);