Use CardView.setRadius(float), see androidx.cardview.
(It is worth noting that this will have no effect if you also invoke setBackgroundColor, make sure to use setCardBackgroundColor instead).
You must specify a pixel size, rather than dp value, e.g. for corner radius of 4dp you can invoke, in Kotlin:
radius = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 4f, context.resources.displayMetrics)