kotlin synthetic Integer cannot be cast to floating button

Question

Why does

val fabOpen = view.findViewById(R.id.fab_open) as FloatingActionButton

work correctly and not error out but the kotlin synthetic of

val fabOpen = R.id.fab_open as FloatingActionButton

gives me the

java.lang.Integer cannot be cast to android.support.design.widget.FloatingActionButton

error? They both show that they are casting as FloatingActionButton. Using the synthetics is not only less code, it's better memory management and I'd prefer to do it this way. Is there something I am missing?

****Update**** I forgot to mention I am trying access the FloatingActionButton inside of a fragment if that makes a difference.

Answer 1

R.id.fab_open is a generated integer value that will be set as the ID of your button at inflation, and that you can look up with findViewById like you've shown.

Casting this to a button won't work, think (FloatingActionButton) 2688664731 in Java terms.

If you wish to use Kotlin Android Extensions and its synthetic properties, those are simply named as the ID itself, but they don't come from the R class - and you don't need to assign them to variables or properties. You can simply use your button like this:

fab_open.setOnClickListener { ... }
fab_open.visibility = View.VISIBLE