Your Second Kotlin Android App - Part 4: Setting up a | Ray Wenderlich

Having a RecyclerView isn't enough. You also need an adapter. Learn about adapters in this video.

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Hi @bdmoakley !

It might be my Android version but I saw that on the MainActivity there are two variables: fab and toolbar that are not declared and assigned through findViewById but auto-referenced with the import:


So the question is:
Which way is more efficient than the other?

Hey @macsimus, any thoughts on this? Cheers! Brian

@macsimus Can you please help with this when you get a chance? Thank you - much appreciated! :]

Hi David and Brian!

Those views are referenced using Kotlin Android Extensions. You can read more about KAE here:

Kotlin Android Extensions utilize a cache and so it’s possible they can be more efficient than using findViewByID(). The best part is you need less code to reference your views, so they are definitely more efficient from a developer-perspective.

Hi, I use latest Android Studio, and when type

class ListSelectionViewHolder(itemView: View?) : RecyclerView.ViewHolder(itemView)

has error : type mismatch, required View found View?, then I change it to just View. Is there any explaination for this since in the view no problem.

@bdmoakley @macsimus Can you please help with this when you get a chance? Thank you - much appreciated! :]

Thanks for the question @telornaga!

As the Android SDK and support libraries evolve, Google has been adding nullability annotations into the Java code. So it will sometimes happen that a constructor, like that here for RecyclerView.ViewHolder, goes from taking a nullable parameter to needing a non-nullable parameter. That seems to be what happened here. You’ll also see this sometimes when you use Ctrl+I to add shells for overrides for an interface you’re implementing, and need to make the parameters non-nullable.

I hope that helps. Thanks again!

@bdmoakley this causing App crash because is not null safe i think so , so what is the solution of it.
Am using android studio 3.5.3.
And even the solution from @macsimus in the below is not working

@bdmoakley @macsimus

just removed TODO() from the method

indent preformatted text by 4 spaces  override fun onBindViewHolder(holder: ListSelectionViewHolder, position: Int) {


What’s the complete code for your method?

It was empty but now it has the below

indent preformatted text by 4 spaces    if (holder != null)
        holder.listpostion.text = (position + 1).toString()
        holder.listTitle.text = listTitles[position] //since it will pass every index so the array will read it one by one

@ankm What error do you get exactly?

I will go ahead and change the nullable View? to a non-nullable View like @telornaga suggested and see what happens later. Crossing my fingers that I don’t get an app crash later like what happened to @bdmoakley. I am also on Android Studio 3.5.3.

@karldusenbery Do you still have issues with this?

This course is out of date - it probably causes some minor issues with Android Studio 3.5 - you can find our latest version over here: