Hi guys, I’ve got a problem with Kotlin, which involves generics. Stick with me though 😉. I have a below generic interface:

interface RecyclerItem<in T : RecyclerView.ViewHolder> {

    fun onBindViewHolder(holder: T?)
}

Generic is defined as

in

because clearly

RecyclerItem

is a consumer, hence I can only assign this instance to its subtype. I also have a below implementations of `RecyclerItem`:

class ItemSuncalcData() : RecyclerItem<ItemSuncalcData.ViewHolder>

and:

class SomeOtherItem() : RecyclerItem<SomeOtherItem.ViewHolder>

Long story short I want to store above two instances in a

List<RecyclerItem<RecyclerView.ViewHolder>>

, but because of

in T

I can’t assign subclasses to its supertypes. What’s my best move here? I’ve written same code in java and only got unchecked cast warning. The only way I could think of was to remove generics from the interface:

interface RecyclerItem {

    fun onBindViewHolder(holder: RecyclerView.ViewHolder?)
}

But before I’ll change 15+ files (because of casting I would have to make) I wanted to ask you first, thanks in advance 😉 Take a look at this github repo I made showcasing my problem above: https://github.com/matdziu/GenericsKotlin. You can switch between two commits - one that uses generic way (compilation error in MainActivity) and the second which is the solution I think I found. Note that on production we’re using multiple different item types and factory pattern to choose a proper view holder (this sample is just a huge simplification).