Hi guys, I’ve got a problem with Kotlin, which inv...

# android

[matt_dziubek]

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).

[mzgreen]

Have you tried List<RecyclerItem<*>> ?

[matt_dziubek]

yes, didn’t work

[matt_dziubek]

could you look at my sample and confirm?

[mzgreen]

class RecyclerView{
    interface ViewHolder
}

interface RecyclerItem<in T : RecyclerView.ViewHolder> {
     fun onBindViewHolder(holder: T?)
} 

class ItemTextView : RecyclerItem<ItemTextView.ViewHolder> {

    override fun onBindViewHolder(holder: ViewHolder?) {}

    class ViewHolder : RecyclerView.ViewHolder {}
}

fun main(args: Array<String>) {
	
    val list = mutableListOf<RecyclerItem<*>>()
    list.add(ItemTextView())
    
    println(list.size) // prints 1
}