Why is a ClassCastException thrown in onBindViewHolder?

Question

Android

override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
        when (holder) {
            is FirstLetterViewHolder -> {
                val firstLetter = dataSource[position] as FirstLetter
                holder.bind(firstLetter.letter.toString())
            }
            is AcronymViewHolder -> {
                val acronym = dataSource[position] as Acronym
                holder.bind(acronym)
            }
        }
    }

Answer 1

Obviously, the logic of working with data is incorrectly described in the adapter class.
There should be synchronism between the model (that you have the `dataSource` field) and the ViewHolder.
Practically, this means that in the method (based on the fact that RecyclerView is used)
`getItemViewType` must return a unique int-id for each model class. for example

override fun getItemViewType(position: Int): Int {
        return when (val item = dataSource[position]) {
            is FirstLetter -> 1
            is Acronym ->2
            else -> throw IllegalArgumentException("unknown item type $item")
        }
    }

and in `onCreateViewHolder` a ViewHolder should be returned that corresponds to a unique identifier for the position (for example, `FirstLetterViewHolder` should be created for `viewType == 1`)
Thus, the synchronism of ViewHolders and the model will be organized.
That is, if `AcronymViewHolder` is passed to the onBindViewHolder method, then `Acronym` will always be in dataSource[position]

Answer 2

Before ghosting a type, you need to check that this is possible.
Try "instanceof" operator in java "if(x instanceof Integer)" or "is" in kotlin "if(x is Integer)"