RecyclerView, ListAdapter and DiffUtil

Views, layouts and RecyclerView Mid-level Kotlin 2.0+ API 24+ Reviewed 31 Oct 2026 By Best Latest Apps

Spot the bug

After toggling a favourite, the row doesn't update. Which line causes it?

private val items = mutableListOf<Item>()
fun toggle(id: Long) {
val i = items.indexOfFirst { it.id == id }
items[i] = items[i].copy(favourite = !items[i].favourite)
adapter.submitList(items)
}

Hint 1 of 1

Is the submitted list a new object?

Choose an answer, then open Show answer below to check it.

Show answer

Answer: line 6 adapter.submitList(items)

The same list instance is submitted again, so ListAdapter sees the same reference and skips the diff. Submit a new list (items.toList(), or currentList.map { ... }).

30-second answer

RecyclerView creates only enough ViewHolders to fill the screen plus a few, and rebinds them with new data as you scroll. ListAdapter with a DiffUtil.ItemCallback compares the old and new list on a background thread and dispatches exact insert, remove, move and change events, so only affected rows rebind and animations work. notifyDataSetChanged() rebinds everything and kills animations.

Practise saying it · 35 s. No recording, just a timer

Explain RecyclerView recycling and DiffUtil.

Say your answer out loud, then check which of these points you covered:

ViewHolders are reused
DiffUtil computes minimal changes
areItemsTheSame vs areContentsTheSame
Submit a new list

Deep dive

areItemsTheSame compares IDs; areContentsTheSame compares contents.

submitList must get a new list instance; submitting the same mutated list does nothing.

Use setHasStableIds only with real stable IDs.

Shared RecycledViewPool helps nested horizontal lists reuse views.

ListAdapter with DiffUtil Compile-checked · Kotlin 2.4.21

data class Item(val id: Long, val title: String, val favourite: Boolean)

object ItemDiff : DiffUtil.ItemCallback<Item>() {
    override fun areItemsTheSame(old: Item, new: Item) = old.id == new.id
    override fun areContentsTheSame(old: Item, new: Item) = old == new
}

class ItemAdapter : ListAdapter<Item, ItemAdapter.Holder>(ItemDiff) {
    class Holder(val text: TextView) : RecyclerView.ViewHolder(text)
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) = Holder(TextView(parent.context))
    override fun onBindViewHolder(holder: Holder, position: Int) {
        holder.text.text = getItem(position).title
    }
}

Common mistakes

Calling notifyDataSetChanged on every update.
Mutating the current list and calling submitList with it.
Comparing whole objects in areItemsTheSame.

Senior take & follow-ups

Explain the two callbacks precisely and the same-instance trap.

Follow-up questions

Think of your answer first, then open the hint.

Why does submitList with the same list do nothing?
Show hint
ListAdapter sees the same reference and skips diffing.
What's a payload?
Show hint
Partial change info passed to onBindViewHolder to update part of a row.

Compare on iOS: UICollectionView vs UITableView and diffable data sources Diffable data sources do the same diffing for UIKit lists.

Suggest a fix if something here is wrong or out of date.