Item keys for performance in Android Kotlin - Mini App: Build & Ship

Build: Simple Item List

This screen shows a scrollable list of items with unique keys to improve performance during updates.

Target UI

-------------------------
| Simple Item List       |
|-----------------------|
| • Item 1              |
| • Item 2              |
| • Item 3              |
| • Item 4              |
| • Item 5              |
|                       |
-------------------------

Display a vertical list of 5 items labeled 'Item 1' to 'Item 5'.

Use RecyclerView with an Adapter that assigns a unique stable ID (key) to each item.

Enable stable IDs in the adapter to optimize RecyclerView performance.

Ensure smooth scrolling and proper item recycling.

Starter Code

Android Kotlin

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val recyclerView = findViewById<RecyclerView>(R.id.recyclerView)
        recyclerView.layoutManager = LinearLayoutManager(this)

        val items = listOf("Item 1", "Item 2", "Item 3", "Item 4", "Item 5")
        val adapter = ItemAdapter(items)
        recyclerView.adapter = adapter

        // TODO: Enable stable IDs and implement getItemId() in adapter
    }
}

class ItemAdapter(private val items: List<String>) : RecyclerView.Adapter<ItemAdapter.ViewHolder>() {

    class ViewHolder(val view: android.widget.TextView) : RecyclerView.ViewHolder(view)

    override fun onCreateViewHolder(parent: android.view.ViewGroup, viewType: Int): ViewHolder {
        val textView = android.widget.TextView(parent.context)
        textView.textSize = 20f
        textView.setPadding(16, 16, 16, 16)
        return ViewHolder(textView)
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.view.text = items[position]
    }

    override fun getItemCount(): Int = items.size

    // TODO: Override getItemId() and enable stable IDs
}

Task 1

Task 2

Solution

Android Kotlin

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val recyclerView = findViewById<RecyclerView>(R.id.recyclerView)
        recyclerView.layoutManager = LinearLayoutManager(this)

        val items = listOf("Item 1", "Item 2", "Item 3", "Item 4", "Item 5")
        val adapter = ItemAdapter(items)
        recyclerView.adapter = adapter
    }
}

class ItemAdapter(private val items: List<String>) : RecyclerView.Adapter<ItemAdapter.ViewHolder>() {

    init {
        setHasStableIds(true)
    }

    class ViewHolder(val view: android.widget.TextView) : RecyclerView.ViewHolder(view)

    override fun onCreateViewHolder(parent: android.view.ViewGroup, viewType: Int): ViewHolder {
        val textView = android.widget.TextView(parent.context)
        textView.textSize = 20f
        textView.setPadding(16, 16, 16, 16)
        return ViewHolder(textView)
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.view.text = items[position]
    }

    override fun getItemCount(): Int = items.size

    override fun getItemId(position: Int): Long {
        return position.toLong()
    }
}

We enabled stable IDs by calling setHasStableIds(true) in the adapter's init block. This tells RecyclerView that each item has a unique ID that won't change.

Then, we override getItemId(position) to return a unique long value for each item. Here, we simply use the position as the unique ID.

This helps RecyclerView optimize item animations and recycling, improving performance especially when the list updates.

Final Result

Completed Screen

-------------------------
| Simple Item List       |
|-----------------------|
| • Item 1              |
| • Item 2              |
| • Item 3              |
| • Item 4              |
| • Item 5              |
|                       |
-------------------------

User can scroll the list smoothly up and down.

RecyclerView reuses item views efficiently without flicker.

Performance is improved because each item has a stable unique key.

Stretch Goal

Add a button to shuffle the list items and update the RecyclerView while preserving smooth animations.

💡 Hint

Use notifyItemMoved() or DiffUtil with stable IDs to animate changes smoothly.