Handle IndexOutOfBoundsException in Kotlin

Dealing with IndexOutOfBoundsException is a common challenge in Kotlin, especially for Android developers using lists or arrays. This error occurs when you try to access an index that doesn’t exist in a collection, such as a List, Array, or MutableList.

In this guide, you’ll learn what causes this exception, how to reproduce it, and the best practices to prevent and handle it efficiently in Android Studio.

What Is IndexOutOfBoundsException?

It’s a runtime exception thrown when trying to access an illegal index in a collection. For example:

val numbers = listOf(1, 2, 3)
println(numbers[5]) // Throws IndexOutOfBoundsException

How to Prevent IndexOutOfBoundsException in Kotlin

Use .getOrNull(index)
This safely returns null if the index is out of range.

val value = numbers.getOrNull(5) ?: "Not Found"
println(value) // Output: Not Found

Check Size Before Accessing

if (index < numbers.size) {
    println(numbers[index])
} else {
    println("Index is out of bounds.")
}

Use .elementAtOrElse()

val result = numbers.elementAtOrElse(5) { -1 }
println(result) // Output: -1

Example in Android Studio Project

Let’s create a simple example with a button that accesses a list.

MainActivity.kt

package com.example.indexerror

import android.os.Bundle
import android.widget.Button
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity

class MainActivity : AppCompatActivity() {

    private val colors = listOf("Red", "Green", "Blue")

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

        val checkButton = findViewById<Button>(R.id.btnCheckColor)

        checkButton.setOnClickListener {
            val index = 5 // intentionally out of bounds
            val color = colors.getOrNull(index) ?: "No color found"
            Toast.makeText(this, color, Toast.LENGTH_SHORT).show()
        }
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    android:padding="24dp">

    <Button
        android:id="@+id/btnCheckColor"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Check Color" />
</LinearLayout>

Additional Resource

For deeper Kotlin list handling techniques, refer to Kotlin List Documentation.

Best Practices

• Avoid hardcoded indexes.
• Always check the size of the list before accessing.
• Use safe functions like getOrNull() or elementAtOrElse().
• Write unit tests to check edge cases involving empty or short lists.