Exploring Kotlin’s Immutable Collections Library

Kotlin’s standard collections (List, Set, Map) are mutable by default, which can lead to unintended modifications. To enforce immutability at the API level, JetBrains introduced the Kotlin Immutable Collections library. This library provides a set of truly immutable collection types that prevent accidental modifications and enhance safety in concurrent or multi-threaded environments.

Why Use Immutable Collections?

While Kotlin already has listOf(), setOf(), and mapOf() for read-only collections, they are not truly immutable. The underlying collection can still be modified if it’s referenced elsewhere. Example:

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val list = mutableListOf("A", "B", "C")
val readOnlyList: List<String> = list  
list.add("D")  // Modifies the original list  
println(readOnlyList) // Output: [A, B, C, D]  

(readOnlyList as MutableList).add("E")
println(readOnlyList) // Output: [A, B, C, D, E]

To solve this, the Immutable Collections library provides collections that guarantee immutability at runtime.

Key Features

1. Truly Immutable – Once created, they cannot be changed.
2. Safe for Multi-threading – Avoids unintended modifications in concurrent environments.
3. Optimized for Performance – Uses structural sharing to prevent unnecessary copying.

How to Use Kotlin Immutable Collections

1. Add the Dependency

inFirst, include the Immutable Collections dependency in your build.gradle.kts:

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dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-collections-immutable:0.3.5")
}

2. Creating Immutable Collections

The library provides persistentListOf(), persistentSetOf(), and persistentMapOf() to create immutable collections:

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import kotlinx.collections.immutable.*

val immutableList = persistentListOf("A", "B", "C")
val immutableSet = persistentSetOf(1, 2, 3)
val immutableMap = persistentMapOf("key1" to 100, "key2" to 200)

3. Adding and Removing Elements

Since these collections are immutable, modifying operations return a new modified copy instead of changing the original collection:

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val newList = immutableList.add("D") // Creates a new list
println(newList)  // Output: [A, B, C, D]

val newMap = immutableMap.put("key3", 300)
println(newMap)   // Output: {key1=100, key2=200, key3=300}

The original immutableList and immutableMap remain unchanged!

Performance Considerations

Unlike regular immutable collections (which require full copies for modifications), persistent collections use structural sharing. This means that modifications create a new collection while reusing unchanged parts of the original, improving performance and memory efficiency.

For example, adding an item to a persistent list does not create a full copy but instead reuses most of the existing structure:

Original:  [A, B, C]  
New List:  [A, B, C, D] (Only "D" is newly allocated)  

This makes immutable collections efficient even for large datasets.

Benefits in Jetpack Compose

Immutable collections are particularly useful in Jetpack Compose because they optimize state management and recompositions. Here’s why they matter in Compose applications:

1. Avoid Unnecessary Recompositions

• Compose tracks state changes to decide when to recompose UI elements.
• Mutable lists, sets, or maps might trigger unnecessary recompositions even when data hasn’t changed.
• Immutable collections ensure that state remains stable, preventing redundant recompositions.

Example:

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@Composable
fun MyListScreen(items: List<String>) {
    LazyColumn {
        items(items) { item ->
            Text(text = item)
        }
    }
}

If items is a mutable list, even reassigning the same values triggers recomposition. Using an immutable collection like PersistentList ensures that Compose recognizes when data is unchanged:

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val items = remember { persistentListOf("A", "B", "C") }
MyListScreen(items)

2. State Stability for Performance

• Compose optimizes rendering by skipping recompositions when state objects are stable.
• Immutable collections use structural sharing, meaning that modifications only affect the changed part while reusing the rest.
• This leads to better performance in large lists or complex UI hierarchies.

3. Predictable UI Behavior

• Since immutable collections cannot be modified after creation, they prevent accidental mutations that might lead to unpredictable UI updates.
• This is especially useful in state-driven architectures (MVI, Redux, etc.), ensuring UI updates only when necessary.

4. Thread Safety

• In Compose apps using coroutines (Flows, LiveData, etc.), immutable collections prevent race conditions when multiple threads update state.
• They ensure safe data flow between ViewModels, repositories, and UI components.

When to Use Immutable Collections?

✅ Functional Programming – Encourages immutability for safer data transformations.
✅ Thread Safety – Prevents unintended modifications in multi-threaded environments.
✅ Prevent Bugs – Reduces unexpected side effects due to accidental mutation.
✅ State Management in Compose – Helps optimize recompositions and improve UI performance.

Conclusion

The Kotlin Immutable Collections library provides truly immutable, efficient, and safe collections, making them a great choice for functional programming, concurrent applications, and Jetpack Compose development. By leveraging persistent collections, you can write safer and more predictable Kotlin code.

🚀 Would you use immutable collections in your projects?