Functional Programming in Kotlin: Immutability

Functional Programming in Kotlin: Immutability

Giovanni Laquidara's photo
Giovanni Laquidara
ยทMar 31, 2023ยท

4 min read

Table of contents

  • Benefits of Immutability
  • Applying Immutability in Kotlin

Immutability is an important concept in functional programming. In an immutable programming paradigm, data cannot be changed once it is created. This is in contrast to a mutable programming paradigm, where data can be modified in-place.

In Kotlin, there are several ways to enforce immutability, such as using val instead of var, which creates a read-only variable.

Let's explore some of the benefits of immutability and how to apply it in Kotlin.

Benefits of Immutability

  1. Predictable behavior: Immutable objects behave predictably, which can make code easier to reason about and debug. Since the values cannot change, there is no need to worry about side effects that may occur in mutable objects.

  2. Thread safety: Immutable objects can be safely accessed from multiple threads, as they cannot be modified concurrently. This can eliminate the need for complex synchronization mechanisms.

  3. Improved performance: Immutable objects can be optimized by the compiler and runtime, as they don't need to account for changes to their values.

Applying Immutability in Kotlin

To create immutable objects in Kotlin, we can use the following techniques:

  1. Use val instead of var: As mentioned before, val creates a read-only variable that cannot be reassigned.

  2. Data classes: Kotlin provides data classes that generate equals, hashCode, and toString methods for you. These classes can also be made immutable by using val for their properties.

  3. Functional programming concepts: Concepts such as pure functions and higher-order functions can encourage immutability by requiring that functions have no side effects and always return a new value.

here are some code samples in Kotlin to illustrate immutability:

Declaring an immutable variable:

val x: Int = 5

Creating an immutable list:

val list: List<Int> = listOf(1, 2, 3)

Using a function to return an immutable value:

fun getImmutableValue(): String {
    return "Hello, world!"

Using a data class to create an immutable object

data class Person(val name: String, val age: Int)

Declaring an immutable map:

val map: Map<String, Int> = mapOf("one" to 1, "two" to 2, "three" to 3)

Creating an immutable set

val set: Set<Int> = setOf(1, 2, 3)

Mutability VS Immutability

// No immutability
data class Person(var name: String, var age: Int)

fun main() {
    val person = Person("John", 30)
    println("Before mutation: $person")
    println("After mutation: $person")

fun mutatePerson(person: Person) { = "Jane"
    person.age = 25

In this code sample, we have a Person class with mutable properties name and age. In the main function, we create a Person instance and then pass it to the mutatePerson function, which mutates the person's properties. When we print the person instance before and after the mutation, we can see that the mutation has affected the original object, which can cause unexpected behavior and bugs.

To fix this issue, we can make the Person class immutable by making its properties val instead of var. Here's the updated code:

// With immutability
data class Person(val name: String, val age: Int)

fun main() {
    val person = Person("John", 30)
    println("Before mutation: $person")
    val newPerson = mutatePerson(person)
    println("After mutation: $person")
    println("New person: $newPerson")

fun mutatePerson(person: Person): Person {
    return person.copy(name = "Jane", age = 25)

In this code sample, we've updated the Person class to have immutable properties. Instead of mutating the original Person instance in the mutatePerson function, we're returning a new instance of Person with the updated properties using the copy method. In the main function, we create a Person instance and pass it to the mutatePerson function. We then print the original person instance and the new Person instance returned from the mutatePerson function to show that the original instance has not been mutated.

Wrap it up!

Immutability is a key concept in functional programming, and the last sample code demonstrates why. By using immutable data structures and avoiding mutation, we can avoid issues that can arise when multiple threads or processes are accessing and modifying the same data. Not only does this lead to more reliable and predictable code, but it also allows for better performance, as immutable data structures can be shared safely between multiple threads without the need for locking or synchronization mechanisms. Additionally, using immutable data structures can simplify reasoning about code, making it easier to understand and maintain over time.

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