Why immutability by default matters in Kotlin

Introduction

Immutability means values cannot change once created. This helps avoid mistakes and makes programs easier to understand and safe.

When you want to keep data safe from accidental changes.

When multiple parts of a program use the same data without interfering.

When you want your program to be easier to test and debug.

When working with multiple threads to avoid conflicts.

When you want to write clear and predictable code.

Syntax

Kotlin

val name: String = "Alice"  // Immutable variable
var age: Int = 30               // Mutable variable

val creates an immutable variable that cannot be changed after assignment.

var creates a mutable variable that can be changed later.

Examples

You cannot change a val once set.

Kotlin

val greeting = "Hello"
// greeting = "Hi"  // This will cause a compile error

var allows changing the value.

Kotlin

var counter = 0
counter = 1  // This is allowed because counter is mutable

Immutable collections cannot be changed after creation.

Kotlin

val list = listOf(1, 2, 3)
// list.add(4)  // Error: list is immutable

Mutable collections can be changed.

Kotlin

val mutableList = mutableListOf(1, 2, 3)
mutableList.add(4)  // This works because the list is mutable

Sample Program

This program shows how val and var work. Trying to change val causes an error, but var can be changed.

Kotlin

fun main() {
    val name = "Bob"
    // name = "Alice"  // Uncommenting this line causes an error

    var age = 25
    println("Name: $name, Age: $age")

    age = 26  // Changing mutable variable
    println("Updated Age: $age")
}

Important Notes

Using val by default helps prevent bugs caused by unexpected changes.

Immutability makes your code easier to read and reason about.

Mutable variables (var) should be used only when necessary.

Summary

Immutability means values cannot change after creation.

Kotlin uses val for immutable and var for mutable variables.

Using immutability by default makes code safer and easier to understand.