Immutability

Learn immutability with val and data class.

Immutability and data class

Immutability means an object does not change after creation. In Kotlin it is achieved with data class and val properties. A data class auto-generates the copy method.

Example: data class User(val name: String, val age: Int). To change a property you call user.copy(age = 31); this returns a new User. The original stays unchanged. This pattern reduces concurrency bugs and makes code more predictable.

The val vs var difference

Variables declared with val are assigned once and cannot be reassigned. var can be reassigned. For immutability, val is preferred.

Example: after val name = "Alex", writing name = "Sam" gives a compile error. var name = "Alex" can be reassigned. In functional style, val is preferred; it reduces side effects and improves readability.

Exercise Update age with copy

Define a data class User with properties val name: String and val age: Int. In main create val u = User("Alex", 30), create u2 with u.copy(age = 31) and print println(u2).

Show example solution

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

fun main() {
    val u = User("Alex", 30)
    val u2 = u.copy(age = 31)
    println(u2)
}

Test yourself

Which method is used to change a property of a data class instance?
- update
- set
- mutate
- copy
Show answer

copy — copy returns a new modified instance.

Complete the data class definition

____ User(val name: String, val age: Int)

Show answer

data User(val name: String, val age: Int)

Match the immutability parts

data class

Immutable class
val

Single assignment
copy(age=31)

New object
var

Reassignable

Next lesson: Pure functions

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