mutable vs. immutable

There’s a distinction between mutable and immutable data structures in Kotlin.

When referring to the concept of a List, it’s usually referring to the Kotlin’s MutableList. Unlike List, MutableList also has functions for adding and removing elements. Kotlin doesn’t allow a List to be changed in any way.

To change a data structure, you must express this intent by using the Mutable version of that data structure. These data structures have functions for adding and removing elements.

So why would you ever use the immutable version? For safety.

Whenever you need to pass your data structure as a parameter, and you want to be sure that the function doesn’t produce a side effect, you should use an immutable collection as the parameter.

fun noSideEffectList(names: List<String>) { println(names)}fun sideEffectList(names: MutableList<String>) { names.add("Cem")}fun mutableVsImmutable() { val people = mutableListOf("Kaan", "Fırat", "İlknur") noSideEffectList(people) // [Kaan, Fırat, İlknur] sideEffectList(people) // Adds Cem to the list noSideEffectList(people) // [Kaan, Fırat, İlknur, Cem]}

The sideEffectList function adds a Cem to it. These kind of side-effects are usually the ones generating bugs. Avoiding them by using a List instead of a MutableList is preferred.