What is the difference between val and var in Kotlin?

Short answer

val declares a reference that cannot be reassigned (read-only), while var declares a variable that can be modified and reassigned (mutable).

Explanation

In Kotlin it is always preferable to use val to promote immutability and avoid side effects across multiple threads. Note that val prevents re-pointing the reference to another object, but if the object itself is mutable (such as a MutableList) its internal contents can still be modified.

Code example

val name = "Android" // Cannot be reset
// name = "Kotlin" // Compile-time error

var counter = 0
counter += 1 // Allowed

val list = mutableListOf("A", "B")
list.add("C") // Allowed because the object itself is mutable

Common mistakes

• Believing that val means the object's contents are completely unchangeable (deep immutability), like const.
• Using var with no real need to modify the value, which reduces code clarity and raises the chance of bugs.

Follow-up questions

• What is the difference between const val and val in Kotlin?

Sources

• Kotlin Docs — Basic syntax / Variables