Kotlin - Keywords

Kotlin keywords are predefined, reserved words used in Kotlin programming that have special meanings to the compiler. These words cannot be used as an identifier (variables names, package names, function names etc.) and if used then compiler will raise an exception.

Kotlin uses fun keyword to define a function, so if we we will try to use it as a variable name then it will be an exception. For example:

fun main() {
   var fun = "Zara Ali"  // Not allowed, throws an exception
   var age = 19          // Valid variable name

   println("Name = $fun")
   println("Age = $age")

When you run the above Kotlin program, it will generate the following output:

main.kt:2:7: error: expecting property name or receiver type
   var fun = "Zara Ali"  // Not allowed, throws an exception
main.kt:2:11: error: expecting '('
   var fun = "Zara Ali"  // Not allowed, throws an exception
main.kt:5:21: error: keyword cannot be used as a reference
   println("Name = $fun")

Kotlin keywords have been categorised into three broad categories: (a) Hard Keywords (b) Soft Keywords (c) Modifier Keywords

As a good programming practice, it is highly recommended not to use any of the mentioned keywords to name any identifiers while coding in Kotlin.

(a) Kotlin Hard Keywords

Following is a list of hard keywords and they cannot be used as identifiers:

as as? break class
continue do else false
for fun if in
!in interface is !is
null object package return
super this throw true
try typealias typeof val
var when while

(b) Kotlin Soft Keywords

Following is the list of keywords (soft) in the context when they are applicable and can be used as identifiers in other contexts:

by catch constructor delegate
dynamic field file finally
get import init param
property receiver set setparam
value where

(c) Kotlin Modifier Keywords

Following is the list of tokens which act as keywords in modifier lists of declarations and can be used as identifiers in other contexts:

actual abstract annotation companion
const crossinline data enum
expect external final infix
inline inner internal lateinit
noinline open operator out
override private protected public
reified sealed suspend tailrec

Quiz Time (Interview & Exams Preparation)

Q 1 - Which of the following is a hard keyword in Kotlin:

A - var

B - val

C - for

D - All of the above

Answer : D


All the mentioned keywords are from a list of hard keywords in Kotlin.

Q 2 - Identify which line of the following program will raise an error:

   var name = "Zara Ali"
   var age = 19
   var class = "6th"
   var height = 5.3

A - First Line

B - Second Line

C - Third Line

D - Last Line

Answer : C


Here 3rd line will raise an error because we have used hard keyword class to define a variable.