Keywords and operators

Hard keywords

The following tokens are always interpreted as keywords and cannot be used as identifiers:

as
- is used for type casts.
- specifies an alias for an import
as? is used for safe type casts.
break terminates the execution of a loop.
class declares a class.
continue proceeds to the next step of the nearest enclosing loop.
do begins a do/while loop (a loop with a postcondition).
else defines the branch of an if expression that is executed when the condition is false.
false specifies the 'false' value of the Boolean type.
for begins a for loop.
fun declares a function.
if begins an if expression.
in
- specifies the object being iterated in a for loop.
- is used as an infix operator to check that a value belongs to a range, a collection, or another entity that defines a 'contains' method.
- is used in when expressions for the same purpose.
- marks a type parameter as contravariant.
!in
- is used as an operator to check that a value does NOT belong to a range, a collection, or another entity that defines a 'contains' method.
- is used in when expressions for the same purpose.
interface declares an interface.
is
- checks that a value has a certain type.
- is used in when expressions for the same purpose.
!is
- checks that a value does NOT have a certain type.
- is used in when expressions for the same purpose.
null is a constant representing an object reference that doesn't point to any object.
object declares a class and its instance at the same time.
package specifies the package for the current file.
return returns from the nearest enclosing function or anonymous function.
super
- refers to the superclass implementation of a method or property.
- calls the superclass constructor from a secondary constructor.
this
- refers to the current receiver.
- calls another constructor of the same class from a secondary constructor.
throw throws an exception.
true specifies the 'true' value of the Boolean type.
try begins an exception-handling block.
typealias declares a type alias.
typeof is reserved for future use.
val declares a read-only property or local variable.
var declares a mutable property or local variable.
when begins a when expression (executes one of the given branches).
while begins a while loop (a loop with a precondition).

Soft keywords

The following tokens act as keywords in the context in which they are applicable, and they can be used as identifiers in other contexts:

by
- delegates the implementation of an interface to another object.
- delegates the implementation of the accessors for a property to another object.
catch begins a block that handles a specific exception type.
constructor declares a primary or secondary constructor.
delegate is used as an annotation use-site target.
dynamic references a dynamic type in Kotlin/JS code.
field is used as an annotation use-site target.
file is used as an annotation use-site target.
finally begins a block that is always executed when a try block exits.
get
- declares the getter of a property.
- is used as an annotation use-site target.
import imports a declaration from another package into the current file.
init begins an initializer block.
param is used as an annotation use-site target.
property is used as an annotation use-site target.
receiver is used as an annotation use-site target.
set
- declares the setter of a property.
- is used as an annotation use-site target.
setparam is used as an annotation use-site target.
value with the class keyword declares an inline class.
where specifies the constraints for a generic type parameter.

Modifier keywords

The following tokens act as keywords in modifier lists of declarations, and they can be used as identifiers in other contexts:

abstract marks a class or member as abstract.
actual denotes a platform-specific implementation in multiplatform projects.
annotation declares an annotation class.
companion declares a companion object.
const marks a property as a compile-time constant.
crossinline forbids non-local returns in a lambda passed to an inline function.
data instructs the compiler to generate canonical members for a class.
enum declares an enumeration.
expect marks a declaration as platform-specific, expecting an implementation in platform modules.
external marks a declaration as implemented outside of Kotlin (accessible through JNI or in JavaScript).
final forbids overriding a member.
infix allows calling a function using infix notation.
inline tells the compiler to inline a function and the lambdas passed to it at the call site.
inner allows referring to an outer class instance from a nested class.
internal marks a declaration as visible in the current module.
lateinit allows initializing a non-nullable property outside of a constructor.
noinline turns off inlining of a lambda passed to an inline function.
open allows subclassing a class or overriding a member.
operator marks a function as overloading an operator or implementing a convention.
out marks a type parameter as covariant.
override marks a member as an override of a superclass member.
private marks a declaration as visible in the current class or file.
protected marks a declaration as visible in the current class and its subclasses.
public marks a declaration as visible anywhere.
reified marks a type parameter of an inline function as accessible at runtime.
sealed declares a sealed class (a class with restricted subclassing).
suspend marks a function or lambda as suspending (usable as a coroutine).
tailrec marks a function as tail-recursive (allowing the compiler to replace recursion with iteration).
vararg allows passing a variable number of arguments for a parameter.

Special identifiers

The following identifiers are defined by the compiler in specific contexts, and they can be used as regular identifiers in other contexts:

field is used inside a property accessor to refer to the backing field of the property.
it is used inside a lambda to refer to its parameter implicitly.

Operators and special symbols

Kotlin supports the following operators and special symbols:

+, -, *, /, % - mathematical operators
- * is also used to pass an array to a vararg parameter.
=
- assignment operator.
- is used to specify default values for parameters.
+=, -=, *=, /=, %= - augmented assignment operators.
++, -- - increment and decrement operators.
&&, ||, ! - logical 'and', 'or', 'not' operators (for bitwise operations, use the corresponding infix functions instead).
==, != - equality operators (translated to calls of equals() for non-primitive types).
===, !== - referential equality operators.
<, >, <=, >= - comparison operators (translated to calls of compareTo() for non-primitive types).
[, ] - indexed access operator (translated to calls of get and set).
!! asserts that an expression is non-nullable.
?. performs a safe call (calls a method or accesses a property if the receiver is non-nullable).
?: takes the right-hand value if the left-hand value is null (the elvis operator).
:: creates a member reference or a class reference.
.., ..< create ranges.
: separates a name from a type in a declaration.
? marks a type as nullable.
->
- separates the parameters and body of a lambda expression.
- separates the parameters and return type declaration in a function type.
- separates the condition and body of a when expression branch.
@
- introduces an annotation.
- introduces or references a loop label.
- introduces or references a lambda label.
- references a 'this' expression from an outer scope.
- references an outer superclass.
; separates multiple statements on the same line.
$ references a variable or expression in a string template.
_
- substitutes an unused parameter in a lambda expression.
- substitutes an unused parameter in a destructuring declaration.

For operator precedence, see this reference in Kotlin grammar.

Last modified: 27 November 2023