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 (loop with postcondition)
• else defines the branch of an if expression which 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 the ‘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 the ‘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 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 (loop with precondition)

Soft Keywords

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

• by
  • delegates the implementation of an interface to another object
  • delegates the implementation of 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
• receiveris 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
• where specifies constraints for a generic type parameter

Modifier Keywords

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

• actual denotes a platform-specific implementation in multiplatform projects
• abstract marks a class or member as abstract
• 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 not in Kotlin (accessible through JNI or in JavaScript)
• final forbids overriding a member
• infix allows calling a function in infix notation
• inline tells the compiler to inline the function and the lambdas passed to it at the call site
• inner allows referring to the outer class instance from a nested class
• internal marks a declaration as visible in the current module
• lateinit allows initializing a non-null 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 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 corresponding infix functions)
• ==, != - 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-null
• ?. performs a safe call (calls a method or accesses a property if the receiver is non-null)
• ?: takes the right-hand value if the left-hand value is null (the elvis operator)
• :: creates a member reference or a class reference
• .. creates a range
• : separates a name from a type in declarations
• ? 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