Type Mapping

swift4j maps Swift types to their JVM equivalents automatically. The tables below show the mapping for each supported category.

Primitive types

Swift	Kotlin	Java
Bool	Boolean	boolean
Int	Long	long
Int32	Int	int
Int16	Short	short
Int8	Byte	byte
UInt8	Byte	byte
Float	Float	float
Double	Double	double

Int maps to Long because Swift’s Int is 64-bit on all platforms swift4j targets.

Strings

Swift	Kotlin	Java
String	String	String

Arrays

Swift	Kotlin	Java
[Int]	LongArray	long[]
[Double]	DoubleArray	double[]
[Float]	FloatArray	float[]
[Bool]	BooleanArray	boolean[]
[String]	Array<String>	String[]
[T] where T is @jvm	Array<T>	T[]

Optionals

Swift	Kotlin	Java
T? where T is a class	T?	@Nullable T
Int?	Long?	Long (boxed)
String?	String?	@Nullable String
(() -> Void)?	(() -> Unit)?	@Nullable Runnable

Closures

Swift	Kotlin	Java
() -> Void	() -> Unit	Runnable
(T) -> Void	(T) -> Unit	Consumer<T>
(T) -> R	(T) -> R	Function<T, R>
(T, U) -> R	(T, U) -> R	BiFunction<T, U, R>

All types in the closure signature must be mapped types or @jvm-annotated types.

Async

Swift	Kotlin / Java
async func f() -> T	CompletableFuture<T>
async func f() throws -> T	CompletableFuture<T> (exception in future)

Exported types

Classes, structs, and enums annotated with @jvm map to their generated proxy class by name. The package is the target name (hyphens replaced with underscores).

Swift	Kotlin
@jvm class Foo	MyTarget.Foo
@jvm struct Bar	MyTarget.Bar
@jvm enum Baz (no assoc. values)	MyTarget.Baz (enum)
@jvm enum Baz (with assoc. values)	MyTarget.Baz (sealed class)

Unsupported types

Types not listed above cannot be exported. Attempting to use them in an exported method signature causes a compiler warning from @jvm. Use @nonjvm to suppress the warning for members that intentionally use unexportable types.