Supported Types

Type Mapping

Type	As param	As return	As property	Notes
Int, Long, Double, Float, Boolean, Byte, Short	✅	✅	✅	Direct pass-through
String	✅	✅	✅	UTF-8 output-buffer pattern
ByteArray	✅	✅	—	Pointer + size; suspend, callbacks, DC fields, collections
enum class	✅	✅	✅	Ordinal mapping
data class	✅	✅	✅	Fields: primitives, String, ByteArray, Enum, Object, nested DC, List, Set, Map
Object (class instances)	✅	✅	✅	Opaque StableRef handle, lifecycle tracked
Nested classes	✅	✅	✅	Exported as Outer_Inner, up to 3+ nesting levels
T? (nullable)	✅	✅	✅	Sentinel-based null encoding
List<T>, Set<T>	✅	✅	✅	All element types incl. DataClass, ByteArray, nested collections
Map<K, V>	✅	✅	✅	Parallel key + value arrays
List<List<T>>	✅	✅	—	Nested collections via StableRef handles
(T) -> R (lambda)	✅	✅	—	FFM upcall/downcall stubs; nullable ((T) -> R)? supported
suspend fun	—	✅	—	All return types: primitives, String, ByteArray, DataClass, List, Set, Map
Flow<T>	—	✅	—	All element types: primitives, String, ByteArray, DataClass, List, Set, Map

Declarations

Feature	Notes
Classes	StableRef lifecycle, AutoCloseable on JVM
Open / abstract classes	open class Shape → JVM open class Shape, hierarchy mirrored
Inheritance	class Circle : Shape → JVM class Circle : Shape(handle), multi-level (3+)
Interfaces	interface Measurable → JVM interface Measurable, multi-interface impl
Sealed classes	sealed class Result → JVM sealed class, subclass ordinal bridges
Extension functions	fun Shape.displayName() → real Kotlin extension on JVM proxy
Constructor val/var params	Exposed as properties with getters (and setters for var)
Companion objects	Static methods and properties on JVM proxy
Top-level functions	Grouped into a singleton object on JVM

Not yet supported

Feature	Notes
Generics (class Box<T>)	Use concrete types or collections
Interface / sealed class as return type	Methods must return the concrete type
Operator overloading, infix functions	Use named methods
ByteArray in collections / data class fields / callback params	Use List<Int> or Base64 String
Subclassing from JVM	Subclass on native side instead
CInterop types in public API (CPointer, etc.)	Wrap behind a clean Kotlin API