class MyExample {
abstract class Field<in E, T : Any>(private val getter: (E) -> T?) {
fun get(entity: E): T? { return getter(entity) }
}
interface Meta<T: Any> {
fun getFields(): List<MyExample.Field<T, *>>
}
interface MetaRepository {
fun <T : Any> getMeta(klass: KClass<T>): Meta<T>?
}
lateinit var metaRepository: MetaRepository
fun <T : Any> doSomthing(entity: T?) {
val meta = metaRepository.getMeta(entity!!::class)!!
meta.getFields().forEach { field ->
val fieldValue = field.get(entity) // <-- Error Kotlin: Type mismatch: inferred type is T? but Nothing was expected
Unit
}
}
}
Does anyone know why the compilation error "Error Kotlin: Type mismatch: inferred type is T? but Nothing was expected" is produced for the code above?
How can I solve this error?
Look at
val meta = metaRepository.getMeta(entity!!::class)!!
What type do you expect for meta? Probably Meta<T>. But if you try to annotate it, you'll see that's wrong. It's actually inferred to be Meta<out T> (basically because entity could actually belong to some subclass of T, so entity!!::class is KClass<out T>).
So field is Field<Nothing, out Any> and field.get needs Nothing as its argument.
The solution is reified type parameters to get KClass<T> instead:
inline fun <reified T : Any> doSomthing(entity: T?) {
val meta = metaRepository.getMeta(T::class)!!
meta.getFields().forEach { field ->
val fieldValue = field.get(entity!!)
// Unit at the end isn't needed
}
}
Unfortunately, getMeta itself can't use reified because it's an interface method and there's nothing to inline, but you can make a helper method to simplify calls to it:
inline fun <reified T : Any> MetaRepository.getMeta1() = getMeta(T::class)
...
val meta = metaRepository.getMeta1<T>()!!
Side note: if you need entity not to be null anyway (using entity!!), there's probably no good reason to make its type T? instead of T.