Kotlin - get T::class.java from class generic type parameter

Question

I'm trying to build a generic repository around Corda's vault. It should look something like this:

class VaultRepository<out T : ContractState>(private val services: CordaRPCOps) {

    fun getUnconsumedStateByExternalId(externalId: String): T {
        return services.vaultQuery<T>(...).states.single().state.data
    }
}

But I get this error:

Cannot use 'T' as reified type parameter

I could use another vaultQuery function, which takes a Class<T> instead as a parameter, but I need to be able to get it from VaultRepository<T> first.

Is there a way to get T from a classes generic type parameter list?

Answer 1

My go-to solution is to add a Class<T> parameter to the primary constructor and a constructor-like function which gets the Class from reified T.

class VaultRepository<out T : ContractState>(
    private val services: CordaRPCOps,
    private val clazz: Class<T>
) {
    fun getUnconsumedStateByExternalId(externalId: String): T {
        return services.vaultQuery(..., clazz).states.single().state.data
    }
}

fun VaultRepository<reified T : ContractState>(services: CordaRPCOps) =
    VaultRepository(services, T::class.java)

To anticipate a specific comment: some people think funs shouldn't look like constructors. The Kotlin team doesn't agree.