QueryBus pattern implementation - FAILURE

GetBookQueryHandler.kt

data class GetBookQuery(val page: Int = 1, val limit: Int = 10): Query<BookDto?>

class GetBookQueryHandler: QueryHandler<BookDto?, GetBookQuery> {
    override fun handle(query: GetBookQuery): BookDto {
        return BookDto("Dune")
    }

    override fun listenTo(): String = GetBookQuery::class.toString()
}

ListBooksQueryHandler.kt

data class ListBooksQuery(val page: Int = 1, val limit: Int = 10): Query<List<BookDto>>

class ListBooksQueryHandler: QueryHandler<List<BookDto>, ListBooksQuery> {
    override fun handle(query: ListBooksQuery): List<BookDto> {
        return listOf(BookDto("Dune"), BookDto("Dune II"))
    }

    override fun listenTo(): String = ListBooksQuery::class.toString()
}

Main.kt

package test


data class BookDto(val name: String)

// Run it!

fun main(args: Array<String>) {
    // Initializing query bus

    val queryHandlers = listOf(
            ListBooksQueryHandler(),
            GetBookQueryHandler()
    )

    val dispatcher = QueryDispatcherMiddleware(queryHandlers)

    // Calling query bus
    val query = ListBooksQuery()

    // Result should be List<BookDto>
    val result = dispatcher.dispatch(query)

    print(result)
}

QueryBus.kt

interface QueryBusMiddleware {
    fun <R, Q : Query<R>> dispatch(query: Q): R
}

class QueryDispatcherMiddleware constructor(handlers: List<QueryHandler< ... , ...>>) : QueryBusMiddleware {
    private val handlers = HashMap<String, QueryHandler<..., ...>>()

    init {
        handlers.forEach { handler -> this.handlers[handler.listenTo()] = handler }
    }

    override fun <R, Q : Query<R>> dispatch(query: Q): R {
        val queryClass = query::class.toString()
        val handler = handlers[queryClass] ?: throw Exception("No handler listen to the query: $queryClass")
        return handler.handle(query)
    }
}

QueryHandler.kt

interface Query<R>

interface QueryHandler<R, Q : Query<R>> {
    fun handle(query: Q): R
    fun listenTo(): String
}