types.kt

import com.google.common.reflect.TypeToken

inline fun <reified T> typeToken(): TypeToken<T> = object : TypeToken<T>() { }

private val iterableParamType = Iterable::class.java.typeParameters[0]
private val mapKeyType = Map::class.java.typeParameters[0]
private val mapValueType = Map::class.java.typeParameters[1]

fun <T, I : Iterable<T>> TypeToken<I>.elementType(): TypeToken<T> {
    @Suppress("UNCHECKED_CAST")
    return resolveType(iterableParamType) as TypeToken<T>
}

fun <K, M : Map<K, *>> TypeToken<M>.keyType(): TypeToken<K> {
    @Suppress("UNCHECKED_CAST")
    return resolveType(mapKeyType) as TypeToken<K>
}

fun <V, M : Map<*, V>> TypeToken<M>.valueType(): TypeToken<V> {
    @Suppress("UNCHECKED_CAST")
    return resolveType(mapValueType) as TypeToken<V>
}

fun main(args: Array<String>) {
    val listType = typeToken<List<*>>()
    val listElementType = listType.elementType()
    
    val mapType = typeToken<Map<*, *>>()
    val mapKeyType = mapType.keyType()
}