obtain class from nested type parameters in kotlin

Question

I have a val built like this

val qs = hashMapOf<KProperty1<ProfileModel.PersonalInfo, *> ,Question>()

How can I obtain the class of ProfileModel.PersonalInfo from this variable?

In other words what expression(involving qs of course) should replace Any so that this test passes.

@Test
fun obtaionResultTypeFromQuestionList(){
    val resultType = Any()
    assertEquals(ProfileModel.PersonalInfo::class, resultType)
}

Thank you for your attention

hluhovskyi · Accepted Answer · 2018-05-26 12:39:13Z

4

There is no straight way to get such information due to Java type erasure. To be short - all information about generics (in your case) is unavailable at runtime and HashMap<String, String> becomes HashMap.

But if you do some changes on JVM-level, like defining new class, information about actual type parameters is kept. It gives you ability to do some hacks like this:

val toResolve = object : HashMap<KProperty1<ProfileModel.PersonalInfo, *> ,Question>() {
    init {
        //fill your data here
    }
}

val parameterized = toResolve::class.java.genericSuperclass as ParameterizedType
val property = parameterized.actualTypeArguments[0] as ParameterizedType
print(property.actualTypeArguments[0])

prints ProfileModel.PersonalInfo.

Explanation:

We define new anonymous class which impacts JVM-level, not only runtime, so info about generic is left
We get generic supperclass of our new anonymous class instance what results in HashMap< ... , ... >
We get first type which is passed to HashMap generic brackets. It gives us KProperty1< ... , ... >
Do previous step with KProperty1

Kotlin is tied to the JVM type erasure as well as Java does. You can do a code a bit nice by moving creation of hash map to separate function:

inline fun <reified K, reified V> genericHashMapOf(
        vararg pairs: Pair<K, V>
): HashMap<K, V> = object : HashMap<K, V>() {
    init {
        putAll(pairs)
    }
}

...

val hashMap = genericHashMapOf(something to something)

edited May 26, 2018 at 12:39

answered May 26, 2018 at 6:48

hluhovskyi

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3 Comments

saiedmomen Over a year ago

yeah that works. Thanks! doesn't kotlin offer a simpler way?

hluhovskyi Over a year ago

Unfortunately, the coastline is tied the same as Java to type erasure, so basically no way to retrieve generics without some voodoo. I've put some improvements to the answer, maybe it will be helpful.

saiedmomen Over a year ago

yeah that makes sense. Thanks

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