I try to create a function to identify data types ```fun <T> checkType(args: T): String { ...

# getting-started

[Kiki Abdullah]

I try to create a function to identify data types

fun <T> checkType(args: T): String {
    return when(args) {
        Int -> "Yes! it's Integer"
        String -> "Yes! it's String"
        Boolean -> "Yes! it's Boolean"
        Double -> "Yes! it's Double"
        listOf<Any>() -> "Yes! it's List"
        mapOf<Any, Any>() -> "Yes! it's Map"
        else -> "Can't identify"
    }
}

But, when I call this function like checkType("I have 3 cats") the output always display "Can't identify" . What's wrong with my logic? Thanks in advance

[Dominaezzz]

String -> is String

[Sam]

You need to use is to perform a type check. What you're doing here is actually checking for equality.

[Sam]

The only reason this compiles at all is because String ends up referring to the companion object of the String class. It checks whether args is equal to String.Companion 😞

[Sam]

By the way, listOf<Any>() will also end up doing an equality check, so it will only match against empty lists. Same logic applies; you would need is List<*> to match all lists.

[Javier]

you already have typeOf<SomeType>()

[Sam]

Another note: the generic type argument isn't serving any purpose in your example. Just use fun checkType(args: Any).

[Javier]

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.reflect/type-of.html

[Sam]

☝️ typeOf could help if the type is known at compile time, but not if you need to figure out the type of some unknown object at runtime. Depends on the use case, but this scenario seems more like the latter.

[Javier]

Are you sure about that? I think it should work in runtime too

[Sam]

It uses a reified type parameter, which means the type is actually inferred at compile time. If you were to try and write something like

fun <T> checkType(args: T) = typeOf<T>()

you'd just get a compile-time error saying "can't use T as a reified type parameter"

[Kiki Abdullah]

Thanks for all of the replies. I've seen the code run correctly except on mapOf<Any, Any>() I've tried to change Any with * but it return an error

[Sam]

Same as the List one, you need to use is Map<*, *>

[Kiki Abdullah]

Ok, thank you all

[Kiki Abdullah]

Sorry for this stupid question. I'm a beginner

[Michael de Kaste]

No question is stupid