How to get generic parameter class in Kotlin

Question

Firebase\'s snapshot.getValue() expects to be called as follows:

snapshot?.getValue(Person::class.java)

However I would like t

Answer 1

For a class with generic parameter T, you cannot do this because you have no type information for T since the JVM erases the type information. Therefore code like this cannot work:

class Storage {
    val snapshot: Snapshot? = ...

    fun retrieveSomething(): T? {
        return snapshot?.getValue(T::class.java) // ERROR "only classes can be used..."
    }
}

But, you can make this work if the type of T is reified and used within an inline function:

class Storage {
    val snapshot: Snapshot? = ...

    inline fun  retrieveSomething(): T? {
        return snapshot?.getValue(T::class.java)
    }
}

Note that the inline function if public can only access public members of the class. But you can have two variants of the function, one that receives a class parameter which is not inline and accesses private internals, and another inline helper function that does the reification from the inferred type parameter:

class Storage {
    private val snapshot: Snapshot? = ...

    fun  retrieveSomething(ofClass: Class): T? {
        return snapshot?.getValue(ofClass)
    }

    inline fun  retrieveSomething(): T? {
        return retrieveSomething(T::class.java)
    }
}

You can also use KClass instead of Class so that callers that are Kotlin-only can just use MyClass::class instead of MyClass::class.java

If you want the class to cooperate with the inline method on the generics (meaning that class Storage only stores objects of type T):

class Storage  {
    val snapshot: Snapshot? = ...

    inline fun  retrieveSomething(): R? {
        return snapshot?.getValue(R::class.java)
    }
}

The link to reified types in inline functions: https://kotlinlang.org/docs/reference/inline-functions.html#reified-type-parameters