How to get generic parameter class in Kotlin

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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<T: Any> {
    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 <reified T: Any> 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 <T: Any> retrieveSomething(ofClass: Class<T>): T? {
        return snapshot?.getValue(ofClass)
    }
    
    inline fun <reified T: Any> 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 <T: Any> {
    val snapshot: Snapshot? = ...

    inline fun <reified R: T> 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

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