How to get class of generic type parameter in Kotlin

Question

I would like get the class property from a generic type T. I've decided to extend to Any but I'm getting an error. https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/index.html#extension-properties

I have the following code:

class FirebaseDBRepo<T : Any>(val child:String) {

private var callback: FirebaseDatabaseRepositoryCallback<T>? = null
private val ref: DatabaseReference
private val listener = object : ValueEventListener {
    override fun onDataChange(dataSnapshot: DataSnapshot) {

        //T::class.java is showing the error cannot use t as reified type parameter use class instead

        val gameDS = dataSnapshot.getValue(T::class.java)
        callback!!.onSuccess(gameDS!!)
    }

    override fun onCancelled(databaseError: DatabaseError) {

    }
}

init {
    ref = FirebaseDatabase.getInstance().reference.child(child)
}


fun addListener(callback: FirebaseDatabaseRepositoryCallback<T>) {
    this.callback = callback
    ref.addValueEventListener(listener)
}

fun removeListener() {
    ref.removeEventListener(listener)
}

}

Answer 1

You can only get the class on reified variables. The same thing happens in java, but with a slightly different message:

public <T> void x(){
    T t = T.class.newInstance();
}

In Java, you'd solve this like:

public <T> void x(Class<T> cls){
    T t = cls.newInstance();
}

The same applies to Kotlin, and any calls. You'd need to get a class instance in most cases. However, Kotlin supports reified generics using a keyword, but only on inline generic functions. You could pass a class, but in functions, it's really easy just using the reified keyword.

As in you can't declare a class with reified generics, which means this is invalid:

class SomeClass<reified T>

But it is valid for inline functions, meaning you can do:

inline fun <reified T> someFunction()

So you have two options. But since you extend a listener, the first option of adding the generics to the function isn't an option. You can't override a non-generic method with generics. It won't compile.

Which leaves the second option, which unfortunately is rather hackish; passing the class to the constructor. So it should look like this:

class FirebaseDBRepo<T : Any>(val child: String, private val cls: Class<T>) {

Now, I don't use Firebase, so I have no clue what classes you'd pass, so for this next example, I just use String.

Kotlin supports some type minimization without going over to raw types. This:

val t = FirebaseDBRepo<String>("", String::class.java)

Could be shortened to this:

val t = FirebaseDBRepo("", String::class.java)

The inferred type in both cases is FirebaseDBRepo<String>.