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 to substitute Person with a generic parameter that is passed into the class via the class declaration i.e.

class DataQuery<T : IModel> 

and use that generic parameter to do something like this:

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

but when I attempt that I get an error stating that

only classes can be used on the left-hand side of a class literal

Is it possible to provide a class constraint on the generic parameter like in C# or is there some other syntax I can use to get the type info for the generic param?

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<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

Answer 2

What you need is reified modifier for your generic param, you can read about it here. https://kotlinlang.org/docs/reference/inline-functions.html#reified-type-parameters So if you do something like that:

inline fun <reified T : Any>T.logTag() = T::class.java.simpleName

you will get name of the actual caller class, not "Object".

Answer 3

you can get its class type like this

snapshot?.getValue((this.javaClass
                        .genericSuperclass as ParameterizedType)
                        .actualTypeArguments[0] as Class<T>)

Answer 4

Using typeOf is another option. (Added in Kotlin 1.3)

Example:

@OptIn(ExperimentalStdlibApi::class)
inline fun <reified T> someMethod(data: T) {

    val typeClassifier = typeOf<T>().classifier

    if (typeClassifier == List::class) {
        //Do something
    }

}

Answer 5

1. Using class comparison. See https://stackoverflow.com/a/61756761/2914140.

    inline fun <reified T> SharedPreferences.getData(key: String, defValue: Any? = null): T? =
        when (T::class) {
            Integer::class -> {
                val value = getInt(key, (defValue as? Int) ?: 0) as T
                if (value == 0) defValue as? T else value
            }
            Long::class -> {
                val value = getLong(key, (defValue as? Long) ?: 0L) as T
                if (value == 0) defValue as? T else value
            }
            Boolean::class -> {
                val value = getBoolean(key, (defValue as? Boolean) ?: false) as T
                if (value == false) defValue as? T else value
            }
            String::class -> getString(key, defValue as? String) as T?
            else -> throw IllegalStateException("Unsupported type")
        }
   

2. Using isAssignableFrom. See https://dev.to/cjbrooks12/kotlin-reified-generics-explained-3mie.

    inline fun <reified T : Number> SharedPreferences.getData(key: String): T? {
        val cls = T::class.java
        return if (cls.isAssignableFrom(Integer::class.java)) {
            getInt(key, 0) as T
        } else if (cls.isAssignableFrom(Long::class.java)) {
            getLong(key, 0) as T
        } else {
            throw IllegalStateException("Unsupported type")
        }
    }
   

For Double in SharedPreferences see https://stackoverflow.com/a/45412036/2914140.

Use:

val s: String? = preferences.getData("key", "")
val i = preferences.getData<Int>("key")

Answer 6

This is what I have.

class ClubsViewModel<T>(clazz: Class<T>) :  BaseViewModel<T>(clazz) {
    private var _mClubs = MutableLiveData<List<T>>()

     listenToFireStoreCollection("Clubs", _mClubs)
...
}

class BViewModel<T>(clazz: Class<T>) :  BaseViewModel<T>(clazz) { 
  
    private var _mBs = MutableLiveData<List<T>>()
    listenToFireStoreCollection("Bname", _mBs)
...
}

class BaseViewModel<T>(val clazz: Class<T>) {
    protected val mFirestore = Firebase.firestore

    protected  fun listenToFireStoreCollection(val collectionName: String, liveData: MutableLiveData<List<T>>) 
        mFirestore.collection(collectionName).addSnapshotListener { snapshot, e ->
            if (e != null) {
                return@addSnapshotListener
            }
            if (snapshot != null) {
                liveData.value = snapshot.documents.mapNotNull { it.toObject(clazz) }
            }
        }
    }
}
//FRAGMENT EXAMPLES.
class ClubsFragment : Fragment() {

    private val mClubsViewModel: ClubsViewModel<ClubsFSEntity> by viewModels()
...
}

class BsFragment : Fragment() {

    private val mBsViewModel: BsViewModel<BsFSEntity> by viewModels()
...
}

I have a similar problem here: Hilt Dependency injection with Class<T> in ViewModel