How to get generic parameter class in Kotlin

generics kotlin
Jaja Harris picture Jaja Harris · Dec 6, 2015 · Viewed 74.5k times · Source

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

Jayson Minard picture Jayson Minard · Dec 25, 2015

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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