In Kotlin there is a limited form of reified generics. Is there any way to use reification to filter for a generic type without using getClass()
or as
or any kind of weird annotation, ie. just by using the is
keyword? For example, I have the following structure:
import java.util.*
internal class Layout<out T : LayoutProtocol>(val t: T) {
fun getName(): String {
return t.getName()
}
}
interface LayoutProtocol {
fun getName(): String
}
internal class Vertical : LayoutProtocol {
override fun getName(): String {
return "Vertical"
}
}
internal class Horizontal : LayoutProtocol {
override fun getName(): String {
return "Horizontal"
}
}
fun main(args: Array<String>) {
val layouts = LinkedList<Layout<*>>()
layouts.add(Layout<Horizontal>(Horizontal()))
layouts.add(Layout<Vertical>(Vertical()))
println("Horizontal layouts:")
layouts.filterIsInstance<Layout<Horizontal>>().forEach { println(it.getName()) }
}
This outputs:
Horizontal layouts:
Horizontal
Vertical
I would like it to output the following. Is there any way to get:
Horizontal layouts:
Horizontal
If we look at the source code for filterIsInstance(...)
, Kotlin does some tricky stuff to circumvent type erasure, but still does not work:
/**
* Returns a list containing all elements that are instances of specified type parameter R.
*/
public inline fun <reified R> Iterable<*>.filterIsInstance(): List<@kotlin.internal.NoInfer R> {
return filterIsInstanceTo(ArrayList<R>())
}
/**
* Appends all elements that are instances of specified type parameter R to the given [destination].
*/
public inline fun <reified R, C : MutableCollection<in R>> Iterable<*>.filterIsInstanceTo(destination: C): C {
for (element in this) if (element is R) destination.add(element)
return destination
}
If this is not possible in Kotlin, is there any language (JVM or non-JVM) that lets me do something like the following:
inline fun <reified R: LayoutProtocol> filterVerticals(from: Iterable<Layout<R>>): Iterable<Layout<Vertical>> {
val dest = ArrayList<Layout<Vertical>>()
for (element in from)
if (element is Layout<Vertical>)
dest.add(element)
return dest
}
There is no simple way to do this because of type erasure, but if you really want and performance/readability/error-proneness is not something you are worried about, you can do a few tricks:
First, let's add a factory method to Layout
to preserve erased type
open internal class Layout<T : LayoutProtocol>(val t: T) {
...
companion object {
inline fun <reified T: LayoutProtocol> create(instance: T): Layout<T> {
return object: Layout<T>(instance) {}
}
}
}
(note: here I removed out variance for simplicity)
Second, you need a helper class
open class TypeLiteral<T> {
val type: Type = getSuperclassTypeParameter(javaClass)
companion object {
fun getSuperclassTypeParameter(subclass: Class<*>) =
(subclass.genericSuperclass as ParameterizedType).actualTypeArguments[0]
}
}
(note: the same approach is used by Guice DI, it contains a production-ready TypeLiteral
implementation )
And, finally our own filter method
inline fun <reified R> Iterable<*>.genericFilterIsInstance() where R : Any =
filterIsInstance<R>()
.filter { object : TypeLiteral<R>() {}.type == it.javaClass.genericSuperclass }
And now it prints exactly what you want
fun main(args: Array<String>) {
val layouts = LinkedList<Layout<*>>()
layouts.add(Layout.create(Horizontal()))
layouts.add(Layout.create(Vertical()))
println("Horizontal layouts:")
layouts.genericFilterIsInstance<Layout<Horizontal>>().forEach { println(it.getName()) }
/* prints:
Horizontal layouts:
Horizontal
*/
}
But please, don't use this answer in production code. In real life passing a class instance for filtering would always be preferable.