I'm having an issue with Kotlin generics and generic java classes. I'm trying to write a function fo...

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[Marko Mitic]

I'm having an issue with Kotlin generics and generic java classes. I'm trying to write a function for a DI lib that will takes two Class instances, one of interface and one of class implementing the interface (akin to Dagger's @Binds). So far, I can make it work with Kotlin types and even Java ones but it fails for generic Java types (like List<*> and ArrayList<*>). Is this limitation of generics system or am I doing something wrong?

function: fun <C : I, I: Any> bind(clazz: Class<C>, iface: Class<I>)

usage:

bind(ArrayList::class.java, List::class.java) //kotlin types, works fine
bind(java.util.ArrayList::class.java, java.util.List::class.java) // error: inferred type ArrayList<*> is not a subtype of List<*>

[marstran]

What error do you get?

[Marko Mitic]

added it to "usage" part

[Marko Mitic]

Full error

Type parameter bound for C in fun <C : I, I : Any> bind(clazz: Class<C>, iface: Class<I>): Unit
 is not satisfied: inferred type ArrayList<*> is not a subtype of List<*>

[marstran]

Right. My guess is that it’s because the type parameter on the Java types are invariant.

[marstran]

While the Kotlin ones are covariant.

[Marko Mitic]

Any idea for a workaround?

[Marko Mitic]

Interestingly enough, calling the function from java works fine 😄

[marstran]

So the workaround is to use Kotlin types in Kotlin and Java types in Java? 😛

[Marko Mitic]

looks so 😄

[molikuner]

Just overload the function:

inline fun <C : I, I: Any> bind(clazz: KClass<C>, iface: KClass<I>) = bind(clazz.java, iface.java)

If you need to do this often, you may want to publish an extra artifact with those overloaded functions.

[Marko Mitic]

That gives the same error unfortunately

Type parameter bound for C in inline fun <C : I, I : Any> bind2(clazz: KClass<C>, iface: KClass<I>): Unit
 is not satisfied: inferred type ArrayList<*> is not a subtype of List<*>

[Marko Mitic]

Called like this bind2(java.util.ArrayList::class, java.util.List::class)

[Timmy]

For a similar problem the following solution was suggested to me: Instead of having one function, split it into two parts like so:

fun <I: Any> bind(iface: Class<I>) = BindOp(iface)
class BindOp<T>(val t: T)
infix fun <I:Any, C:I> BindOp<I>.to(clazz: Class<C>): Unit = TODO()