Compilation error with bounded wildcards using Java classes in Scala

Question

In Java, we have defined an ObservableCollection.java like this:

public class ObservableCollection<T> implements Collection<T> {

   public SubscriptionHandle onElementAdded(Consumer<T> onAdded) {
     // ... 
   }
}

And an AgentService.java that returns an ObservableCollection:

public interface AgentService {

    ObservableCollection<? extends Agent> getAgents();

}

Now, I am trying to use this ObservableCollection.java in a Scala project like this:

  def test(service: AgentService): Unit = {
    val onAdded: Consumer[_ <: Agent] = ???
    service.getAgents.onElementAdded(onAdded)
  }

Trying this results in the following compilation error:

type mismatch;
 found   : java.util.function.Consumer[_$1] where type _$1 <: com.xxxx.xx.xx.agent.Agent
 required: java.util.function.Consumer[?0] where type ?0 <: com.xxxx.xx.xx.agent.Agent
    service.getAgents.onElementAdded(onAdded)
                                     ^
one error found

This does not make much sense to me. Is there a way I can get this running?

Edit: Using a Cosumer[Agent] results in the following error:

type mismatch;
 found   : java.util.function.Consumer[com.xxxx.xx.xx.agent.Agent]
 required: java.util.function.Consumer[?0] where type ?0 <: com.kuka.cc.si.agent.Agent
Note: com.xxxx.xx.xx.agent.Agent >: ?0, but Java-defined trait Consumer is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: ?0`. (SLS 3.2.10)
    service.getAgents.onElementAdded(onAdded)
                                     ^
one error found

Is the error real or just in some IDE? - What happens is you have a Consumer[Agent]? - What happens if you do a dirty cast like service.getAgents.asInstanceOf[ObservableCollection[Agent]] (plus the previous point)? — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Aug 24, 2020 at 13:40
Yes, the error is real. Regarding the usage of Consumer[Agent]: see edited question. With the dirty cast it works, though! — Florian Baierl
– Florian Baierl, Commented Aug 24, 2020 at 14:20

Dmytro Mitin · Accepted Answer · 2020-08-24 14:45:25Z

The thing is not in Scala-Java interop. The following Scala code doesn't compile either

import java.util.function.Consumer
import java.util

trait Agent
trait SubscriptionHandle
trait AgentService {
  def getAgents: ObservableCollection[_ <: Agent]
}
trait ObservableCollection[T] extends util.Collection[T] {
  def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
}

def test(service: AgentService): Unit = {
  val onAdded: Consumer[_ <: Agent] = ???
  val agents: ObservableCollection[_ <: Agent] = service.getAgents
  agents.onElementAdded(onAdded)
//                      ^^^^^^^
}

//type mismatch;
// found   : java.util.function.Consumer[_$2] where type _$2 <: App.Agent
// required: java.util.function.Consumer[_$3]

You misuse existential types (wildcard generics). The following code can't compile

trait X[T]
trait Y[T] {
  def foo(x: X[T]) = ???
}
val x: X[_] = ???
val y: Y[_] = ???
y.foo(x) // doesn't compile

Both x and y have existential types but foo accepts x of type X[T], where T must be the same as T in the type of y, i.e. Y[T], so you can't guarantee that T are the same.

One way to fix compilation is to add generics to AgentService

trait Agent
trait SubscriptionHandle
trait AgentService[T <: Agent] {
  def getAgents: ObservableCollection[T]
}
trait ObservableCollection[T] extends util.Collection[T] {
  def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
}

def test[T <: Agent](service: AgentService[T]): Unit = {
  val onAdded: Consumer[T] = ???
  val agents: ObservableCollection[T] = service.getAgents
  agents.onElementAdded(onAdded)
}

or to its method

trait Agent
trait SubscriptionHandle
trait AgentService {
  def getAgents[T <: Agent]: ObservableCollection[T]
}
trait ObservableCollection[T] extends util.Collection[T] {
  def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
}

def test[T <: Agent](service: AgentService): Unit = {
  val onAdded: Consumer[T] = ???
  val agents: ObservableCollection[T] = service.getAgents
  agents.onElementAdded(onAdded)
}

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