What is the recommended way to implement Java interface with Java collections in Scala ?

trait ScalaInterface {  def entries: List[String] }

import collection.JavaConverters._
trait JavaInterface {  self: ScalaInterface =>   def getEntries = entries.asJava}

class MyClass extends ScalaInterface with JavaInterface {  val entries: List[String] = List(1,2,3) }

import collection.JavaConverters._
trait JavaInterface {   self: MyClass =>  def getEntries = entries.asJava}

class MyClass extends JavaInterface {   val entries: List[String] = List(1,2,3)}

Hi all,
the task:

Implement a Java interface in Scala. The implementation shall be used both in Java and in Scala.

It looks like an easy task. And it was, when the interface contained only String, Integer and Boolean types.
Once the interface has got Java collections it became a disaster.
The problem is not that it is difficult to implement, the problem is that the implementation is ugly. Java collections penetrate to the Scala implementation and I constanly have to translate collections forth and back.

 What is the recommended way to implement Java interface with Java collections in Scala ?

I think to accept the following way:
1) Deliver 2 independent identical interfaces - a Java interface and a Scala trait.
2) Implement only Scala trait (using only Scala :)
3) Java interface is implemented as an adaptor around the Scala implementation from 2). The main purpose of this adaptor is to translate the collections.

Is there a better way ?

-
Andrey