client.scala

// Java library
public final class SomeObjectFactory {
  public SomeObject getInstance(String someParam) { ... }
}

// Scala client
class SomeClientClass(sof: SomeObjectFactory) {
  def foo(param: String) {
    val someObj = sof.getInstance(param)
  }
}

duck.scala

object SomeClientClass {
  type SomeObjectFactory = { def getInstance(someParam: String): SomeObject }
}

class SomeClientClass(sof: SomeClientClass.SomeObjectFactory) {
  def foo(param: String) {
    val someObj = sof.getInstance(param)
  }
}

test.scala

trait FakeSomeObjectFactory {
  def getInstance(someParam: String): SomeObject
}

class TestSomeClientClass extends Specification with Mockito {
  private val mockFactory = mock[FakeSomeObjectFactory]
  mockFactory.getInstance("some param") returns ...
}