Scala testable code with inheritance and mixins

I've developed a lot of code in Java and dabbled in Groovy and Haskell which has now led me to Scala.

I feel relatively comfortable with the functional side of Scala, but I'm finding myself a bit shaky on object oriented design in Scala, because it feels a bit different to Java, in particular due to traits/mix-ins.

I aim to write code that is as testable as possible, which in my Java development has always translated into focus on

• Immutability where possible


• Prefer injection of state


• ALWAYS go for composition instead of inheritance (heavily influenced by, and likely an over-reaction to this post on SO)

Now I'm trying to land on my feet in this new Scala territory, and I'm having a hard time figuring out what approach I should go for here, in particular whether I should start using inheritance for some purposes.

Programming Scala (Wampler and Payne; O'Reilly, 2nd Edition) has a section with considerations ("Good Object-Oriented Design: A Digression"), and I've read a number of posts on SO, but I haven't seen explicit mentions of the design consideration of testability. The book offers this advice on using inheritance:

1. An abstract base class or trait is subclassed one level by concrete classes, including case classes.


2. Concrete classes are never subclassed, except for two cases:
   
   
   
   • Classes that mix in other behaviors defined in traits (...)
   
   
   • Test-only versions to promote automated unit teting.
   
   
   
   


3. When subclassing seems like the right approach, consider partitioning behaviors into traits and mix in those traits instead.


4. Never split logical state across parent-child type boundaries.

So, in essence, can I write maximally testable code in Scala incorporating inheritance, and are there common cases where it would be inappropriate to favour composition?