When to use mutable vs immutable classes in Scala

Question

Much is written about the advantages of immutable state, but are there common cases in Scala where it makes sense to prefer mutable classes? (This is a Scala newbie question fr

Answer 1

I think the short answer is most likely: Yes, immutable data structures are far more usable and efficient than you realize.

The question you've posed is a bit ambiguous because the answer depends less on the motor you've described than on the software system that you haven't described. The great mistake of how OOP is always taught, in my opinion, is recommending bottom-up design of "domain" classes prior to considering how the classes will be used. Maybe your system even needs more than one data structure holding the same information about a motor in different ways.

The "old way" of doing OOP in Java or C# using classes to encapsulate mutable state seems to fit the motor example very well.

The "new way" (arguably), in support of multithreaded systems, is to encapsulate mutable state within actors. An actor that represents the current state of a motor would be mutable. But if you were to take a "snapshot" of the motor's state and pass that information to another actor, the message needs to be immutable.

In that [immutable] case, would 'speed' be represented internally as a 'val' instead of a 'var', and the 'setSpeed' method return a new instance of the class?

Yes, but you don't actually have to write that method if you use a case class. Suppose you have a class defined as case class Motor(speed: Speed, rpm: Int, mass: Mass, color: Color). Using the copy method, you could write something like motor2 = motor1.copy(rpm = 3500, speed = 88.mph).