Question

I get the coding in that you basically provide an “object SomeClass” and a “class SomeClass” and the companion class is the class declaration and the object is a singleton. Of which you cannot create an instance. So… my question is mostly the purpose of a singleton object in this particular instance.

Is this basically just a way to provide class methods in Scala? Like + based methods in Objective-C?

I’m reading the Programming in Scala book and Chapter 4 just talked about singleton objects, but it doesn’t get into a lot of detail on why this matters.

I realize I may be getting ahead of myself here and that it might be explained in greater detail later. If so, let me know. This book is reasonably good so far, but it has a lot of “in Java, you do this”, but I have so little Java experience that I sort of miss a bit of the points I fear. I don’t want this to be one of those situations.

I don’t recall reading anywhere on the Programming in Scala website that Java was a prerequisite for reading this book…

Answer 1

Yes, companion singletons provide an equivalent to Java’s (and C++’s, c#’s, etc.) static methods.

(indeed, companion object methods are exposed via “static forwarders” for the sake of Java interop)

However, singletons go a fair way beyond this.

• A singleton can inherit methods from other classes/traits, which can’t be done with statics.
• A singleton can be passed as a parameter (perhaps via an inherited interface)
• A singleton can exist within the scope of a surrounding class or method, just as Java can have inner classes
• It’s also worth noting that a singleton doesn’t have to be a companion, it’s perfectly valid to define a singleton without also defining a companion class.

Which helps make Scala a far more object-oriented language that Java (static methods don’t belong to an object). Ironic, given that it’s largely discussed in terms of its functional credentials.