Programming Scala, 2nd Edition

Chapter 13. Visibility Rules

Scala goes well beyond Java’s four-tiered visibility rules: public, protected, private, and default package visibility. Visibility rules in object-oriented languages are designed to expose only the essential public abstraction of a type and encapsulate implementation information, hiding it from view.

This chapter covers the details in depth and to be honest, it’s dry stuff. Superficially, they are just like Java’s rules with the exception that public visibility is the default in Scala. Scala adds more sophisticated scoping rules, but you don’t see them used a lot in every-day Scala code. So, consider skimming the first few sections of this chapter and the concluding section, “Final Thoughts on Visibility”. Save the rest of the chapter for when you need to know the particulars, especially when you start writing your own libraries.

Public Visibility: The Default

Unlike Java, but more like many other object-oriented languages, Scala uses public visibility by default. It’s common to declare type members private, when you want to limit their visibility to the type only, or to declare them protected to limit visbility to subclasses. But Scala gives you more options. This chapter covers the details.

You’ll want to use public visibility for anything that users of your objects should see and use. Keep in mind that the set of publicly visible members form the abstraction exposed by the type, along with the type’s name itself.

Tip

The art of good object-oriented ...