Private and protected constructor in Scala

I've been curious about the impact of not having an explicit primary constructor in Scala, just the contents of the class body.

In particular, I suspect that the private or protected constructor pattern, that is, controlling construction through the companion object or another class or object's methods might not have an obvious implem开发者_开发问答entation.

Am I wrong? If so, how is it done?

You can declare the default constructor as private/protected by inserting the appropriate keyword between the class name and the parameter list, like this:

class Foo private () { 
  /* class body goes here... */
}

Aleksander's answer is correct, but Programming in Scala offers an additional alternative:

sealed trait Foo {
 // interface
}

object Foo {
  def apply(...): Foo = // public constructor

  private class FooImpl(...) extends Foo { ... } // real class
}