Private and protected constructor in Scala

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
}

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... */
}