Private and protected constructor in Scala

111
votes

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 implementation.

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

2
scala
You could have a Scala singleton (with the object keyword, that is), and define your class as private within that singleton, and have methods of the singleton for constructing your objects. – Paggas
@Paggas, unfortunately when you return an instance of a class marked private out of it's scope it won't compile, even when returned from a method of it's in scope companion object. – Don Mackenzie
This is done quite profusely throughout the Scalaz source code. The concept is also known as an abstract algebraic data type. – Tony Morris

2 Answers

195
votes

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... */
}
65
votes

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
}