Scala case class private constructor but public apply method

Here’s the technique to have a private constructor and a public apply method.

trait Meter {
  def m: Int
}

object Meter {   
  def apply(m: Int): Meter = { MeterImpl(m) }
  private case class MeterImpl(m: Int) extends Meter { println(m) }
}

object Application extends App {
  val m1 = new Meter(10) // Forbidden
  val m2 = Meter(10)
}

Background information private-and-protected-constructor-in-scala