Scala singletons

Scala has singleton objects called companion objects. A companion object is an object with the same name as a class. A companion object also can access private methods and fields of its companion class. Both a class and its companion object must be defined in the same source file. The companion object is where the apply() factory method may be defined. Let's have a look at the following example of a companion class:

class Singleton {  // Companion class
  def m() {
    println("class")
  }         
}

And then its companion object as:

object Singleton { // Companion Object
  def m() {
    println("companion")
  }         
}

It is that simple, when a case class is defined, Scala automatically generates a companion object for it.

The apply() factory method

If a companion object defines an apply() method, the Scala compiler calls it when it sees the class name followed by (). So, for example, when Scala sees something like:

                 Singleton(arg1, arg2, …, argN) // syntactic...