Scala class/object companion pattern

A Scala companion object is an object with the same name as a it own class class, hence the "companion" name. The object has access to all of that class’ members, including the private ones.

Typical usage:

class Apple private (val color:String) {
    // Stuff goes here...
}

object Apple {
    def apply(color:String):Apple = new Apple(color)
}

val greenApple = Apple("blue")
val redApple   = Apple("white")

One of the main reason to use the companion pattern is to overload the constructors and redefine the behavior of the apply method. This turns very useful when you want to define a factory or when you want to treat an object as a primitive type.

Furthermore, objects serve the same (and some additional) purposes as the static methods and fields in Java.