Companion objects

Companion objects are declared within a class by utilizing the companion and object keywords. You can use functions that are static within a companion object:

class Printer {
  companion object DocumentPrinter {
    fun printDocument() = println("Document printing successful.")
  }
}

fun main(args: Array<String>) {
  Printer.printDocument() // printDocument() invoked via companion object
  Printer.Companion.printDocument() // also invokes printDocument() via 
                                    // a companion object
}

Sometimes, you may want to give an identifier to a companion object. This can be done by placing the name after the object keyword. Consider the following example:

class Printer {
  companion object DocumentPrinter { // Companion object identified by DocumentPrinter
    fun printDocument() = println("Document printing successful.")
  }
}

fun main(args: Array<String>) {
  Printer.DocumentPrinter.printDocument() // printDocument() invoked via
                                          // a named companion object
}