The object Keyword

Most likely, there have been developers over the years who have wondered why object-oriented programming languages do not have any sort of object keyword.

The designers of Kotlin may have been among those developers, as they have given Kotlin such an object keyword. This gets used in a few different places in Kotlin syntax

Singletons

One way to declare singletons in Kotlin is to use a top-level val declaration:

class Transmogrifier {
  fun transmogrify() {
    println("Presto, change-o!")
  }
}

val QUASI_SINGLETON = Transmogrifier()

fun main() {
  QUASI_SINGLETON.transmogrify()
}

In truth, though, we can have as many instances of Transmogrifier as we want. QUASI_SINGLETON is merely a global instance of Transmogrifier. We cannot prevent others from making their own Transmogrifier instances, as then we could not create the quasi-singleton. For example, we cannot make the constructor be private, like this:

class Transmogrifier private constructor() {
  fun transmogrify() {
    // TODO
  }
}

val QUASI_SINGLETON = Transmogrifier()

QUASI_SINGLETON.transmogrify()

as that results in a compile error:

error: cannot access '<init>': it is private in 'Transmogrifier'
val QUASI_SINGLETON = Transmogrifier()
                      ^

To create a true singleton, all we need to do is:

• Replace the class with object
• Call functions on that object based on its name, rather than creating some instance ourselves

object Transmogrifier {
  fun transmogrify() {
    println("Presto, change-o!")
  }
}

fun main() {
  Transmogrifier.transmogrify()
}

An object declared this way can have properties and functions like a regular class. It cannot have a constructor, which makes sense, since there will only ever be one instance. However, if it extends classes, it can (and must) call the superclass constructors:

open class Base {
  // TODO stuff here
}

object Transmogrifier : Base() {
  fun transmogrify() {
    println("Presto, change-o!")
  }
}

fun main() {
  Transmogrifier.transmogrify()
}

As with singletons in any programming environment, though, the ones that we declare via object will live for the life of our process, so be careful that you are not creating a memory leak by having an object hold onto more and more stuff over time.