Companion objects

Objects declared inside a class/interface can be marked as companion objects. Observe the use of companion objects in the following code:

class Cupcake(flavour: String) : BakeryGood(flavour), Bakeable {
  override fun name(): String { 
    return "cupcake" 
  } 
  
  companion object { 
    fun almond(): Cupcake { 
      return Cupcake("almond") 
    } 

    fun cheese(): Cupcake { 
      return Cupcake("cheese") 
    } 
  } 
}

Now, methods inside the companion object can be used directly, using the class name without instantiating it:

fun main(args: Array<String>) {
    val myBlueberryCupcake: BakeryGood = Cupcake("Blueberry")
    val myAlmondCupcake = Cupcake.almond()
    val myCheeseCupcake = Cupcake.cheese()
    val myCaramelCupcake = Cupcake("Caramel")
}

Companion object's methods can't be used from instances:

fun main(args: Array<String>) {
    val myAlmondCupcake = Cupcake.almond()
    val myCheeseCupcake = myAlmondCupcake.cheese() //Compilation error: Unresolved reference: cheese
}

Companion objects can be used outside the class as values with the name Companion:

fun main(args: Array<String>) {
    val factory: Cupcake.Companion = Cupcake.Companion
}

Alternatively, a Companion object can have a name:

class Cupcake(flavour: String) : BakeryGood(flavour), Bakeable {
    override fun name(): String {
        return "cupcake"
    }

    companion object Factory {
        fun almond(): Cupcake {
            return Cupcake("almond")
        }

        fun cheese(): Cupcake {
            return Cupcake("cheese")
        }
    }
}

fun main(args: Array<String>) {
    val factory: Cupcake.Factory = Cupcake.Factory
}

They can also be used without a name, as shown in the following code:

fun main(args: Array<String>) {
    val factory: Cupcake.Factory = Cupcake
}

Don't be confused by this syntax. The Cupcake value without parenthesis is the companion object; Cupcake() is an instance.