Static Factory Method in Kotlin

We've already discussed the object keyword earlier in the Singleton section. Now we'll see another use of it is a companion object.

In Java, Static Factory Methods are declared static. But in Kotlin, there's no such keyword. Instead, methods that don't belong to an instance of a class can be declared inside a companion object:

class NumberMaster {
    companion object {
        fun valueOf(hopefullyNumber: String) : Long {
            return hopefullyNumber.toLong()
        }
    }
}

Companion objects may have a name: companion object Parser, for example. But this is only for clarity of what the goal of this object is.

Calling a companion object doesn't require instantiating a class:

println(NumberMaster.valueOf("123")) // Prints 123

Moreover, calling it on an instance of a class simply won't work, unlike Java:

println(NumberMaster().valueOf("123")) // Won't compile

The class may have only one companion object.