Why use companion object to make singleton in Kotlin?

Question

I'm studying creation pattern(Singleton pattern) in android kotlin.

I have question about difference in making singleton object by companion object and object.

In some example, singletons are made like this.

class ABC {
  companion object {
    private var sInstance: ABC? = null
    
    fun getInstance(): ABC {
        if (sInstance == null) sInstance = ABC()
        return sInstance ?: throw IllegalStateException("")
    }
  }
}

but with above method,

// a and b are not same object
val a = ABC()
val b = ABC.getInstance()

println(a == b) // false

but what I know, singleton in kotlin is just object.

So, my question is "Why, When use companion object to make singleton object"

See the first few sentences of [this answer](https://stackoverflow.com/a/51835156/2550406). Does that answer your question? — lucidbrot, Jan 16 '22 at 10:07

broot · Answer 1 · 2022-01-16T10:22:05.140

getInstance() is just a "traditional" way of creating singletons. It is a common technique in e.g. Java, because contrary to Kotlin, Java doesn't have singletons provided by the language itself. For this reason you will see getInstance() from time to time in Kotlin as well. It may be the code ported from Java, it may be developed by a person who has more experience with Java, so they don't know there is a better alternative in Kotlin, etc.

Additionally, object is pretty much static. If we have more complicated logic for creating a singleton, it may be required to create it by some kind of factory (e.g. companion object).

So I would say, the rule of thumb is to use object as a default and only if this is not possible, create it manually. And in the latter case, it is more "Kotlin-ish" to use a property, not a getInstance() function.

Why use companion object to make singleton in Kotlin?

1 Answers1