Create Kotlin Singletons using delegated properties

A typical java-like singleton would look like this in Kotlin:

class Singleton private constructor() {

     private object SingletonHolder {
            val INSTANCE = Singleton()
     }

    companion object {
            val instance : Singleton by lazy {
                        SingletonHolder.INSTANCE
            }
    }

}

The above code:

marks the constructor as private so you cannot create a new "Singleton" instance directly
contains a private object which can access the private constructor and so holds an instance of Singleton()
contains a companion object to return the instance

The important note here is the "by lazy" delegate property. by lazy is thread safe and hence avoids the double check code we normally use in Java to create a singleton. In addition, the companion object is itself a singleton, so we can shorten the above to:

class Singleton private constructor() {

    companion object {
            val instance : Singleton by lazy {
                        Singleton()
            }
    }

}

We simply got rid of the private SingletonHolder object, Furthermore, Kotlin shorthands all this to a single line:

object Singleton

dvas0004 / NerdNotes

Create Kotlin Singletons using delegated properties #106