Kotlin Object class instance get()

Question

public class Singleton {
    private static Singleton instance = null;
    private Singleton(){
    }

    public synchronized static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        doSomeInitializationIfRequired();
        return instance;
    }
}

here getInstance() method is called whenever we request for the Instance where we can add code if we want to do something when the instance is called from any where everytime.

is there some way to override instance variable get() like this with Kotlin Objects

e.g.

object SomeSingleton {
    get() = {
        doSomeInitializationIfRequired()
    }
}

I know i can write

init {
}

but that will be called only once.

Answer 1

I wouldn't use object for this type of singleton. You can do it more like the Java way.

class Singleton private constructor() {

    companion object {
        private var _instance = Singleton()
        val instance: Singleton 
            get() = synchronized(this) {
                doSomeInitializationIfRequired()
                _instance
            }
    }

}

If you need parameters for the constructor or for doSomeInitializationIfRequired(), you can replace val instance with a getInstance function. This is common on Android where your singleton might need a reference to the Application instance.

class Singleton private constructor(val application: Application) {

    companion object {
        private val instance: Singleton? = null

        fun getInstance(application: Application): Singleton = synchronized(this) {
            val value = instance ?: Singleton(application).also { instance = it }
            doSomeInitializationIfRequired(application)
            value
        }
    }

}