Singleton in Kotlin - Companion Object and Operator fun invoke

Java Way : 

public class Singleton {

    private static Singleton INSTANCE = null;

    // other instance variables can be here

    private Singleton() {};

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

    // other instance methods can follow
}

2.

class Manager private constructor() {

// We do not have static keyword in Kotlin. 

// We use Companion Object in Kotlin which is a replacement of static

    companion object {

        private var instance : Manager? = null

        // Already instantiated? - return the instance
        // Otherwise instantiate in a thread-safe manner
        fun getInstance() = synchronized(this) { // This makes Thread safe

            if(instance == null)

                instance = Manager()

            instance

        }
    }
}

fun main() {

    val m = Manager() -> This cannot be accessed here

    Manager.getInstance()

    println(Manager.getInstance())

    println(Manager.getInstance())
    println(Manager.getInstance()) // For all the 3 instances called, only 1 instance is created

}

2.

class Manager private constructor() {
// We do not have static keyword in Kotlin. 
// We use Companion Object in Kotlin which is a replacement of static
    companion object {
        private var instance : Manager? = null

        // Already instantiated? - return the instance
        // Otherwise instantiate in a thread-safe manner
        operator fun invoke() = synchronized(this) { // This makes Thread safe
            if(instance == null)
                instance = Manager()
            instance
        }
    }
}
fun main() {
    Manager() => Manager.invoke() // For all the 3 instances called, only 1 instance is created
}

Kotlin Way :
object Manager {
    init{
        println("Manager Block Initialised")
    }
}

fun main() {
    println(Manager)
    println(Manager)
}