Hands-On Object-Oriented Programming with Kotlin

Singleton

The singleton pattern is used when we want to be sure that only one instance of a certain class is ever created. According to this pattern, a class is responsible for providing a reference to the same underlying instance. The following example shows the easiest way to implement the singleton pattern in Java:

public class Singleton {

     private Singleton() {}     private static final Singleton INSTANCE = new Singleton();

     public static Singleton getInstance() {
        return INSTANCE;
     }
}

However, Kotlin supports the concept of object declaration. For instance, we need to create an object that represents a user who is currently logged in:

object User {    var firstName: String? = null    var lastName: String? = null}

The version of this code that is ...