Kotlin Programming: The Big Nerd Ranch Guide, First Edition

Delaying Initialization

Wherever it is declared, a class property must be initialized when the class instance is constructed. This rule is an important part of Kotlin’s null safety system, because it means that all non-nullable properties of a class are initialized with a non-null value when the constructor for that class is called. When you instantiate an object, you can immediately reference any property on that object, from within or outside of the class.

Despite its importance, you can bend this rule. Why would you? You do not always have control over how or when a constructor is called. One such case is in the Android framework.

Late initialization

On Android, a class called Activity represents a screen in your application. ...