How to have a class which can only be instantiated by another specific class
I want to have a class that can only be instantiated by another class. I know I have to make the constructor private otherwise everyone can instantiate it.
class Root private constructor() { } class RootMaker: Application() { fun createRoot() = Root() } // but this doesn't work because Root's constructor is private one workaround is to make the maker class, the inner class of the Root class.
class Root private constructor() { class RootMaker: Application() { fun createRoot() = Root() } } but I really don’t want to do this because the maker class is my application class in android. so what is the better way?
If you want only one instance of an object you can use object keyword in Kotlin. It implements Singleton pattern:
class App : Application { val root = Root } object Root { fun createObject(): Any {} } Now we can access to one instance of Root class either by a property in App class or via Root class: Root.createObject()
UPDATE:
To implement a singleton that only one specific class has access to, we can use an interface and hide its implementation in that specific class (the maker class):
interface IRoot { // ... methods of creation different objects for dependency injection } class App : Application { val root: IRoot = Root // hide implementation of `IRoot` interface in `App` class private object Root : IRoot { // ... implementation of methods of creation different objects for dependency injection } } 
