CODEWITHSUNDEEP
javaandroidkotlin
Mohsen
Mohsen

Reputation: 1389

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?

Upvotes: 0

Views: 1001

Answers (1)

Sergio
Sergio

Reputation: 30655

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
    }
}

Upvotes: 1

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