Variable in base class is null when initialized from derived class but called from super

Question

I'm having trouble converting an inherited singleton pattern from Java to Kotlin.

open class ThumbnailManager {

   lateinit var data: Data

   companion object{
       fun getInstance(): ThumbnailManager{
           return ThumbnailManager()
       }
     }
     open fun setContent()
     open fun setData(){
         data = ...// data is indeed set 
     }
}

class ThumbnailManagerLite : ThumbnailManager(){

   companion object{
       fun getInstance(): ThumbnailManagerLite{
           return ThumbnailManagerLite()
       }
   }

   override fun setContent(){
       setData()
   }
}

From one of the main classes (not lite) I call ThumbnailManager.getInstance().gameData and gameData is null, even though it was definitely initialized through ThumbnailManagerLite. I notice that when I call ThumbnailManagerLite.getInstance() there is no call through to super.

you can use an object keyword instead of class for singleton in kotlin — Shalu T D, Jul 07 '20 at 10:06

Sergio · Answer 1 · 2020-07-07T11:42:37.923

2

Every time you call ThumbnailManager.getInstance() you create a new instance of ThumbnailManager. In Kotlin you can use object keyword to implement a Singleton pattern:

object ThumbnailManager {
    
    lateinit var data: Data

    fun setContent() { 
    }

    fun setData() {
    }
}

But it doesn't support inheritance.

You can try to use Delegation instead of inheritance in singleton:

interface IThumbnailManager {
    fun setContent()
    fun setData()
}

object ThumbnailManager : IThumbnailManager {

    lateinit var data: Data


    override fun setContent() {
    }

    override fun setData(){
    }
}

object ThumbnailManagerLite : IThumbnailManager by ThumbnailManager {

    override fun setContent(){
        setData()
    }
}

edited Jul 07 '20 at 11:42

answered Jul 07 '20 at 10:50

Sergio

27,326
8
128
149

Usually Singletons are not inheritable. – Sergio Jul 07 '20 at 11:27
Please check my edited answer. I've added information about Delegation instead of inheritance in singleton. It may help. – Sergio Jul 07 '20 at 11:43

Variable in base class is null when initialized from derived class but called from super

1 Answers1