Singleton with parameter in Kotlin

Issue

I am trying to convert an Android app from Java to Kotlin. There are a few singletons in the app. I used a companion object for the singletons without constructor parameters. There is another singleton that takes a constructor parameter.

Java code:

public class TasksLocalDataSource implements TasksDataSource {

    private static TasksLocalDataSource INSTANCE;

    private TasksDbHelper mDbHelper;

    // Prevent direct instantiation.
    private TasksLocalDataSource(@NonNull Context context) {
        checkNotNull(context);
        mDbHelper = new TasksDbHelper(context);
    }

    public static TasksLocalDataSource getInstance(@NonNull Context context) {
        if (INSTANCE == null) {
            INSTANCE = new TasksLocalDataSource(context);
        }
        return INSTANCE;
    }
}

My solution in kotlin:

class TasksLocalDataSource private constructor(context: Context) : TasksDataSource {

    private val mDbHelper: TasksDbHelper

    init {
        checkNotNull(context)
        mDbHelper = TasksDbHelper(context)
    }

    companion object {
        lateinit var INSTANCE: TasksLocalDataSource
        private val initialized = AtomicBoolean()

        fun getInstance(context: Context) : TasksLocalDataSource {
            if(initialized.getAndSet(true)) {
                INSTANCE = TasksLocalDataSource(context)
            }
            return INSTANCE
        }
    }
}

Am I missing anything? Thread safety? Laziness ?

There were a few similar questions but I don't like the answers :)

Solution

Here's a neat alternative from Google's architecture components sample code, which uses the also function:

class UsersDatabase : RoomDatabase() {

    companion object {

        @Volatile private var INSTANCE: UsersDatabase? = null

        fun getInstance(context: Context): UsersDatabase =
            INSTANCE ?: synchronized(this) {
                INSTANCE ?: buildDatabase(context).also { INSTANCE = it }
            }

        private fun buildDatabase(context: Context) =
            Room.databaseBuilder(context.applicationContext,
                    UsersDatabase::class.java, "Sample.db")
                    .build()
    }
}

Answered By - fal