How can I make this singleton simpler in Kotlin?

Question

How can I make this singleton simpler in Kotlin for the Android room database initialization?

@Database(entities = arrayOf(Book::class, User::class), version = 1)
abstract class AppDatabase : RoomDatabase() {

    abstract fun bookModel() : BookDao
    abstract fun userModel() : UserDao

    companion object {
        private var INSTANCE: AppDatabase? = null

        fun getInMemoryDatabase(context: Context): AppDatabase {
            if (INSTANCE == null) {
                INSTANCE = Room.inMemoryDatabaseBuilder(context.applicationContext, AppDatabase::class.java).build()
            }
            return INSTANCE!!
        }

        fun destroyInstance() {
            INSTANCE = null
        }
    }
}

Answer 1

You can use an array literal ([]) instead of arrayOf and you can use the elvis operator for the null check. See here.

@Database(entities = [Book::class, User::class], version = 1)
abstract class AppDatabase : RoomDatabase() {

    abstract fun bookModel() : BookDao
    abstract fun userModel() : UserDao

    companion object {
        private var INSTANCE: AppDatabase? = null

        fun getInMemoryDatabase(context: Context): AppDatabase { 
            INSTANCE = INSTANCE ?: Room.inMemoryDatabaseBuilder(context.applicationContext, AppDatabase::class.java).build()
            return INSTANCE!!
        }
    
        fun destroyInstance() {
            INSTANCE = null
        }
    }
}

Since you need the instance you have to save it somewhere, using a companion object seems like a reasonable solution to me.

If you somehow don't want to save the intance inside AppDatabase, you can also use an object (which is a singleton in Kotlin).

object AppDatabaseProvider {
    private var INSTANCE: AppDatabase? = null

    fun getInMemoryDatabase(context: Context): AppDatabase {
       // ...
    }

    fun destroyInstance() {
        INSTANCE = null
    }
}

These are both options to deal with static data in Kotlin, but you won't get it much shorter than that.