Call Kotlin function in Java

Question

I am a neophite in Kotlin. I need to call a method in a class created in Kotlin from a Java class. The class in question concerns the creation of the db.

@Database(entities = arrayOf(Indirizzo::class, Dispositivo::class), version = 1, exportSchema = false)
abstract class WppDb : RoomDatabase() {
    abstract fun DispositivoDao(): DispositivoDao
    abstract fun IndirizzoDao(): IndirizzoDao

    private var INSTANCE : WppDb? = null

    fun getInstance(context: Context): WppDb? {
        if (INSTANCE == null) {
            synchronized(WppDb::class) {
                INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                            WppDb::class.java, "weather.db")
                           .build()
            }
        }
        return INSTANCE
    }

    fun destroyInstance() {
        INSTANCE = null
    }
}

I need to call the getInstance() method from a Java Activity.

Answer 1

If you want the equivalent of what Room samples usually show with a static Java field and static getter method for it, you can place those functions in the companion object of your class:

@Database(entities = arrayOf(Indirizzo::class, Dispositivo::class), version = 1, exportSchema = false)
abstract class WppDb : RoomDatabase() {
    abstract fun DispositivoDao(): DispositivoDao
    abstract fun IndirizzoDao() : IndirizzoDao

    companion object {
        private var INSTANCE : WppDb? =  null

        @JvmStatic
        fun getInstance(context: Context): WppDb? {
            if (INSTANCE == null) {
                synchronized(WppDb::class) {
                    INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                                    WppDb::class.java, "weather.db")
                                   .build()
                }
            }
            return INSTANCE
        }

        @JvmStatic
        fun destroyInstance() {
            INSTANCE = null
        }
    }
}

You can then call WppDb.getInstance(context) from either Kotlin or Java code. Note the @JvmStatic annotations which make these calls nicer in Java - without them, you'd have to use WppDb.Companion to get the companion object, and then call the getInstance function on that (so WppDb.Companion.getInstance(context) altogether).