I am adapting the Android Room with a View tutorial to my domain. As shown in the picture, I get an

Unresolved reference: Callback

error in the expression

ContactDatabase.Callback

. Why? As shown,

ContactDatabase

extends

RoomDatabase

, and

RoomDatabase.Callback

is defined.

@Database(entities = arrayOf(ContactEntity::class), version = 1)
abstract class ContactDatabase : RoomDatabase() {
    abstract fun contactDao(): ContactDao
    lateinit var callback: RoomDatabase.Callback

    private class ContactDatabaseCallback(
        private val scope: CoroutineScope
    ) : ContactDatabase.Callback() {  <-- Error on this line

I think you meant:

abstract class ContactDatabase : RoomDatabase() {
  [...]

  private class ContactDatabaseCallback(
    private val scope: CoroutineScope
  ) : RoomDatabase.Callback() { <-- Difference is here
  }
}

I discovered that removing the outer class name from the line with the error eliminates the error:

@Database(entities = arrayOf(ContactEntity::class), version = 1)
abstract class ContactDatabase : RoomDatabase() {
    abstract fun contactDao(): ContactDao
    lateinit var callback: RoomDatabase.Callback
    private class ContactDatabaseCallback(
        private val scope: CoroutineScope
    ) : Callback() { <-- No error here

How come?

For your 2nd question, because

ContactDatabase

extends from

RoomDatabase

, it can access its inner class

RoomDatabase.Callback

without specifying the full qualifier. That's a rather odd rule, probably inherited from Java.

@tseisel Why doesn’t

ContactDatabase.Callback

exist, when

ContactDatabase

extends

RoomDatabase

?

Inner classes definitions are not inherited. You have to define yours in `ContactDatabase

Thank you!