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

# android

[Ellen Spertus]

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

[tseisel]

I think you meant:

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

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

[Ellen Spertus]

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?

[Ellen Spertus]

Thank you, @tseisel. That was it.

[tseisel]

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.

[Ellen Spertus]

@tseisel Why doesn’t ContactDatabase.Callback exist, when ContactDatabase extends RoomDatabase?

[tseisel]

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

[Ellen Spertus]

Thank you!