DatabaseClass.kt

@Database(
    entities = [User::class],
    version = 1,
    exportSchema = false
)
abstract class DatabaseClass : RoomDatabase() {
    abstract fun userDao(): UserDao
    companion object {
        private var instance: DatabaseClass? = null

        fun getDatabase(context: Context): DatabaseClass {
            if (instance == null) {
                synchronized(DatabaseClass::class.java) {}

                instance =
                    Room.databaseBuilder(context, DatabaseClass::class.java, "app.db")
                        .fallbackToDestructiveMigration()
                        .build()
            }

            return instance!!
        }
    }
}