Why does a lambda expression need a explicity return in Android Kotlin?

Question

I'm learning Room with the sample code RoomWordsSample at https://github.com/googlecodelabs/android-room-with-a-view/tree/kotlin.

The Code A is from WordRoomDatabase.kt

1: I can't understand why the author need to add the code INSTANCE = instance, so I remove it and Code B works well, can you tell me why?

2: In my mind, the latest expression in a lambda expression will return result, the val instance = Room.databaseBuilder( ... is latest expression in Code C, but Code C cann't pass compile, why?

Code A

@Database(entities = [Word::class], version = 1, exportSchema = false)
abstract class WordRoomDatabase : RoomDatabase() {

    abstract fun wordDao(): WordDao

    companion object {
        @Volatile
        private var INSTANCE: WordRoomDatabase? = null

        fun getDatabase(
                context: Context,
                scope: CoroutineScope
        ): WordRoomDatabase {           
            return INSTANCE ?: synchronized(this) {
                val instance = Room.databaseBuilder(
                        context.applicationContext,
                        WordRoomDatabase::class.java,
                        "word_database"
                )
                        .addCallback(WordDatabaseCallback(scope))
                        .build()

                INSTANCE = instance                
                instance
            }
        }
}

Code B

@Database(entities = [Word::class], version = 1, exportSchema = false)
abstract class WordRoomDatabase : RoomDatabase() {

    abstract fun wordDao(): WordDao

    companion object {
        @Volatile
        private var INSTANCE: WordRoomDatabase? = null

        fun getDatabase(
                context: Context,
                scope: CoroutineScope
        ): WordRoomDatabase {           
            return INSTANCE ?: synchronized(this) {
                val instance = Room.databaseBuilder(
                        context.applicationContext,
                        WordRoomDatabase::class.java,
                        "word_database"
                )
                        .addCallback(WordDatabaseCallback(scope))
                        .build()                        
                instance
            }
        }
}

Code C

@Database(entities = [Word::class], version = 1, exportSchema = false)
abstract class WordRoomDatabase : RoomDatabase() {

    abstract fun wordDao(): WordDao

    companion object {
        @Volatile
        private var INSTANCE: WordRoomDatabase? = null

        fun getDatabase(
                context: Context,
                scope: CoroutineScope
        ): WordRoomDatabase {           
            return INSTANCE ?: synchronized(this) {
                val instance = Room.databaseBuilder(
                        context.applicationContext,
                        WordRoomDatabase::class.java,
                        "word_database"
                )
                        .addCallback(WordDatabaseCallback(scope))
                        .build()                       

            }
        }
}

INSTANCE is a Singleton so it needs to be set the first time you instanciate it, that's why the author sets it INSTANCE = instance. The next time you call it, you will get the same instance instead of creating a new one. — Glenn Sandoval
– Glenn Sandoval, Commented Mar 11, 2020 at 3:23

Yrii Borodkin · Accepted Answer · 2020-03-11 03:40:22Z

5

Code A is implementation of Singleton pattern. It's aim to have only 1 instance of a class. So, Code A checks if INSTANCE is null (was class instance created or not), and if not, it creates instance of WordRoomDatabase class and assigns it to INSTANCE variable. Subsequent calls to getDatabase will return INSTANCE, and not create new WordRoomDatabase each time.

Code B breaks this pattern, not saving instance to static INSTANCE variable. Code will work, but for each subsequent call to getDatabase new instance of WordRoomDatabase will be created.

Code C not working, because getDatabase method must return WordRoomDatabase instance, but it returns nothing (assignments are not expressions in Kotlin)

edited Mar 11, 2020 at 3:40

answered Mar 11, 2020 at 3:34

Yrii Borodkin

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