How can I resolve the error when using Kotlin Flow and deleting data in Android RoomDB?

Question

I am getting the following error after deleting the RoomDB data:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String app.zimablue.artbookfragmentversion.model.ArtEntity.getArtName()' on a null object reference

I think I am getting this error because I am trying to access the deleted data again, because I can delete the data, but I get an error because there is no data that will appear on the screen after it is deleted.

@Dao
interface ArtDao {

    @Insert
    suspend fun insert(artEntity: ArtEntity)

    @Delete
    suspend fun delete(artEntity: ArtEntity)

    @Query("SELECT * FROM `art-table`")
    fun getArtWithNameAndId():Flow<List<ArtEntity>>

    @Query("SELECT * FROM `art-table` WHERE id = :id")
    fun getArtById(id: Int): Flow<ArtEntity>

@Entity(tableName = "art-table")
class ArtEntity(

    @ColumnInfo(name = "name")
    @NonNull var artName : String,

    @NonNull @ColumnInfo(name = "artistname")
    var artistName: String,

    @NonNull @ColumnInfo(name = "year")
    var year : String,

    @NonNull @ColumnInfo(name = "image")
    var image : ByteArray

) {
    @PrimaryKey(autoGenerate = true)
    var id : Int = 0
}

the line where the error is pointed.

private fun oldArtDetails(art : ArtEntity){
        artFromMain = art




        art.artName.let {
            binding.artText.setText(it)
        }
        art.artistName.let {
            binding.artistText.setText(it)
        }
        art.year.let {
            binding.yearText.setText(it)
        }
        art.image.let {
            val bitmap = it.let {
                BitmapFactory.decodeByteArray(it,0, it.size) }
            binding.imageView.setImageBitmap(bitmap)
        }


    }

I have uploaded the version of the application where the error is occurring to Github: https://github.com/mertparlak-zima/ArtBookFragment

Translation: I checked if "art.name" and "art.artistName" are null, but it didn't work. I made it nullable in the database, but it still didn't work.

Answer 1

In your ArtDao return type is set to non-nullable, so dao returns ArtEntity that actually null but it can't be checked that it is not null.

Solution:

@Query("SELECT * FROM `art-table` WHERE id = :id")
fun getArtById(id: Int): Flow<ArtEntity?>