Kotlin Android Room 错误：枚举列表无法识别 TypeConverter

Room库无法识别我为List枚举创建的TypeConverter.但是，当我将其更改为ArrayList个枚举时，它工作得很好.有没有人知道为什么，我能做些什么来让List起作用？(使用Kotlin中的List更容易，我真的不想仅仅因为这个而来回转换到ArrayList).

Here is my code:

My model:

@Entity
data class Example(@PrimaryKey val id: String?,
                   val name: String,
                   var days: List<DayOfWeek>?)

DayOfWeek is an enum:

enum class DayOfWeek {

    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY;

    val value: Int
        get() = ordinal + 1


    companion object {

        private val ENUMS = DayOfWeek.values()

        fun of(dayOfWeek: Int): DayOfWeek {
            if (dayOfWeek < 1 || dayOfWeek > 7) {
                throw RuntimeException("Invalid value for DayOfWeek: " + dayOfWeek)
            }

            return ENUMS[dayOfWeek - 1]
        }

    }

}

我的TypeConverter:

private const val SEPARATOR = ","

class DayOfWeekConverter {

    @TypeConverter
    fun daysOfWeekToString(daysOfWeek: List<DayOfWeek>?): String? {
        return daysOfWeek?.map { it.value }?.joinToString(separator = SEPARATOR)
    }

    @TypeConverter
    fun stringToDaysOfWeek(daysOfWeek: String?): List<DayOfWeek>? {
        return daysOfWeek?.split(SEPARATOR)?.map { DayOfWeek.of(it.toInt()) }
    }

}

And I set it in my DB class like this:

@Database(entities = arrayOf(Example::class), version = 1)
@TypeConverters(DayOfWeekConverter::class)
abstract class AppDatabase : RoomDatabase() {

    abstract fun exampleDao(): ExampleDao

}

我的刀是这样的:

@Dao
interface ExampleDao {

    @Query("SELECT * FROM example")
    fun getAll(): LiveData<List<Example>>

    @Insert(onConflict = REPLACE)
    fun save(examples: List<Example>)

}

我使用此代码得到的错误是:

error: Cannot figure out how to save this field into database. You can consider adding a type converter for it.
e: 

e:     private java.util.List<? extends com.example.DayOfWeek> days;

就像我上面说的，如果我将days属性更改为ArrayList<DayOfWeek>(并在DayOfWeekConverter中更改为ArrayList)，那么一切正常.如果有人能帮我解决这个问题，并告诉我如何在这里使用List，这将是非常有帮助的，它让我发疯:/.