Android Room错误：无法识别枚举列表的TypeConverter

这是我的代码：

我的模特：

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

DayOfWeek是一个枚举：

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()) }
    }

}

我将它设置在我的DB类中,如下所示：

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

    abstract fun exampleDao(): ExampleDao

}

我的DAO看起来像这样：

@Dao
interface ExampleDao {

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

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

}

我用这段代码得到的错误是：

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中对ArrayList进行更改)然后一切正常.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它会有很大的帮助,它让我发疯：/.

解决方法

@Entity
data class Example(@primaryKey val id: String,var days: MutableList<DayOfWeek>?)

class DayOfWeekConverter {
    companion object {

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

        @TypeConverter
        @JvmStatic
        fun StringToDaysOfWeek(daysOfWeek: String?): MutableList<DayOfWeek>? =
                daysOfWeek?.split(SEPARATOR)?.map { DayOfWeek.of(it.toInt()) }?.toMutableList()
    }
}