如何为属性为val类型的Kotlin data class实例正确赋值

Kotlin中属性为val的不可变data class，不能在实例创建后修改属性值，符合规范的实现方式有两种：

方案一：先收集所有参数，一次性构造实例（推荐） #

不可变对象的最佳实践是创建时就确定所有属性值，你可以先把各时段的Time、date值存在临时变量里，遍历完所有时段数据后再直接构造Day实例，完全不需要修改属性：

val days = ArrayList<Day>()
val listOfDays = this
    .groupBy { it.getDayOfMonth() }
    .map { WeatherForDay(it.value) }
    .take(FIVE_DAYS_FORECAST)

listOfDays.forEach { weatherForDay ->
    // 临时变量暂存各时段数据
    var date: Int = 0
    var night: Time? = null
    var morning: Time? = null
    var noon: Time? = null
    var evening: Time? = null

    weatherForDay.weather.forEach { weather ->
        val time = Time(
            weather.main.temp,
            weather.main.feels_like,
            weather.wind.speed,
            weather.wind.gust,
            weather.wind.deg,
            weather.main.pressure,
        )
        // 匹配时段赋值给临时变量
        when (weather.getTime()) {
            NIGHT_DAYTIME -> night = time
            MORNING_DAYTIME -> morning = time
            DAY_DAYTIME -> noon = time
            EVENING_DAYTIME -> evening = time
        }
        date = weather.dt
    }
    // 所有数据收集完成后一次性构造Day实例
    days.add(Day(date, night, morning, noon, evening))
}

方案二：使用data class自带的copy方法分步构建 #

如果业务场景确实需要分步修改属性，可以用data class默认生成的copy方法，该方法会返回一个修改了指定属性的新实例，原实例保持不变：

listOfDays.forEach {
    var day = Day()
    it.weather.forEach { weather ->
        val time = Time(
            weather.main.temp,
            weather.main.feels_like,
            weather.wind.speed,
            weather.wind.gust,
            weather.wind.deg,
            weather.main.pressure,
        )
        // 每次修改都生成新的Day实例
        day = when (weather.getTime()) {
            NIGHT_DAYTIME -> day.copy(night = time)
            MORNING_DAYTIME -> day.copy(morning = time)
            DAY_DAYTIME -> day.copy(noon = time)
            EVENING_DAYTIME -> day.copy(evening = time)
            else -> day
        }
        day = day.copy(date = weather.dt)
    }
    days.add(day)
}

该方案会生成多个中间实例，适合属性少、修改次数少的场景，性能不如一次性构造方案。

内容的提问来源于stack exchange，提问作者SoulReaver313