java - HashMap(它)会做什么？

Question

我正在学习《Kotlin for androiddevelopments》的示例代码，地址:https://github.com/antoniolg/Kotlin-for-Android-Developers

代码中.parseList { DayForecast(HashMap(it)) } ，我不明白函数 HashMap(it) 会做什么。 HashMap() 是一个类并接受参数 it ？

还有，我认为类DayForecast(...)..的完整代码是代码A，对吗？

同样，如果我创建一个对象 var myDayForecast=DayForecast(10L,"Desciption",10,5,"http://www.a.com",10L) ，myDayForecast.map 会是空的吗？

代码A

class DayForecast(var map: MutableMap<String, Any?>) {
    var _id: Long by map
    var date: Long by map
    var description: String by map
    var high: Int by map
    var low: Int by map
    var iconUrl: String by map
    var cityId: Long by map

    constructor(date: Long, description: String, high: Int, low: Int, iconUrl: String, cityId: Long)
            : this(map: MutableMap<String, Any?>=HashMap()) {
        this.date = date
        this.description = description
        this.high = high
        this.low = low
        this.iconUrl = iconUrl
        this.cityId = cityId
    }
}

原始代码

override fun requestForecastByZipCode(zipCode: Long, date: Long) = forecastDbHelper.use {

        val dailyRequest = "${DayForecastTable.CITY_ID} = ? AND ${DayForecastTable.DATE} >= ?"
        val dailyForecast = select(DayForecastTable.NAME)
                .whereSimple(dailyRequest, zipCode.toString(), date.toString())
                .parseList { DayForecast(HashMap(it)) }

        val city = select(CityForecastTable.NAME)
                .whereSimple("${CityForecastTable.ID} = ?", zipCode.toString())
                .parseOpt { CityForecast(HashMap(it), dailyForecast) }

        city?.let { dataMapper.convertToDomain(it) }
}


class DayForecast(var map: MutableMap<String, Any?>) {
    var _id: Long by map
    var date: Long by map
    var description: String by map
    var high: Int by map
    var low: Int by map
    var iconUrl: String by map
    var cityId: Long by map

    constructor(date: Long, description: String, high: Int, low: Int, iconUrl: String, cityId: Long)
            : this(HashMap()) {
        this.date = date
        this.description = description
        this.high = high
        this.low = low
        this.iconUrl = iconUrl
        this.cityId = cityId
    }
}

Answer 1

我认为从答案here ，你应该已经明白 parseList 提供了闭包 { DayForecast(HashMap(it)) }与 Map 对象。

但是正如您现在显示的定义 DayForecast

class DayForecast(var map: MutableMap<String, Any?>)

需要 MutableMap

MutableMap 之间的主要区别和一个 Map那是Map无法更改，而 MutableMap可以改变。

原因DayForecast需要 MutableMap是因为在辅助构造函数中传入了对象，称为map被改变(突变)。这是您最近的其他部分question .

HashMap是 MutableMap 的基于哈希表的实现接口(interface)，因此可以使用 MutableMap预计。

总结一下:

DayForecast() 期望 MutableMap对象传递给它的主构造函数，但 parseList 仅提供 Map到它收到的闭包，所以解决方案是插入 HashMap()创建 MutableMap .

这也应该回答您在评论中提出的问题，为什么 parseList 不能只采用闭包 { DayForecast(it) }而不是{ DayForecast(HashMap(it)) } ？这是因为如上面所示，DayForecast() 的构造函数需要 MutableMap ，其中it不是(它是 Map )而 HashMap(it)是 MutableMap .