Why Isn't Mydayforecast.map Empty?

I create a object myDayForecast with secondary construction, I think the var bb will be empty because the parmater of secondary construction which pass to main construction is Hash

Solution 1:

You're using a map to store your property data in it. This is done via by map and works for getting and setting. By assigning a value to the properties you add elements to the map. So initially your HashMap is empty, but at the end of the constructor, you've added values to it implicitly. Kotlin doesn't create another field or mechanism to store the values otherwise.

Solution 2:

In at the end if the secondary constructor definition line:

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

: this(HashMap()) is a call to the primary constructor:

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

but with an empty HashMap as the argument.

So when you make the call :

var myDayForecast=DayForecast(15L,"Desciption",10,5,"http://www.a.com",10L)

one of the first things that happens is that the primary constructor is called with the empty HashMap as noted above.

This is as if you called the primary constructor like this:

DayForcast(map = HashMap())

So now map has been set to an empty HashMap.

In the secondary constructor, each of the fields are marked with bymap, where map is a MutableMap property of DayForecast. as seen here:

classDayForecast(var map: MutableMap<String, Any?>) {
    var _id: Longby map
    var date: Longby map
    var description: String by map
    var high: Intby map
    var low: Intby map
    var iconUrl: String by map
    var cityId: Longby map 
    ...
}   

This means that any access to those fields is delegated to the object referred to by map, which in this case is a MutableMap object. For a MutableMap object this means that the compiler will translate calls such as this.date = 15L into something like this this.map.put("date", 15L) and references like blah = this.date will be translated into something like, blah = this.map.get("date")

Next, after the primary constructor has been called, the second part of the secondary constructor is run.

this.date = date
    this.description = description
    this.high = high
    this.low = low
    this.iconUrl = iconUrl
    this.cityId = cityId

Now since each of these properties are declared as var propXYZ by map each of these calls are translated into calls like this.map.put("date", date), which will fill in the initially empty HashMap with values, so that by the time you call

var bb=myDayForecast.map , map is now a filled in HashMap.

Please take a look at the delegate-properties section of the Kotlin documentation, if this is still confusing.