Kotlin ArrayList won't append object using 'add' method

Question

When trying to 'add' a custom object to an ArrayList, the ArrayList remains null

I've tried modifying the data class initialization by using MutableLists, among other things

Here is the data class that I'm using:

data class WYRStatistics(val team: String, val league: String, val gp: String, val g: String, val a: String, val pt: String, val pim: String, val pm: String)

This is where I'm using the data class to construct on object and try to add the object to an ArrayList

var n = 0
        var statsArr: ArrayList<WYRStatistics>? = null

        while (n < tempStats.length()) {

            val statObject = tempStats.getJSONObject(n)

            val singleStat = WYRStatistics(statObject.getString("Team"),
                                statObject.getString("League"),
                                statObject.getString("GamesPlayed"),
                                statObject.getString("Goals"),
                                statObject.getString("Assists"),
                                statObject.getString("Points"),
                                statObject.getString("PenaltyMinutes"),
                                statObject.getString("PlusMinus")
                            )
            println(singleStat)
            statsArr?.add(singleStat)
            println(statsArr)
            tempPlayer.stats?.add(singleStat)
            println(tempPlayer)
            n++
        }

The utilization is inside of a function that handles the asynchronous task of decoding JSON. This is all working fine.

tempStats is a JSONArray - when printed it is formatted properly

When I println singleStat, the object is printed properly

However, when I 'add' singleStat to statsArr, the statsArr returns null

Well, yes. You're explicitly initializing to null, and never assign any no null value to the variable. list?.add() means: if list if not null, then add, otherwise do nothing. Avoid nulls as much as you can. If you need a list, create a list. And make it a val, too, since it's not supposed to be reassigned, is it? — JB Nizet
– JB Nizet, Commented Jul 13, 2019 at 16:04
Does that mean you can never 'add' anything to a list that is null? — Mason Black
– Mason Black, Commented Jul 13, 2019 at 16:27
That sentence doesn't even make sense. A list can't be null. A variable can be null. If a variable's value is null, then it means that it doesn't refer to any object, so there is no list at all. You can't add anything to a list that doesn't exist. null doesn't mean "an empty list". null means "no list at all". — JB Nizet
– JB Nizet, Commented Jul 13, 2019 at 16:30

John Joe · Accepted Answer · 2019-07-13 16:08:51Z

1

You are initializing arrayList to nullas what JB mentioned.

The correct way should be

val statsArr =  arrayListOf<WYRStatistics>()

edited Jul 13, 2019 at 16:08

answered Jul 13, 2019 at 16:05

John Joe

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2 Comments

JB Nizet Over a year ago

Why make it nullable? Why use a var?

JB Nizet Over a year ago

: should be = .

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