Sorting custom object List in kotlin

Question

interface Feed

class Memo : Feed {
    var memoId: String? = null
    var updateTime: String? = null
//    other fileds
}

class Activity : Feed {
    var activityId: String? = null
    var updateTime: String? = null
//    other fileds
}

As above code, I have Created one interface and extend it in the other two classes.

   var mainFeedList: ArrayList<Feed> = ArrayList()
   var memoList: ArrayList<Memo> = ArrayList()
   var ActivityList: ArrayList<Activity> = ArrayList()

    fun arrangeFeedDataOrder() {
   
        mainFeedList.addAll(memoList)
        mainFeedList.addAll(ActivityList)

        mainFeedList.sortedBy {
            when (it) {
                is Activity -> {
                    it.updateTime
                }
                is Memo -> {
                    it.updateTime
                }
                else -> {
                    null
                }
            }
        }
//        Not work above code for sorting
    }

I have the main list which contains memos and activity data. I want to sort this list with updateTime filed. I used sortedBy function but it does not work in my case. Anyone has an idea how to sort the list?

Why doesn't sortedBy work for you? You're not actually assigning the result to anything in your code, it's not an in-place sort. — Henry Twist
– Henry Twist, Commented Apr 16, 2021 at 15:34

cactustictacs · Accepted Answer · 2021-04-16 15:36:20Z

4

You want sortBy (sorts the list in place) instead of sortedBy (returns a sorted copy, which you're basically discarding)

Also, why not put updateTime in the interface if it's something common to all Feeds? That way you don't need to do any type checking - you have a list of Feeds, they have an updateTime, so you can just sort by it.updateTime on all of them:

interface Feed {
    var updateTime: String?
}

class Memo : Feed {
    var memoId: String? = null
    override var updateTime: String? = null
}

class Activity : Feed {
    var activityId: String? = null
    override var updateTime: String? = null
}

...

mainFeedList.sortBy { it.updateTime } // or sortBy(Feed::updateTime)

or if you have a superclass for those classes, you could just define the property in there with the default null value (can't do that in an interface, which is why you need to override and provide a value in each class that implements it)

answered Apr 16, 2021 at 15:36

cactustictacs

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5 Comments

Tenfour04 Over a year ago

You can't sort by a nullable.

cactustictacs Over a year ago

@Tenfour04 You can? Nulls end up first in the list - pl.kotl.in/SKDs9HlkB

Tenfour04 Over a year ago

Wow, I never realized. Never assume.

cactustictacs Over a year ago

Kotlin is doing good work rehabilitating the humble null!

anoop ghildiyal Over a year ago

@cactustictacs Thanks to kotlin.collections indeed... hahahaha the true life saver while working with lot's of data manipulation.

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