Sort list in kotlin by variable property given as parameter

Question

I know it's possible to sort a list of person by a property using following syntax:

persons = persons.sortedBy { person: Person-> person.name }

Now i want to write a sort function which accepts the property by which i want the list to be sorted by as an parameter.

In JavaScript i would write something like this:

fun sort(property: String) { 
        persons = persons.sortedBy { person: Person-> person[property]} 
}

But i cannot write "person[property]" in Kotlin.

So how can i achieve such a sorting function in Kotlin?

CommonsWare · Accepted Answer · 2020-08-29 17:27:27Z

4

Now i want to write a sort function which accepts the property by which i want the list to be sorted by as an parameter.

Let's hope that by "the property", you can settle for something like a function reference:

data class Person(val name: String, val age: Int)
  
var people = listOf(Person("Jane Doe", 25), Person("John Smith", 24), Person("Jen Jones", 37))

fun <R: Comparable<R>> sort(selector: (Person) -> R) {
  people = people.sortedBy(selector)
}

fun main() {
  println(people)
  sort(Person::name)
  println(people)
}

I could also use sort(Person::age) if I wanted to sort by age.

If your pseudocode is more literal, and you want a String parameter for the property name, you should start by asking yourself "why?". But, if you feel that you have a legitimate reason to use a String, one approach would be:

data class Person(val name: String, val age: Int)
  
var people = listOf(Person("Jane Doe", 25), Person("John Smith", 24), Person("Jen Jones", 37))

fun sort(property: String) {
  people = when(property) {
    "name" -> people.sortedBy(Person::name)
    "age" -> people.sortedBy(Person::age)
    else -> throw IllegalArgumentException("and this is why you should not be doing this")
  }
}

fun main() {
  println(people)
  sort("name")
  println(people)
}

edited Aug 29, 2020 at 17:27

answered Aug 29, 2020 at 16:56

CommonsWare

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2 Comments

flaxel Over a year ago

There is also a description for all possible variants. It is also possible to sort by several fields.

gidds Over a year ago

Yes, a selector function is the standard way of doing this; you can see many examples in the standard library. As you demonstrate, providing a function name is awkward and restrictive, as well as being a lot less flexible. (You could also do something with reflection, but that's usually a very bad idea.)

Dharman · Accepted Answer · 2020-08-29 17:24:59Z

0

class CustomObject(val customProperty: String, val value:Int)

 val list = ArrayList<CustomObject>()
        list.add(CustomObject("Z",5))
        list.add(CustomObject("A",4))
        list.add(CustomObject("B",1))
        list.add(CustomObject("X",6))
        list.add(CustomObject("Aa",7))

        var alphabeticallySortedList = list.sortedWith(compareBy { it.customProperty })
        var numberWiseSortedList = list.sortedWith(compareBy { it.value })

edited Aug 29, 2020 at 17:24

Dharman♦

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answered Aug 29, 2020 at 17:19

Muhammad Ali

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