kotlin

Sort collection by multiple fields in Kotlin

Let's say I have a list of People which I need to sort by Age first and then by Name.

Coming from a C#-background, I can easily achieve this in said language by using LINQ:

var list = new List<Person>();
list.Add(new Person(25, "Tom"));
list.Add(new Person(25, "Dave"));
list.Add(new Person(20, "Kate"));
list.Add(new Person(20, "Alice"));

// will produce: Alice, Kate, Dave, Tom
var sortedList = list.OrderBy(person => person.Age).ThenBy(person => person.Name).ToList();

How does one accomplish this using Kotlin?

This is what I tried (it's obviously wrong since the output of the first "sortedBy" clause gets overridden by the second one, which results in a list sorted by Name only)

val sortedList = ArrayList(list.sortedBy { it.age }.sortedBy { it.name })) // wrong
Solution

  • sortedWith + compareBy (taking a vararg of lambdas) do the trick:

    val sortedList = list.sortedWith(compareBy({ it.age }, { it.name }))

    You can also use the somewhat more succinct callable reference syntax:

    val sortedList = list.sortedWith(compareBy(Person::age, Person::name))