62. Sorting & Comparators

The collection extensions sorted and sortedDescending work on anything Comparable. The result is a new list; the original is left untouched.

fun main() {
    val nums = listOf(3, 1, 4, 1, 5, 9, 2)
    println(nums.sorted())
    println(nums.sortedDescending())
}

To sort objects, pick a key with sortedBy / sortedByDescending. For multi-level ordering, build a Comparator with compareBy { }.thenBy { }, then pass it to sortedWith. You can also write a Comparator by hand for fully custom logic.

data class Person(val name: String, val age: Int)

fun main() {
    val people = listOf(
        Person("Bob", 30),
        Person("Alice", 30),
        Person("Carol", 25),
    )

    println(people.sortedBy { it.age })            // youngest first
    println(people.sortedByDescending { it.age })  // oldest first

    // age ascending, ties broken by name ascending
    val byAgeThenName = people.sortedWith(compareBy<Person> { it.age }.thenBy { it.name })
    println(byAgeThenName)

    // custom comparator: by name length
    val byNameLength = Comparator<Person> { a, b -> a.name.length - b.name.length }
    println(people.sortedWith(byNameLength))
}

Running it:

$ kotlin run
[1, 1, 2, 3, 4, 5, 9]
[9, 5, 4, 3, 2, 1, 1]
[Person(name=Carol, age=25), Person(name=Bob, age=30), Person(name=Alice, age=30)]
[Person(name=Bob, age=30), Person(name=Alice, age=30), Person(name=Carol, age=25)]
[Person(name=Carol, age=25), Person(name=Alice, age=30), Person(name=Bob, age=30)]
[Person(name=Bob, age=30), Person(name=Alice, age=30), Person(name=Carol, age=25)]

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