How can I sort an ArrayList of custom objects?

Question

I want to sort the ArrayList by the age and then, if two people have the same age, by their name in alphabetical order.

Here's my code:

import java.util.*
import kotlin.collections.ArrayList

fun main(args:Array<String>){
    var listName=ArrayList<person>()
    listName.add(person("ahmed",25))
    listName.add(person("Fethi",28))
    listName.add(person("abdou",28))

    Collections.sort(listName)
    for(person in listName){
        println("the name:${person.name} and the age is:${person.age}")

    }

}


class person(
    var name:String?=null,
    var age:Int=null
):Comparable<person> {
    override fun compareTo(other: person): Int {
        return this.age!!-other.age!!

    }
}

I expect the following output:

1. ahmed 25
2. abdou 28
3. fethi 28

Answer 1

According to the example you provided, you want to sort the collection by age (ascending) and then by name in alphabetical order.

You can easily achieve that in Kotlin by using the sortedWith extension function (that accepts a Comparator ):

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

fun main() {
    val people = listOf(
        Person("Ahmed", 25),
        Person("Fethi", 28),
        Person("Abdou", 28)
    )

    val comparator = Comparator.comparingInt(Person::age).thenComparing(Person::name)
    val sortedPeople = people.sortedWith(comparator)
    println(sortedPeople)
}

Result:

[Person(name=Ahmed, age=25), Person(name=Abdou, age=28), Person(name=Fethi, age=28)]

Note: I slightly changed the Person class and the main method to follow Kotlin conventions and best practices around mutable variables and nullable types