Kotlin Comparator & Sort List of objects example

In this tutorial, I will show you how to create Kotlin Comparator and custom Comparator, then we’re gonna use Comparator to sort List of objects.

Related Posts:
– Kotlin – Sort List of custom Objects
– Kotlin – Compare Objects with Comparable Example
– Kotlin Priority Queue tutorial with examples

Kotlin Comparator

Kotlin provides Comparator interface to order the objects of user-defined classes.

There are two ways to create a Comparator object:

• using compareBy() (and thenBy() for comparing more fields)
• creating custom Comparator

If you want to reverse the ordering of the Comparator (sort by Descending, for example), just use reversed().

Comparator compareBy()

These are ways to create Comparator object for Date(year,month,day) class:

compareBy<Date> { it.year }.thenBy { it.month }.thenBy { it.day }
compareBy<Date>({ it.year }, { it.month }, { it.day })

Custom Comparator

We’re gonna follow these steps:
– Implement Comparator interface for the class that you use for handling sorting.
– Override compare(object1: T, object2: T) method and:

• return zero if object1 is equal object2
• a negative number if object1 is less than object2
• a positive number if object1 is greater than object2

Sort List of objects using Comparator

Create Data Class for objects to be sorted

We’re gonna create a data class with 3 fields: year, month & day.

Date.kt

package com.bezkoder.kotlin.comparator

data class Date(val year: Int, val month: Int, val day: Int) {
}

Create Class for handling sorting

This is the main part, DateComparator class overrides Comparator.compare() method.

DateComparator.kt

package com.bezkoder.kotlin.comparator

class DateComparator {

	companion object : Comparator<Date> {

		override fun compare(a: Date, b: Date): Int = when {
			a.year != b.year -> a.year - b.year
			a.month != b.month -> a.month - b.month
			else -> a.day - b.day
		}
	}
}

Create test function

For checking our Comparator objects, we’re gonna create a List of Date objects and sort it using sortedWith() method.

app.kt

package com.bezkoder.kotlin.comparator

fun main(args: Array<String>) {
  val dates = mutableListOf(
    Date(2020, 4, 3),
    Date(2021, 5, 16),
    Date(2020, 1, 29)
  )
  
  println("--- ASC ---")
  dates.sortedWith(compareBy { it.year }.thenBy { it.month }.thenBy { it.day })
       .forEach { println(it) }
       
//  dates.sortedWith(compareBy({ it.year }, { it.month }, { it.day })).forEach { println(it) }

  println("--- DESC ---")
  dates.sortedWith(compareBy({ it.year }, { it.month }, { it.day }).reversed())
       .forEach { println(it) }

  println("--- ASC ---")
  dates.sortedWith(DateComparator).forEach { println(it) }

  println("--- DESC ---")
  dates.sortedWith(DateComparator).reversed().forEach { println(it) }
}

The Output in console will look like this:

--- ASC ---
Date(year=2020, month=1, day=29)
Date(year=2020, month=4, day=3)
Date(year=2021, month=5, day=16)
--- DESC ---
Date(year=2021, month=5, day=16)
Date(year=2020, month=4, day=3)
Date(year=2020, month=1, day=29)

Further Reading

• Kotlin Comparator API
• https://kotlinlang.org/docs/reference/collection-ordering.html