Kotlin – Sort List of custom Objects

bezkoder Kotlin

In this tutorial, I will show you several useful ways to sort List of custom Objects for Ascending and Descending order in Kotlin. You’re gonna know how to:

  • sort List of Objects by one field or multiple fields
  • use your custom Comparator to sort List of Objects
  • implement Comparable interface to make sorting List of Objects more convenient

Related Posts:
– Kotlin List & Mutable List tutorial with examples
– Kotlin – Compare Objects with Comparable Example


Sort List of custom Objects by field in Kotlin

sortBy

We can sort a List of Objects by one field by using sortBy(): sortBy {it.field}

For example, if we have Date class like this:

package com.bezkoder.kotlin.sortlist

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

This is how we sort List of Date objects by field year–

package com.bezkoder.kotlin.sortlist

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

The result:

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

sortBy Descending

You can use sortByDescending {it.field} for descending order.

println("--- DESC ---")
dates.sortByDescending { it.year }
dates.forEach { println(it) }

The result:

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

sortedBy

If you don’t want to change the original List, you can use sortedBy() method that return another sorted List.

/* dates
Date(year=2020, month=4, day=3)
Date(year=2021, month=5, day=16)
Date(year=2020, month=1, day=29) */

println("--- ASC ---")
val sortedDates = dates.sortedBy { it.year }
dates.forEach { println(it) }
println("------")
sortedDates.forEach { println(it) }

The result:

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

sortedBy Descending

And sortedByDescending() for descending order.

println("--- DESC ---")
val sortedDatesDescending = dates.sortedByDescending { it.year }
dates.forEach { println(it) }
println("------")
sortedDatesDescending.forEach { println(it) }

The result:

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

With sortBy() & sortedByDescending(), you can see that we are able to sort by ONLY ONE field – year.

How about continue sorting by month and day fields? We have sortWith() method.

Sort List by multiple fields in Kotlin

sortWith

We can sort a List of Objects by multiple fields passing a Comparator object to sortWith() method.

The Comparator object is created by using compareBy() and thenBy() method.

val dates = mutableListOf(
	Date(2020, 4, 3),
	Date(2021, 5, 16),
	Date(2020, 1, 29)
)

println("--- ASC ---")
dates.sortWith(compareBy<Date> { it.year }.thenBy { it.month }.thenBy { it.day })
dates.forEach { println(it) }

Now, check the result, you will see that if years are equal, the program will continue to compare month, then day.

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

sortWith Descending

If you want to sort for descending order, just put additional method of Comparator named reverse().

For example:

println("--- DESC ---")
dates.sortWith(compareBy<Date> { it.year }.thenBy { it.month }.thenBy { it.day })
dates.reverse()
dates.forEach { println(it) }

The result:

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

sortedWith

sortedWith() returns a sorted List but doesn’t change the original List. We continue to pass Comparator object to the method.

val dates = mutableListOf(
	Date(2020, 4, 3),
	Date(2021, 5, 16),
	Date(2020, 1, 29)
)

println("--- ASC ---")
val sortedDates = dates.sortedWith(compareBy<Date> { it.year }.thenBy { it.month }.thenBy { it.day })
dates.forEach { println(it) }
println("------")
sortedDates.forEach { println(it) }

In the result, dates was not changed.

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

sortedWith Descending

For descending order, just put additional method named reversed().

For example:

println("--- DESC ---")
val sortedDatesDescending =
	dates.sortedWith(compareBy<Date> { it.year }.thenBy { it.month }.thenBy { it.day }).reversed()
dates.forEach { println(it) }
println("------")
sortedDatesDescending.forEach { println(it) }

Check the result in console-

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

Kotlin Sort List of Objects with custom Comparator

Overview

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

– Use sortedWith(comparator: Comparator) method that returns a List.

Create 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.sortlist

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

Create Class for handling sorting

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

CompareDates.kt

package com.bezkoder.kotlin.sortlist

class CompareDates {

	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
		}
	}
}

Check the result

Now we can sort the List easily by using sortedWith(CompareDates).

app.kt

package com.bezkoder.kotlin.sortlist

fun main(args: Array<String>) {
	val dates = mutableListOf(
		Date(2020, 4, 3),
		Date(2021, 5, 16),
		Date(2020, 1, 29)
	)

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

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

The result 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)

Sort List of custom Objects with Kotlin Comparable

Overview

We’re gonna follow these steps:
– Implement Comparable interface for the class of objects you want to sort.
– Override compareTo(other: T) method and:

  • return zero if this object is equal other
  • a negative number if the object is less than other
  • a positive number if it is greater than other

– Use sorted() method that returns a List.

Create Comparable Class

Let’s create a class that implements Comparable.

MyComparableDate.kt

package com.bezkoder.kotlin.sortlist

import kotlin.Comparable

data class MyComparableDate(val year: Int, val month: Int, val day: Int) : Comparable<MyComparableDate> {

	override fun compareTo(other: MyComparableDate) = when {
		year != other.year -> year - other.year
		month != other.month -> month - other.month
		else -> day - other.day
	}
}

Check the result

Our work becomes simple now, we just need to use sorted() method for ascending order or sortedDescending() for descending order.

app.kt

package com.bezkoder.kotlin.sortlist

fun main(args: Array<String>) {
	val myComparableDates = listOf(
		MyComparableDate(2021, 4, 3),
		MyComparableDate(2020, 7, 29),
		MyComparableDate(2021, 2, 26)
	)

	println("--- ASC ---")
	myComparableDates.sorted().forEach { println(it) }

	println("--- DESC ---")
	myComparableDates.sortedDescending().forEach { println(it) }
}

You can see the result in console.

--- ASC ---
MyComparableDate(year=2020, month=7, day=29)
MyComparableDate(year=2021, month=2, day=26)
MyComparableDate(year=2021, month=4, day=3)
--- DESC ---
MyComparableDate(year=2021, month=4, day=3)
MyComparableDate(year=2021, month=2, day=26)
MyComparableDate(year=2020, month=7, day=29)

Conclusion

Today we’ve learned many ways to sort a List of custom Objects in Kotlin, from simple to more complicated way. You’ve known how to sort by field and multiple fields, or by using Comparator/Comparable.

Happly Learning! See you again.

Further Reading

5 thoughts to “Kotlin – Sort List of custom Objects”

  2. WOW! One of the most complete tutorial for Kotlin List – sort custom objects. Clear and detailed!

    Thank you =)

  4. Thanks in advance! This is a complete reference for Sorting List in Kotlin. I’m looking forward to your Kotlin filter List tutorial.

  5. Wow, you’ve shown many ways to sort list of Objects in Kotlin, just complete reference. Thanks.

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