How to Sort a List of Custom Objects in Kotlin

Learn how to efficiently sort a List of custom objects by a specific property in Kotlin using built-in functions.
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Sorting a List of Custom Objects in Kotlin

When working with collections in Kotlin, you often find yourself needing to sort a list based on specific attributes of the objects within that list. For example, let’s say you have a class representing the edges of a graph, and you need to sort a list of these edges based on their weight. In this guide, we’ll explore how to achieve that in Kotlin.

Defining the Problem

Imagine you have a Kotlin class defined as follows:

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This class helps you create weighted edges for a graph. Now suppose you have a mutable list of Edge objects defined like this:

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You populate it with several Edge instances:

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Your goal is to sort this list in ascending order based on the weight property so that the sorted list looks like:

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The question is: Is there a built-in function to sort this list, or do you need to implement a sorting method manually?

The Solution

Kotlin provides several built-in functions that can help you sort collections easily and efficiently. Here’s how to sort your MutableList<Edge>:

Using sortBy

For a mutable list, you can use the sortBy function. This method sorts the original list in place, modifying the order of the elements based on the specified attribute.

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Using sortedBy

If you’re working with an immutable list, you’ll want to use sortedBy instead. This function returns a new list that is sorted based on the specified property.

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Sorting in Descending Order

If you need to sort the list in descending order, Kotlin provides companion functions:

sortByDescending: Sorts a mutable list in place

sortedByDescending: Returns a new sorted list

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or

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Summary

In Kotlin, sorting a list of custom objects is straightforward thanks to functions like sortBy and sortedBy. Here’s a brief recap:

Use sortBy for mutable lists to sort in place.

Use sortedBy for immutable lists to create a new sorted list.

Use sortByDescending and sortedByDescending for descending order sorting.

With these tools at your disposal, you can efficiently manage the order of your collections, allowing you to focus on more strategic aspects of your programming tasks. Happy coding!