Updating a List in Kotlin using Another List

Описание: Discover how to efficiently update a list of objects in Kotlin by merging data from another list. Learn the best practices with clear examples and explanations.
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This video is based on the question https://stackoverflow.com/q/72552058/ asked by the user 'MakiX' ( https://stackoverflow.com/u/11762148/ ) and on the answer https://stackoverflow.com/a/72552101/ provided by the user 'Some random IT boy' ( https://stackoverflow.com/u/9248718/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Updating a List in Kotlin using Another List: A Step-by-Step Guide

In Kotlin, managing lists effectively is crucial for creating dynamic applications. A common scenario developers face is updating a list using another list, particularly when dealing with objects that may overlap in data.

In this post, we'll tackle a specific problem: updating a list of Skill objects where the updated entries come from a second list of Skill objects. You'll learn how to seamlessly merge two lists while ensuring that relevant data from the second list overrides data in the first list when necessary.

The Problem

You have two lists:

AllSkills: This list contains all the skills available in your application.

UserSkills: This list contains the skills that a user has acquired, along with specific information such as a note.

Your goal is to replace any skill in AllSkills that also exists in UserSkills — meaning they have the same ID — with the skill from UserSkills. For example, if a user has a skill with ID 1 that carries a note, it should replace the corresponding skill in the AllSkills list.

Example Scenario

Given the following lists:

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The target outcome after the update should look like this:

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The skill with ID 1 in UserSkills should take precedence and replace the skill in the AllSkills list.

The Solution

Step 1: Create a Map from UserSkills

To efficiently replace skills in AllSkills, we can convert UserSkills into a Map where each skill is associated with its ID. This allows for quick lookups.

Here's how to do it:

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Step 2: Update AllSkills Using the Map

Next, we will iterate over AllSkills, checking if each skill exists in the userSkillsMap. If it does, we'll take the Skill from UserSkills; otherwise, we'll keep the existing Skill from AllSkills.

We can use the Elvis operator (?:) to accomplish this succinctly:

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This one-liner effectively:

Looks up the current skill in the userSkillsMap.

Returns the corresponding user skill if it exists, or the original skill otherwise.

Bringing It All Together

Here's the complete code snippet:

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Final Thoughts

By utilizing Kotlin's collection functions and the powerful map structure, you can efficiently update lists based on specific criteria. This approach not only keeps your code clean but also enhances performance by minimizing iteration overhead.

If you encounter similar problems in your Kotlin applications, remember this method for merging lists effectively. Happy coding!