SOLID Principles – Separation of Concern

# SOLID Principles – Separation of Concern

In this blog post, I'm starting a series to explain the famous SOLID principles of software development. These are five core principles that help us write better, cleaner, and more maintainable code. I'll break the series down into individual posts for each principle so we can focus on understanding one at a time.

Whether you're just starting out or already working as a senior engineer, knowing these principles will help you design better systems. And let's be honest — writing code can feel frustrating, stressful, and even boring if it's messy. But when done right, it becomes fun, satisfying, and creative.

So let's begin this journey with the first one:

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## Separation of Concern (SoC)

Even though SoC is not the "S" in SOLID (we'll get to that in the next blog), it's a very foundational concept that supports all of the SOLID principles. SoC is about organizing your code in a way that each part of your program does only one kind of task or is responsible for only one thing.

In simple terms:

> "Don't put everything into one place."

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## Why is Separation of Concern important?

Imagine you're building a small app that shows a list of users.
Now think about everything you need to do:

Fetch users from the internet
Parse the data
* Display it on screen

If you do all of this inside one function or one class, it becomes hard to read, test, or update in the future. That's what SoC helps you avoid.

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## A Simple Kotlin Example

Let's take an example in Kotlin:

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// BAD: Everything in one place
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class UserManager {
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    fun loadAndShowUsers() {
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        // Fetching data
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        val response = URL("https://api.example.com/users").readText()
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        // Parsing data (simplified)
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        val users = response.split(",").map { it.trim() }
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        // Showing data
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        println("Users: $users")
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    }
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}

In the above code, fetching, parsing, and displaying — everything is inside one function. This breaks the SoC principle.

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## Let's Fix It with Separation of Concern

Let's now split responsibilities:

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// 1. Responsibility: Fetching users
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class UserService {
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    fun fetchUsers(): String {
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        return URL("https://api.example.com/users").readText()
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    }
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}
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// 2. Responsibility: Parsing users
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class UserParser {
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    fun parse(rawData: String): List<String> {
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        return rawData.split(",").map { it.trim() }
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    }
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}
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// 3. Responsibility: Displaying users
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class UserPresenter {
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    fun show(users: List<String>) {
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        println("Users: $users")
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    }
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}
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// 4. Coordinator class that brings them together
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class UserManager(
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    private val userService: UserService,
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    private val userParser: UserParser,
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    private val userPresenter: UserPresenter
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) {
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    fun loadAndShowUsers() {
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        val rawData = userService.fetchUsers()
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        val users = userParser.parse(rawData)
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        userPresenter.show(users)
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    }
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}

Now:

UserService only fetches data.
UserParser only parses it.
UserPresenter only displays it.
UserManager coordinates everything.

Each class has a single concern, and that's the beauty of SoC.

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## Benefits of Separation of Concern

✅ Easier to understand code
✅ Easier to test (e.g. you can test UserParser independently)
✅ Easier to change or extend (e.g. switch from console output to a UI without touching parsing logic)
✅ Better teamwork (team members can work on separate parts)

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## Final Thoughts

Separation of Concern is a principle that sets the foundation for good architecture and clean code. It's not just for big projects — even small apps can benefit from writing code with clear separation between responsibilities.

In the next blog post, I'll cover the Single Responsibility Principle, which builds on the same idea and is the "S" in SOLID.

Let's keep writing clean, enjoyable code ✨

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This post is part of my series on SOLID principles. Check out my other posts on software design, mobile engineering, and building better teams.