Expense Tracker Workshop — Session 1

Kotlin & Spring Boot Basics

Workshop for DevOps engineers who want to understand the backend applications they deploy. No prior programming experience required.

By the end of the workshop you will have a working REST API with documentation that you can call from a browser.

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Workshop structure

Step	Topic	What we build
Step 1	Variables and functions	val, var, fun, string templates
Step 2	Data class and instances	data class Expense, copy(), toString()
Step 3	List and HashMap	In-memory database, null safety ?.
Step 4	Service class	ExpenseService — logic in one place
Step 5	Spring Boot Controller	REST API at localhost:8080
Step 6	SpringDoc / Swagger UI	API documentation in the browser

Each step has:

• start — project with TODO comments (your tasks)
• final — complete solution with explanatory comments

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How to work with this repository

Option A — Folders (recommended for beginners)

You don't need to know git. Just download and open the folders.

1. Download the repository:

Click the green Code button → Download ZIP on this page. Extract the ZIP file.

2. Getting started — Step 1:

1. Open IntelliJ IDEA
2. File → Open → select the step-01-start folder
3. Wait for dependencies to download
4. Run: ./mvnw spring-boot:run
5. You will see the output in the IntelliJ terminal

3. When you're done or get stuck:

Open step-01-final/ — there is the complete solution with explanation.

4. Continue to the next step:

Open step-02-start/ and repeat the process.

Step by step flow:

step-01-start/  →  fill in TODOs  →  compare with  →  step-01-final/
step-02-start/  →  fill in TODOs  →  compare with  →  step-02-final/
step-03-start/  →  fill in TODOs  →  compare with  →  step-03-final/
step-04-start/  →  fill in TODOs  →  compare with  →  step-04-final/
step-05-start/  →  fill in TODOs  →  compare with  →  step-05-final/
step-06-start/  →  fill in TODOs  →  compare with  →  step-06-final/

For steps 1-4: You will see the output in the IntelliJ terminal (CommandLineRunner).

For steps 5-6: Open in your browser http://localhost:8080/expenses

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Option B — Git branches (for advanced users)

Each step has its own branch. The branch contains ONLY the files for that step (at the root level).

1. Clone the repository:

git clone https://github.com/UnityInFlow/expense-tracker-workshop-01.git
cd expense-tracker-workshop-01

2. Switch to the first step:

git checkout step-01-start

You will now see the complete Maven project directly in the repository root.

3. Run the project:

./mvnw spring-boot:run

For steps 1-4: you will see the output in the terminal. For steps 5-6: open http://localhost:8080/expenses in your browser.

Work on the tasks:

Open the project in IntelliJ IDEA and edit the files — fill in the code where you see // TODO:.

4. When you're done or get stuck:

git stash                     # saves your changes aside
git checkout step-01-final    # switches to the solution

5. Continue to the next step:

git checkout step-02-start    # next step

All available branches:

step-01-start    step-01-final
step-02-start    step-02-final
step-03-start    step-03-final
step-04-start    step-04-final
step-05-start    step-05-final
step-06-start    step-06-final

Tip — comparing your work with the solution:

git diff step-01-final -- src/main/kotlin/dev/workshop/expense/Playground.kt

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Tools

Tool	Usage
IntelliJ IDEA Community	All steps
JDK 21	Compilation and running

Detailed installation guide: SETUP.md

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Follow-up workshop

Session 2 — SQLite + Repository + Validation https://github.com/UnityInFlow/expense-tracker-workshop-02

Data from Session 1 disappears after restart (HashMap = memory). In Session 2 we fix this with an SQLite database.