-When I first started learning Java, my banking application was nothing more than a console program. Everything happened inside a main() method—creating accounts, depositing money, withdrawing funds, and transferring money between accounts.
- While that taught me the fundamentals of Java, I knew real backend applications worked differently.
Over the past few weeks, I've been transforming that console application into a Spring Boot backend, and it's been an eye-opening experience.
What I've Built So Far
Core Java Foundation
Before touching Spring Boot, I focused on mastering Java fundamentals:
- Object-Oriented Programming
- Inheritance
- Polymorphism
- Interfaces
- Abstract Classes
- Exception Handling
- Collections
- Streams API
- Enums
- JUnit Testing
- Gradle
- Git & GitHub
That foundation made learning Spring much easier.
Enter Spring Boot
My first Spring Boot application felt surprisingly simple:
@SpringBootApplication
public class BankingApiApplication {
public static void main(String[] args) {
SpringApplication.run(BankingApiApplication.class, args);
}
With just a few annotations, I had a web server running.
Building My First REST API
The first endpoint I created was:
@RestController
public class HomeController {
@GetMapping("/")
public String home() {
return "Welcome to my Banking API!";
}
Seeing text returned in a browser instead of the console was a memorable milestone.
Understanding Dependency Injection
One concept that really changed how I think about software design was Dependency Injection.
Instead of writing:
BankService service = new BankService();
Spring creates and injects the service for me:
@RestController
public class AccountController {
private final BankService bankService;
public AccountController(BankService bankService) {
this.bankService = bankService;
}
This keeps the code cleaner, easier to test, and more maintainable.
Creating REST Endpoints
I added endpoints to:
- Retrieve all accounts
- Create new accounts
- Search accounts by account number
- Testing them with Postman helped me understand how frontend and backend applications communicate through HTTP and JSON.
- Learning Proper Error Handling
- Instead of returning null when an account doesn't exist, I implemented custom exceptions and global exception handling.
- This allowed my API to return meaningful HTTP responses such as:
- 404 Not Found
- Structured JSON error messages
- This made the API much more professional.
- Moving from Memory to PostgreSQL
Initially, my application stored data inside an ArrayList.
Eventually, I replaced it with:
- PostgreSQL
- Spring Data JPA
- JpaRepository
Instead of manually managing collections, Spring now handles persistence through repositories.
Introducing DTOs
One lesson that stood out was why professional applications don't expose database entities directly.
I learned to separate:
- Request DTOs
- Response DTOs
- Database Entities
This makes the API cleaner, safer, and easier to evolve over time.
Looking Back
It's interesting to compare where I started and where I am now.
- Then
- Console application
- System.out.println()
- ArrayList
- Manual object creation
- Now
- Spring Boot
- REST APIs
- Dependency Injection
- Controllers
- Services
- Repositories
- PostgreSQL
- DTOs
- Postman
- JUnit
The concepts I learned in Core Java didn't disappear—they became the foundation for everything I built with Spring Boot.
What's Next?
I'm continuing to expand this banking API by learning:
- Bean Validation
- Spring Security
- JWT Authentication
- Docker
- Integration Testing
- Deployment
Every feature brings the project closer to what you'd expect in a production backend application.
If you're learning Java, my advice is simple:
Don't rush into frameworks. Build a strong Core Java foundation first. When you eventually learn Spring Boot, you'll understand not just how things work, but why they work that way.
Thanks for reading! If you're on a similar backend development journey, I'd love to connect and hear what you're building.
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