Dependency Injection: Why `new` Is Not Always Your Friend

Early in my career, I used the `new` keyword everywhere. `new UserService(new UserRepository(new Database()))`. I thought I was being efficient. Then came the day we needed to switch from MySQL to MongoDB. I had to go through every file that instantiated `UserRepository` and change the constructor. It was a nightmare. Dependency Injection (DI) solves this by inverting the control. Instead of the class creating its dependencies, they are provided (injected) from the outside. Using Spring’s IoC container, I now just annotate with `@Autowired` and let the framework manage the lifecycle. It makes the code decoupled, testable (because I can inject mocks), and flexible. If you’re starting with Java, learn Spring just for this principle alone. It enforces good architecture by making you think about how components connect.