What is Dependency Injection and what problem does it solve? डिपेंडेंसी इंजेक्शन क्या है और यह किस समस्या को हल करता है?

Dependency Injection (DI) is a design pattern where an object's dependencies are supplied by an external source (the Spring IoC container) rather than the object creating them itself with new. Instead of a class instantiating its own collaborators, those collaborators are "injected" through the constructor, a setter, or a field.

DI solves tight coupling: without it, classes are hard-wired to specific implementations, making unit testing difficult (you can't easily substitute a mock) and changes ripple across the codebase. With DI, classes depend on abstractions, implementations can be swapped without touching consumer code, and testing becomes straightforward since dependencies can be mocked and injected directly.

@Service
public class OrderService {
    private final PaymentGateway paymentGateway; // injected, not "new"ed

    public OrderService(PaymentGateway paymentGateway) {
        this.paymentGateway = paymentGateway;
    }
}