"Why make something private? Just make everything public. If the coder knows which method to call, they call it. Problem solved, right?" That was me a week ago. I was so wrong. Here's my journey from "public/private is just fancy nonsense" to "holy crap, this actually makes sense!" 🤯
"Why create InvalidCredentialsException extending RuntimeException? Isn't it just to get a descriptive name instead of generic RuntimeException?" No! The name is only a small part. The real power is type-safe error handling - allowing Spring to distinguish between different errors and handle them differently without string parsing. Let's see why custom exceptions are essential, not just fancy naming.
"But don't we still have if-else statements with interfaces?" Yes! But here's the critical difference: scattered if-else in 10 files vs centralized if-else in 1 factory vs zero if-else with registry pattern. Let's see the evolution.
"Why do we need both generateToken() (public) and createToken() (private) instead of just one method?" Because separation of concerns makes your code extensible. The public method handles what claims to add (business logic), while the private method handles how to build the JWT (technical details). This lets you add new token types without repeating code. Let's understand why this design pattern is essential.
"Can't we just create a utility class with static methods instead of using interfaces? Isn't the interface approach just fancy?" Great question! Yes, you could use utility classes - but you'd be solving only 50% of the problem. Let's see what happens in practice and why interfaces aren't fancy, they're necessary.