[Asooka]

Where/how does one learn to structure programs like this? Obviously I won't write fizzbuzz like this, but the point is it won't occur to me to structure a fizzbuzz solution like this. And if I can't produce such a structure for the dead simple case, then I can't trust myself to be able to architect an actual complex system where one does need to apply all these abstractions.

Are these skills only attainable from years of real-world experience?

[LoSboccacc]

people loves enterprise architecture jokes but all starts with some questions: what if I need to use the client datasource for the input numbers? what if a client ask to send the output to a network endpoint? to a printer?

so you start decoupling things. this here is played for fun, but if you need to build something that lasts 10 years across a multitude of clients instead of just as an one off project those question starts to be legitimate.

[auspex]

You should first learn about design patterns and then read Patterns of Enterprise Architecture by Martin Fowler. The book basically outlines how to think like this.

[nbevans]

Just research SOLID and IOC and practice these in combination with a Java or C# or other OOP-inspired language. That will get you pretty close.

[organsnyder]

Effective Java is the best book I've read on the subject.