It was another normal day on the IT department at a big manufacturing company I was working back then when my boss ask me to see him on his office. It turns out we were starting a new project and needed some extra hands. My boss wanted me to interview potential candidates. It was my first time as a recruiter and a fun experience. I was tasked with finding some experienced OO developers, which can be tricky to say the least. One night it occurred to me that the best way to gauge a developers OO skills are to evaluate his/her design and thinking skills: it’s faster, and you cannot cheat on it (googling it or whatever). That’s how I came up with the calculator challenge.
Design a program that given a string such as “(1+2)/3*4” returns the correct result. It has to be extensible, meaning that you can add further operations without having to modify the structure of the program to do so (open/closed principle).
I didn’t require the developers to code such a program, just to design it and explain it. It shouldn’t be that hard for a seasoned OO developer to complete the design, but for the guys I interviewed back then, it was puzzling. They could not figure out the SQL to do that. Some other suggested using web services, but no one could ever solve it. My coworker, who was in charge of evaluating the SQL part told me that he wasn’t sure he could do it either. I never told him the answer 😛
So, how would you solve this? Leave your answer in the comments.