Overview

Mutation Testing: Watching the Watchmen

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You can’t do without automated (unit) tests if you want to stay on top of the ever increasing complexity of software projects. A mutation testing framework ‘watches the watchmen’ by inserting small changes into your compiled byte code and then validating your test suites against these intentional bugs. As a quality safeguard it’s much more effective than traditional source code validation. It is a even a challenging way to improve your coding skills and makes writing tests suites fun again.

As a largely self-taught programmer I am still thankful for the millennial internet boom, when a linguistics graduate could get hired with nothing more than a thirst for coding and no professional experience or relevant diplomas to show for. I like to think my coding skills have improved in the fifteen years since, starting out – like most glorified amateurs – with the anti-bureaucratic “just get it done even if it requires duct-tape” attitude of coding, where writing unit tests only gets in the way of shipping code. Read Joel Spolsky about Netscape pioneer Jamy Zawinsky. Nowadays I believe in the school of “You can have good, cheap or fast. Pick two.” I don’t do duct tape anymore, except for small home repairs.

If a job is worth doing, it is worth doing well. Good code written by fallible humans needs automated tests. Too bad it’s not really the most fun part of programming. For many who write code on a daily basis test driven development still doesn’t come natural. Plenty of lip service is paid, but we’re just not that motivated to do it properly unless we’re harassed with minimum thresholds for test coverage. Tests feel like unexciting pieces of code making sure that other code which puts a blue ball in a red box has indeed put a blue ball in a red box, and not a green ball in an orange box. A pedantic bureaucratic requirement. “I can write working code, you know. Let me just get on making customers happy. They don’t care about stupid unit tests”.

That’s a dangerous attitude. Unit tests matter. Integration tests matter even more. You should get motivated. Test cases are a yardstick for quality while you’re designing your code, not something to add when you have time to spare. Code that cannot be properly unit-tested is a likely candidate for some serious overhaul. However, full rewrites are a waste of time and money. Agile development means software is continuously thought out, written down, re-thought and re-written until the user is happy or the money runs out, whichever comes first. Adopting such a just-in-time approach means that a code base is not only being expanded with new code, but the existing code adapts to the more complex software architecture without breaking the functionality it already provides. This change is happening all the time, ideally from day one, but no later than day three. Good tests make this incremental refactoring possible. Bad tests make it impossible.

Greater minds than myself have written great books about the need for refactoring and why meaningful tests should cover your entire code basis. I shouldn’t have to convince you. Let me just add my own argument here:

If you can’t test some piece of code, I don’t trust you to understand it.

Understanding how each line of code relates to the whole is a major challenge. Here’s something to put Moore’s Law into practical perspective. I am writing this post on a fancy Macbook with 16 Gb memory. In the early eighties I saved up my precious pocket money to buy a 16 Kb memory module for my Vic 20 home computer. That’s a million times more memory in thirty years. Human short-term memory is stuck on a measly seven items and has probably been since Socrates. Yes, we have better tools and faster compilers, but we have fixed-size brains that have to deal with ever-more complex projects. The only way to comprehend these is to break them down into manageable, testable chunks.

Jasper joined codecentric NL in 2015 but has been coding since the early eighties. Having a background in English and linguistics, he always likes to stress the importance of human language in software.

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