Monday, June 15, 2009

Debugging friendly code

When you often take over other people’s code, you often start getting your own pet issue which you focus on. Well, my pet issue is debugging, or rather ease of debugging. I don’t want to have to know and understand the whole business domain, program or component when I want to fix an error (bug). Most of the time, it should be possible to fix errors by stepping through the code while debugging, and finding the error.

For this to be possible, however, requires the code to be debugging friendly. By this I mean that each class, method and even code line should have well defined responsibilities, which leaves no doubt where and how the error occurred.
This sounds all well and fine on the abstract plan, but how does it relate to real code? Well, that’s of course a little harder to say, but I can give some general guidelines which should be followed to achieve this.

  • Methods should be limited in scope. E.g. if you need to implement a method which fetches something from the database, the method should not also be responsible for putting the data in the cache or convert the data into a different data type. If those things are necessary, a method should be made for each of those functionalities.

  • Methods should be generalized as much as possible. Instead of copy and pasting methods and then modifying them to suit your needs, see if it isn’t possible to generalize the functionality in some way or other, so just one function is responsible for it.

  • Methods, parameters, classes, variables etc. should have telling names. Don’t pass x, y, z along as parameters in a method call. On the other hand, don’t make long names explaining the exact circumstances when it’s obvious from the scope what it is. The id of a customer object doesn’t need to get called customer Id.

  • Use local variables! Martin Fowler might disagree, but he probably doesn’t have to debug other peoples’ code very often. When you call methodX(methodY(Z)) it’s not possible to easily see whether it’s method or methodY which causes the null pointer exception.

  • Make unit tests for, at least, the critical methods.

  • Comment the code. Don’t explain the obvious, but rather focus on explaining the assumptions behind what you’re doing.

  • Check parameters for illegal values. If your parameter should never be null, then check for that – in case it is null, then throw an exception, explaining the problem. This shows other people (or a later you) that someone thought of the possibility, and didn’t just forget to handle null values as input parameters.

  • Ensure that your method behaves in a uniform way, i.e. giving the same parameters the method should always behave the same way (barring other dependencies). I once experienced a ToString() method in a class which always appended something at the end of the string, causing the behavior to different dependent on whether the method had been called before or not.

All of these things might seem simple, but when the project is 3 months over time, your customer or project leader (or both!) is breathing down your neck, then it’s easy to cut corners. You might also know the system and/or domain very well, which allows you to make some assumptions which are not obvious for others – not only does this make it harder for others to debug, but it also might cause people to use the code in the wrong way.

So, what happens if you come across code, or inherit code, which doesn’t conform to my guidelines? Well, be bold and refactor. Do it one step at the time – if method calls are used as parameters, make local variables. If methods are responsible for several things, split it into several methods with distinct responsibilities, and so on. And of course, make sure that there are unit tests.

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