Previously I wrote:
Sometimes you work with something for so long that you forget that other people aren’t familiar with it. This happened recently talking to a friend of mine — “I’ve got no need for object orientation”, he said, and I was shocked. […] What, I wondered to myself, were the benefits of OO that he was missing? How could I persuade him that it does have a point? So here is my attempt to trawl my memory and see how I’ve changed in response to OO, and why I think it serves me (and my projects) better. In the end I’ve found that the benefits are real, but nothing I would have guessed at when I first started in this field.
Another great benefit of OO is better testability. The simplest kind of testing you can do is unit testing — taking a single class and testing it s methods in isolation. By being able to instantiate individual objects and ensuring there are minimal dependencies it becomes fairly easy to test almost every branch of your code. Fairly easy, but not trivial — there’s a difference between writing merely working code and writing testable code. Writing testable code is a very good skill to acquire, and test-driven development is a further discipline that’s certainly improved my code (and my peace of mind) no end.
There are further aids to testing that OO gives you, most notably stubs and mocks. Stubs are dummy implementations when you need a non-critical dependency (such as Repository for an Article, but you’re not actually testing the Repository). Mocks are dummy implementations which simulate a given interaction and allow you to check that interaction afterwards. Both of these are easier with dependency injection.
Personally I think the greatest step forward in testing is unit testing, and automatic unit testing — have a look at NUnit for .NET languages and JUnit for Java. Even coding for fun, I’ve found a great sense of relief in not having to think about the correctness of my code, but leaving that to my unit testing framework. It means I can leave my code at any point and not wonder about whether or not it really works — because I know it does — and not have to hold anything in my head for when I return, because everything will be verified by the tests. Pedants will say there’s a lot more to testing than unit testing, and they’d be right, but to go from next-to-no testing, or infrequent testing, to automated unit tests is a huge step forward, and I cannot recommend it highly enough.