13 Nov
2010

Could sealing a class be a sign of a good design

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I recently attended this years Øredev conference and one of the things I had the good fortune of doing was to meet-up with a long time twitter friend, Philip Laureano. One of the days me and Philip started talking about a previous discussion he had with another attendee (whom shall remain nameless since I do not know the full details of his exact opinions). Anyway, the short version is that the person suggested that classes should be sealed by default, or at least have the developer explicitly state if the class should be sealed or closed.

My immediate reaction was that this was a terrible idea and that I had been struck down too many times by sealed classes before. But then I started thinking that maybe it was not such a bad idea after all. Maybe it even was a sign of a well designed class and that more developers would be better of by sealing their classes.

Now let me inform you that I am still on the ropes about this, but I would like your thoughts on it. In fact I am hoping that the most interesting part of this post will end up being the discussion in the comment section.

So when you take a moment to think about the S.O.L.I.D principles, most specifically the Open-Closed Principle and Dependency Inversion Principle, a long with the old design principle of ‘Favor object composition over class inheritance’ then maybe it is not such a bad thing after all. Throw in interfaces into the mix and program to an interface and not an implementation, and it will enables you to create different branches if needed. If your classes can flourish while being sealed, chances are that you have some pretty nice structures code in your hands.

There probably are some legit reasons to not seal classes at time, despite the reasoning above, so I am not going to be definitive and say that is never the case. Voice your thoughts in the comments and let us see where this ends up – who knows, I might be left standing as a fool!

As always, you can find me on twitter by the name of @thecodejunkie

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The opinions and content expressed here are my own and not those of my employer.