Blog.CBojar.net

We've all heard the mantra that we should favor composition over inheritance, and in most cases this is good advice. Occasionally, though, we really do need an inheritance relationship. Inheritance provides that useful “is-a” relationship that sometimes fits a situation so well. The pitfall we have to watch out for is using inheritance for code sharing between classes that should otherwise not be related, and not for polymorphism, that “is-a” relationship. What makes code sharing through inheritance so bad in the first place? An inheritance relationship is one of the tightest dependencies that could be made between two classes. It is hard-compiled in, and cannot be swapped out at runtime. It's an all-or-nothing relationship, and anything the parent wants to expose, the child must also expose. The child class may be able to override some of the parent behaviors, but the parent may finalize some methods and prevent this, locking the child class in even more tightly. This may be...

Sometimes, it's not just a method that needs refactoring, or even a class. Sometimes it's several large packages with a spider web of dependents that just need a complete "rehacktoring." This can happen when years of legacy code build up in a system. It is complicated by the fact that the thing you want to change or delete or rename or move has tendrils of dependents spread all across the system, sometimes into classloaders and reflection and dynamic scripts that you cannot begin to search-and-replace all of. It is going to be a big, long slog into the swamp. We don't want our massive change to be a surprise to our team, though. We want to commit and merge the bits as we're going along. They don't represent complete refactorings. We might have pulled out a class just to put some code in a separate place, knowing that down the road we're going to merge it together with some other class and move that other method somewhere else. We're not quite su...