How many constructors should a class have?

Question

I\'m currently modifying a class that has 9 different constructors. Now overall I believe this class is very poorly designed... so I\'m wondering if it is poor design for a clas

Answer 1

A class should do one thing and one thing only. If it has so many constructors it seems to be a tell tale sign that it's doing too many things.

Using multiple constructors to force the correct creation of instances of the object in a variety of circumstances but 9 seems like a lot. I would suspect there is an interface in there and a couple of implementations of the interface that could be dragged out. Each of those would likely have from one to a few constructors each relevant to their specialism.

Answer 2

9 constructors and 6000 lines in class is a sign of code smell. You should re-factor that class. If the class is having lot of responsibilities and then you should separate them out. If the responsibilities are similar but little deviation then you should look to implement inheritance buy creating a interface and different implementations.

Answer 3

As little as possible,
As many as necessary.

Answer 4

Your problem isn't the number of constructors. Having 9 constructors is more than usual, but I don't think it is necessarily wrong. It's certainly not the source of your problem. The real problem is that the initial design was all static methods. This is really a special case of the classes being too tightly coupled. The now-failing classes are bound to the idea that the functions are static. There isn't much you can do about that from the class in question. If you want to make this class non-static, you'll have to undo all that coupling that was written into the code by others. Modify the class to be non-static and then update all of the callers to instantiate a class first (or get one from a singleton). One way to find all of the callers is to make the functions private and let the compiler tell you.

At 6000 lines, the class is not very cohesive. It's probably trying to do too much. In a perfect world you would refactor the class (and those calling it) into several smaller classes.

Answer 5

Enough to do its task, but remember the Single Responsibility Principle, which states that a class should only have a single responsibility. With that in mind there are probably very few cases where it makes sense to have 9 constructors.

Answer 6

It's not just this class you have to worry about re-factoring. It's all the other classes as well. And this is probably just one thread in the tangled skein that is your code base. You have my sympathy... I'm in the same boat. Boss wants everything unit tested, doesn't want to rewrite code so we can unit test. End up doing some ugly hacks to make it work. You're going to have to re-write everything that is using the static class to no longer use it, and probably pass it around a lot more... or you can wrap it in a static proxy that accessses a singleton. That way you an at least mock the singleton out, and test that way.