Should I include private methods into UML class diagram?

Question

So the question is simple. I just do not find the answer on the internet. I think private methods are useless in diagram, not telling much. But maybe I think wrong.

Answer 1

Private methods are an implementation detail of the class, not its public API.

No:

If you are using the diagram to show the external interface of the class, or some subsystem that the class is part of, then there is no reason to include them. They will just add visual noise and confuse the reader.

Yes/Maybe:

On the other hand, if you are trying to communicate the internal design of the class itself, or its tightly coupled collaboration with another class (think of friend in C++), then it might be appropriate.

Answer 2

Opinion:

UML is not* a 4GL programming language. You use it to convey an idea, design, architecture. You only add information that serves that purpose. It does not have the concept of being "complete". Like text in a book, you only use words necessary for the purpose of conveying the message.

If it is used as part of system documentation, and it needs to show all members, then we just generate it afterwards.

UML is not the goal, it is just a modelling language to communicate with people.

So if it serves the purpose you add it, otherwise you leave it out.

*) some attempts have been made at that, but have not been very succesful (togetherJ springs to mind)