Noel Rappin had something to say about private methods (in ruby):
I would consider making these methods private if:
- The class has a specific other meaning for public methods, like a Rails controller or generator. In both cases, making a method public makes it something more than just a method, so in those cases I’m more likely to use privacy.
- There is direct reason to believe that calling the method at the wrong time would cause problems. This doesn’t happen to me often, but I could imagine a class that interacted with a billing service, or which was doing some other thing with external state where you’d really want to lock down the context in which a method is called.
- I have unusual reason to believe that I don’t want to maintain this method in a public-facing API. Again, this doesn’t happen to me often, but I can imagine a case were there was a system metaphor that I really wanted to keep clean, or a case where I knew there would be multiple users in other code bases that I don’t control. Again, this would for me go beyond just “this makes the class harder to maintain” – if I think the method is useful, it’s worth the cost to maintain.