Lots of developers think that testing private methods is a bad idea. However, all examples I’ve found were based on the idea that private methods are private because calling them could break internal object’s state. But that’s not only reason to hide methods.
Let’s consider Facade pattern. My class users need the 2 public methods. They would be too large. In my example, they need to load some complex structure from the database’s BLOB, parse it, fill some temporary COM objects, run user’s macro to validate and modify these objects, and serialize modified objects to XML. Quite large functionality for the single metod 🙂 Most of these actions are required for both public methods. So, I’ve created about 10 private methods, and 2 public methods do call them. Actually, my private methods should not necessarily be private; they’ll not break the internal state of instance. But, when I don’t wont to test private methods, I have the following problems:
- Publishing them means complexity for users (they have a choice they don’t need)
- I cannot imagine TDD style for such a large public methods, when you’re to write 500+ lines of code just to return something (even not real result).
- Data for these methods is retrieved from database, and testing DB-related functionality is much more difficult.
When I’m testing private methods:
- I don’t publish details that would confuse users. Public interface includes 2 methods.
- I can work in TDD style (write small methods step-by-step).
- I can cover most of class’s functionality using test data, without database connection.
Could somebody describe, what am I doing wrong? What design should I use to obtain the same bonuses and do not test private methods?
UPDATE: It seems to me I’ve extracted everything I was able to another classes. So, I cannot imagine what could I extract additionally. Loading from database is performed by ORM layer, parsing stream, serializing to XML, running macro – everything is done by standalone classes. This class contains quite complex data structure, routines to search and conversion, and calls for all mentioned utilities. So, I don’t think something else could be extracted; otherwise, its responsibility (knowledge about the data structure) would be divided between classes.
So, the best method to solve I see now is dividing into 2 objects (Facade itself and real object, with private methods become public) and move real object to somewhere nobody would try to find it. In my case (Delphi) it would be a standalone unit, in other languages it could be a separate name space. Other similar option is 2 interfaces, thanks for idea.
I think you are putting too many responsibilities (implementations) into the facade. I would normally consider this to be a front-end for actual implementations that are in other classes.
So the private methods in your facade are likely to be public methods in one or more other classes. Then you can test them there.