A perhaps non-obvious question then is, SHOULD we have private methods? Could it be better if everything was public? Why? Well you wrote those private methods to help you. They are helpful to you so isn't it conceivable they would be helpful to other programmers as well? Why not share them with the rest of the world? Why not make them public?
I can see three (3) reasons why not:
1. They would clutter your public interface, making it harder to understand, harder to use.
2. If others can call upon your private methods, you can no longer easily replace them with something better. You have entered into an implicit contract with your callers about having a set of methods available to them. Once you've given them "out" you can't take them back if others (including other classes YOU have written) have started to use them already . Which means you can't improve the implementation of your original public interface, without the risk of breaking classes that now call it.
3. An Object-Oriented "class", which more generally means any software component, should be *"single purpose"*. Why? Because then it best serves those who need it just for that single purpose, without additional baggage they would have to pay for either in money, or in time to learn to use it.
For instance a Car has a single purpose: to transport you in comfort between places. If you're a car-manufacturer and you add to your latest model also the feature of being able to plow the fields, your "class" is no longer single-purpose. Which means it will unlikely to be useful to people who just need a transport. If you also need to plow, you can pick a different object, different class for that purpose.
The purpose of your internal implementation is not the same as the purpose of your public interface. The implementation serves YOUR purpose of providing a public service to your callers. Therefore if you expose your implementation to the public, by making it 'public', your class will no longer be "single-purpose". And therefore it is likely to be less useful to anybody who just needs something for that single purpose. Such a class will not win in market-place.
© 2014 Panu Viljamaa. All rights reserved