Part one of my Three Steps to Code Quality via TDD series.
We always default to public class when creating a new class. Why? The concept of visibility in OO languages appears very early on in programming books, yet more often than not most of the classes we create default to public visibility.
@simonbrown stated that each time you make something public you need to make a donation to charity. In other words we should think more about why the class we are making should be visible to everyone. I really like this idea that the use of the public keyword should be a well thought out decision.
Server side development has a part to play in the lack of concern given to visibility issues. Library or framework developers on the other hand must carefully consider what is part of the public API. Any changes made after are considered breaking and require careful consideration. Yet in the land of server side development this is see as a non issue. This is wrong. If we treat our tests as consumers of our public API, constantly updating them or modifying them should be a warning symbol.
Use internal or private classes for details and public classes for your API. The beauty of this is that TDD drives your public API, which should be fairly stable. Internally however you want the ability to refactor without a suite of tests breaking, otherwise what is the point of writing automated tests?
Implementation details are introduced as part of the refactor step. Ideally they should never be introduced without a passing test in place, as previously the simplest possible thing should have been done.
What Should be Public Then?
- Application services (use cases) that adapters talk to.
- Adapters - controllers, console application, presentation layer.
- Domain concepts - money or customer for example
- Strategies - things you need to inject, repositories, third parties
Doesn't this lead to god classes?
No. As part of the TDD cycle, when refactoring you can extract implementation details. There is no reason why the public types should suffer.
Doesn't this lead to large tests on the public types?
No. You'll use less test doubles (stubs, mocks, fakes) and in turn reduce setup. For any logic that appears to be painful or common you can introduce domain concepts which can and should be public. The tests can be wrote at this level then. Just find the right test seam.
What is the benefit?
The ability to switch implementation details without breaking public functionality. A whole world of refactoring options are available, inline method, extract method, extract class, inline class, replace with library and so forth. As long as the tests pass, you can be confident.