The Open-Closed Nature of Independent Objects

In his book Object-Oriented Software Construction, Bertrand Meyer introduced the Open-Closed Principle. This principle may be one of the most confusingly stated ideas in all of computing and has led to a whole sub-industry of articles and podcasts explaining how a ShapeRenderer can draw Squares and Circles (of course, I have also partaken of such, and will continue here).

The Open-Closed Principle says that a module (an object, in our case) should be open to extension – it should be possible to extend its behavior for new purposes – and yet closed to modification – you should not need to change it. This design principle comes with a cost, as you need to design your objects to support extensibility along lines that are not yet known (or at least, to make it clear which lines are or are not going to be fruitful) in return for the benefit that maintainers and users of the objects know that they are going to be stable and will...