Friday, 1 April 2011

Getters and Setters are Evil

Update: There is a new version of this post.

I've been programming with OO languages since I was seventeen yet in the last week I've had what is without doubt one of the biggest learning experiences since I've started.

Numerous developers that I've worked with claimed that we aren't doing OO properly. By we I mean software developers as a whole. Their argument being having all your code defined in classes does not mean you are obeying OO principles. By this they are often referring to the "Tell Don't Ask" principle. One particular individual at Codeweavers introduced me to idea that getters and setters are evil. While not true at face value, this statement is to get you thinking about what you expose to the outside world. Consider one of the founding pillars of OO programming; encapsulation.

Encapsulation states that an objects internal state should be just that, internal. If we want a object to do something we should tell it. We shouldn't care how its done either. The more I begin to think about what I'm programming the more I begin to question myself. In a recent programming session this was even more apparent. I stumbled across a situation in which I wished to hide a objects internal state, and in turn tell the object to do stuff. The problem I encountered was how the hell do I display the state of this object to the user (say on a GUI) without adding a load of properties (getters/setters).

Had I added the properties to the object I could ignore the methods on the object and just dig down and fiddle the objects internal state from the outside. This was not right, alarms bells were going off yet I was unsure how to solve this. Thankfully some inspiration from a helpful StackOverflow user and advice from a collegue pointed me in the right direction.

The solution was simple and can be summarised in the following pseudo code:

A more encapsulated approach could be:

A business object should property free, or at least only able to be updated from the outside world by asking it to do something. Its internal state may be internal to the class itself, or there may be a DTO passed in at construction it matters not. The Writer in this case is an abstraction around some form of output. We could have a console writer, HTML writer, Json writer and so on. By inverting the dependency we can avoid adding properties to the business object. Any consumer of this object must invoke the business object's methods - aka tell the object to do stuff. There is no way of the outside world modifying this objects internal state without abiding by the business rules. What's nice about this revelation?

There are many examples of this pattern at Codeweavers, yet I was unaware of the problem it was solving. By being burned by this issue personally the reason for patterns such as the one detailed above become much clearer and stand out. Whats better is when this problem crops up again I'll be able to handle it.

Properties or accessors have their place. They are required for DTOs, frameworks and certain language features, yet as with any tool their usage should always be considered. Blindly adding properties to a object, or worse, having the IDE auto generate accessors to an objects state is a clear problem. The biggest lesson I've took away is that even for input/output the use of accessors is not required. As usual the Codeweavers saying of "if it feels wrong, it probably is" still holds true and on that I'm off to try and write some proper OO code for the first time in six years...