Simplicity does not precede complexity, but follows it. - Alan Perlis
The Single Responsibility Principle (SRP) - one of the SOLID principles - is definitely one of my favourites principles in Object-Oriented Design. I find its simplicity and, at the same time, its complexity fascinating. I have known this principle for years and wrote about it for the first time over one year ago. Still, I can't stop thinking about how hard it is to apply it. Speaking to and pairing with many developers over the years, I noticed that every single one of them understands what the principle is about but when it comes to write code, either they forget it or they simply can't find a way to adhere to it. I'll give you my view about the problem and what I've seen happening in many occasions.
The outside view
Every class and method should have just one responsibility
This is what I call outside view and naming plays a big role here.
This view is generally used when we need to add some totally new code. We think what the new code is about, create a class and/or a method, give it an appropriate name that represents what the new code does and we are done. We also use this outside view when reading code. When looking at a class or method name, it should be clear to us what they do.
I don't think anyone ever had a problem understanding this principle.
However, if it is so simple to understand, why don't we all follow it? Why do we still find classes and methods with hundreds, if not thousands of lines? A few things come to my mind to explain that. Maybe we are just bad at naming classes and methods. Maybe the names we give are too generic or too broad, allowing us to write loads of code in there. Maybe some of us just don't care.
The inside view
Classes and methods should have one, and only one, reason to change
This is what I call inside view and it is the other (often forgotten) side of the Single Responsibility Principle.
This view is generally used when we need to change some existing code or add more code to it. That's when we look inside each class and method to see exactly what they do. More often than not, we get frustrated because they do much more than they should do and we need to go through many lines of frustration and WTFs to understand the bloody thing.
However, it is much easier to adhere to the SRP when we keep the inside view in mind instead of the outside view. That means, always think about the number of reasons to change a class or method instead of thinking purely in their responsibilities.
So, when and where do things go wrong?
NOTE: In general I find more SRP violations in systems where developers are not doing TDD and refactoring.
Here is some food for thought. I'm risking to say that the majority of SRP violations are found in the classes and methods that are closer to the interface of the system. For example, in a web application, it's very common to find large controllers, actions or backing bean classes. For swing applications, we tend to find large event handlers. For systems where the entry point is a queue, we tend to find large methods that are responsible for processing the messages.
In general, this happens because the classes and methods closer to the interface of the system have a more generic and broader responsibility. Quite often, these methods are responsible to trigger loads of business rules and/or complex workflows.
Imaging that the input to our system is a huge XML file representing a trade. Also imagine that the first method invoked to handle that is called "processTrade(tradeXML)", located in a TradeService class. What is the responsibility of this method? It is to process a trade, right? Should it have a different name? Our system is expected to "process" all trades received so it is fair to say that the first method handling the request (or reading from a queue) should be called processTrade.
In another example, imagine that an user added a few items to her shopping basket (web application), provided some payment details and clicked on the "place order" button. In our back end, we will have a method handling this request that probably will be called something like placeOrder(order). Fair, right?
Developing an idea
In general, the closer the code is to the system's interface, the broader and more generic its responsibility will be. The further away the code is from the system's interface, the narrower and more specific its responsibility will be.
In both examples above, by their names, you can argue that the methods processTrade and placeOrder have a single responsibility. One processes the incoming trade and the other places a customer order. So, when developers take into account just the outside view of the SRP, they feel comfortable to add as much code as they need to satisfy these responsibilities.
The problem is that processing a trade or placing an order may be extremely complicated tasks, where a huge workflow will be triggered, many business rules and requirements need to be satisfied and we will need to write hundreds, if not thousands, of lines for that. So clearly, adding all these lines to a single method doesn't just simply violates the SRP. It's also plain stupid.
So, in order to make our code compliant to the SRP, we need to have just a single reason to change it. This leads to a complementary idea.
In general, the closer the class is to the system's interface, the more delegation the class will do. The further way the class is from the system's interface, less delegation the class will do.
A general example would be, in a traditional Java web application, the controllers, that are closer to the UI, have a broad responsibility and tend to delegate all the business logic to other objects. They just control the flow. At the other end, we have the DAOs that have a very specific and narrow responsibility, almost never delegating anything to another class. In the middle, we have services, that do some business logic of their own but also delegate work to other collaborator classes. Services tend to have a narrower responsibility when compared to a controller but a broader responsibility when compared to DAOs. Still, each class and method have a single responsibility.
Asking a different question
When mentoring and/or pair-programming with other developers, quite often we end up discussing about the amount of code in a method or methods in a class. Just using the SRP's outside view as an argument sometimes is not enough to convince some developers that the code is doing too much. That's why I find the inside view more useful. Instead of asking how many responsibilities a method or class have, I now ask how many reasons we would have to change them.