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Previously I wrote:

Sometimes you work with something for so long that you forget that other people aren’t familiar with it. This happened recently talking to a friend of mine — “I’ve got no need for object orientation”, he said, and I was shocked. [...] What, I wondered to myself, were the benefits of OO that he was missing? How could I persuade him that it does have a point? So here is my attempt to trawl my memory and see how I’ve changed in response to OO, and why I think it serves me (and my projects) better. In the end I’ve found that the benefits are real, but nothing I would have guessed at when I first started in this field.

I’ve already written about data hiding. Now read on…

Separation of concerns

This is one of those things that is a consequence of OO fundamentals, but by no means obvious. The root of so many problems in software is dependencies. A depends on B, B depends on C, and before you know it everything depends on everything else. It makes things very difficult to manage: changes become slower and slower, and riskier and riskier. Separation of concerns is about moving away from this: by forcing things into classes it encourages each class to do its own specific job. This breaks the dependencies and makes the management of your code much easier.

But it’s not that easy or obvious, and requires a fair bit of imagination to see it working really well. Let’s take a very simple example.

Suppose Guardian Unlimited has an Article class, and articles can be saved into a Repository (probably a database). Then we have two classes, each with a separate concern; the Article class has a save() method which uses the Respository to save to the backing store. But hold on there — we’ve still got the chance for it to go wrong.

An easy mistake would be to have the Article manage the setup of the Repository: when you create an Article it sets up the Repository all by itself. In some ways this is nice (it’s a form of encapsulation) because you can create and save an article without worrying about how the save is done, or even that there is a Respository at all. But there is a nasty dependency here: the Article depends on the Repository: not only are we unable to set up an Article without having a Respository, but it’s the Article which determines what the Repository is — if your application requires a working SQL Server database then it will always require a working SQL Server database, even if you just want to do a little test.

Enter dependency injection [1, 2, 3]. This is a technique that is by no means an obvious consequence of object orientation, but is facilitated by it.

In our example the Article depends on the Repository. It also specifies exactly what the Repository is, because it sets it up internally. Dependency injection says we shouldn’t allow the Article to set up the Repository; rather, we should set up the Repository externally and inject it into the Article (for example by calling a setRepository() method). The dependency isn’t broken, but the dangerous element is: the particular kind of Repository is no longer a concern of the Article.

What this means in practice is that you can feed any kind of Repository into the Article and it will still work. You can even inject a “do nothing” Repository just in case you want to do a quick test which doesn’t require saving it into the real database. Or you could inject a disposable in-memory Repository which only lasts for a limited time.

All of this is possible with another feature of OO: abstraction. If we ensure Repository is only an abstraction (rather than a specific concrete class) then we can have all kinds of Repository implementations: a SQL Server one, an in-memory one, one which always throws an error (so you can check how your application handles errors), and so on. In VB.NET you’d use an interface for this particular example.

Dependency injection forces concerns to stay separate and makes your code much more flexible and easy to manage. But that wouldn’t be obvious to you if you’re just starting out in OO development, and if you’re just looking at the concept of objects, inheritance, and so on.

All installments:

• Part I of V, data hiding
• Part II of V, separation of concerns (this installment)
• Part III of V, better testability
  
• Part IV of V, refactoring
• Part V of V, evolving the product

Sometimes you work with something for so long that you forget that other people aren’t familiar with it. This happened recently talking to a friend of mine — “I’ve got no need for object orientation”, he said, and I was shocked. Yet I couldn’t easily explain why I thought OO was far superior to procedural languages. And thinking about it afterwards I realised that its advantages for me weren’t obvious — almost none of them are clear from OO’s fundamentals (inheritance, abstraction, etc), and almost all of the advantanges are things that have been learnt by most of us recently, years after OO first appeared.

My friend spends most of his time writing embedded software, but occasionally is called upon to write a front-end interface. His front-ends are Windows-based and for his last GUI project he chose to try out VB.NET rather than his usual VB6. He was baffled by the verbosity and he felt it didn’t add anything. It all seemed rather unnecessary. And yet to me it’s long been the only way.

What, I wondered to myself, were the benefits of OO that he was missing? How could I persuade him that it does have a point? So here is my attempt to trawl my memory and see how I’ve changed in response to OO, and why I think it serves me (and my projects) better. In the end I’ve found that the benefits are real, but nothing I would have guessed at when I first started in this field.

Not the point of OO

DNJ Online has an article on migrating from VB6 to VB.NET, and it says that from this point of view the OO language is “unlike anything you’ve used before”. This is helpful in its honesty, but that particular article explains the low level differences rather than the high level aims. Try/catch blocks and namespaces are definite steps forward, but they hardly justify learning an entirely new paradigm, let alone phasing out a well-established one.

Similarly, common justifications of OO is that objects promote code re-use and allow better modelling of user requirements, but it’s hard to see how this is obvious. Regards the former, procedural languages allow code reuse via libraries. Regards the latter, it’s just as easy to see user requirements as procedural things as object-based things — easier, arguably.

For me, the advantages of OO are not obvious. But they are tangible once experienced, given the right environment. For me, those advantages are…

Data hiding

Even I’m horrified by how little fits into my tiny brain, and the more I have to keep in my head at any one time the more likely I am to make mistakes. In terms of development, the more attention I have to pay to the intricacies of my software the less attention I can pay to solving whatever the immediate problem happens to be.

Fortunately OO’s concept of data hiding (or encapsulation, if you like long words) is for people like me. By making explicit what I need to know (public methods/functions) and hiding what I don’t (private methods/functions and fields/variables) I am freed to focus on the task in hand. Private fields and methods allow my classes to manage themselves without having to burden the user with the details. You can look at the class (or its auto-generated documentation [1, 2, 3, 4]) and focus on what you need to know, without getting distracted by the details.

Anything which makes me think less is huge benefit.

Of course, data hiding by itself isn’t enough to switch programming paradigm, and if you’re writing in VB6 you’ll already have the “private” keyword. But this is only the start…

All installments, which I’ll post over the next few days:

• Part I of V, data hiding (this installment)
• Part II of V, separation of concerns
• Part III of V, better testability
  
• Part IV of V, refactoring
• Part V of V, evolving the product

Compare and contrast two blog entries that popped up in my RSS reader on the same day last week. In the blue corner Joel O’Software, fighting for private offices. And in the red corner, Martin Fowler, battling it out for continuous collaboration between developers and their customers.

Naturally, they’re not really at opposite ends of a spectrum — in fact, their posts are about different things — and they’ll find a lot to agree with each other. But look at a select part of each of their text and see the contrast. Here’s Joel:

Not every programmer in the world wants to work in a private office. In fact quite a few would tell you unequivocally that they prefer the camaradarie and easy information sharing of an open space. Don’t fall for it. They also want M&Ms for breakfast and a pony.

And here’s Martin:

One of Kent’s suggested names for ‘Agile’ was conversational software development - the point being that it’s a two way communication. This isn’t something like a telecoms protocol that you can define, but the back and forth discussions about how software can enhance the business are where the real value lives. Much of this conversation is of half-baked ideas, some of which grow into valuable features - often ones that aren’t things that the customer originally thought of.

It’s notable how two people renowned for being leaders in software can be veering apart on what should be a fundamental issue: how should people interact? Joel is for concentration, Martin is for conversation.

It’s notable also that they do different things in the software world. Joel produces shrink-wrapped products on behalf of his own company. So does Microsoft, the company Joel used to work for and who he praises for “putting literally everyone in individual, private offices”. Martin is a gun for hire (via his employer), called in to consult on a variety of projects for different companies, no doubt 90% of the time producing in-house software for each client.

I can’t escape the feeling that their respective backgrounds inform their respective views, though wouldn’t for a second think that implies that one approach always suits one kind of output.

So, should one prefer concentration or conversation? Obviously[*] it depends on several factors, and here’s the way I see it…

First, it must come from whoever you start with. If Joel O’Software starts a one-man band and likes to work in silence, then takes on his first employee, he’s not going to want his hire to keep piping up with questions every two minutes. Similarly, if you’ve created your business by extolling the virtues of pair programming then you’ll be looking exclusively for developers who will continue that good work. If you start with one introvert or extravert, you’ll grow from there.

Second, it depends on where you think your strengths are as an organisation. Joel talks a lot about productivity and algorithms, while Martin tends to talk of people and methodologies. Each also has an interest in the other’s topics, but their chosen hot subjects are where they see the biggest gaps, and where they think they can most make a difference.

Third, it’s about how you see your team. I suspect Joel has very low staff turnover, hires developers very infrequently, and there’s no doubt he puts a lot of effort into picking the cream of the crop: he’s in a buyer’s market, and his developers will all be very smart. Martin will inevitably work with a much broader range of companies. While they will of course have made a very smart decision to hire him and his colleagues (ahem) they will tend more towards the market norm, and will also tend to be fairly large development teams — even if individual project teams are smaller. Thus Martin is going to be much more concerned about sharing information between developers, evolving designs collaboratively, establishing standards and keeping those standards refreshed.

Finally, it’s about how you see the long term. Again, low staff turnover and a tight-knit team means Joel is less concerned about knowledge silos, but an average corporate team will have average turnover and will have its average share of crises. Knowledge sharing and reducing single points of failure is essential in these cases.

All of that is why I favour conversational development. Knowledge sharing and evolving ideas is key to me as a general principle, all the more so that Guardian Unlimited is such a diverse site that there’s just so much to know. That doesn’t mean it’s easy for everyone, but for me a typical team will be stronger if it keeps every last bit of information flowing round, ideas constantly exchanged and checked, and experience continuously refreshed and revised. Concentration is often needed, but too often the price paid is too high, and is only found when one person is seconded to another project, has left, or is holiday.

[*] I think there’s a progression whenever you ask “Is A or B better?” Naive or inexperienced people will always pick one or the other. Eventually they’ll come to hedge their bets because they realise things are more subtle than they previously thought, or else it just makes them sound wiser. Finally they may reach a point when they are (or regard themselves as) leaders in their field and act as evangelists or iconoclasts pushing one or other opinion heavily. You’ll see a lot of hedging on this blog.