Hmm, SOLID is very well-known and the principles are ubiquitous good practices for object-oriented programming and also useful for software dev in general.
Certainly, asking about them and , especially, asking the candidate to critique them is a very good question.
Perhaps there’s a difference between knowing and having internalized these concepts, versus having heard of them via a particular acronym, and being expected to remember that acronym and these specific names.
“Liskov substitution principle” for instance is something I had to look up. I can read it and say, “oh, duh, yes that’s a key part of what interfaces are even for.” I’ve built with it for decades now.
The term SOLID was apparently introduced in 2004. It would be a shame to reject good programmers who can opine intelligently on these principles, but who have not internalized this particular jargon.
Yes, there is quite a difference in my opinion. What does SQL stand for? No clue, but I can write you an SQL query. Same thing with HTML, CSS, PHP, etc.
That's funny. I wrote SQL daily for 10 years, read the ISO standards when working on SQL transpilers, persistence frameworks for multiple databases, etc. Even wrote a new storage engine for MySQL once.
But I just had to look up what the acronym stands for! Apparently it's "structured".
It’s generally not what it claims. Single purpose is a good idea, but the rest…
The idea that one class changing won’t change others is a pipe dream.
Substitution only matters if you’re using inheritance, but if you’re inheriting, you likely have bigger problems as few real world problems are naturally represented by inheritance.
The interface rule is redundant with the single principle idea. Maybe it should have been SOLD instead.
Dependency injection is a guideline at best. If you actually do what it says, you get enterprise fizzbuzz. It should only be used occasionally and sparingly at specific boundaries otherwise the cure becomes worse than the disease.
More controversially, when you avoid inheritance in favor of composition in a language like Java, you lose polymorphism while keep a leaky abstraction and really bad encapsulation while the language still retains all the mental and syntactic complexity of the inheritance bits.
> The idea that one class changing won’t change others is a pipe dream
Reality is never black and white but that's a good principle for good software design and worth at least aiming for.
If anything that has been adopted and generalised in microservices, etc. because encapsulation, modularity, design by contract all hinge of this same idea, which is powerful and useful.
That's why when discussing any "principles" the important is to get the core idea behind them instead of sticking to the letter and discarding them as useless.
Encapsulation is extremely important and pairs with single responsibility, but that’s completely orthogonal to OOP.
Once you are solidly in the composition camp, functions are immediately superior. They tend to be naturally single purpose and have mathematical rules that make them easy to compose.
Most function-heavy languages also adopted modules which provide a better, less-leaky abstraction too and that’s without getting into all the other advantages in the type system or better syntactic ergonomics from dropping all the OOP baggage.
Even in Uncle Bob's version of the single responsibility principle, 'one reason to change' is about people.
He words it poorly, but it is more about not commingling DBA, UX, core domain logic in a module.
People miss that and end up having low cohesion.
While SRP and interfaces do have an intersection, they are not the same.
Interfaces leaking implementation details is an example.
Composition, dependency inversion, and dependency injection are all forms of L.
Polymorphism is problematic in most cases but thinking it is the only way to accomplish that looser coupling goal is a misunderstanding of where we have grown with the concepts.
I tend to prefer the ports and adapters pattern, which ensures client components that invoke an operation on a server component will meet the preconditions specified as required for that operation.
That is equivalent to LSP or design by contract.
SOLID is just a set of principles that are intended to make loose coupling and high cohesion the ideal default.
This is in contrast to procedural code that we know makes unmaintainable systems.
The problem with SOLID is the same as Agile, XP etc...
People ignore the core ideas that are descriptive in nature and try to use them as oversimplified prescriptivist rules.
To be fair Robert Martin wasn't great at selling this and that is why I tend to point people to authors like Martin Fowler who are more careful about sounding like a preacher handing out laws.
But Robert Martin developed SOLID in the context of cohesion, and if you read it in that context it is valuable.
He didn't invent the SOLID tool to remember the rules, but it seems that blog posts and Wikipedia pages are as far as people get before trying to implement it or dismiss it.
Take a look at Uncle Bob’s code some day. You will then realize that all of his writing was synthesized solely to make him consulting money. His actual code is rubbish. This has been true all the way back to the 90’s with his inane questions to the OOP Usenet groups.
That's also my unpopular opinion about him, I thought I was alone. I watched some of his clean code videos in the past and thought his advice looked dated and very "enterprise" styled.
Back in the 1800's and early 1900's medicine was a very new science (in a modern sense), and snake oil salesmen abounded. Laymen had very little information to discern legitimate medical advice from quacks hocking piss and ink as a cure all.
We have the same problem now with software systems, although at the least we are advancing beyond the primitive stages and the charlatans are becoming more apparent.
The problem is that Uncle Bob made up SOLID and other ideas more or less by cobbling random ideas from other people and selling them on the market as his own. As many others have mentioned here, pieces of SOLID kinda-sorta make sense, but it doesn't hang together cohesively because it was never created in a cohesive manner.
ALWAYS look behind the curtain at what software consultants are selling to what they have actually built, and then you can ascertain if they are selling you hard won engineering knowledge or a bunch of marketing BS.
As I want to learn, outside of the problems with cargo culting, why having the SOLID principals as the default _ideal_ when programming cause problems.
Sure he was pulling from Structural design ideas, which even Tom DeMarco, the author of the canonical book on SD, now says should have been iterative and Agile-like, but he was also pulling from papers like the following from 1972 about modularity.
So is your reason for discrediting him that he aspired to be a professional writer, or that he tried to collect concepts from the wider tent into a more domain specific presentation?
Because structural design is an example where the founders realized that not paying attention to that first paper was problematic.
"We have tried to demonstrate by these examples that
it is almost always incorrect to begin the decomposition
of a system into modules on the basis of a flowchart.
We propose instead that one begins with a list of
difficult design decisions or design decisions which are
likely to change. Each module is then designed to hide
such a decision from the others. Since, in most cases,
design decisions transcend time of execution, modules
will not correspond to steps in the processing. To
achieve an efficient implementation we must abandon
the assumption that a module is one or more sub-
routines, and instead allow subroutines and programs
to be assembled collections of code from various
modules."
How about addressing Robert Martin's position, instead of irrelevantly attacking random aspects of the person who is making the argument. A.K.A Ad Hominem fallacies.
In the context of this thread, if a candidate could only express their objections to _ideals_ that are intended to be the defaults, and couldn't express their objections in a way that related to business and customer needs, I would be a hard no on the hiring position.
It is OK to not agree with these ideals, but to only offer personal attacks to dismiss them, it indicates someone who will probably be a difficult team member and lack customer focus.
Ad Hominem's never address the real concerns or needs of a project and are simply not constructive.
We know that EA failed, we know SOA works better if imperfect, we know that aiming for loosely coupled and highly cohesive code results in systems that are easier to maintain and scale, and we know that with the rise of the cloud most needs will be distributed. All of those apply the same principles at a different scale or domain.
Hand waving away those well known concepts because you have a distaste for an author won't change that. But if you share ideas that address the limits of those ideas perhaps it can help move us further.
>The SOLID principles are a specific application of computer science principles to OO.
There's little to no hard science and serious research behind them, aside from Uncle Bob writing about them and some hazy papers.
The Liskov substitution principle was developed by an actual computer scientist working on such research, but the general field of programming methodology Liskov worked in remains scientifically something like what psychology was before Freud.
Not saying Freud is scientific - saying that programming methodology is even less well established and researched scientifically in 2024 that psychology was not just at the time of Freud, but even before Freud.
Dijkstra himself, in many of his notes regarding methodology, was basically 90% opinion, even when he was right.
> Substitution only matters if you’re using inheritance, but if you’re inheriting, you likely have bigger problems as few real world problems are naturally represented by inheritance.
No, it matters when you implement interfaces (in the Java sense) too.
Tons of real world problems are naturally solved by having multiple implementations of interfaces. Also, Dependency Inversion doesn't even make sense without it.
It’s well known in a specific setting, possibly? I’ve managed to never hear of it in 18 years.
If asking “well known to some people” stuff is kosher, should I be able to quiz candidates on FFP design principles in Haskell, or maybe how to implement collision detection in a video game? Both of those are well known in swaths of the industry.
I guess if you’re hiring for a specific role, writing OOP business applications, you will have no shortage of candidates who know about SOLID, but you’re probably passing on everyone who is changing domains.
The only part of SOLID that is unquestionably correct is "L" - it is the only sane way to use subclassing in OO languages.
"D", when used judiciously, can be useful because it can make unit tests much easier to write. But when used too much, it results in incomprehensible code.
But as for "I", in my humble opinion, when you find yourself reaching for it, it's a sign that the code base is already poorly organized.
S (Single responsibility principle) is very important, not only for OOP but for good, maintainable software design in general (yes, even if you do firmware stuff).
If you think those principles got out of date then indeed I would not hire you.
Something like the "single responsibility principle" is just good software design to anyone who ever had to maintain any piece of software.
Quite extraordinary how commenters in this thread discard things as old without even understanding that core principles are as valid now as they were then.
>If you think those principles got out of date then indeed I would not hire you
If you think your hiring criteria are some kind of general yardstick, I wouldn't want to be hired by you in the first place. What's with people using their position of power (hiring people) as some kind of argument - or perhaps threat?
>Something like the "single responsibility principle" is just good software design to anyone who ever had to maintain any piece of software.
The "single responsibility principle" is an ad hoc, ill defined idea. A bad idea for many real world cases, that has to be applied with discretion - not taken as a core guiding principle. Not to mention that neither "reason to change", nor "change" is well defined by Martin.
It's as bad one-size-fits-all advice as advising about the ideal function being "two to four lines of code long" (also by Martin) - leading to crappy, hard to follow code, and needless abstraction.
A principle always has to be applied with discretion.
Hence why I think why the OP's idea to ask candidates to critique is good. It shows whether the candidate blindly follow things as the gospel or is able to articulate limitations but also the reason behind a "principle" because there are very good reasons behind them.
Certainly, asking about them and , especially, asking the candidate to critique them is a very good question.