The Open-Closed Principle is Often Not What You Think it Is

jOOQ is a library that loves making everything internal final and package private. We have tons of classes like these:

final class Concat extends AbstractFunction<String> {
    // ...

The class implements the semantics of SQL string concatenation. Clearly, you shouldn’t need to tamper with it (or even know about it), because it is “protected” behind the corresponding public API in the DSL class:

// You can see this:
public class DSL {

    // You can see this but not override it:
    public static Field<String> concat(Field<?>... fields) {

        // But you cannot do this, yourself:
        return new Concat(nullSafe(fields));

Now, in the past decades, there have been a lot of software design movements that were contrary to the concept of encapsulation in some ways. The driving powers of that were:

A fun to read example of “slightly” (i.e. completely) exaggerated advocacy of extreme application of object orientation is Yegor Bugayenko’s blog:

Through exaggeration, he makes some really interesting points that make you think. Of course, you have to be able to accept the hyperboles as non-facts. Not everyone can do that, so don’t get angry reading 😉

Let’s look at the open-closed principle

The open-closed principle claims, according to Wikipedia:

In object-oriented programming, the open/closed principle states “software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification”; that is, such an entity can allow its behaviour to be extended without modifying its source code.

This is a very desireable aspect of some software entities. For instance, it is always true for an SPI (Service Provider Interface), by design, of course. Let’s read the Wikipedia definition of an SPI:

Service Provider Interface (SPI) is an API intended to be implemented or extended by a third party. It can be used to enable framework extension and replaceable components

Perfect. For instance, a jOOQ Converter is a SPI. We’ve just published a recent post about how to use the Converter API in a strategy pattern style with lambdas – the strategy pattern works really well with SPIs.

In fact, the strategy pattern isn’t even strictly an object oriented feature, you can get it for free in functional programming without giving it a fancy name. It’s just any ordinary higher order function.

Another fine example of what could be considered an SPI is an Iterable. While Iterable subtypes like List are more often used as APIs (user is the consumer) rather than SPIs (user is the implementor), the Iterable API itself is more of a way of providing the functionality required to run code inside of a foreach loop. For instance, jOOQ’s ResultQuery implements Iterable, which allows it to be used in a foreach loop:

for (MyTableRecord rec : DSL
    .orderBy(MY_TABLE.COLUMN)) { // Automatic execution, fetching

So, clearly, it can be said that:

  • Iterable follows the open-closed principle as it models an entity that is open for extension (I can produce my own iterable semantics), but closed for modification (I won’t ever modify the Java compiler and/or the foreach loop semantics
  • The Liskov substitution principle is also followed trivially, as the foreach loop doesn’t care at all about how I implement my Iterable, as long as it behaves like one (providing an Iterator)

That was easy

But when does it not apply?

In a lot of situations. For instance, jOOQ is in many ways not designed for object oriented extension. You simply should not:

  • Mock the concat() function.
    You might be tempted to do so, as you might think that you need to unit test everything, including third party libraries, and then you have to mock out the string concatenation feature inside of your database. But it doesn’t work. The DSL.concat() method is static, and the implementation hidden. No way you could replace it with ordinary means (there are some dirty tricks).

    But hold on for a second. Why are you even doing this? Aren’t integration tests the better way here? Do you really have time (and want to spend it) on replacing entire complex implementations with your mocks? I don’t think so. That hardly every works

  • Modify the concatenation behaviour for some use-case.
    While you may think that sometimes, you’d just like to tweak an implementation a little bit to get a quick win, that is certainly not the intent of the authors of the open-closed principle or the Lishkov substitution principle. We as API designers don’t want you to extend all of our functionality. As simple as that. Why? Because we want you to get in touch with us to help us improve our software for everyone, rather than you tweaking something for a quick win.

Let this sink in – especially the latter.

The premise that everything should be object oriented and everything should be extensible is wrong. Object orientation (and all the philosophies connected to it) are a tool. They’re a very powerful tool, for instance, when we as API/SPI designers want to allow users to extend our software. (mostly through SPIs). And we spend a lot of time thinking about really good, generic, useful, powerful SPIs that solve 99% of all extensibility problems in a way that we can control and keep backwards compatible. For some examples, check out these blog posts:

And sometimes, yes, we did not foresee a justified request for extensibility. Nothing is perfect. You have a feature request, and you cannot implement it right away. Then you start exploring. You look into ways how you can inject some behaviour into jOOQ. And as we Java developers like object orientation, we’re looking into writing subclasses to override existing behaviour. That’s what we were taught. That’s what we’re doing all the time. That’s what the combination of the open-closed principle and the Liskov substitution principle suggest.

Let me shock you for a moment.

Haskell (and many other languages) doesn’t support subtype polymorphism

Yes. There are entire ecosystems out there, that don’t have the luxury of bikeshedding the fact that if a class cannot be (easily) extended through subtype polymorphism and overriding of methods, it must be ill-designed. An entire ecosystem that never worries about something being final, and thus “closed for extension” (through subtype polymorphism).

Alternative definitions

Given the historic context, both principles are very interesting things. But their object-oriented context is something we should free our minds of. Here’s a better definition:

  • open-closed principle:
    Systems should strive for openness for extension, but not at any price. Some parts of a system / module / perhaps class should be open for extension. Those parts should be very well designed and kept very backwards compatible. And the vendor of those parts should listen to its consumers to better identify the required extension points. Consumers on the other hand shouldn’t blindly assume that everything can be extended. If they’re extending (through unexpected subtype polymorphism) random parts, then they’re hacking in the same way as if they would be actually modifying the system / parts. There’s no more benefit to extending.
  • Liskov substitution principle:
    Subtype polymorphism is just a tool, and in 2017, we have long started understanding that it’s a very wrong tool for many things. The composition over inheritance concept has shown that we’ve regretted the subtype polymorphism hype from the 90s. So, forget about your mocks through subtype overriding. Start looking for alternative interpretations of this principle. I like Jessica Kerr’s finding:

    Therefore, the Liskov Substition Principle says, “Don’t surprise people.”

    That’s a much better credo to follow, than the one that is strictly related to an aspect of object orientation and in particular to subtype polymorphism.


Yes. Package private, final classes mean, you cannot extend them. The open-closed principle is “violated”. Because that part of the system was not designed for you to know about (it’s encapsulated).

Sometimes, you think that if just you could override such an entity, you might get a quick win and inject your desired behaviour into a third party library / entity / class / module / system. My claim here is that: Mostly, you’ll deeply regret your desire for a quick win later on. You shouldn’t argue about open-closed or Liskov substitution. These principles simply don’t apply here. They do not at all, in particular, apply to badly designed legacy software. Once software is “badly designed”, no principles will help you.

Instead, do get in touch with the vendor if you run into a bump. There’s always an interesting idea for a great new feature hidden in such a limitation. And for the time being, accept that your overriding of what was not meant to be overridden is just the same thing as actually modifying that entity. You’re patching the library. Let’s do that and move on.

This is the Final Discussion!

Pun intended… Let’s discuss Java final.

Recently, our popular blog post “10 Subtle Best Practices when Coding Java” had a significant revival and a new set of comments as it was summarised and linked from JavaWorld. In particular, the JavaWorld editors challenged our opinion about the Java keyword “final“:

More controversially, Eder takes on the question of whether it’s ever safe to make methods final by default:

“If you’re in full control of all source code, there’s absolutely nothing wrong with making methods final by default, because:”

  • “If you do need to override a method (do you really?), you can still remove the final keyword”
  • “You will never accidentally override any method anymore”

Yes, indeed. All classes, methods, fields and local variables should be final by default and mutable via keyword.

Here are fields and local variables:

    int finalInt   = 1;
val int finalInt   = 2;
var int mutableInt = 3;

Whether the Scala/C#-style val keyword is really necessary is debatable. But clearly, in order to modify a field / variable ever again, we should have a keyword explicitly allowing for it. The same for methods – and I’m using Java 8’s default keyword for improved consistency and regularity:

class FinalClass {
    void finalMethod() {}

default class ExtendableClass {
            void finalMethod      () {}
    default void overridableMethod() {}

That would be the perfect world in our opinion, but Java goes the other way round making default (overridable, mutable) the default and final (non-overridable, immutable) the explicit option.

Fair enough, we’ll live with that

… and as API designers (from the jOOQ API, of course), we’ll just happily put final all over the place to at least pretend that Java had the more sensible defaults mentioned above.

But many people disagree with this assessment, mostly for the same reason:

As someone who works mostly in osgi environments, I could not agree more, but can you guarantee that another api designer felt the same way? I think it’s better to preempt the mistakes of api designers rather than preempt the mistakes of users by putting limits on what they can extend by default. – eliasv on reddit


Strongly disagree. I would much rather ban final and private from public libraries. Such a pain when I really need to extend something and it cannot be done.

Intentionally locking the code can mean two things, it either sucks, or it is perfect. But if it is perfect, then nobody needs to extend it, so why do you care about that.

Of course there exists valid reasons to use final, but fear of breaking someone with a new version of a library is not one of them. – meotau on reddit

Or also…

I know we’ve had a very useful conversation about this already, but just to remind other folks on this thread: much of the debate around ‘final’ depends on the context: is this a public API, or is this internal code? In the former context, I agree there are some good arguments for final. In the latter case, final is almost always a BAD idea. – Charles Roth on our blog

All of these arguments tend to go into one direction: “We’re working on crappy code so we need at least some workaround to ease the pain.”

But why not think about it this way:

The API designers that all of the above people have in mind will create precisely that horrible API that you’d like to patch through extension. Coincidentally, the same API designer will not reflect on the usefulness and communicativeness of the keyword final, and thus will never use it, unless required by the Java language. Win-win (albeit crappy API, shaky workarounds and patches).

The API designers that want to use final for their API will reflect a lot on how to properly design APIs (and well-defined extension points / SPIs), such that you will never worry about something being final. Again, win-win (and an awesome API).

Plus, in the latter case, the odd hacker will be kept from hacking and breaking your API in a way that will only lead to pain and suffering, but that’s not really a loss.

Final interface methods

For the aforementioned reasons, I still deeply regret that final is not possible in Java 8 interfaces. Brian Goetz has given an excellent explanation why this has been decideed upon like that. In fact, the usual explanation. The one about this not being the main design goal for the change 😉

But think about the consistency, the regularity of the language if we had:

default interface ImplementableInterface {
            void abstractMethod   () ;
            void finalMethod      () {}
    default void overridableMethod() {}

(Ducks and runs…)

Or, more realistically with our status quo of defaulting to default:

interface ImplementableInterface {
          void abstractMethod   () ;
    final void finalMethod      () {}
          void overridableMethod() {}


So again, what are your (final) thoughts on this discussion?

If you haven’t heard enough, consider also reading this excellent post by Dr. David Pearce, author of the whiley programming language

A dirt-ugly hack to modify private final fields in Java

We all use reflection from time to time. We may even tamper with visibility through Java’s Field.setAccessible() and similar methods. But this post here takes things to the extreme and shows how to modify private (static) final fields in Java. Think twice, when choosing this tool 😉

Wikileaks To Leak 5000 Open Source Java Projects With All That Private/Final Bullshit Removed

Hilarious. Not even written on April 1! Taken from here:

EYJAFJÖLL, ICELAND — Java programmers around the globe are in a panic today over a Wikileaks press release issued at 8:15am GMT. Wikileaks announced that they will re-release the source code for thousands of Open Source Java projects, making all access modifiers ‘public’ and all classes and members non-‘final’.