Why composition is often better than inheritance

http://joostdevblog.blogspot.com/2014/07/why-composition-is-often-better-than.html

by exch • 12 years ago

192 90 12 years ago

joostdevblog.blogspot.com

chton8• 12 years ago

While it's a well-written article, it really seems like beating a dead horse. Composition over inheritance is a basic rule of OO programming, so much so that it has its own wikipedia page (http://en.wikipedia.org/wiki/Composition_over_inheritance)

Torn2• 12 years ago

This horse still needs to be beaten.

Messy inheritance still plagues product Java, C#, and, with the increasing proliferation of MVC frameworks, JS these days.

Hierarchies usually start out small. But, when new features are added and scope creeps, they get deeper and more abstract and messier.

Substitutability (i.e. the L in SOLID principles) does require more boiler-plate when using composition though. Interfaces and mixins (if available in your language) go some way to helping.

dtech2• 12 years ago

Do you have examples? Most modern Java libraries I know (e.g. Guava) implement interfaces and heavily use composition.

The "inheritance based" things mostly got deprecated/replaced when Java 1.5 introduced generics and most libraries needed to be heavily changed anyways.

chton8• 12 years ago

I haven't really noticed that this particular horse needs much beating (I work mainly in the C# world). It's drilled so well into beginner programmers that they tend to forget other principles like Single Responsibility.

tonyedgecombe• 12 years ago

I wish we could move beyond this idea that just because someone uses a particular language their code is going to be poorly written.

kyllo2• 12 years ago

Not to mention Ruby on Rails, that codebase is a huge mess of inheritance.

EdwardDiego• 12 years ago

Hierarchies usually start out small. But, when new features are added and scope creeps, they get deeper and more abstract and messier.

We have real issues with some complex class hierarchies that another team likes - they keep adding yet another abstract subclass of an abstract subclass to handle more cases, so you can end up following logic up and down multiple levels of classes when reading code, and missing one overridden statement can dramatically change the outcome.

dtech2• 12 years ago

I agree, but it is a well written article with interesting (simplified) real-life examples, so might be good to show the concept to an unaware or beginner programmer.

And everyone likes to read stuff that reinforces their own belief :)

chton8• 12 years ago

That's true, but there are a huge amount of resources about this particular thing already. It's pissing in the ocean by this point.

achr2• 12 years ago

Your wikipedia link actually does a good job of showing why the composition shown in the article is poorly conceived. The whole point of 'composition over inheritance' is that your object is still polymorphic on the composed-of classes.

chton8• 12 years ago

that's a good point, in the process of trying to use composition the author has made a (very) leaky abstraction.

jerf2• 12 years ago

Many of us still learned about OO in school, where inheritance was all but beaten into us, and composition was not mentioned. And based on what I've seen in interviews, this is still a common model taught in school. Composition is in the process of winning as the default method of composing things together in OO (see, for instance, Go), but it has not won yet.

And on the topic of Go, note how most common OO languages in use still have more convenient support for inheritance than composition, where "inheritance" is one token in the right place but "composition" takes a lot more boilerplate because there's no support built in, or you have to add a third-party library to make the boilerplate go away. Again, changing over time, but languages change slowly.

judk2• 12 years ago

Composition always takes more boilerplate, because inheritance is noting more than composition with a bunch of default choices for a privileged component (single inheritance) or arbitrary sorting of components (multiple inheritance) made so the programmer doesn't have to write them (but also loses control).

benihana• 12 years ago

While it's a well-written article, it really seems like beating a dead horse

I graduated college about 10 years ago, and I was never taught anything near composition over inheritance. I was told about inheritance, but had to learn from other coworkers and experience that composition is much favored over inheritance. Now I mentor a number of junior developers and they need to be told composition over inheritance often. This isn't a problem that has fixed itself, and it's most certainly not beating a dead horse when it needs to be repeated for so many young programmers.

This isn't just a problem with formal CS courses - code schools like Hacker School and Flatiron School have this issue as well. It seems more natural and intuitive to inherit than compose. So this horse needs to continue being being as it's very much alive.

judk2• 12 years ago

Effective Java is the textbook of junior professional programming. All your new hires shoyld read it.

timr2• 12 years ago

"I was told about inheritance, but had to learn from other coworkers and experience that composition is much favored over inheritance."

You weren't taught that, because it isn't true. Inheritance is appropriate sometimes, composition at other times.

In particular, when a two objects are modeled by an ISA relationship, you should use inheritance. When they're modeled by HASA, you should use composition.

A Dog is not an Animal that includes Barkable and PeeOnHydrantable, and a SpaceShuttle is not a descendant of Airplane.

moonchrome• 12 years ago

It seems more natural and intuitive to inherit than compose

Especially when you finally figure out inheritance and OO you fall in to that "when all you have is a hammer".

Composition is not an obvious way to approach this problem when you've been taught to solve problems with inheritance - and it fits "so nicely".

kazinator• 12 years ago

It's not a rule of OO programming; it's a useful guideline for using certain programming languages that purport to be OO.

jiaweihli• 12 years ago

I think this is a tooling issue. People initially tend to favor inheritance because it looks cleaner than composition. Mixing a lot of unrelated code in the same class makes things hard to find. (which method applies to which composed object?)

In languages that build in a concept of traits/mixins however, this isn't an issue.

k__2• 12 years ago

I think so too.

In JS it's rather easy to compose objects. In Java it feels clunky, you code looks kinda wrong when you done.

zo12• 12 years ago

I don't quite understand this. In my opinion JS "classes", if you want to call them that, look, feel and act very clunky. My opinion, of course... Please could you explain to me how you feel Java "classes" feel clunky and look wrong? I'm genuinely curious, and open to being convinced.

Note I substituted your usage of "objects" with classes, because they're vastly different concepts. I know, murky water when it comes to JS, but still.

falcolas3• 12 years ago

I disagree that mixing remove this issue. Even with mixins, you will still have places where all of that boilerplate from composition comes back, just with a concept like Python's `super` thrown back in.

And mixins in the wild are rarely so pure as people like to think they are - they often inherit from their own parents or other mix-ins, creating the diamond (or worse!) inheritance problem outlined in the article.

andybak• 12 years ago

The Wikipedia article chton mentions (https://en.wikipedia.org/wiki/Composition_over_inheritance) ends with the following when discussing the drawback of composition (boilerplate for forwarding methods): "This drawback can be avoided by using traits or mixins."

Now this is where things get a little blurry for me.

Mixins can help with the main drawback of Composition - but Mixins ARE inheritance - so isn't this a contradiction?

If I use PhysicsObjectMixin in my CharacterComposition class then I have to inherit from it. So aren't we back with the perils of inheritance?

chton8• 12 years ago

the idea is that you create a mixin that has only the reference to the physics object and the forwarding methods. If you implement that mixin, it will be exactly equivalent as writing them out in your class, but you are spared from having to do it for every single class with a physics object.

Remember that a mixin, by definition, isn't the same as inheritance. The methods, fields and properties are compiled into the class, not inherited. It's essentially a fancy way to 'include' a code file.

seanmcdirmid6• 12 years ago

Mixins can be implemented in a variety of ways, they don't need to be unmodularly inlined into class/object definitions as in scala. Also, mixin-style inheritance is by definition linearized multiple inheritance (at least according to cook/bracha, things get weirder with the Gabriel/Common Lisp definition).

kissgyorgy• 12 years ago

In Python, we use mixins. Mixins can only inherit from 'object' and nothing else, like this:

    class PhysicsobjectMixin(object):
        def update_physics(self):
            pass
        
        def apply_konckback(self, force):
            pass

        def get_position(self):
            pass


    class FightMixin(object):
        def attack(self):
            pass

        def defend(self):
            pass

    
    class TalkMixin(object):
        def say_something(self):
            pass


    class Character(PhysicsobjectMixin, FightMixin, TalkMixin):
        pass


    class Pickup(PhysicsobjectMixin):
        pass


    class Projectile(PhysicsobjectMixin):
        pass

it's still inheritance, but the classes will be flat; every class only inherits one deep, so there will be no diamond problems and no repeating code.

jhasse2• 12 years ago

I don't see the difference to interfaces.

skj2• 12 years ago

The stuff you mix in has code. Interfaces do not.

zo12• 12 years ago

The difference is that those aren't interfaces. I'm confused by the question?

Walkman4• 12 years ago

With interfaces, the above example would look like this:

    from abc import ABCMeta   

    class PhysicsobjectMixin(ABCMeta):
        @abstractmethod
        def update_physics(self):
            pass
        
        @abstractmethod
        def apply_konckback(self, force):
            pass

        @abstractmethod
        def get_position(self):
            pass


    class FightMixin(ABCMeta):
        @abstractmethod
        def attack(self):
            pass

        @abstractmethod
        def defend(self):
            pass

    
    class TalkMixin(ABCMeta):
        @abstractmethod
        def say_something(self):
            pass


    class Character(PhysicsobjectMixin, FightMixin, TalkMixin):
        def update_physics(self):
            pass

        def apply_konckback(self, force):
            pass

        def get_position(self):
            pass

        def attack(self):
            pass

        def defend(self):
            pass

        def say_something(self):
            pass


    class Pickup(PhysicsobjectMixin):
        def update_physics(self):
            pass
        
        def apply_konckback(self, force):
            pass

        def get_position(self):
            pass


    class Projectile(PhysicsobjectMixin):
        def update_physics(self):
            pass
        
        def apply_konckback(self, force):
            pass

        def get_position(self):
            pass

This is not DRY at all. I like mixins much better.

heylo• 12 years ago

Is that just multiple inheritance, or are mixins something else?

Walkman4• 12 years ago

Multiple inheritance with your discipline to not do more complicated things. Only inherit from base 'object', nothing else, so you will have no complicated inheritance problems.

lispm4• 12 years ago

That's just normal multiple-inheritance. Nothing like mixins.

Mixins would be if the classes actually contribute pieces which combine in an interesting way. Like a border-mixin added to a button class would add to the drawing and to the geometry of the object.

clubhi• 12 years ago

Actually some cases of mixins are equivalent to multiple inheritance.

kyllo2• 12 years ago

I don't know if you've ever used Django, but the entire thing is based on inheritance. When they introduced Class Based Views it got so complex that someone had to make this site just for exploring the inheritance tree: http://ccbv.co.uk

Walkman4• 12 years ago

There is a good discussion about it here: http://lukeplant.me.uk/blog/posts/djangos-cbvs-were-a-mistak...

teamhappy• 12 years ago

It's pretty much the same example I used here a couple of days ago: https://news.ycombinator.com/item?id=7976227

Game development seems to be the poster child for composition over inheritance.

Here's a lengthy article that explains it way better (IMHO): http://gameprogrammingpatterns.com/component.html

userbinator• 12 years ago

While it's possible to overuse inheritance, I don't think replacing it with composition is all that much better, and in addition all those forwarding methods that do nothing more than call another (could they even be optimised out?) are a great example of code that would need to be written, consuming resources like programmer time, but otherwise serves no true useful purpose to the functionality of the software. The complexity only changes form, so instead of tracing the flow through an inheritance hierarchy you're just doing it through chains of forwarding methods. It's for this same reason I don't believe so much his argument for readability and short classes - breaking everything up does not make things simpler, it makes the complexity spread out over a larger area; while it may be true that it is easier to understand an individual piece, it becomes more difficult to understand the system as a whole. This is especially important when debugging, where "can't see the forest for the trees" is a big hindrance.

I think his example of flexibility is the strongest argument for composition, because in that case the forwarding methods are not a waste - they would need to do (useful) work to determine which of the multiple composited objects they would need to work with.

Being mostly a C programmer who does OO-things, I use inheritance when it's obvious that most of the "methods" will be passthroughs to the "superclass", and composition when there is something more that needs to be done. Also, as I am not constrained by the OO model/conventions of the language, it's more flexible in that I can do things like "inherit" multiple times and even change that at runtime, so there is really no strict separation between composition and inheritance; to me, it's just "which function do I set this to point to."

twic2• 12 years ago

The complexity only changes form, so instead of tracing the flow through an inheritance hierarchy you're just doing it through chains of forwarding methods. It's for this same reason I don't believe so much his argument for readability and short classes - breaking everything up does not make things simpler, it makes the complexity spread out over a larger area; while it may be true that it is easier to understand an individual piece, it becomes more difficult to understand the system as a whole.

Preach it! Whilst immense monolithic classes are bad, smashing a system up into a million tiny bits is just as much of a barrier to understanding. It is baffling to me that this is not immediately obvious to everyone.

See also the microservices movement!

rybosome• 12 years ago

That's a fair point, although long chains of forwarded methods could be indicative of poor design - writing inheritance via composition in much the same way that one may write Lisp via Java. Granted, it's very easy to say that this is a bad idea, but that's part of the art of good OO design. :-)

I ran into this at work recently when refactoring an inheritance hierarchy. My initial instincts led me towards many forwarding calls, but after taking some time to reflect on the problem further, I found that many were no longer needed given the smaller scope of each class.

yayitswei• 12 years ago

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_pmf_• 12 years ago

The burden of proof should fall onto the user of inheritance to justify his decision. Interfaces plug delegates is more tedious to implement, but greatly reduces the chances of an architecture turning into a complete train wreck.

I often wonder why declarative delagating is not a first class concern in programming languages.

fithisux2• 12 years ago

Why often and not always? Can you provide an exceptional case?

arunix• 12 years ago

Exception hierarchies?

fithisux2• 12 years ago

Thank you very much. I will do my homework.

kstenerud• 12 years ago

In this case I'd argue that the roles are a bit messy and Character has too much knowledge. Character should not know that physics objects can be updated, and certainly shouldn't be calling updatePhysics. You could end up with an updated Character interacting with a Character whose physics state hasn't been updated yet.

applyKnockback: Character -> Physics object -> Physics engine

updatePhysics: Physics engine -> Physics object -> Character new position (x, y)

updateCharacter: Character reacts to change

javinpaul• 12 years ago

Couldn't agree more than this. I have also shared my 2 cents on Why composition is better than Inheritance for Java Programmers here http://javarevisited.blogspot.sg/2013/06/why-favor-compositi...

pllbnk• 12 years ago

When I try to choose between the two, I often like to think if the object I try to inherit from is from the same domain/context and solves a related problem. In the example in the article a PhysicsObject solves the problem of calculating coordinates in space and from the beginning it was not designed as something to be used in the game by itself. While the character participates in the actual game and executes the game logic. The character does not 'inherit' from PhysicsObject, it merely knows that PhysicsObject represents it in the space.

kilemensi• 12 years ago

I think one of the biggest reasons why most libraries/frameworks/apps/etc. use inheritance over composition is the easy with which the underlying languages allow the use of inheritance as opposed to composition. Most of these software writers know SOLID and other OO principles but when they're faced with the actual implementation, inheritance is just too damned easy to implement.

zak_mc_kracken• 12 years ago

We've known this since at least 1994, when the GoF book [1] famously said:

"Favor object composition over class inheritance"

[1] http://www.amazon.com/Design-Patterns-Elements-Reusable-Obje...

drumdance• 12 years ago

Ha, that's one of the few things I remember from that book. (Not knocking the book, just my poor memory.) I remember it being a big "a ha!" for a project I was working on.

known• 12 years ago

Sounds like https://en.m.wikipedia.org/wiki/Triarchy_%28theory%29

al2o3cr• 12 years ago

Meh. I think this phrase has been repeated until it has lost any connection with the original intent and turned into a generic "INHERITANCE BAD! COMPOSITION GOOD!" without much meaning attached to either word.

I haven't found the original source, but I've always presumed the statement originally referred to some of the bizarro "inheritance-as-composition" stuff in the early C++ days: for instance, you might have a class 'Window' and a class 'Button', then combine them with multiple inheritance to get a 'WindowWithButton', then inherit from that and a 'Scrollbar' class to get 'WindowWithButtonAndScrollbar'.

I can't imagine anybody thinking of that as a "good" pattern today, but remember it was the '90s. :)

Nowadays, the basic statement has been dogmatized to the point where you get code like this:

https://github.com/elm-city-craftworks/broken_record/blob/ma...

This code re-implements Ruby's built-in method lookup algorithm, but with per-instance objects and none of the optimizations available to the real thing. It basically remakes inheritance, slowly and poorly, using composition.

The other one that makes me scratch my head: people who rail against inheritance, then suggest mixins as an alternative. At least in Ruby, the two are equivalent. Check the `ancestors` property on a class with mixins sometime if you don't believe me.

TL;DR (too late) - use your damn brain to make decisions, not just parrot slogans.

joevandyk• 12 years ago

You can only inherit from one class in ruby. You can mixin multiple modules into a class. So they aren't entirely equivalent.

stouset• 12 years ago

Under the hood, mixing in modules actually is inheritance. An anonymous class is created that has all the module's methods, and this class is added to the inheritance hierarchy.

They are quite literally identical, even if Ruby tries it's best to hide that fact.

SEMW• 12 years ago

the basic statement has been dogmatized to the point where you get code like this:

From the readme of that repo: "It is not suitable for any real purpose ... it may be a fun starting point for palying around with design strategies ... [the compositional design] is probably a bad idea for a number of reasons, but is worth investigating."

When someone explains at length that some code they've put up on github is experimental, probably-a-bad-idea, not-for-serious-use, just-playing-around-with-design-strategies code, deep-linking to it in order to hold it up as an example of 'bad things people are doing nowadays' seems a little uncharitable.

It certainly makes me think twice about putting my own just-for-fun code experiments up on github in the future (without a disclaimer at the top of every file, anyway).

taeric• 12 years ago

I would prefer this with "why a shallower abstraction pool is better than a deep one." I've seen some compositional concoctions that were just as terrible to deal with as inheritance based ones. I think I've even contributed/originated some.

millstone• 12 years ago

Consider how these things get stored. In the inheritance model, you might have a big quad tree or some other data structure of PhysicsObjects, and just run through and call updatePhysics() on all of them.

In the composition model, we now have multiple classes (Character, Pickup, Projectile), each with an unrelated updatePhysics(). This means code duplication to call the relevant method on each separate class.

We could relate them all via an interface, instead of inheritance; now we can store IEntity or whatever. We will soon discover three needs that are awkward to address:

1. Whenever we want to add some new method (say `fall`), we must go back and implement it separately in each class.

2. Different classes will want to share implementations. For example, both Characters and Pickups bounce on fall.

3. Some classes will want to specialize an implementation to do more. Characters bounce on fall, but also take damage.

In practice you may end up with both: an interface that your engine talks to, but also a common base class that provides sane defaults.

So while interfaces allow uniform interactions with disparate classes, inheritance provides that and also the ability to share and specialize the implementations. So inheritance solves some problems that interfaces cannot.

See also default methods in Java, which makes an interface more like a class, and implementing an interface more like inheritance. The documentation even says that a class that implements an interface inherits its default methods.

ufmace• 12 years ago

I've seen this problem in projects I've worked on recently. It's gotten me more interested in Ruby mixins as a solution, though I haven't worked on any really complex object structures in Ruby. The project in question is in C#, and you can sorta-almost do something like it by creating interfaces and putting the methods that should be shared as extension methods on it, but it feels very hacky.

yeureka• 12 years ago

In my opinion the best arguments against inheritance were written by Richard Gabriel in his book Patterns of Software: http://dreamsongs.net/Files/PatternsOfSoftware.pdf

Why composition is often better than inheritance

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