Some months ago, I stumbled into an OOP inheritance rabbithole which got me thinking the same:
- Isn't the (biological) concept of inheritance built at it's core around the idea of "generations"? How does that make any sense in OOP?
- Why do Java beginner courses still teach Inheritance like it was 1995 with all the bells and whistles and dogs and cats and mammals?
- Why do they teach Inheritance and later introduce "favor composition to inheritance"? Doesn't this just confuse everyone?
- Why does Inheritance have first-class syntax support in an OOP language like Java ("extends") whereas Composition usually needs to be "engineered" using some more complex patterns, e.g. DI?
Most of it is probably for historical reasons, i guess.
"Why does Inheritance have first-class syntax support in an OOP language like Java ("extends") whereas Composition usually needs to be "engineered" using some more complex patterns, e.g. DI?"
I think the main reason is that DI is seen as an orthogonal concept by language designers, so you'd need new two first-class features in a language.
If you do composition without injection (e.g.: by having new ChildObject() in the constructor), you don't really require that many more lines of code compared to inheritance.
class Car {
private Engine engine;
public Car() {
engine = new Engine();
}
public void drive() {
engine.start();
System.out.println("Driving...");
engine.stop();
}
}
Of course it's much less flexible and less testable than composition + injection, but not that inflexible when compared to inheritance. And first-class support for only that would make the feature a bit useless without DI...
> Why does Inheritance have first-class syntax support in an OOP language like Java ("extends") whereas Composition usually needs to be "engineered" using some more complex patterns, e.g. DI?
Yep. You've got to remember that Java is nearly 30 years old; at the time making a distinction between interfaces and abstract classes was seen as a radical move. Newer languages have better support for more decoupled ways of doing things, e.g. Rust's use of typeclasses (which it calls traits for some reason) or Kotlin's built-in support for delegation.
The thing that Rust calls traits is quite different from the thing that Scala and PHP call traits, and much more like the thing that many languages call typeclasses.
If every class inherits Object then you probably need to understand the concept still, even if it’s no longer favored for organizing your own business logic. Besides, you will encounter other people’s code that doesn’t follow all the best practices eventually. And I’m not sure anyone has come up with a better way of teaching inheritance in the past few decades.
I don't know for Java currently but for example in Ruby composition is simply a matter of putting your code in a module rather than a class. Then you can extend any class with this module. You can even extend instances. That is 3.extends(Some_module).method_from_some_module is perfectly valid.
Of the many models of “composition” that are possible I think Ruby’s free-for-all blend of mix-ins and monkey patches is the only one that can drive a maintenance programmer more insane than a deep hierarchy of inheritance of Java classes.
irb(main):001:0> "bar".foo
Traceback (most recent call last):
4: from /usr/bin/irb:23:in `<main>'
3: from /usr/bin/irb:23:in `load'
2: from /Library/Ruby/Gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
1: from (irb):1
NoMethodError (undefined method `foo' for "bar":String)
Did you mean? for
irb(main):002:0> class String
irb(main):003:1> def foo
irb(main):004:2> "foobar!"
irb(main):005:2> end
irb(main):006:1> end
=> :foo
irb(main):007:0> "bar".foo
=> "foobar!"
irb(main):008:0>
Yes! Let's modify the core library String class on the fly to add new functions to it.
There are things about ruby that truly scare me if my goal was to write secure and reasonable code.
This doesn’t leverage on the more subtle but all the more elegant type system that Ruby provides. As told just above, since all object also have their own class instance you can extend this instance specific class without polluting the general class it derives from.
irb(main):001:1* module Awesomeness
irb(main):002:1* def awesome? = :yes
irb(main):003:0> end
=> :awesome?
irb(main):004:0> ?Ô.awesome?
(irb):4:in `<main>': undefined method `awesome?' for "Ô":String (NoMethodError)
from /Users/someone/.asdf/installs/ruby/3.1.2/lib/ruby/gems/3.1.0/gems/irb-1.4.1/exe/irb:11:in `<top (required)>'
from /Users/someone/.asdf/installs/ruby/3.1.2/bin/irb:25:in `load'
from /Users/someone/.asdf/installs/ruby/3.1.2/bin/irb:25:in `<main>'
irb(main):005:0> ?Ô.extend(Awesomeness).awesome?
=> :yes
irb(main):005:0> ?Ô.awesome?
(irb):5:in `<main>': undefined method `awesome?' for "Ô":String (NoMethodError)
from /Users/someone/.asdf/installs/ruby/3.1.2/lib/ruby/gems/3.1.0/gems/irb-1.4.1/exe/irb:11:in `<top (required)>'
from /Users/someone/.asdf/installs/ruby/3.1.2/bin/irb:25:in `load'
from /Users/someone/.asdf/installs/ruby/3.1.2/bin/irb:25:in `<main>'
Right, that’s monkey patching. You run into the real problems when you have multiple people wanting to add them.
Ruby isn’t alone in supporting this functionality (you can fiddle with JavaScript prototypes, for instance), but I think it is unique how much it is encouraged (at least in Rails world). I think extension methods are a much better model to achieve something similar.
Consider where you can slip in a block invocation and the following code:
def mal(&block)
block.call
block.binding.eval('a = 43')
end
a = 42
puts a
mal do
puts 1
end
puts a
and that that means that you've got full access to be able to inspect and manipulate all of the variables in scope at the time of the invocation. While that example is rather obvious, it can be done much more subtly too.
The power of ruby to do meta programming and by extension some really neat DSLs also provides it with some dangerous tools that are otherwise rather difficult to track down.
This is the Smalltalkiness of Ruby. Why have a separate place to put a function that operates only on strings when you can put it in String?
Of course, in Smalltalk you might have #trim defined by two different packages and they might conflict. Then what? Maybe you have to manage that complexity. However, it's not much different from having conflicting functions in the global namespace in C, but actually easier to resolve in a given situation.
Yeah, more modern Smalltalks are able to put things in various namespaces so they don't conflict but still generally work as expected. In Smalltalk, adding methods doesn't weigh down a class like in other languages, because method lookup is on-the-fly like in Objective-C, not precalculated and placed in a table.
Because it's an object-oriented language, and so its primary paradigm is object-orientation. Various style guides will tell you that object-orientation is bad and you should try writing in a procedural style instead, but the correct answer when you pine for procedural programming is to stop using Java.
