alt Hacker NewsDo yourself a favor and wear yourself off all this SOLID, Uncle Bob, Object Oriented, Clean Code crap.
Don't ever use inheritance. Instead of things inheriting from other things, flip the relationship and make things HAVE other things. This is called composition and it has all the positives of inheritance but none of the negatives.
Example: imagine you have a school system where there are student users and there are employee users, and some features like grading that should only be available for employees.
Instead of making Student and Employee inherit from User, just have a User class/record/object/whatever you want to call it that constitutes the account
data class User (id: Int, name: String, email: String)
data class Student (userId: Id, blabla student specific attributes)
data class Employee (userId: Id, blabla employee specific attributes)
and for those users who really are both (after all, students can graduate and become employees, and employees can enroll to study), THE SAME USER can be BOTH a Student and an Employee! (this is one of the biggest footguns with inheritance: in the inheritance world, a Student can never be an Employee, even though that's just an accident of using inheritance and in the real world there's actually nothing that calls for that kind of artificial, hard segregation)
Once you see it you can't unsee it. The emperor has no clothes. Type-wise functional programmers have had solutions for all these made up problems for decades. That's why these days even sane Object Oriented language designers like Josh Bloch, Brian Goetz, the Kotlin devs etc are taking their languages in that direction.
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Just because you see OO languages starting to favor composition over inheritance does not mean inheritance has no place, and indeed, interfaces as a form of composition have existed in many bog-standard OO languages for decades.
Your example dosn't compute, at least in most languages, because derived objects would not have the same shape as one another, only the shape of the base class. I.e. functions expecting a User object would of course accept either an Employee or a Student (both subclasses of a User), but functions expecting a Student object or an Employee object would not accept the other object type just because they share a base class. Indeed, that's the whole point. And as another poster mentioned, you are introducing a burden by now having no way to determine whether a User is an Employee or a Student without having to pass additional information.
Listen, I'll be the first to admit that the oo paradigm went overboard with inheritance and object classification to the n-th degree by inventing ridiculous object hierarchies etc, but inheritance (even multiple inheritance) has a place- not just when reasoning about and organizing code, but for programmer ergonomics. And with the trend for composition to disallow data members (like traits in Rust), it can seriously limit the expressiveness of code.
Sometimes inheritance is better, and if used properly, there's nothing wrong with that. The alternative is that you wind up implementing the same 5-10 interfaces repeatedly for every different object you create.
It should never be all or nothing. Inheritance has its place. Composition has its place.
And if you squint just right they're two sides of the same coin. "Is A" vs "Can Do" or "Has".
> Don't ever use inheritance. Instead of things inheriting from other things, flip the relationship and make things HAVE other things. This is called composition and it has all the positives of inheritance but none of the negatives.
Bah. There are completely legitimate uses of inheritance where it's a really great fit. I think you'll find that being dogmatic about avoiding a programming pattern will eventually get you twisted up in other ways.
Inheritance can be used in a couple of ways that achieve a very-specific kind of code reuse. While I went through the early 2000's Java hype cycle with interfaces and factories and builders and double-dispatch and visitors everywhere, I went through a period where I hated most of that crap and swore to never use the visitor pattern again.
But hey, within the past two years I found an unbeatable use case where the visitor pattern absolutely rocks (it's there: https://github.com/titzer/wizard-engine/blob/master/src/util...). If you can come up with another way by which you can deal with 550 different kinds of animals (the Wasm instructions) and inherit the logic for 545 and just override the logic for 5 of them, then be my guest. (And yes, you can use ADTs and pattern-matching, which I do, liberally--but the specifics of how immediates and their types are encoded and decoded just simply cannot be replicated with as little code as the visitor pattern).
So don't completely swear off inheritance. It's like saying you'll never use a butter knife because you only do pocket knives. After all, butter knives are dull and not good for anything but butter.
I will agree that the problem that class hierarchies attempt to solve is a problem one usually does not really have, but in your example you have not solved it at all.
It matches a relational database well, but once you have a just a user reference, you can not narrow it down to an employee without out of band information. If a user should be just one type that can be multiple things at once, you can give them a set of roles.
It's sad that late-binding languages like Objective-C never got the love they should have, and instead people have favored strongly (or stronger) typed languages. In Objective-C, you could have your User class take a delegate. The delegate could either handle messages for students or employees. And you could code the base User object to ignore anything that couldn't be handled by its particular delegate.
This is a very flexible way of doing things. Sure, you'll have people complain that this is "slow." But it's only slow in computer standards. By human standards—meaning the person sitting at a desktop or phone UI—it's fast enough that they'll never notice.
Inheritance is nothing more or less than composition + implementing the same interface + saving a bit of boilerplate.
When class A inherits from class B, that is equivalent to class A containing an object of class B, and recoding the same interface as class B, and automatically generating method stubs for every method of the interface that call that same method on your B.
That is, these two are perfectly equivalent:
class B {
public void foo() {
}
}
class A_inheritance extends B {
}
class A_composition implements B {
private B super;
public void foo() {
super.foo();
}
The problem with inheritance is that people get taught to use it for modeling purposes, instead of using it for what it actually does - when you need polymorphism with virtual dispatch, which isn't all that common in practice. That is, inheritance should only be used if you need to have somewhere in you code base a collection of different objects that get called in the same way but have to execute different code, and you can only find out at runtime which is which.
For your students and employees and users example, the only reason to make Student and Employee inherit from User would be if there is a reason to have a collection of Users that you need to interact with, and you expect that different things will happen if a User is an Employee than if that User is a Student. This also implies that a Student can't be an Employee and vice versa (but you could have a third type, StudentEmployee, which may need to do something different from either a Student or an Employee). This is pretty unlikely for this scenario, so inheritance is unlikely to be a good idea.
Note also that deep class hierarchies are also extremely unlikely to be a good idea by this standard: if classes A and B derive from C, that doesn't mean class D can derive from A - that is only useful if you have a collection of As that can be either A or D, and you actually need them to do different things. If you simply need a third type of behavior for Cs, D should be a subclass of C instead.
I'd also note that my definition applies for inheritance in the general case - whether the parent is an interface-only class or a concrete class with fields and methods and everything. I don't thing the distinction between "interface inheritance" and "implementation inheritance" is meaningful.
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Never using inheritance is pushing the dogma to the other extreme imo. The whole prefer composition over inheritance is meant to help people avoid the typical OO over use of inheritance. It doesn't mean Is-A relation doesn't/shouldn't exist. It just means when there is data, prefer using composition to give access to that data.
There will be times when you want to represent Is-A relationship - especially when you want to guarantee a specific interface/set of functions your object will have - irrespective of the under the hood details.
Here what you care about is notifier providing a set of functions (sendNotification, updateNotification, removeNotification) - irrespective of what the implementation details are - whether you're using desktop notifications or SMS notifications.