Of course there are the so-called five concepts of object oriented design, which says a design that meets the following criteria is considered object-oriented (OO):
- Object/Class
- Information Hiding
- Inheritance
- Interface
- Polymorphism
But I'm not so sure I'm convinced. A coworker accused me of trying to re-define it:
Coworker: So now we're redefining what OO is too... care to define common words as well?
Me: I'm not redefining it per se... just calling bullshit where I see bullshit.
Coworker: Excuse while I go outside and laugh loudly.
Me: Sorry my name isn't Martin Fowler so my opinion probably doesn't matter much, but I still smell bullshit.
Me: I'm not redefining it per se... just calling bullshit where I see bullshit.
Coworker: Excuse while I go outside and laugh loudly.
Me: Sorry my name isn't Martin Fowler so my opinion probably doesn't matter much, but I still smell bullshit.
If we think about programming in C for a moment, a language that isn't considered to support OO practices, we can still encapsulate our code and provide an interface for data manipulation. Member variables are placed in a struct, public methods are functions listed in a header file, and private methods exist only in the code file. Encapsulating in this case depends on an understanding of variable and file scoping rules of the language, not the existence of some class, public, protected, and private keywords.
We have to pass a pointer to the instance class into the functions, but that still happens in languages that are considered to be OO languages too; it's just syntactic sugar that automatically passes it and assigns it to this. It's difficult to ignore the similarity between foo.doSomething() and doSomething(foo).
Java has developers define the method functions in the same code unit as the member variables. Others allow defining them separately, and then associate them in some way later. C++ does this through file and namespacing, and Go does this with interfaces.
If we were to think of instances of objects in terms as payload structs, then is inheritance really necessary? We don't extend a class for example because we have a protected integer member we want to reuse, we do it for the functionality provided by method functions. Inheritance supposedly helps with code-organization and promotes re-use, but I'd argue not essential for OO. In fact, the proliferation of OO far exceeds the amount of code we actually end up reusing.
Polymorphism probably isn't really needed either for OO. Data typing is all about the underlying semantic meaning of data and what operations are allowed to be performed on it. What it means to "add" two strings together and what it means to "add" two integers together are different even though both are aggregate operations. Typing is less relevant in dynamic languages like PHP and Ruby. We don't care what type an object is so long as we can call a method on it. The concept of polymorphism is only important in statically-typed languages, and promoting it as a core OO concept means a true dynamically typed language could never be object oriented.
The way I see it, encapsulation begets abstraction. Inheritance is about reusability, not objects. Interface is a design pattern that attempts to overcome difficulty for reusing code from different objects. Pure object oriented programming seems really to be about encapsulation and message passing, while the concepts above seem more concerned about structure.
It can be argued that the software industry is in the mess it's in right now because of the over-definition and application of intolerant ideas about OO, and a refusal to acknowledge other schools of thought on the subject. But if I'm taking on the responsibility to create a new language, then I'm going to take the time to think about such things and not just follow the herd. I want to create something better, not just a better Java.