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How to Summarily Name Classes and Interfaces?

A naming conundrum: how to collectively refer to classes and interfaces? For instance, what should you call a variable that could contain either a class or an interface name? What should be used instead of $class?

One might consider the term type ($type), but this is quite generic because a type can also be a string or an array. From the perspective of the language, a type could be something more complex, such as ?array. Moreover, it's debatable what constitutes the type of an object: is it the class name, or is it object?

However, there indeed exists a collective term for classes and interfaces: it is the word class.

How so?

  1. From a declaration standpoint, an interface is essentially a stripped-down class. It can only contain public abstract methods, which also implies that objects cannot be created. Therefore, interfaces are a subset of classes. If something is a subset, we can refer to it by the name of the superset. Just as a human is a mammal, an interface is a class.
  2. Nevertheless, there's also the usage perspective. A class can inherit from only one class but can implement multiple interfaces. However, this limitation pertains to classes, not to the interfaces themselves. Similarly, a class cannot inherit from a final class, but we still perceive the final class as a class. Also, if a class can implement multiple interfaces (i.e., classes, see 1.), we still regard them as classes.

And what about traits? They simply do not belong here, as they do not exist from an OOP standpoint.

Thus, the issue of naming classes and interfaces together is resolved. Let’s simply call them classes.

classes + interfaces = classes

Well, but a new problem has arisen. How to refer to classes that are not interfaces? That is, their complement. What was referred to at the beginning of the article as classes. Non-interface? Or “implementations”#Class_vs._type)? ?

That's an even bigger nut to crack. It’s a tough nut indeed. You know what, let's forget that interfaces are also classes and again pretend that every OOP identifier is either a class or an interface. It will be easier.

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