Why classes implicitly derive from only the Object Class?

Question

I do not have any argument opposing why we need only a single universal class. However why not we have two universal classes, say an Object and an AntiObject Class. In nature and in science we find the concept of duality - like Energy & Dark Energy; Male & Female; Plus & Minus; Multiply & Divide; Electrons & Protons; Integration & Derivation; and in set theory. There are so many examples of dualism that it is a philosophy in itself. In programming itself we see Anti-Patterns which helps us to perform work in contrast to how we use Design patterns. I am not sure, but the usefulness of this duality concept may lie in creating garbage collectors that create AntiObjects that combine with free or loose Objects to destruct themselves, thereby releasing memory. Or may be AntiObjects work along with Objects to create a self-modifying programming language - that allows us to create a safe self modifying code, do evolutionary computing using genetic programming, do hiding of code to prevent reverse engineering.

We call it object-oriented programming. Is that a limiting factor or is there something fundamental I am missing in understanding the formation of programming languages?

Answer 1

This is only an answer to answer the question in the title.

Languages such as Java have every class deriving from Object for two reasons.

Firstly, to increase the amount of polymorphism available. This was particularly required before generics were added to the language. Without Object, collection classes would be impossible to write in a useful fashion.

Secondly, there are many methods that classes are expected to have or are useful, and these are collected in Object. By ensuring that all classes inherit from Object, all classes will implement the same minimal interface.

As mentioned in a comment, C++ does not have a class like Object. C++ is in many ways untyped, so the issues I mention above are not applicable. Also, C++ templates provide a lot of polymorphism and are used to implement collections.

Answer 2

I think the accepted answer covers pretty much the original question, but I would like to complement it slightly by throwing in (quite informally) a few ideas regarding the secondary questions.

From an inheritance point of view, nothing prevents the class hierarchy to have several roots. As it has been pointed out by other peoples, C++, as well as many OO languages don't constraint expressiveness to a single root ancestor class.

However, from a type theoretic point of view (recall that inheritance and subtyping are not the same thing, so I am probably stepping out of the frame of the main question here), a single "top" supertype makes a lot of sense (depending on the type theory of course). For instance, in OCaml, there is a common supertype to all objects (whether class instances, or immediate objects), written < > to denote that the object type is empty. This seems indeed the most general object type we can define, since we cannot remove anything from it to make it more general. Hence, in this conception of object types, there is necessarily a supertype to all object.

Regarding the dual of the root object, scala sports a class called Nothing, which curiously is also empty, and is the subtype of every other classes. It cannot be instantiated, but holds enough useful semantic to implement the empty list, which is called Nil, and is equal to List[Nothing] (as pointed out in comments, it is possible that programmer never directly use that value in most case, making it seemingly not as useful as it is). Nothing could be considered a dual of the root type - called Any in scala, however these types not only cover classes, but also primitives types, so that everything can be upcasted to Any, for instance.