(Part 1, Part 2.)

One more thing about the typing debate in today’s expert panel. Dave Thomas said, at one point, “Java and C++ have equated an object’s type with it class, and that’s wrong. The type of an object isn’t its class; it’s what the object can do.”

I agree with that, and I think it’ll be interesting to go revive a four-year-old piece I wrote as part of a WikiWikiWeb debate on the merits of multiple inheritance.

I think multiple inheritance is relatively unimportant and rarely useful, and I’m happy to be working in a language (Java) that does not support it. In part, I’ve come to believe that inheritance is given far too much importance by most OO languages, designs, developers, and pundits.

My thoughts about inheritance have been evolving since I learned Java. The revelations I’ve had may not seem like much to a Smalltalk programmer, but they represent a complete shift in my thinking.

Nearly every introduction to OO concepts I’ve ever read or seen has dealt with inheritance very early, and then moved on to a cursory discussion of “polymorphism” as a sort of nice side-effect of inheritance. Partly as a result of this (and partly because of the underlying misunderstanding) the word “inheritance” is usually used to refer to some combination of behavior inheritance and subtyping.

Java is the first language I have used that mostly separates those two concepts. Extending or implementing an interface represents a subtyping relationship, whereas extending a class represents the more traditional combination of subtyping and behavior inheritance. The process of using and designing with interfaces has brought subtyping out of the shadows and into the foreground of my thinking.

I’ve begun teaching the concepts of inheritance and polymorphism the other way ‘round: polymorphism and interfaces come first, as the fundamental issue, and behavior inheritance comes later, as a nice facility (more, but not much more, than a convenience) when two subtypes share significant portions of their behavior or implementation. It seems to work well. Many of the common inappropriate uses of inheritance never occur to programmers who learn it this way. I’ve found that the method also helps in the explanation of abstract classes and when to use them.

Consider a language that makes the separation even more clear: subtyping uses one mechanism, and behavior inheritance can be specified without also producing a subtype relationship. (I’m speaking hypothetically, but I won’t be surprised to learn that such a language actually exists.) Behavior inheritance becomes a kind of specialized composition or delegation facility. Some of the traditional objections and implementation complexities of MI disappear.

My conclusion, then, is that the strongly-typed OO community may have been going down a side path for most of its history, conflating two concepts that are actually separate. Perhaps, once we’ve corrected that misdirection, the time will come for MI to move back to center stage. For now, though, I’m pleased that Java avoids MI, if only because having to do without it has helped me to understand the fundamental issues more clearly. (From talking to colleagues, I believe it is having a similar effect on others, too.)

I realized almost immediately after posting that that Smalltalk and its ilk (including, for example, Ruby) have essentially the characteristic I was talking about, where subtyping and inheritance are separate concepts. With those languages in particular, that distinction is present because there is no subtyping at all … the notion really doesn’t exist in those languages, because typing as Java and C++ folks think of it doesn’t exist.