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Divide and Conquer + Hierarchical Composition

Bill Venners: To go back to the subject of complexity, I think there's an algorithm for dealing with complexity that in general people use and in software in particular it's used. If you have this massive complex thing, you chop it up in pieces to start with.

James Gosling: Right. Divide and conquer. It's been with us for years.

Bill Venners: And each piece is then a understandable amount of complexity. And that's one of the ways I think about what a class or an object should encapsulate. It should encapsulate an amount of complexity that if there is someone working on the guts, working on the implementation, they can understand it.

James Gosling: Right.

Bill Venners: And then you have interfaces to those parts. And in those interfaces, you raise the level of abstraction. You abstract away some of the details.

James Gosling: Right.

Bill Venners: For example, if someone peels of the back of my television set, they could focus on and understand the guts of the TV. They don't have to think about and understand the entire television broadcast system all at once.

James Gosling: Right.

Bill Venners: But when I use my TV, I just push on. And maybe when I push on, 30 things happen inside the TV. And maybe over on this other TV when I push on, 50 things happen inside it that are different. I don't have to know all that. I just think in more abstract terms of on - off. The external interface to the TV abstracts away much of the details of its implementation. So, if you think about pulling all these pieces together again, I can understand this complex whole in terms of the simpler, more abstract interfaces to its parts.

James Gosling: Right. It's the pair of intellectual tools that people have used since the dawn of time. One is the divide and conquer, splitting things up in pieces, so they are individually understandable. And then there is hierarchical composition, where you take these things and you wrap them and put them together in these more and more abstract concepts.

There are often flaws in that, though. There's a fancy word in here that I'm forgetting. But there is often this view that with large complex systems, you can deconstruct them, decompose them into their parts and then analyze them individually. In biology, that kind of view is one of the things that led to a lot of major mistakes. Because you have to take this ecological view, of viewing the entire system. Because if you just look at a leaf or a frog, it's almost impossible to understand that leaf in isolation. You have to understand the whole ecology surrounding the leaf.

And so, there's this dilemma. You really do need to split things down into pieces, but when you split them down into pieces, you have to understand that they don't make any sense in isolation. You can't forget the ecology within which they live.

And so this business of on the one hand decomposition, dividing and conquering and hierarchical composition, they actually go hand in hand because hierarchical composition, putting things together, is really the only way that we see large systems of things, because it lets us encapsulate their behavior. And in order to understand a little piece, we have to understand the rest of it and so, you may end up with this funny way of working where you never forget the whole system, but you might have some parts that you're thinking of decomposed all the way down to the iron. Other parts are conceptually more aggregated, so that you don't lose the perspective of how this thing fits into the world.

A certain amount of that manifests itself in the way that a lot of designs become iterative. You do one design, you see how it all comes together and you say, nyeah, I don't think so and you go back again. Because in an awful lot of systems, they might be these big hairy systems, but there's a nut in the middle, around which it all kind of rotates. There's one little technological detail that kind of makes it all work. 99% of the code you ever write is kind of do-whatever-you-want code, but there's something in the middle that's really crucial. You just have to get that one algorithm, that one data structure, absolutely right. And often you find that these things pervade entire systems.

Bill Venners: What things pervade? That little kernel of importance?

James Gosling: That one little kernel of importance. If you're building a text editor it's often either like the document representation or the redisplay algorithm. And the rest of the work rotates around that.