97 Things Every Programmer Should Know

Chapter 11. Code in the Language of the Domain

Dan North

PICTURE TWO CODEBASES. In one, you come across:

if (portfolioIdsByTraderId.get(trader.getId())
    .containsKey(portfolio.getId())) {...}

You scratch your head, wondering what this code might be for. It seems to be getting an ID from a trader object; using that to get a map out of a, well, mapof-maps, apparently; and then seeing if another ID from a portfolio object exists in the inner map. You scratch your head some more. You look for the declaration of portfolioIdsByTraderId and discover this:

Map<int, Map<int, int>> portfolioIdsByTraderId;

Gradually, you realize it might have something to do with whether a trader has access to a particular portfolio. And of course you will find the same lookup fragment—or, more likely, a similar but subtly different code fragment—whenever something cares whether a trader has access to a particular portfolio.

In the other codebase, you come across this:

if (trader.canView(portfolio)) {...}

No head scratching. You don’t need to know how a trader knows. Perhaps there is one of these maps-of-maps tucked away somewhere inside. But that’s the trader’s business, not yours.

Now which of those codebases would you rather be working in?

Once upon a time, we only had very basic data structures: bits and bytes and characters (really just bytes, but we would pretend they were letters and punctuation). Decimals ...