Shalloway’s Law and Shalloway’s Principle

August 30, 2007 — Posted by Al Shalloway

In my earlier post I mentioned these. OK, I know it was a tease, but now that someone has asked for what they are I figured I’d better fess up.

A few years ago somebody came up to me in one of my classes and said, "you're the CEO of a successful company, co-author of a successful book (Design Patterns Explained: A New Perspective on Object-Oriented Design), … you ought to have something named after yourself." So I, of course, immediately like this guy and think - hmm, what should that be?  And I came up with this:

Shalloway’s Law:

“When N things need to change and N>1, Shalloway will find at most N-1 of these things.”

Hey, I didn’t say it was complimentary. It’s a law! I have to follow it. That’s the problem. BTW: They didn’t ask me about gravity either when I was born! I really would like to break that law at times too! Laughing

So I came up with Shalloway’s Principle:

“Avoid situations where Shalloway’s Law applies”

What situations are these? When N > 1. In other words, avoid redundancy. But when you can’t, make sure you can find everything with a tool or something. For example, I may have redundancy in my interfaces. But I also have a cool “to do list generator” (some people call it a compiler) that when I change a method’s interface it tells me what I need to update. Better not to have redundancy at all, but if you do, make sure Shalloway’s law does not apply.

Al Shalloway

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About the author | Al Shalloway

Al Shalloway is the founder and CEO of Net Objectives. With over 40 years of experience, Alan is an industry thought leader in Lean, Kanban, product portfolio management, SAFe, Scrum and agile design.


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