Where does the idea that an object should only do one thing come from?

Question

I'm not sure I agree with it, so I'd like to find the book or journal article behind this idea so that I can check that I understand exactly what they're saying and what context they mean it.

I think I understand the idea - I just want to know the source so I can check where the idea comes from.

why I'm asking:

The term "do one thing" is vague and could mean lots of things including "only have one method per class" (ridiculous)... I think it could mean a single responsibility (i.e. lots of methods.) It's also not particularly useful because you have to judge when a single responsibility becomes complicated enough to need refactoring out into several responsibilities with some kind of delegation...

Answer 1

SOLID Principles from Bob Martin.

Single Responsibility Principle to be exact.

Although, in the first page of the chapter on the Single Responsibility Principle, he states:

This principle was described in the work of Tom DeMarco and Meilir Page-Jones. They called it cohesion.

The references for the work he mentioned are:

• Structured Analysis and System Specification, Tom DeMarco, Yourdon Press Computing Series, 1979
• The Practical Guide to Structured Systems Design, 2d. ed., Meilir Page- Jones, Yourdon Press Computing Series, 1988

Other sources (from S.Lott in comments) include:

• Wikipedia article on GRASP
• c2 article on Allocation of Responsibility

Answer 2

Single responsibility principle - Check here for some information on the subject.

Answer 3

What ever the source, I don't think it's a common idea in OO. An object might do many things.

Answer 4

I'm pretty sure it's more of a function idea. Every object I've ever seen does more than one thing. Functions (methods) should only do one thing.

Answer 5

The man who invented the idea of modularity in software was Dr David Parnas. The classic paper is On the Criteria To Be Used in Decomposing Systems into Modules

While he doesn't talk about OO in general (as it wasn't around yet), the ideas of OO extend naturally from Dr Parnas' work. And part of this work is analysis on how to decompose your software into modules, and that modules should be single purpose.

Answer 6

The Single Responsibility Principle (SRP) is quite common on the Java world. The references mentioned here are good.

The principle can apply pretty much to both classes and methods because it's a good idea for both.

The results of being aware of the SRP and applying it when possible are usually cleaner simpler code, but at the cost of more classes/methods. This can be of great value for reuse, testing and the next programmer to see it.