Tight on the heels of my article on the evils of ‘extends’ and Scriptaculous, Bernard Sumption, the author of Animator.js, has expounded on why he thinks it is evil and must be destroyed.

All of the pain caused by inheritance can be traced back to the fact that inheritance forces ‘is-a’ rather than ‘has-a’ relationships. If class R2Unit extends Droid, then a R2Unit is-a Droid. If class Jedi contains an instance variable of type Lightsabre, then a Jedi has-a Lightsabre.

The difference between is-a and has-a relationships is well known and a fundamental part of OOAD, but what is less well known is that almost every is-a relationship would be better off re-articulated as a has-a relationship.

As I’ve already pointed out, composition is to be preferred over subclassing in most cases, and that the GoF patterns are all about replacing implementation inheritance (extends) with interface inheritance (implements). Combine this with Larman’s GRASP patterns, and you see that much of the thrust of OOAD is about working around the limitations of "extends."

Technorati Tags: ajax, javascript, ooad, patterns

Share/Bookmark