Facets

I first started thinking about facets after reading an article on java.net nearly two years ago.

I kind of liked the idea that object can gain attributes and behaviours and indeed gain and lose types through it's life cycle.

In normal OO practices where an object is defined having all the types that it is ever going to have (A String is an Object and a CharArray and that is all it will ever be). Facets would allow an object to gain and lose types (A panel in Swing could become a type of Window or a scrollable viewport in another Window). In the real world when a person joins a footbal team they gain the facet of being a footbal player and when they leave the team they lose it. The facet of being a football player brings a team association and a shirt number with it plus a lot of interesting behaviours involving a ball and a pitch.

A number of similar patterns exist such as Mixins, but mixins only really bring behaviour not type information. Unfortunately such a useful word has already been used here.

In fact the idea has already been had by this bunch, and unfortunately they have patented it as US Patent 6,513,157. I did nothing more about facets for quite some time after finding this.

A couple of weeks ago I came across some of the code that I had knocked together to play with facets and decided to bite the bullet and see if I could do something with it. I went through the patent again and realised that the patent covered an implementation of facets that did not need changes to source code. As long as I made sure that what I implemented required changes to existing objects to add the functionality then I would be fine.

I'm finally getting around to completing my implementation and when I've had a chance to explore it I'm going to make it available and blog its uses.

One thing has come out of my work on Facets - Abstract super classes are a snare and a delusion. Take an interface 'Faceted', implement it with a concrete class called 'FacetedSupport'. 'FacetedSupport' can be used as an ancestor of 'Faceted' objects OR it can be used as a support class for objects that are implementing the 'Faceted' interface. If you made it an abstract super class then you would be forced to create a sub-class to make use of its functionality.