The Object Graph
Now, what did Sandi mean about the “edges of your object graph”? Let’s not use the word “edge”, since it’s overloaded in the context of graphs. Let’s use the word “boundaries”. She’s talking about the tiniest leaves of our tree. Polymorphic objects are okay by Sandi as long as they are only referred to, instead of doing any referring. And this makes a lot of sense! Polymorphic classes are dangerous because small changes in them ripple to all of their subclasses and users. By making sure your inheritance hierarchies stay small and well managed, you can contain that complexity. When you refer to other objects, you’re more fragile. If you’re only referred to, a stable interface is the only thing you need to keep from breaking anything else.
In addition to your object graph, you also have a dependency graph. Here, your nodes are classes instead of instances, and your links are #imports instead of object ownership. […]
It makes me think that it would be great to have two types of objects, separated into layers.
