I just got an affirmation that a subtype hierarchy is a conceptually difficult thingy. A subtype hierarchy uses the is-a ("a truck is a vehicle") relation to build the hierarchy:
But when you want to add tires and a motor to the truck you could put it like this:
Which looks like a component hierarchy.
In the case of a subtype hierarchy those tires and motor would end up in entirely different parts of the hierarchy, as tires aren't vehicles.
When you see a complete subtype hierarchy tree it turns out that many people have a lot of problems to stay focussed on the fact that it's a subtype hierarchy. They keep seeing parts...
Conclusion: I should be very careful in displaying subtype hierarchies in user interfaces... Preferably I should hide them as much as possible.
My name is Reinout van Rees and I work a lot with Python (programming language) and Django (website framework). I live in The Netherlands and I'm happily married to Annie van Rees-Kooiman.
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