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Re: OO should break when broken(was re: the benefits of immutability)
> On Monday, September 1, 2003, at 06:14 AM, Colin Putney wrote:
> > An ellipse is a circle with additional capabilities - it can have
> > different radii along each of its dimensions.
> This reminds me of something that bothers me about OO: the
> accumulation of unused stuff. When I extend Circle to make Ellipse, I
> add x-radius and y-radius, but the original radius instance variable is
> still there. Since I'm not going to use Circle's radius any more, I
> wish I could just ask it to go away, if the compiler or runtime could
> verify it isn't needed by methods I decide not to override.
When you want things in the superclass to "go away", it points to bad
design. It can be very confusing when you come across a system that where
subclasses are designed to ignore various things they inherit.
I don't think an ellipse is a kind of circle. It would make more sense for a
circle to be a kind of ellipse. The problem seems to be that the "extends"
relationship doesn't mean the same thing as "is-a", "a-kind-of" or