"If two of them were Squares, I would expect height and width to mirror each other. Your saying you wouldn't expect it."
No, I wouldn't expect it, because I don't know about Squares. Don't be misled because this example are uses shapes we all learn in kindergarten. The real lesson is about all the abstractions we make up all day long building applications in the real world. Think of it as Frobulator and the false subclass MagicFrobulator.
Caches happen everywhere. Going back to squares and rectangles, I stated the collection was a column. Each rectangle rests on the one before it in the collection. When I narrow the column, I set the width of every Rectangle in the collection, but now unbeknownst to me some elements have changed their height. If the column happened to be in a collection of columns sorted by height, it would now be out of sort order. The original programmer would make a reasonable assumption (well, duh) that the column sort order would not be affected by a SetWidth call.
"Its still a Square (by my definition). The new Squares she has are still Rectangles too. Why did this programmer assume that changing a rectangle's length would never effect its width? This invalid assumption could have been made prior or after the introduction of Squares."
The definition of Rectangle states that changing the width would not change the height. I don't understand your objection.
I think using something as familiar as shapes is confusing the point.
Again, go back to Frobulator and MagicFrobulator. In 2005, I invent Frobulator which has two properties foo and bar that are mutually exclusive. I release Frobulator to the world and make a million dollars.
In 2008 you have a revelation that you could make a billion dollars if you could just simplify Frobulator. You decide that you only really need foo, and you can kind of obsolete bar by making it dependent on foo. You then market this as MagicFrobulator with the pitch, "Throw out your Frobulators! MagicFrobulator is the perfect replacement."
Customers buy your MagicFrobulators and plug them in and discover to their horror that they explode in a shower of Star Trek induced sparks. As it turns out, other systems have regulated themselves by setting and getting foo and bar independently. Now, Widgets set foo and screw up Gadgets who did not expect bar to have changed. Bar just happens to control the amperage, and so the Gadgets explode.
No, I wouldn't expect it, because I don't know about Squares. Don't be misled because this example are uses shapes we all learn in kindergarten. The real lesson is about all the abstractions we make up all day long building applications in the real world. Think of it as Frobulator and the false subclass MagicFrobulator.
Caches happen everywhere. Going back to squares and rectangles, I stated the collection was a column. Each rectangle rests on the one before it in the collection. When I narrow the column, I set the width of every Rectangle in the collection, but now unbeknownst to me some elements have changed their height. If the column happened to be in a collection of columns sorted by height, it would now be out of sort order. The original programmer would make a reasonable assumption (well, duh) that the column sort order would not be affected by a SetWidth call.
"Its still a Square (by my definition). The new Squares she has are still Rectangles too. Why did this programmer assume that changing a rectangle's length would never effect its width? This invalid assumption could have been made prior or after the introduction of Squares."
The definition of Rectangle states that changing the width would not change the height. I don't understand your objection.
I think using something as familiar as shapes is confusing the point.
Again, go back to Frobulator and MagicFrobulator. In 2005, I invent Frobulator which has two properties foo and bar that are mutually exclusive. I release Frobulator to the world and make a million dollars.
In 2008 you have a revelation that you could make a billion dollars if you could just simplify Frobulator. You decide that you only really need foo, and you can kind of obsolete bar by making it dependent on foo. You then market this as MagicFrobulator with the pitch, "Throw out your Frobulators! MagicFrobulator is the perfect replacement."
Customers buy your MagicFrobulators and plug them in and discover to their horror that they explode in a shower of Star Trek induced sparks. As it turns out, other systems have regulated themselves by setting and getting foo and bar independently. Now, Widgets set foo and screw up Gadgets who did not expect bar to have changed. Bar just happens to control the amperage, and so the Gadgets explode.