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Looking at the requirements, item is definitely a relevant element
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for my system, so it is appropriate to model item as
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a class. Sale, on the other hand, is more of
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a characteristic of an item, an attribute of an item, rather
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than a class in itself. So, we're not going to mark
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this one. Shopping cart sounds, as well, as an important element
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for my system. Time can be an important system in some
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contexts, but in this case we're measuring time just because more
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than one item at a time can be added to the shopping cart.
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So, time really doesn't have any reason for being modeled as a class.
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And finally, user also seems to also have an important role to play
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in the system, and therefore we will model user as a class as well.
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