relationship between a category and a item

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OOP

Hey guys,

this seems to be a trivial question, but it blows away my brain when i'm thought a lot about it.

IS-A category a list of things or HAS-A category a list of things?
Me and my co-worker aren't sure at all how to answer the question correctly.

After discussion we are thinking both is right but we aren't 100% sure.

I think it is right to say a category is like a box. You can put in some letters, close the box and put the box into a rack. So the box has letters of a category inside.
BUT
I think you could also argument that you put the letters on a stack and the stack is the category.

And the same thing with the rack.
The rack could have a list of boxes
BUT
The rack can consists of boxes

I know it's a little bit confusing.

But i hope someone could write down his perspective and point out why.