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OK I like the way this class diagram is

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coming along. So at this point I think we

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have all the classes that we need. For each

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class we specified the attributes or the characteristics of the

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class. And we also specified the operations so we

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know what the class can do And, I like to

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kind of move forward on this, but I first

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want to see that you're fine with the class structure.

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So that's the way the class structure is going

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to be in terms of attributes and operations. So anything

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that bothers you? Well, one thing I didn't understand

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is how come you put check out over where

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the patron when it's really the item being checked

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out? Right. Okay. So that actually is you know, is a perfect

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segway for the next thing that really wanted

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to model. Because what you're talking about is basically a

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relationship between two classes which is something we haven't touched on

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yet. So we haven't, haven't looked at individual classes. But now, it

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is typical, now we are looking more at requirements, we're starting to

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find more things about our system, and what you're pointed out right

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now is the fact that patron and item are somehow related. So

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this checkout operation is not really something that belongs only on in

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the patron, because it needs to know about the item. And it

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doesn't belong only on the item because it needs to know about

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the patron. So, it's something that associates the patron and

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the item. Okay. And that's exactly the way we call

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in the UML which is the notation that we're using

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here this kind of relationship. So, we're going to represent that

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by drawing a line between these two classes that tells

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us there is an association. And we're also going to

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give a name to this. Since this refers to the

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fact of checking out items. We're just going to call

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it, checkout. Gotcha. And notice that this basically you

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know,eventually will end up kind of replacing this attribute. Because

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the existence of this association will tell us that

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this is checked out. We're, we're not going to, you know,

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do it right now, but in the final cleanup

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or the diagram, this name will disappear. Okay. Okay.

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And so since we started talking about relationships and

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associations, is there any other kind of relationship that you

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see here? Well, what you just did with checked

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out is, it seems similar to the whole issue

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of requests. It is, it is. So a request

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is something else that happens in both, you know, in

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the patron and in the item, it involves both.

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And in fact in a request, I would definitely

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represent this as an additional association. So I will

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just draw an another line between these two that represent

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that specific kind of relationship and I will call it

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request. So that indicates that this association refers to a

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request that also connects the patron with an item. Okay.

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And, let's see. Any, anything else that jumps at you?

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Yeah, well, how about all these ones down at the

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bottom? I mean book and item's got to be

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related, right? A book is a kind of item, And

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audiovisual... are there associations between them? Can you repeat

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that, you said that the book, yeah? Is

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a kind of item. Perfect, that's exactly what we're

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modeling next, which is, this, what we call the

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is-a relationship. So you said, a book is an item?

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A book is an item. And, we can model that in the diagram. So, we do that

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using another kind of relationship between the classes. So

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we're going to represent that as a specialization we call it.

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And, a specialization is indicated in this way. Okay?

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With this arrow at the end, so a solid

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with this kind of arrow at the end. And

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we can do the same for book, magazine, reference book

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and audiovisual material. So we're all going to connect,

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we're going to connect all of them, to the

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item, using the same kind of connection. And now

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that we have connected all these four, with item

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and indicated them in subclasses. That's something else that we can

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do. So we can make this kind of cleaner. And I'll

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tell you what I mean by that. So now we have

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this loanable attribute that refers to item, but it seems to

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me from what you were saying before, that loanable is not

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really an attribute of an item. Right? It's more of a

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characteristic of the type of item. Right. Is that right? Right.

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Books, and audio/visual are loanable but the others aren't.

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Okay, and so representing it here, it's okay to,

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it will work. But it's not really right so from

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the style standpoint it doesn't really you know,

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it's not the best way of modeling this. What we're going to

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do instead, we're going to use this specialization relationship to make

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that more explicit. To make it cleaner. Okay, so what

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I'm doing here is I'm going to take this hierarchy

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of classes, this is just on two levels now, and I'm

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going to kind of make it a little richer.

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So I'm going to add an intermediate set of

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classes. And in particular I'm going to have these two

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classes that I'm going to call non loanable item and loanable

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item. So, they're both items but they tell me

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clearly that some items are loanable and some items

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are not. Okay. Okay. And then I'm simply going to

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put book and audio video material as subclasses of loanable

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item and reference book and magazine as subclasses of non-loanable

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item. So, if we look at this diagram now it's

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pretty clear what is loanable and what is not. And

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it's actually is a very clean, much cleaner

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design. And, and I see you've, gotten rid of

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the loanable attribute, too. I did. Because at

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this point this is already represented by the fact of

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having these two classes. And actually, something else that I did

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is that I moved all these attributes, value,

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due date, renewed and checked out, that makes

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sense only for loanable item. From item to

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loanable item. So at this point, this really

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is telling me that, you know, these

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characteristics are just meaningful for the loanable item,

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and not for the other ones. Well, speaking of that, the way that you got the

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lines going in the diagram here is you still have

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request and checked out going to item, even though you

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can't request non loanable Items. You can't check out non

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loanable Items. Oh, you were right actually. You got me on

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that one. You're exactly right. So this associations are between

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the two wrong classes. So, I guess, at this point,

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you can probably go and fix the diagram yourself. Well,

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can we just make the lines go from patron to loanable

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item instead of to item? That's exactly the way in which we

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are going to fix it. So, we're going to move these two associations down

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here. And at this point, this will represent the right relationships in

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the, in the diagram, and in the system. Makes sense to me.