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Okay, so this is our first, class diagram.

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>> So, let me ask you something about.

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>> Okay.

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>> What we've done so far. I also sent, in what

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I sent you, I also had some stories about how the actual

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>> Library is used. You asked me to do

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that and are we going to take, use that here?

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>> Glad you asked actually. yeah. Those are, you know, what we call use

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cases, or what we will use as scenarios kind of things that we will

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use to derive use cases. And they're also a very good

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way of extracting requirements. We're not going to look at them right

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now because now, because we're more working on kind of the static

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structure of the system. But after we're done with the class diagram,

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you know, we will do it at a different time. But

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we're going to use those to see how the

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libraries actually use them, and see whether we can get more

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information that we can use to refine our requirements based on that.

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>> Okay.

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>> Okay,

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So, for now, we'll just focus in on the,

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structure, but, just so you know, I'm,

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I'm glad you sent them, because they were going

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very useful as well.

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Okay. So let's see. Well, first of all, let

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me, seems like that this is already pretty crowded,

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right? We have a number of, classes. So let's

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see if there's, some class that may be superfluous and

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we can model in a different way. So, for

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example, you, while, while thinking of this I was thinking,

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the library card, it doesn't really contain much

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information, right? So is it basically just the number?

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The card has a number on it. We have a separate vendor that does that for us so.

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>> Oh.

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>> We don't need, it doesn't need to be part of this system,

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we just have to make sure that every patron has a library card.

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>> Okay, so basically for you, in a sense, the library

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card is just an ID that gets associated with a patron.

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>> That's right.

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>> So I think that the best way to represent this, I mean, unless you

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need an entity because you are creating it

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yourself, but it seems like you are not.

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I would just remove this one and I would like to put this,

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basically to take the library card number and add it to the pattern.

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>> Okay, makes sense.

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>> Okay, so I'll add it here. And as

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an additional attribute. Okay, and it will eliminate this class.

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>> Okay.

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>> Okay.

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Oh, and, wait a second, so I guess

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also the child needs a library card number, right?

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>> Child needs a library card number, but let me ask you about that. Is,

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is child a separate class, or is it just another kind of patron?

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>> Oh, I see, I see. Because, yeah, it

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is sort of a special patron, right? And, so

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maybe we should, maybe we should represent it as

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a kind of a refinement of the patron.

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Hm, but then that made me think. So what is

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the only thing that characterizes children? Is it just the age?

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>> Well, if they're, that they can't check out more than five books.

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>> Okay. And the, and the only difference is the

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fact that they are less than, you know, twelve years old.

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>> Twelve or less, right.

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>> Twelve or less. So, I guess, you know, I would probably

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like to represent this by making the age explicit in the patron rather

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than to represent it as a class. And I'll tell you why,

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because one, one of the issues, and you know, that might happen

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again, is that, basically, there are patrons that are children. And they're

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no longer children, when they come you know 13 or older right.

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>> Right.

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>> And if we represent them with a separate class in a sense, then we

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cannot really change the type of an instance of these classes.

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So we're left to kind of destroy the patron, create

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a new one, so that means we also have to transfer

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any history we want to keep history and so on.

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So I, I think I kind of like better the idea

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that I represent the age exclusively in

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the patron, and then I'll behave differently, based on whether the

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patron is 12 years old, or younger, or 13 or,

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13 or older. This, do you see any problem with that?

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>> It makes things a little simpler.

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>> Okay,

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and we actually, it allows us also to eliminate

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one class here. So I'm going to proceed this way.

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I'm going to eliminate the children class, and I'm going to

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put the age in the patron. Okay, and let me

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see. But in this spirit, actually, something else that jumps

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at me is this idea of the bestseller, because I

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kind of feel like, we might have the same

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problem. So, what is the story? What is a bestseller.

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>> Well it's

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an item that we want to restrict how

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long people can keep, because there is such demand for it.

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>> I see, and so basically a book that's a

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bestseller, like the New York Times bestseller, is a bestseller forever?

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00:04:30,450 --> 00:04:32,683
>> No, no, no it's hot for

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awhile, and then it becomes just a regular item.

98
00:04:35,940 --> 00:04:38,318
>> I see. Hm. Then I guess it's a

99
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similar situation to the one I was mentioning before, right?

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>> Okay.

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00:04:40,980 --> 00:04:41,800
>> That if we have a book,

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it will kind of have to change its type if it becomes a best seller.

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Then we have to change its type again, if it's no longer a best seller.

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>> Right.

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>> So it seems to me that a better

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way to represent this, is just to eliminate this BestSeller

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class and instead, I'm going to put the best seller

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attribute, which would just be a Boolean in the book.

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>> Okay, what do you mean by Boolean?

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>> Right. We don't know what Boolean is, right? The Boolean is

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basically just a number. It can have two values, right? True or false.

112
00:05:04,940 --> 00:05:05,380
>> Okay.

113
00:05:05,380 --> 00:05:06,830
>> So we usually, normally use it

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in this in this case. Imagine one, zero,

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right? Then it's just kind of the basic.

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>> Okay.

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>> You know, the bits, right?

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>> Okay.

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>> So, this is just telling us, it's like a flag

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that is telling this book is a best seller, or not.

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>> Okay.

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>> It's very easy to change this value and make a book a best

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seller or not a best seller, than

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just creating and destroying instances of these classes.

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>> Okay, makes sense.

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>> Okay, so at this point, this already looks better, right? Because we have,

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less classes, and I think we did, yeah, we

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did some serious cleanup. That's good. Okay, so now that

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we eliminated some of this, what I would like to

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do, as I said, we are going to both clean

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up, but also refine. I would like to go

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back to our, requirements and see whether we can identify

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additional attributes for this, class that maybe are not as

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obvious as the one that we saw so far, okay?