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1
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OK I like the way this class diagram is
2
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coming along. So at this point I think we
3
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have all the classes that we need. For each
4
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class we specified the attributes or the characteristics of the
5
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class. And we also specified the operations so we
6
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know what the class can do And, I like to
7
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kind of move forward on this, but I first
8
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want to see that you're fine with the class structure.
9
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So that's the way the class structure is going
10
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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
14
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out? Right. Okay. So that actually is you know, is a perfect
15
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segway for the next thing that really wanted
16
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to model. Because what you're talking about is basically a
17
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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
19
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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
21
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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,
37
00:01:53,932 --> 00:01:55,428
do it right now, but in the final cleanup
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00:01:55,428 --> 00:01:58,750
or the diagram, this name will disappear. Okay. Okay.
39
00:01:58,750 --> 00:02:02,190
And so since we started talking about relationships and
40
00:02:02,190 --> 00:02:05,280
associations, is there any other kind of relationship that you
41
00:02:05,280 --> 00:02:08,805
see here? Well, what you just did with checked
42
00:02:08,805 --> 00:02:12,009
out is, it seems similar to the whole issue
43
00:02:12,009 --> 00:02:16,090
of requests. It is, it is. So a request
44
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is something else that happens in both, you know, in
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00:02:19,580 --> 00:02:22,090
the patron and in the item, it involves both.
46
00:02:22,090 --> 00:02:24,070
And in fact in a request, I would definitely
47
00:02:24,070 --> 00:02:26,950
represent this as an additional association. So I will
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00:02:26,950 --> 00:02:30,730
just draw an another line between these two that represent
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00:02:30,730 --> 00:02:33,804
that specific kind of relationship and I will call it
50
00:02:33,804 --> 00:02:37,330
request. So that indicates that this association refers to a
51
00:02:37,330 --> 00:02:41,660
request that also connects the patron with an item. Okay.
52
00:02:41,660 --> 00:02:45,710
And, let's see. Any, anything else that jumps at you?
53
00:02:45,710 --> 00:02:47,590
Yeah, well, how about all these ones down at the
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00:02:47,590 --> 00:02:50,080
bottom? I mean book and item's got to be
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00:02:50,080 --> 00:02:52,920
related, right? A book is a kind of item, And
56
00:02:52,920 --> 00:02:55,860
audiovisual... are there associations between them? Can you repeat
57
00:02:55,860 --> 00:02:58,790
that, you said that the book, yeah? Is
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a kind of item. Perfect, that's exactly what we're
59
00:03:02,550 --> 00:03:04,700
modeling next, which is, this, what we call the
60
00:03:04,700 --> 00:03:07,900
is-a relationship. So you said, a book is an item?
61
00:03:07,900 --> 00:03:13,346
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.
64
00:03:21,210 --> 00:03:24,890
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
67
00:03:30,800 --> 00:03:34,160
we can do the same for book, magazine, reference book
68
00:03:34,160 --> 00:03:36,200
and audiovisual material. So we're all going to connect,
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00:03:36,200 --> 00:03:37,950
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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00:03:43,190 --> 00:03:47,200
that we have connected all these four, with item
72
00:03:47,200 --> 00:03:50,620
and indicated them in subclasses. That's something else that we can
73
00:03:50,620 --> 00:03:54,370
do. So we can make this kind of cleaner. And I'll
74
00:03:54,370 --> 00:03:56,360
tell you what I mean by that. So now we have
75
00:03:56,360 --> 00:04:00,190
this loanable attribute that refers to item, but it seems to
76
00:04:00,190 --> 00:04:03,100
me from what you were saying before, that loanable is not
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00:04:03,100 --> 00:04:05,490
really an attribute of an item. Right? It's more of a
78
00:04:05,490 --> 00:04:09,240
characteristic of the type of item. Right. Is that right? Right.
79
00:04:09,240 --> 00:04:13,120
Books, and audio/visual are loanable but the others aren't.
80
00:04:13,120 --> 00:04:16,450
Okay, and so representing it here, it's okay to,
81
00:04:16,450 --> 00:04:20,579
it will work. But it's not really right so from
82
00:04:20,579 --> 00:04:23,590
the style standpoint it doesn't really you know,
83
00:04:23,590 --> 00:04:26,100
it's not the best way of modeling this. What we're going to
84
00:04:26,100 --> 00:04:30,130
do instead, we're going to use this specialization relationship to make
85
00:04:30,130 --> 00:04:33,000
that more explicit. To make it cleaner. Okay, so what
86
00:04:33,000 --> 00:04:35,300
I'm doing here is I'm going to take this hierarchy
87
00:04:35,300 --> 00:04:38,130
of classes, this is just on two levels now, and I'm
88
00:04:38,130 --> 00:04:40,380
going to kind of make it a little richer.
89
00:04:40,380 --> 00:04:43,470
So I'm going to add an intermediate set of
90
00:04:43,470 --> 00:04:45,756
classes. And in particular I'm going to have these two
91
00:04:45,756 --> 00:04:48,306
classes that I'm going to call non loanable item and loanable
92
00:04:48,306 --> 00:04:51,790
item. So, they're both items but they tell me
93
00:04:51,790 --> 00:04:55,490
clearly that some items are loanable and some items
94
00:04:55,490 --> 00:04:59,392
are not. Okay. Okay. And then I'm simply going to
95
00:04:59,392 --> 00:05:03,192
put book and audio video material as subclasses of loanable
96
00:05:03,192 --> 00:05:07,980
item and reference book and magazine as subclasses of non-loanable
97
00:05:07,980 --> 00:05:10,510
item. So, if we look at this diagram now it's
98
00:05:10,510 --> 00:05:13,650
pretty clear what is loanable and what is not. And
99
00:05:13,650 --> 00:05:16,070
it's actually is a very clean, much cleaner
100
00:05:16,070 --> 00:05:18,980
design. And, and I see you've, gotten rid of
101
00:05:18,980 --> 00:05:21,560
the loanable attribute, too. I did. Because at
102
00:05:21,560 --> 00:05:24,130
this point this is already represented by the fact of
103
00:05:24,130 --> 00:05:28,570
having these two classes. And actually, something else that I did
104
00:05:28,570 --> 00:05:31,820
is that I moved all these attributes, value,
105
00:05:31,820 --> 00:05:34,220
due date, renewed and checked out, that makes
106
00:05:34,220 --> 00:05:37,452
sense only for loanable item. From item to
107
00:05:37,452 --> 00:05:40,450
loanable item. So at this point, this really
108
00:05:40,450 --> 00:05:42,600
is telling me that, you know, these
109
00:05:42,600 --> 00:05:46,410
characteristics are just meaningful for the loanable item,
110
00:05:46,410 --> 00:05:53,800
and not for the other ones. Well, speaking of that, the way that you got the
111
00:05:53,800 --> 00:05:57,270
lines going in the diagram here is you still have
112
00:05:57,270 --> 00:05:59,920
request and checked out going to item, even though you
113
00:05:59,920 --> 00:06:02,610
can't request non loanable Items. You can't check out non
114
00:06:02,610 --> 00:06:05,630
loanable Items. Oh, you were right actually. You got me on
115
00:06:05,630 --> 00:06:10,940
that one. You're exactly right. So this associations are between
116
00:06:10,940 --> 00:06:13,440
the two wrong classes. So, I guess, at this point,
117
00:06:13,440 --> 00:06:16,450
you can probably go and fix the diagram yourself. Well,
118
00:06:16,450 --> 00:06:19,120
can we just make the lines go from patron to loanable
119
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item instead of to item? That's exactly the way in which we
120
00:06:21,640 --> 00:06:25,550
are going to fix it. So, we're going to move these two associations down
121
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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.
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