1 00:00:08,630 --> 00:00:11,330 Here we are, in the second week of the class. 2 00:00:11,330 --> 00:00:16,590 And starting on module four, which is about classes and objects. 3 00:00:16,590 --> 00:00:20,140 Just to let you know the learning objectives for module four. 4 00:00:20,140 --> 00:00:20,760 Here they are. 5 00:00:21,860 --> 00:00:26,570 This is the only module that has an objective about creating. 6 00:00:26,570 --> 00:00:30,809 We're going to be designing a class together at the end of this module. 7 00:00:30,809 --> 00:00:33,824 But we'll also be applying the knowledge you learned 8 00:00:33,824 --> 00:00:36,840 by using existing classes that I'll provide to you. 9 00:00:40,530 --> 00:00:43,158 In this module, we're going to talk about 10 00:00:43,158 --> 00:00:47,820 the foundational ideas behind the object oriented paradigm. 11 00:00:47,820 --> 00:00:49,490 So here's the big idea. 12 00:00:49,490 --> 00:00:52,930 The big idea is that software can be implemented 13 00:00:52,930 --> 00:00:58,050 as a collection of objects that interact with each other. 14 00:00:58,050 --> 00:01:01,725 So, this paradigm is very well suited to game development 15 00:01:01,725 --> 00:01:05,325 because if you think about it, games are virtual world, 16 00:01:05,325 --> 00:01:10,201 where there's a bunch of different entities that interact with each other. 17 00:01:10,201 --> 00:01:13,533 If you're walking along in the virtual world and you see an 18 00:01:13,533 --> 00:01:17,514 Orc, you might decide to interact with that Orc in a particular way. 19 00:01:17,514 --> 00:01:21,348 With your bare hands, or your sword of massive cutting or whatever. 20 00:01:21,348 --> 00:01:25,716 If you see a teddy bear in the game world, you may decide to interact 21 00:01:25,716 --> 00:01:30,552 to that teddy bear in a particular way, so you might hug the teddy bear, or 22 00:01:30,552 --> 00:01:34,608 you might explode the teddy bear, or you might light the teddy 23 00:01:34,608 --> 00:01:39,180 bear on fire, or you'd interact with that object in some way. 24 00:01:39,180 --> 00:01:43,960 And even non-player characters, right, the NPCs interact with each other. 25 00:01:43,960 --> 00:01:46,640 In with you as part of the game world. 26 00:01:46,640 --> 00:01:51,280 So there's lots of interactions between objects or entities in game 27 00:01:51,280 --> 00:01:55,920 world, so modeling those game worlds in our software as interacting 28 00:01:55,920 --> 00:01:59,760 objects is a really powerful technique that we can use. 29 00:02:01,920 --> 00:02:05,351 We'll start off with an in-lecture quiz so you can tell 30 00:02:05,351 --> 00:02:10,010 me a little bit about what you already understand about software objects. 31 00:02:16,630 --> 00:02:17,770 Okay. 32 00:02:17,770 --> 00:02:21,834 So, software objects have three different things. 33 00:02:21,834 --> 00:02:27,660 They have state, they have behavior, and they have identity. 34 00:02:27,660 --> 00:02:31,040 And we can relate this to objects in the real world as well. 35 00:02:31,040 --> 00:02:35,970 So the state of an object is the characteristics of that object. 36 00:02:35,970 --> 00:02:37,380 Think of yourself right now. 37 00:02:37,380 --> 00:02:39,690 You have a number of different characteristics. 38 00:02:39,690 --> 00:02:42,120 You have a height, you have a weight. 39 00:02:42,120 --> 00:02:46,850 You have a degree of hungriness, and perhaps a degree of boredom. 40 00:02:46,850 --> 00:02:48,170 How cruel. 41 00:02:48,170 --> 00:02:51,329 But these are different characteristics that 42 00:02:51,329 --> 00:02:54,430 you have that potentially change over time. 43 00:02:55,720 --> 00:02:58,779 You might grow taller or you might shrink. You might gain or lose weight and so on. 44 00:03:00,220 --> 00:03:06,080 So the state of an object is really all about the characteristics of that object. 45 00:03:07,360 --> 00:03:09,770 Objects also have behavior. 46 00:03:09,770 --> 00:03:13,570 So there are things that we can do to objects, 47 00:03:13,570 --> 00:03:17,329 like we could take a teddy bear and throw it. 48 00:03:18,510 --> 00:03:20,880 And there are things that we can tell 49 00:03:20,880 --> 00:03:23,724 objects to do to themselves, so when we get 50 00:03:23,724 --> 00:03:26,805 to XNA in particular we'll tell an object to, 51 00:03:26,805 --> 00:03:30,330 perhaps we tell a teddy bear to draw itself. 52 00:03:30,330 --> 00:03:32,631 And so we don't have to worry about the details 53 00:03:32,631 --> 00:03:36,140 of how a teddy bear draws itself. We just tell it to do so. 54 00:03:36,140 --> 00:03:39,195 Similarly, if we had a deck of cards, we might just tell 55 00:03:39,195 --> 00:03:43,260 that deck, go shuffle yourself and we don't care how it happens. 56 00:03:43,260 --> 00:03:46,130 It's just that it happens internally to the object. 57 00:03:46,130 --> 00:03:48,370 So, objects have behavior as well. 58 00:03:48,370 --> 00:03:52,550 They have things they can do or that we can do to them. 59 00:03:52,550 --> 00:03:57,734 And finally, objects have identity. We will end up or we'll discover that 60 00:03:57,734 --> 00:04:04,130 we create objects, as we go along in our software as our software program runs. 61 00:04:04,130 --> 00:04:09,270 And each of those objects lives, if you will in a different place in memory. 62 00:04:09,270 --> 00:04:14,690 And so each of them are distinct from each other and so they have identity. 63 00:04:14,690 --> 00:04:16,380 And that's good too, right. 64 00:04:16,380 --> 00:04:19,530 If you are in a traditional classroom and somebody 65 00:04:19,530 --> 00:04:22,755 wanted to call on a particular student, you can't 66 00:04:22,755 --> 00:04:28,030 just yell out the question, you have to first say which student you mean. 67 00:04:28,030 --> 00:04:30,080 And then ask your question. 68 00:04:30,080 --> 00:04:33,284 Similarly with objects, if we want them to 69 00:04:33,284 --> 00:04:36,755 do something, we need to specify which object 70 00:04:36,755 --> 00:04:42,860 we expect to do that thing before we, sort of access the behavior of that object. 71 00:04:42,860 --> 00:04:46,776 So identity is really important as well. So all objects have 72 00:04:46,776 --> 00:04:52,437 state, behavior and identity. That's a language-independent, 73 00:04:52,437 --> 00:04:58,500 object oriented set of concepts that really has nothing to do with C#. 74 00:04:58,500 --> 00:05:04,160 The next slide uses some C# specific terminology. 75 00:05:04,160 --> 00:05:08,610 So let's actually have a concrete example. We're going to have a playing card object. 76 00:05:08,610 --> 00:05:13,220 And that playing card object is going to have state, behavior and identity. 77 00:05:13,220 --> 00:05:18,070 And the state of that playing card will be stored in fields. 78 00:05:18,070 --> 00:05:21,931 That's a C# piece of terminology, but I'll 79 00:05:21,931 --> 00:05:26,360 tell you right now fields are just variables. 80 00:05:26,360 --> 00:05:31,936 We learned in the last module how we can declare and use variables while the fields 81 00:05:31,936 --> 00:05:38,310 in an object are just variables. Externally from the object the 82 00:05:38,310 --> 00:05:43,370 way we'll let sort of consumers of this object. 83 00:05:43,370 --> 00:05:46,404 So the game for example has a playing card in it and 84 00:05:46,404 --> 00:05:50,288 the game needs to know something about the state of this card. 85 00:05:50,288 --> 00:05:58,900 We almost never will let consumers of this object access those fields directly. 86 00:05:58,900 --> 00:06:03,435 That's really bad programming practice. So instead what we'll do is we 87 00:06:03,435 --> 00:06:08,308 will expose, that's the terminology we use, we will expose that state. 88 00:06:08,308 --> 00:06:13,028 Through C# properties if you've programmed in other languages 89 00:06:13,028 --> 00:06:17,428 like Java for example, you've written getters and setters, or 90 00:06:17,428 --> 00:06:20,868 accessors and mutators all the time, in C# we have 91 00:06:20,868 --> 00:06:25,555 this concept called properties, that does exactly the same thing. 92 00:06:25,555 --> 00:06:29,200 It let's us access, read, write or both particular 93 00:06:29,200 --> 00:06:35,020 pieces of state for the particular object. So, that state. 94 00:06:35,020 --> 00:06:38,520 And for a card the particular things we might store for state 95 00:06:38,520 --> 00:06:42,280 are the rank of the card, Ace, King, Queen and so on. 96 00:06:42,280 --> 00:06:45,672 The suit of the card, depending on the game you're playing. 97 00:06:45,672 --> 00:06:48,120 you know, grass, water, fire or perhaps 98 00:06:48,120 --> 00:06:51,112 hearts, clubs, spades and diamonds and whether or 99 00:06:51,112 --> 00:06:54,580 not the card is face up or not which would matter to us if we're 100 00:06:54,580 --> 00:06:56,235 actually drawing the card. 101 00:06:56,235 --> 00:07:00,390 So, you know you draw a face down card nobody can tell what it is. 102 00:07:00,390 --> 00:07:03,640 You draw a face up card and everyone can tell what it is. 103 00:07:03,640 --> 00:07:04,140 Okay. 104 00:07:04,140 --> 00:07:08,654 So that's state, there's also behavior associated with the object, 105 00:07:08,654 --> 00:07:13,120 and in C#, we expose behavior through things called methods. 106 00:07:13,120 --> 00:07:16,968 Other languages used terminology like method or functions, 107 00:07:16,968 --> 00:07:20,002 their methods in C#, and the method that we'll 108 00:07:20,002 --> 00:07:22,889 have for our cargo, will be the flip it over. 109 00:07:23,920 --> 00:07:27,139 And finally cards get their identity when we 110 00:07:27,139 --> 00:07:31,650 create new card objects and that's called instantiation. 111 00:07:35,410 --> 00:07:38,050 So, another opportunity to do an in 112 00:07:38,050 --> 00:07:42,660 lecture quiz addressing particularly the state of objects. 113 00:07:47,550 --> 00:07:52,830 So conceptually for our card what we have is, inside 114 00:07:52,830 --> 00:07:58,110 here, hidden inside the card, we have those fields 115 00:07:58,110 --> 00:08:03,400 that we don't let anyone access externally. 116 00:08:03,400 --> 00:08:08,496 And by the way, having the fields and around the edge here, around 117 00:08:08,496 --> 00:08:13,680 the edge here, all of these different properties and behaviors. 118 00:08:14,950 --> 00:08:18,826 Bundling all that stuff together is called encapsulation, 119 00:08:18,826 --> 00:08:21,714 because we've put the state and the behavior 120 00:08:21,714 --> 00:08:24,298 all into a single capsule, if you will, 121 00:08:24,298 --> 00:08:28,400 that object that carries all the stuff around. 122 00:08:28,400 --> 00:08:34,897 The other thing that's important, though, is as we access things like properties, 123 00:08:34,897 --> 00:08:40,326 like the Suit property here, or as we call a method, the FlipOver method 124 00:08:40,326 --> 00:08:46,378 here, the other thing that we know is that the consumer of this object, doesn't 125 00:08:46,378 --> 00:08:52,640 know anything about the internal details. They have no idea how it's implemented. 126 00:08:52,640 --> 00:08:58,080 They have no idea what data types the Rank, Suit, and FaceUp are. 127 00:08:58,080 --> 00:09:01,346 There are logical choices, but we don't know, and so 128 00:09:01,346 --> 00:09:06,660 this is another important object oriented concept called information hiding. 129 00:09:06,660 --> 00:09:10,550 The implementation details are hidden inside the class. 130 00:09:10,550 --> 00:09:12,500 So, we can change those. 131 00:09:12,500 --> 00:09:15,503 We could change the data type we use to store the 132 00:09:15,503 --> 00:09:19,276 rank of the card, and as long as we didn't actually change 133 00:09:19,276 --> 00:09:24,358 the property, then consumers of this class, or this object, could just keep 134 00:09:24,358 --> 00:09:30,200 using that property without even knowing that the implementation details changed. 135 00:09:30,200 --> 00:09:31,796 And that is fantastic for 136 00:09:31,796 --> 00:09:36,736 when we actually use objects in large software systems over time, because we 137 00:09:36,736 --> 00:09:42,070 make changes all the time when we don't break the rest of the software. 138 00:09:42,070 --> 00:09:45,309 So this conceptual idea of an object where we have 139 00:09:45,309 --> 00:09:48,785 the fields hidden inside and then we sort of have this 140 00:09:48,785 --> 00:09:52,814 boundary of stuff that we expose to people, consumers of the 141 00:09:52,814 --> 00:09:56,922 object to use, is a really powerful object oriented idea that 142 00:09:56,922 --> 00:10:02,893 we will see again and again. Now, I keep talking about objects and 143 00:10:02,893 --> 00:10:10,710 classes, and it's a, there's an important distinction between the two. 144 00:10:10,710 --> 00:10:13,650 And so, let's get that distinction nailed down right now. 145 00:10:13,650 --> 00:10:16,994 The class, is actually the template, for creating 146 00:10:16,994 --> 00:10:20,410 objects, the class is not an object itself. 147 00:10:20,410 --> 00:10:22,048 It's sort of the cookie cutter 148 00:10:22,048 --> 00:10:23,240 if you will. 149 00:10:23,240 --> 00:10:27,083 It tells us what all the cookies will look like and, you know cookies tend 150 00:10:27,083 --> 00:10:29,288 not to have behavior and thus, you know, 151 00:10:29,288 --> 00:10:32,510 you have scary dreams or something about cookies. 152 00:10:32,510 --> 00:10:34,330 But it tells us sort of the shape if 153 00:10:34,330 --> 00:10:37,444 you will, of the objects that you're going to create. 154 00:10:37,444 --> 00:10:39,090 But it's not an object itself. 155 00:10:39,090 --> 00:10:41,050 But the good news is, it's a template. 156 00:10:41,050 --> 00:10:45,870 And we can use the class to create as many of the actual objects as we want. 157 00:10:45,870 --> 00:10:47,146 Just as you can use a cookie 158 00:10:47,146 --> 00:10:50,050 cutter to create as many cookies as you want. 159 00:10:50,050 --> 00:10:54,788 So it's a template that defines the state and behavior 160 00:10:54,788 --> 00:10:59,650 of each of the objects that get created from this class. 161 00:11:00,650 --> 00:11:02,770 The objects are the actual things. 162 00:11:02,770 --> 00:11:03,100 Right? 163 00:11:03,100 --> 00:11:08,180 They're the actual things and memory and so we can have a whole bunch of them. 164 00:11:08,180 --> 00:11:12,330 We can have a whole bunch of playing cards if we wanted to play cards. 165 00:11:12,330 --> 00:11:15,960 Playing cards with one card is kind of a boring game. 166 00:11:15,960 --> 00:11:20,090 And even though each card knows, or if the. 167 00:11:20,090 --> 00:11:21,330 Let me put it a different way. 168 00:11:21,330 --> 00:11:26,294 Even though the class says that each card will store a rank and a suit, and whether 169 00:11:26,294 --> 00:11:28,265 or not it's face up, each of the 170 00:11:28,265 --> 00:11:33,240 individual card objects maintains their own current information, right. 171 00:11:33,240 --> 00:11:38,280 So each of these cards has a rank and a suit, and whether or not it's face up. 172 00:11:38,280 --> 00:11:42,360 But those ranks and suits, and face ups can be different. 173 00:11:42,360 --> 00:11:42,530 Right? 174 00:11:42,530 --> 00:11:45,320 We can have a King of spades as well as a Queen of hearts. 175 00:11:45,320 --> 00:11:50,720 And, they're still all cards, they're just different instances of the cards. 176 00:11:50,720 --> 00:11:56,000 So, classes are a template, objects are the actual things that are instances 177 00:11:56,000 --> 00:12:00,940 of, that's the terminology we use, that are instances of that class. 178 00:12:03,480 --> 00:12:06,270 Now, when we talk about class design. 179 00:12:06,270 --> 00:12:10,020 Not actually using classes externally as consumers of the classes. 180 00:12:10,020 --> 00:12:13,970 But actually designing classes and understanding how they're implemented. 181 00:12:13,970 --> 00:12:17,780 We often use this notation that I've provided here. 182 00:12:17,780 --> 00:12:20,969 And this notation includes the class name. 183 00:12:23,430 --> 00:12:24,830 The fields. 184 00:12:24,830 --> 00:12:28,370 So these are the things that we're always going to make internal to the class. 185 00:12:30,040 --> 00:12:31,495 The properties for the class. 186 00:12:31,495 --> 00:12:35,788 So these are the things, these are the ways that we're going to 187 00:12:35,788 --> 00:12:41,199 provide external consumers of the class to actually find out about the state. 188 00:12:42,260 --> 00:12:43,730 And finally methods. 189 00:12:43,730 --> 00:12:46,610 That behavior stuff, that we've been talking about. 190 00:12:47,620 --> 00:12:49,072 This notation 191 00:12:49,072 --> 00:12:57,590 is called UML and clearly, there's an in lecture quiz about UML and what it means. 192 00:13:01,780 --> 00:13:04,167 Despite what you might have chosen in 193 00:13:04,167 --> 00:13:07,478 that quiz it stands for Unified Modelling Language 194 00:13:07,478 --> 00:13:10,019 and it's really the most commonly used 195 00:13:10,019 --> 00:13:14,970 notation these days for specifying object oriented systems. 196 00:13:14,970 --> 00:13:19,000 Okay, before we finish the lecture because we're almost done we've hit 197 00:13:19,000 --> 00:13:21,145 the big ideas behind object oriented, 198 00:13:21,145 --> 00:13:24,960 implementations the whole object oriented paradigm. 199 00:13:24,960 --> 00:13:27,144 And we'll start using it next time. But before 200 00:13:27,144 --> 00:13:33,140 we leave, I need to point out that classes are not value types. 201 00:13:33,140 --> 00:13:34,910 Classes are reference types. 202 00:13:34,910 --> 00:13:36,886 Remember in the previous module I said well, 203 00:13:36,886 --> 00:13:39,480 we won't worry about reference types until module four. 204 00:13:39,480 --> 00:13:45,520 Well, here we are in module four. So, classes are reference typed and 205 00:13:45,520 --> 00:13:51,188 really classes are each a data type. So, you know we had 206 00:13:51,188 --> 00:13:51,800 [INAUDIBLE] 207 00:13:51,800 --> 00:13:53,500 and float for example. 208 00:13:53,500 --> 00:13:57,950 Well, when you start implementing classes, you're creating your very own data types. 209 00:13:57,950 --> 00:14:03,260 And they're reference types, rather than value types, because. 210 00:14:03,260 --> 00:14:05,802 For value types, the ones and zeros at the memory 211 00:14:05,802 --> 00:14:10,100 location for the variable represent the value of that variable. 212 00:14:10,100 --> 00:14:13,330 For reference types, the ones and zeros 213 00:14:13,330 --> 00:14:17,035 at the memory location for the value represent 214 00:14:17,035 --> 00:14:21,830 a reference to where the object actually is in memory, so. 215 00:14:22,860 --> 00:14:25,848 People try not to use the word pointer, but 216 00:14:25,848 --> 00:14:30,150 it's just a pointer to where the object actually lives. 217 00:14:30,150 --> 00:14:31,020 So a picture might help. 218 00:14:32,380 --> 00:14:33,520 Here we go. 219 00:14:33,520 --> 00:14:37,693 So age is an integer variable, so it's a value 220 00:14:37,693 --> 00:14:42,080 type, so the value 17 the ones and zeros 17, 221 00:14:42,080 --> 00:14:45,720 in that memory location mean ages 17. 222 00:14:45,720 --> 00:14:49,204 The rain message, which is a instance of a message object that 223 00:14:49,204 --> 00:14:53,023 you can read about in the sorry, instance of a message class, which 224 00:14:53,023 --> 00:14:55,837 you can read about in the book if you choose to, but 225 00:14:55,837 --> 00:15:00,240 it just holds the message and let's you print it and so on. 226 00:15:00,240 --> 00:15:05,050 The ones and zeros there, at that memory location represent 44. 227 00:15:05,050 --> 00:15:07,330 But 44 is not the value. 228 00:15:07,330 --> 00:15:09,430 It's a pointer, or a reference to a 229 00:15:09,430 --> 00:15:13,460 different place in memory where the object actually is. 230 00:15:13,460 --> 00:15:16,746 So that's that distinction between reference types and value 231 00:15:16,746 --> 00:15:19,170 types that I said we'd get to in this module. 232 00:15:21,510 --> 00:15:24,550 Okay, to recap the object oriented paradigm 233 00:15:24,550 --> 00:15:27,350 not just C#, just object orientation in 234 00:15:27,350 --> 00:15:31,030 general, provides classes and objects where classes 235 00:15:31,030 --> 00:15:34,580 serve as the template for creating objects. 236 00:15:34,580 --> 00:15:38,719 And objects have state, behavior and identity. 237 00:15:39,760 --> 00:15:43,225 We also learned that objects interact with each other 238 00:15:43,225 --> 00:15:46,536 in the software that we write to actually implement 239 00:15:46,536 --> 00:15:50,630 our game, whatever else we're doing with our software. 240 00:15:50,630 --> 00:15:52,740 It might not be a game, how sad. 241 00:15:54,150 --> 00:15:57,222 Next time, so this was all conceptual stuff, important 242 00:15:57,222 --> 00:16:02,190 foundational conceptual stuff to understand how object orientation works. 243 00:16:02,190 --> 00:16:07,910 But next time, we'll actually dig in and start using this stuff in a C# program.