1 00:00:02,110 --> 00:00:04,440 So now that we know about the difference 2 00:00:04,440 --> 00:00:07,640 between real objects and data containers, 3 00:00:07,640 --> 00:00:10,220 let's dive into another important concept. 4 00:00:10,220 --> 00:00:12,800 And that would be polymorphism. 5 00:00:12,800 --> 00:00:16,120 Now we already had a first look at polymorphism 6 00:00:16,120 --> 00:00:18,430 in the last course section, 7 00:00:18,430 --> 00:00:21,570 where we built a factory function, 8 00:00:21,570 --> 00:00:24,450 which created polymorphic objects 9 00:00:24,450 --> 00:00:26,670 to hold different functions 10 00:00:26,670 --> 00:00:29,210 for handling payments and refunds. 11 00:00:29,210 --> 00:00:32,770 And I explained what polymorphism is back then. 12 00:00:32,770 --> 00:00:35,090 It's simply an object or a method, 13 00:00:35,090 --> 00:00:38,880 which always has the same structure and the same name, 14 00:00:38,880 --> 00:00:41,370 but which then does different things 15 00:00:41,370 --> 00:00:45,770 based on how it was created and how you use it therefore. 16 00:00:45,770 --> 00:00:47,560 And that's a powerful concept, 17 00:00:47,560 --> 00:00:51,910 because it can help us avoid code duplication. 18 00:00:51,910 --> 00:00:54,700 We can also implement polymorphism, 19 00:00:54,700 --> 00:00:58,360 or a use polymorphism when working with classes, 20 00:00:58,360 --> 00:01:01,340 instead of creating objects or dictionaries 21 00:01:01,340 --> 00:01:05,580 on the fly as I did it back in the last section. 22 00:01:05,580 --> 00:01:09,200 So here is another clause, the delivery clause, 23 00:01:09,200 --> 00:01:11,510 which has a private property purchase, 24 00:01:11,510 --> 00:01:14,460 a constructor and then two methods, 25 00:01:14,460 --> 00:01:17,800 deliver product and track product. 26 00:01:17,800 --> 00:01:20,640 And in there you see something similar 27 00:01:20,640 --> 00:01:23,620 as in the last course section, 28 00:01:23,620 --> 00:01:27,010 we have different delivery types on this purchase, 29 00:01:27,010 --> 00:01:28,610 which is part of the delivery, 30 00:01:28,610 --> 00:01:30,740 and based on the different types, 31 00:01:30,740 --> 00:01:33,910 we then dispatch different static methods 32 00:01:33,910 --> 00:01:37,130 on some other object, some are class here. 33 00:01:37,130 --> 00:01:38,700 And this is just some dummy code 34 00:01:38,700 --> 00:01:41,630 both the purchase and the logistics object 35 00:01:41,630 --> 00:01:43,100 doesn't really exist, 36 00:01:43,100 --> 00:01:46,140 but I can still show you what polymorphism is 37 00:01:46,140 --> 00:01:48,850 at this example and how you deal with it 38 00:01:48,850 --> 00:01:50,253 when you using classes. 39 00:01:51,270 --> 00:01:55,030 Because when using classes we can implement polymorphism 40 00:01:55,030 --> 00:01:57,423 in a very, very elegant way. 41 00:01:58,300 --> 00:02:02,340 The solution would be to not just use one class 42 00:02:02,340 --> 00:02:05,370 but to have multiple specialized classes. 43 00:02:05,370 --> 00:02:09,880 In this case, for the different kinds of delivery we have. 44 00:02:09,880 --> 00:02:14,880 Express delivery, ensured delivery and standard delivery. 45 00:02:14,950 --> 00:02:18,170 And we would just have a base delivery class 46 00:02:18,170 --> 00:02:21,150 which holds all the shared logic and properties, 47 00:02:21,150 --> 00:02:24,230 which all specialized classes need as well. 48 00:02:24,230 --> 00:02:26,810 But then it would be the specialized classes 49 00:02:26,810 --> 00:02:29,080 which have the specific deliver product 50 00:02:29,080 --> 00:02:31,810 and track product implementations. 51 00:02:31,810 --> 00:02:34,430 So if we would want to make this class 52 00:02:34,430 --> 00:02:39,180 and these methods use polymorphism and be polymorphic 53 00:02:39,180 --> 00:02:42,870 then we would in the end, create a couple of new classes. 54 00:02:42,870 --> 00:02:46,760 For example, an express delivery class 55 00:02:46,760 --> 00:02:51,400 but also ensured delivery class 56 00:02:51,400 --> 00:02:55,883 and also standard delivery class. 57 00:02:58,840 --> 00:03:01,410 We created these specialized versions 58 00:03:01,410 --> 00:03:03,350 because these are in the end, 59 00:03:03,350 --> 00:03:05,830 the three kinds of deliveries we have 60 00:03:05,830 --> 00:03:08,530 if we have a look at these methods. 61 00:03:08,530 --> 00:03:10,960 We see that the type of delivery seems 62 00:03:10,960 --> 00:03:12,700 to play an important role. 63 00:03:12,700 --> 00:03:16,480 So since this is the delivery class, which is responsible 64 00:03:16,480 --> 00:03:19,210 for delivering a product or for tracking it, 65 00:03:19,210 --> 00:03:22,420 we should have specialized wordings of these classes 66 00:03:22,420 --> 00:03:24,593 for the different types of delivery. 67 00:03:25,590 --> 00:03:28,820 So therefore here, we would have these three classes. 68 00:03:28,820 --> 00:03:32,510 And for example, in the express delivery class here 69 00:03:32,510 --> 00:03:36,830 we would then have a deliver product method. 70 00:03:36,830 --> 00:03:40,720 So we can copy this, but it wouldn't have any if check. 71 00:03:40,720 --> 00:03:44,110 It would just have the code which needs to be executed 72 00:03:44,110 --> 00:03:47,560 to deliver a product with express delivery. 73 00:03:47,560 --> 00:03:50,660 So in this case, I would reach out to logistics 74 00:03:50,660 --> 00:03:53,283 and then call issue express delivery. 75 00:03:54,680 --> 00:03:58,300 Now the purchase property is not accessible here 76 00:03:58,300 --> 00:04:00,960 because at the moment, this is a standalone class 77 00:04:00,960 --> 00:04:03,680 which doesn't have a purchase property. 78 00:04:03,680 --> 00:04:08,610 The solution is to extend the base delivery class here 79 00:04:08,610 --> 00:04:11,530 because when we extend that class 80 00:04:11,530 --> 00:04:14,650 then we also have access to all the properties 81 00:04:14,650 --> 00:04:18,590 this class has including its property. 82 00:04:18,590 --> 00:04:21,040 Now, as a quick side note here, 83 00:04:21,040 --> 00:04:23,320 of course here, I'm using inheritance 84 00:04:23,320 --> 00:04:26,480 and I'm using a couple of concepts we know from classes 85 00:04:26,480 --> 00:04:28,650 and the object oriented world. 86 00:04:28,650 --> 00:04:31,750 Later I will also use an interface. 87 00:04:31,750 --> 00:04:35,210 If this is all something you never heard about, 88 00:04:35,210 --> 00:04:38,180 then you probably never worked with classes. 89 00:04:38,180 --> 00:04:41,100 And then of course this specifically is a concept 90 00:04:41,100 --> 00:04:43,110 you won't be able to utilize 91 00:04:43,110 --> 00:04:45,500 and you won't need in general. 92 00:04:45,500 --> 00:04:47,650 You still learned about polymorphism 93 00:04:47,650 --> 00:04:50,100 in the last course, section though. 94 00:04:50,100 --> 00:04:52,910 As I mentioned earlier, the idea of this course 95 00:04:52,910 --> 00:04:56,220 and of this section is not to teach you classes 96 00:04:56,220 --> 00:04:58,610 or object oriented programming 97 00:04:58,610 --> 00:05:01,680 but to teach you a couple of concepts and ideas 98 00:05:01,680 --> 00:05:03,570 and rules you should be aware 99 00:05:03,570 --> 00:05:07,220 of when working with classes or objects. 100 00:05:07,220 --> 00:05:09,090 And here we're working with classes 101 00:05:09,090 --> 00:05:13,030 so therefore of course this general knowledge about classes 102 00:05:13,030 --> 00:05:16,350 and that includes inheritance, is of course required. 103 00:05:16,350 --> 00:05:19,360 You can of course skip this entire lecture for the moment 104 00:05:19,360 --> 00:05:22,843 if that is something you don't have any experience with. 105 00:05:23,780 --> 00:05:25,940 The only thing here is that this property 106 00:05:25,940 --> 00:05:27,850 is private at the moment, 107 00:05:27,850 --> 00:05:30,520 which means it can really only be used in here. 108 00:05:30,520 --> 00:05:32,880 And if you are using a programming language 109 00:05:32,880 --> 00:05:36,420 which knows the concept of private, public and so on, 110 00:05:36,420 --> 00:05:38,520 then you know that you should change this 111 00:05:38,520 --> 00:05:41,480 from private to protect it. 112 00:05:41,480 --> 00:05:45,090 Which means it can still not be accessed from outside 113 00:05:45,090 --> 00:05:49,080 of the class, but it can be accessed by child classes, 114 00:05:49,080 --> 00:05:51,900 which is exactly what we need here. 115 00:05:51,900 --> 00:05:55,780 And with that, we could then also copy over track product, 116 00:05:55,780 --> 00:05:58,960 like this, and also get rid of the if statement here 117 00:05:58,960 --> 00:06:02,133 and just call the code we need to track a product. 118 00:06:03,090 --> 00:06:05,620 And now we repeat this procedure 119 00:06:05,620 --> 00:06:08,150 for the other classes as well. 120 00:06:08,150 --> 00:06:13,150 So we copy these methods also into ensured delivery, 121 00:06:13,670 --> 00:06:15,350 get rid of all the if checks here 122 00:06:15,350 --> 00:06:17,530 and just keep the code we wanna execute 123 00:06:17,530 --> 00:06:20,050 if we do have an insured delivery. 124 00:06:20,050 --> 00:06:23,970 And the same down here for tracking a product. 125 00:06:23,970 --> 00:06:27,900 And then again, we copy it all into standard delivery 126 00:06:27,900 --> 00:06:30,020 and remove all the if checks 127 00:06:30,020 --> 00:06:33,113 and just keep the standard delivery code. 128 00:06:34,140 --> 00:06:38,090 Of course, standard delivery and insured delivery 129 00:06:38,090 --> 00:06:42,250 now also need to inherit from the base delivery class 130 00:06:42,250 --> 00:06:45,970 which in the case of TypeScript is done like this. 131 00:06:45,970 --> 00:06:48,760 So now we have these specialized classes. 132 00:06:48,760 --> 00:06:52,560 In the base class, we can now remove the deliver product 133 00:06:52,560 --> 00:06:54,773 and track product methods because 134 00:06:54,773 --> 00:06:58,523 that is something our specialized classes will deal with. 135 00:06:59,640 --> 00:07:01,500 Now, of course you might now ask, 136 00:07:01,500 --> 00:07:04,540 that's all great, but where's the if statement? 137 00:07:04,540 --> 00:07:06,870 I mean we need to decide somewhere 138 00:07:06,870 --> 00:07:10,090 which specialized class should be used. 139 00:07:10,090 --> 00:07:12,450 Well, that's something you need to do 140 00:07:12,450 --> 00:07:15,960 in the place where you plan on using these classes. 141 00:07:15,960 --> 00:07:19,750 So if you have some code in some other method or function 142 00:07:19,750 --> 00:07:24,750 it doesn't matter where you wanna use such a delivery class. 143 00:07:25,040 --> 00:07:28,830 You would in the past have created your delivery class 144 00:07:28,830 --> 00:07:31,160 by instantiating a new delivery 145 00:07:31,160 --> 00:07:34,330 and passing in some purchase object. 146 00:07:34,330 --> 00:07:35,910 And then in the past, 147 00:07:35,910 --> 00:07:39,160 before we created this specific versions 148 00:07:39,160 --> 00:07:42,033 you could have called deliver product. 149 00:07:43,000 --> 00:07:44,990 Now that's not possible anymore 150 00:07:44,990 --> 00:07:47,290 because deliver product doesn't exist 151 00:07:47,290 --> 00:07:49,010 on the base class anymore. 152 00:07:49,010 --> 00:07:51,750 Instead you need a specific class now. 153 00:07:51,750 --> 00:07:53,730 And that means that in the place 154 00:07:53,730 --> 00:07:55,300 where this code would execute, 155 00:07:55,300 --> 00:07:58,800 you need to decide which specific version you need. 156 00:07:58,800 --> 00:08:00,780 But that's a decision you typically can make 157 00:08:00,780 --> 00:08:02,440 in these places. 158 00:08:02,440 --> 00:08:06,830 Now you just need to have your if check in that place here. 159 00:08:06,830 --> 00:08:09,460 So here you would have your if check 160 00:08:09,460 --> 00:08:11,620 with these different branches 161 00:08:11,620 --> 00:08:15,120 and you could also use a switch case statement, of course 162 00:08:15,120 --> 00:08:18,540 and then here you would have your delivery variable 163 00:08:18,540 --> 00:08:21,480 which would be of type delivery. 164 00:08:21,480 --> 00:08:23,690 And then here we would assign a value 165 00:08:23,690 --> 00:08:27,903 of new express delivery and pass in our purchase. 166 00:08:31,920 --> 00:08:36,920 Here, we would have a new insured delivery instance. 167 00:08:36,920 --> 00:08:38,140 And down here we would have 168 00:08:38,140 --> 00:08:41,280 a new standard delivery instance. 169 00:08:41,280 --> 00:08:45,620 And then at the bottom we can call deliver product. 170 00:08:45,620 --> 00:08:49,680 And of course, instead of this purchase delivery type, 171 00:08:49,680 --> 00:08:51,590 you would have your purchase object 172 00:08:51,590 --> 00:08:53,610 lying somewhere around here. 173 00:08:53,610 --> 00:08:56,380 where you do execute this if statement. 174 00:08:56,380 --> 00:08:58,990 Let's not worry about this condition at the moment though. 175 00:08:58,990 --> 00:09:02,420 Let's assume we are in some place where we do have access 176 00:09:02,420 --> 00:09:03,730 to this purchase. 177 00:09:03,730 --> 00:09:06,890 Because this is sudo code anyways. 178 00:09:06,890 --> 00:09:08,470 It won't execute anyways 179 00:09:08,470 --> 00:09:11,030 because we don't have a real purchase here. 180 00:09:11,030 --> 00:09:14,600 This is how you would solve these situations though. 181 00:09:14,600 --> 00:09:17,830 How you would create the proper instance 182 00:09:17,830 --> 00:09:20,290 of your delivery class based 183 00:09:20,290 --> 00:09:22,603 on which specific type you need. 184 00:09:23,600 --> 00:09:26,160 Now here actually, my IDE is complaining 185 00:09:26,160 --> 00:09:29,420 that this is not a method of the delivery class. 186 00:09:29,420 --> 00:09:30,560 And that would be correct, 187 00:09:30,560 --> 00:09:32,830 it's the child classes which had it 188 00:09:32,830 --> 00:09:34,160 but of course you can work 189 00:09:34,160 --> 00:09:35,910 around something like this as well. 190 00:09:35,910 --> 00:09:40,173 For example, by utilizing an interface or abstract classes. 191 00:09:41,230 --> 00:09:44,340 So here we could have an interface called delivery 192 00:09:45,570 --> 00:09:50,570 which dictates that we have a deliver product method 193 00:09:53,040 --> 00:09:57,113 and that we have a track product method. 194 00:09:58,080 --> 00:10:01,910 And then this here would be the delivery implementation, 195 00:10:01,910 --> 00:10:03,460 for example, 196 00:10:03,460 --> 00:10:07,170 and we would extend delivery implementation here 197 00:10:09,670 --> 00:10:12,290 And our classes would implement 198 00:10:12,290 --> 00:10:15,143 the delivery interface like this. 199 00:10:16,890 --> 00:10:19,430 This is the TypeScript syntax for doing this, 200 00:10:19,430 --> 00:10:22,620 for extending a class and for implementing an interface. 201 00:10:22,620 --> 00:10:24,900 The exact implementation of course depends 202 00:10:24,900 --> 00:10:27,740 on which programming language you are using. 203 00:10:27,740 --> 00:10:29,460 This is just some way of doing it, 204 00:10:29,460 --> 00:10:31,990 but now we can set the interface 205 00:10:31,990 --> 00:10:34,180 as a type here and we therefore know, 206 00:10:34,180 --> 00:10:37,040 whatever is stored in delivery will be an object 207 00:10:37,040 --> 00:10:39,290 which implements this interface 208 00:10:39,290 --> 00:10:43,350 and which therefore will have a deliver product method. 209 00:10:43,350 --> 00:10:47,040 And therefore, now we can safely call deliver product here. 210 00:10:47,040 --> 00:10:49,770 And it's this if check, which now ensures 211 00:10:49,770 --> 00:10:51,680 that we're executing this method 212 00:10:51,680 --> 00:10:54,610 on the correct type of object. 213 00:10:54,610 --> 00:10:56,850 Now, of course you might have multiple places 214 00:10:56,850 --> 00:10:59,870 in your code where you need such objects. 215 00:10:59,870 --> 00:11:01,570 And therefore of course you could create 216 00:11:01,570 --> 00:11:03,570 a factory function again 217 00:11:03,570 --> 00:11:07,430 or a factory method in some utility class. 218 00:11:07,430 --> 00:11:11,160 And for example, name this create delivery 219 00:11:11,160 --> 00:11:12,570 where you get the purchase 220 00:11:12,570 --> 00:11:14,480 for which it should be created. 221 00:11:14,480 --> 00:11:16,500 And then you move that if code just 222 00:11:16,500 --> 00:11:20,610 into your factory method, you pass the purchase 223 00:11:20,610 --> 00:11:23,320 to these constructor functions 224 00:11:23,320 --> 00:11:25,770 and then wherever you needed such 225 00:11:25,770 --> 00:11:27,800 a specific type of delivery, 226 00:11:27,800 --> 00:11:32,700 you would just call create delivery, pass your object to it 227 00:11:32,700 --> 00:11:35,070 and you would then be good. 228 00:11:35,070 --> 00:11:38,420 Of course, we need to return the created object 229 00:11:38,420 --> 00:11:39,870 in the factory function. 230 00:11:39,870 --> 00:11:42,760 And then this factory function could be used anywhere 231 00:11:42,760 --> 00:11:47,410 in the code where you need this specific type of object. 232 00:11:47,410 --> 00:11:49,690 And now in this factory function, 233 00:11:49,690 --> 00:11:52,290 of course we can get rid of the this key word 234 00:11:52,290 --> 00:11:54,830 and assume the purchase which we got here 235 00:11:54,830 --> 00:11:56,620 has a delivery type. 236 00:11:56,620 --> 00:11:58,760 Again, I just wanna make it really clear, 237 00:11:58,760 --> 00:12:02,300 the object I do pass in doesn't have to this property, 238 00:12:02,300 --> 00:12:05,290 but this again is just some sudo code. 239 00:12:05,290 --> 00:12:08,660 Of course, instead of passing in an empty object down there 240 00:12:08,660 --> 00:12:11,120 where you create your instance, 241 00:12:11,120 --> 00:12:13,410 you would have a concrete purchase object, 242 00:12:13,410 --> 00:12:15,420 which you would be getting from somewhere else 243 00:12:15,420 --> 00:12:17,090 in your application. 244 00:12:17,090 --> 00:12:18,860 It's just not the focus here. 245 00:12:18,860 --> 00:12:22,110 The focus is that we do have such a factory function 246 00:12:22,110 --> 00:12:25,580 which knows how to produce specific instances 247 00:12:25,580 --> 00:12:30,580 of our classes here based on which kind of purchase we get. 248 00:12:30,580 --> 00:12:33,700 And that's how you can use polymorphism 249 00:12:33,700 --> 00:12:37,840 and create polymorphic objects when using classes. 250 00:12:37,840 --> 00:12:41,900 It's basically the same idea as in the last course section 251 00:12:41,900 --> 00:12:44,860 where I essentially created an object 252 00:12:44,860 --> 00:12:48,530 or a dictionary on the fly with the factory function. 253 00:12:48,530 --> 00:12:52,653 Now we're solving the same problem by using classes.