1 00:00:02,150 --> 00:00:06,160 Now when working with classes and objects, 2 00:00:06,160 --> 00:00:09,240 there are a couple of important rules 3 00:00:09,240 --> 00:00:11,810 and principles out there which you might have heard 4 00:00:11,810 --> 00:00:13,740 in the past already. 5 00:00:13,740 --> 00:00:18,450 And all of these rules and principles and laws exist 6 00:00:18,450 --> 00:00:22,350 to help you write more maintainable code. 7 00:00:22,350 --> 00:00:24,160 That is the core idea. 8 00:00:24,160 --> 00:00:27,380 Now, of course, as I already mentioned at the beginning 9 00:00:27,380 --> 00:00:30,990 of this course section, there is a relation 10 00:00:30,990 --> 00:00:34,690 between maintainable and extensible code and clean code, 11 00:00:34,690 --> 00:00:37,540 which primarily focuses on readability 12 00:00:37,540 --> 00:00:41,530 but therefore is also related to maintainability. 13 00:00:41,530 --> 00:00:45,690 There is a connection for sure, but there are some patterns 14 00:00:45,690 --> 00:00:49,130 and principles which are more important for a clean code 15 00:00:49,130 --> 00:00:52,930 which have direct impact on the readability of code 16 00:00:52,930 --> 00:00:56,480 whereas there are other patterns and principles which have 17 00:00:56,480 --> 00:01:00,230 less of an impact there, but which instead really help you 18 00:01:00,230 --> 00:01:03,340 with building large scale extensible, 19 00:01:03,340 --> 00:01:07,150 maintainable applications where a lot of developers 20 00:01:07,150 --> 00:01:10,620 can keep a project up and running over a long time 21 00:01:10,620 --> 00:01:14,180 and where it's easy to add new functionalities ends on 22 00:01:14,180 --> 00:01:17,940 and that is done only loosely related to clean code. 23 00:01:17,940 --> 00:01:21,390 Now, of course, here I will focus on the principles and laws 24 00:01:21,390 --> 00:01:24,810 which have a bigger impact on the cleanness of your code 25 00:01:24,810 --> 00:01:28,720 and for that, we will start with the law of demeter. 26 00:01:28,720 --> 00:01:31,070 Now, what is the law of demeter? 27 00:01:31,070 --> 00:01:33,980 It is related to code like this. 28 00:01:33,980 --> 00:01:38,220 Here in this code which could be in some method 29 00:01:38,220 --> 00:01:42,270 of some class, we are accessing the customer property 30 00:01:42,270 --> 00:01:43,490 of the class. 31 00:01:43,490 --> 00:01:47,980 On that property, the last purchase property, as it seems 32 00:01:47,980 --> 00:01:50,640 and on that last purchase property 33 00:01:50,640 --> 00:01:52,450 which seems to be another object, 34 00:01:52,450 --> 00:01:55,070 we access the date property. 35 00:01:55,070 --> 00:01:58,450 You should avoid code like this. 36 00:01:58,450 --> 00:02:03,260 Drilling deeply into other objects is something 37 00:02:03,260 --> 00:02:06,440 which is discouraged by the law of demeter. 38 00:02:06,440 --> 00:02:09,020 Now, why is it discouraged? 39 00:02:09,020 --> 00:02:11,810 The law of demeter is in the end about the principle 40 00:02:11,810 --> 00:02:16,000 of least knowledge, which means your class shouldn't rely 41 00:02:16,000 --> 00:02:21,000 too much on the internals of strangers of other objects 42 00:02:21,560 --> 00:02:24,810 you are interacting with in your code. 43 00:02:24,810 --> 00:02:28,430 In this example, a customer would be a class property, 44 00:02:28,430 --> 00:02:32,450 so interacting with it is fine and accessing last purchase 45 00:02:32,450 --> 00:02:37,060 is fine but last purchase now is kind of a stranger. 46 00:02:37,060 --> 00:02:40,220 We access it indirectly through customer, 47 00:02:40,220 --> 00:02:42,910 and we shouldn't drill deeper into that. 48 00:02:42,910 --> 00:02:44,690 Now, why shouldn't we do that? 49 00:02:44,690 --> 00:02:47,040 Well, because whenever something changes 50 00:02:47,040 --> 00:02:50,290 about the internal structure of last purchase, 51 00:02:50,290 --> 00:02:54,570 all our code in this place where we have this line of code 52 00:02:54,570 --> 00:02:56,513 needs to change as well. 53 00:02:57,523 --> 00:03:00,730 And that then makes our code harder to extend and maintain 54 00:03:00,730 --> 00:03:04,930 because simple changes can result in a lot of changes 55 00:03:04,930 --> 00:03:07,253 in a lot of places of your application. 56 00:03:08,603 --> 00:03:10,930 And this law is closely related to our concept 57 00:03:10,930 --> 00:03:14,030 of writing clean code, because for one, 58 00:03:14,030 --> 00:03:18,044 when writing clean code, we shouldn't repeat ourselves 59 00:03:18,044 --> 00:03:21,770 and we often might do that if we have statements like this. 60 00:03:21,770 --> 00:03:25,210 And in addition, statements like this are a bit harder 61 00:03:25,210 --> 00:03:29,230 to read, analyzing such lines of code simply takes longer 62 00:03:29,230 --> 00:03:31,563 than short, simple statements. 63 00:03:32,975 --> 00:03:35,060 Of course, this isn't too difficult of a statement, 64 00:03:35,060 --> 00:03:37,363 but it is also just a brief example. 65 00:03:38,720 --> 00:03:41,330 So the law of demeter, which we should therefore 66 00:03:41,330 --> 00:03:45,020 also follow to achieve clean code in the end states 67 00:03:45,020 --> 00:03:50,020 that code in a method may only access the direct internals 68 00:03:50,160 --> 00:03:53,460 which would be properties and methods of the object 69 00:03:53,460 --> 00:03:54,503 it belongs to. 70 00:03:55,406 --> 00:03:57,960 So you can access any property and method of the class 71 00:03:57,960 --> 00:04:00,973 of which this method in which you run the code is part of. 72 00:04:01,956 --> 00:04:04,140 You can access any objects, which are stored in properties 73 00:04:04,140 --> 00:04:05,833 of that object. 74 00:04:05,833 --> 00:04:07,860 So in this case, accessing customer is fine 75 00:04:07,860 --> 00:04:11,840 because it is a property of the class this code belongs to, 76 00:04:11,840 --> 00:04:14,220 as you can tell by the this keyword 77 00:04:14,220 --> 00:04:18,430 and accessing last purchase is of course also fine 78 00:04:18,430 --> 00:04:22,023 because that directly is part of that customer property. 79 00:04:23,356 --> 00:04:25,280 You may also use any objects which you receive 80 00:04:25,280 --> 00:04:28,410 as method parameters, so it's not just about properties 81 00:04:28,410 --> 00:04:30,490 of the class, like customer, 82 00:04:30,490 --> 00:04:33,500 but if customer would be received as a parameter, 83 00:04:33,500 --> 00:04:36,920 we would also be allowed to access data in it 84 00:04:36,920 --> 00:04:41,020 and of course, we also are allowed to interact with objects 85 00:04:41,020 --> 00:04:43,160 which are created in the method. 86 00:04:43,160 --> 00:04:45,630 So if we would instantiate another class, 87 00:04:45,630 --> 00:04:47,950 we can of course, also work with the API 88 00:04:47,950 --> 00:04:50,250 and properties of that class. 89 00:04:50,250 --> 00:04:53,510 That's the idea behind the law of demeter. 90 00:04:53,510 --> 00:04:57,823 And here's this example from the slide now in a 91 00:04:57,823 --> 00:04:59,710 real dummy class. 92 00:04:59,710 --> 00:05:03,170 I'm accessing date on last purchase on customer, 93 00:05:03,170 --> 00:05:07,540 which is a property and only this part here would be okay, 94 00:05:07,540 --> 00:05:11,570 because with this part, I'm directly working on the property 95 00:05:11,570 --> 00:05:15,303 and the data in the property which is part of this class. 96 00:05:16,939 --> 00:05:20,060 Date is not directly data of the customer property, 97 00:05:20,060 --> 00:05:22,070 it's part of some other data, 98 00:05:22,070 --> 00:05:24,070 which is stored in the customer property 99 00:05:24,961 --> 00:05:27,020 and therefore we have only indirect access to it, 100 00:05:27,020 --> 00:05:30,300 which is not allowed by the law of demeter. 101 00:05:31,837 --> 00:05:34,403 Now here, in this example, we are, for example 102 00:05:34,403 --> 00:05:37,310 getting this date to then call the deliver purchases 103 00:05:37,310 --> 00:05:38,533 by date method. 104 00:05:39,618 --> 00:05:40,770 And the obvious question now is 105 00:05:40,770 --> 00:05:43,273 how could we clean this code up? 106 00:05:44,141 --> 00:05:46,991 How could we change it such that we do follow 107 00:05:46,991 --> 00:05:48,670 the law of demeter? 108 00:05:48,670 --> 00:05:52,200 Well, one first simple solution could be 109 00:05:52,200 --> 00:05:56,491 that we simply go to our customer class 110 00:05:56,491 --> 00:05:58,420 so to our customer object and the class defining 111 00:05:58,420 --> 00:06:02,593 that object structure and we add a new method in there. 112 00:06:03,731 --> 00:06:05,210 Instead of this code, which for the moment 113 00:06:05,210 --> 00:06:08,370 I'm commenting out so that we still have it for reference, 114 00:06:08,370 --> 00:06:10,240 instead of this code, it would be better 115 00:06:10,240 --> 00:06:12,440 if our customer class would have 116 00:06:12,440 --> 00:06:15,853 a get last purchase date method. 117 00:06:17,655 --> 00:06:21,352 This code would be allowed because we're directly accessing 118 00:06:21,352 --> 00:06:23,480 a method in our customer property 119 00:06:23,480 --> 00:06:26,660 and now this customer object simply needs such a method 120 00:06:26,660 --> 00:06:30,280 which then internally accesses last purchase date. 121 00:06:30,280 --> 00:06:33,130 And there, this would be okay because last purchase 122 00:06:34,063 --> 00:06:37,130 of course will be a direct property or some data stored 123 00:06:37,130 --> 00:06:38,560 in the customer object, 124 00:06:38,560 --> 00:06:42,590 so then accessing date on that direct last purchase property 125 00:06:42,590 --> 00:06:46,680 would also be fine inside of that customer class. 126 00:06:46,680 --> 00:06:50,280 So this is how we could adjust our code 127 00:06:50,280 --> 00:06:52,289 in the customer class 128 00:06:52,289 --> 00:06:55,520 and then also in the delivery job class to not violate 129 00:06:55,520 --> 00:06:57,410 the law of demeter anymore. 130 00:06:57,410 --> 00:07:00,400 However, this is not a great solution 131 00:07:00,400 --> 00:07:04,560 because now we basically force our code to follow 132 00:07:04,560 --> 00:07:07,790 the law of demeter, but we still have some problems 133 00:07:07,790 --> 00:07:10,530 and we didn't really end up with that much 134 00:07:10,530 --> 00:07:12,160 of a cleaner code. 135 00:07:12,160 --> 00:07:14,360 Yes, this code is a bit easier to read now, 136 00:07:14,360 --> 00:07:17,170 so that is cleaner because it's a clear instruction 137 00:07:17,170 --> 00:07:20,560 which requires no interpretation, but of course we now just 138 00:07:20,560 --> 00:07:23,623 outsourced the problem into customer. 139 00:07:23,623 --> 00:07:25,330 And whilst we technically are not violating 140 00:07:25,330 --> 00:07:28,590 the law of demeter anymore, you'll of course quickly end up 141 00:07:28,590 --> 00:07:30,330 with a lot of helper methods, 142 00:07:30,330 --> 00:07:34,160 like get last purchase date here adjust to follow 143 00:07:34,160 --> 00:07:36,383 the law of demeter on paper. 144 00:07:37,350 --> 00:07:41,600 The better solution for solving problems like this 145 00:07:41,600 --> 00:07:45,015 and the solution, which really makes this clean, 146 00:07:45,015 --> 00:07:46,890 which is why I wanna cover this concept is that 147 00:07:46,890 --> 00:07:50,073 you should tell, don't ask in your code. 148 00:07:51,190 --> 00:07:53,770 Don't ask for information to then use it 149 00:07:53,770 --> 00:07:58,180 but instead, simply tell other classes what to do, 150 00:07:58,180 --> 00:08:00,360 if that is possible. 151 00:08:00,360 --> 00:08:04,680 For example here, instead of getting that date 152 00:08:04,680 --> 00:08:07,170 now with this helper method, just to then forward it 153 00:08:07,170 --> 00:08:11,640 to deliver purchase by date, the better solution would be 154 00:08:11,640 --> 00:08:15,683 to work on the warehouse instead of the customer class. 155 00:08:17,120 --> 00:08:22,120 So that here on this warehouse, we, for example, can call 156 00:08:24,320 --> 00:08:29,320 deliver purchase method instead of purchases by date 157 00:08:31,600 --> 00:08:35,280 and here we pass this customer last purchase 158 00:08:36,350 --> 00:08:40,110 and then it would be the job of the deliver purchase method 159 00:08:40,110 --> 00:08:42,940 to extract the data from that last purchase 160 00:08:42,940 --> 00:08:45,030 which it receives, which is required 161 00:08:45,030 --> 00:08:46,653 to deliver this purchase. 162 00:08:47,670 --> 00:08:50,470 Now we're not violating the law of demeter 163 00:08:50,470 --> 00:08:52,860 because we're only accessing the customer property 164 00:08:52,860 --> 00:08:55,540 and some data which is directly stored in that 165 00:08:55,540 --> 00:09:00,540 and in deliver purchase, we just get a purchase object 166 00:09:00,540 --> 00:09:04,480 which we also can use there to extract any data we want 167 00:09:04,480 --> 00:09:07,070 because you can work with objects 168 00:09:07,070 --> 00:09:09,430 and the data directly stored in them 169 00:09:09,430 --> 00:09:12,613 if they are received as method parameters. 170 00:09:14,265 --> 00:09:17,870 And this is now how you really would wanna solve this. 171 00:09:17,870 --> 00:09:22,140 You wanna tell your code what to do and not ask 172 00:09:22,140 --> 00:09:25,595 and that's all the why I wanted to cover this principle 173 00:09:25,595 --> 00:09:29,650 or this law here because this certainly is clean code. 174 00:09:29,650 --> 00:09:34,310 We don't have to interpret why we want that date 175 00:09:34,310 --> 00:09:36,510 and what we could be doing with it. 176 00:09:36,510 --> 00:09:41,510 Instead we, in the end, got rid of two lines of code, 177 00:09:41,634 --> 00:09:44,210 which we required before, which we had to read 178 00:09:44,210 --> 00:09:47,440 and we just got one line of code, which is 100% clear 179 00:09:47,440 --> 00:09:49,050 about what it does. 180 00:09:49,050 --> 00:09:52,240 That's the law of demeter and how you should follow it 181 00:09:52,240 --> 00:09:55,833 and that's why it matters for writing clean code. 182 00:09:56,690 --> 00:10:01,030 And as a side note and why I also wanted to include 183 00:10:01,030 --> 00:10:04,530 this in the course, which is of course not just about 184 00:10:04,530 --> 00:10:07,350 object-oriented programming and classes, 185 00:10:07,350 --> 00:10:12,350 the law of demeter actually also can be applied to code 186 00:10:12,360 --> 00:10:15,870 that uses just functions and no classes. 187 00:10:15,870 --> 00:10:18,030 I mean, of course you can't apply it like this, 188 00:10:18,030 --> 00:10:21,790 you can't apply the official law with all its definitions, 189 00:10:21,790 --> 00:10:24,060 but you can apply its logic. 190 00:10:24,060 --> 00:10:28,420 If you write code with just functions without any classes 191 00:10:28,420 --> 00:10:31,630 because you are writing code in a procedural way 192 00:10:31,630 --> 00:10:34,840 or because you use functional programming, 193 00:10:34,840 --> 00:10:37,570 if you do either of these two things, 194 00:10:37,570 --> 00:10:41,230 then you still wanna ensure that inside of your functions, 195 00:10:41,230 --> 00:10:44,980 you don't dig too deeply into received data 196 00:10:44,980 --> 00:10:49,200 unless there is kind of a data container only. 197 00:10:49,200 --> 00:10:53,110 And you also wanna tell, not ask, 198 00:10:53,110 --> 00:10:55,250 at least if it makes sense. 199 00:10:55,250 --> 00:10:59,430 You should always ask yourself whether getting some data 200 00:10:59,430 --> 00:11:02,100 to then do something with it in the next step 201 00:11:02,100 --> 00:11:05,020 is the best possible way right now 202 00:11:05,020 --> 00:11:09,150 or if you maybe just instead should tell some other part 203 00:11:09,150 --> 00:11:11,780 of your application what to do next. 204 00:11:11,780 --> 00:11:14,850 This is always something you can consider 205 00:11:14,850 --> 00:11:18,510 because telling instead of asking is always super easy 206 00:11:18,510 --> 00:11:22,930 to understand and automatically leads to shorter code 207 00:11:22,930 --> 00:11:26,680 at least in the part where you are telling. 208 00:11:26,680 --> 00:11:30,100 So the law of demeter is super important especially 209 00:11:30,100 --> 00:11:32,460 when working with classes and objects 210 00:11:32,460 --> 00:11:36,520 but also if you think about the general idea behind it 211 00:11:36,520 --> 00:11:38,093 in other kinds of code.