1 00:00:02,150 --> 00:00:02,983 Let's continue 2 00:00:02,983 --> 00:00:05,770 with these important principles, though, 3 00:00:05,770 --> 00:00:07,980 as mentioned before, we already covered 4 00:00:07,980 --> 00:00:09,910 the two most important ones 5 00:00:09,910 --> 00:00:12,490 when it comes to writing clean code. 6 00:00:12,490 --> 00:00:14,450 Nonetheless, let's next have a look 7 00:00:14,450 --> 00:00:18,010 at the interface segregation principle. 8 00:00:18,010 --> 00:00:20,610 What is this principle about? 9 00:00:20,610 --> 00:00:23,890 The interface segregation principles says 10 00:00:23,890 --> 00:00:28,260 that many client-specific interfaces are better 11 00:00:28,260 --> 00:00:30,950 than one general purpose interface. 12 00:00:30,950 --> 00:00:34,450 Now, again, as always, what does this mean? 13 00:00:34,450 --> 00:00:36,740 I got another example for you. 14 00:00:36,740 --> 00:00:38,200 I got a class SQLDatabase 15 00:00:39,070 --> 00:00:43,260 which implements an interface called Database. 16 00:00:43,260 --> 00:00:45,790 Interfaces are a common thing 17 00:00:45,790 --> 00:00:48,380 in object-oriented programming, of course. 18 00:00:48,380 --> 00:00:51,710 Interfaces are contracts which force classes 19 00:00:51,710 --> 00:00:53,820 to implement certain behaviors. 20 00:00:53,820 --> 00:00:56,660 In this case, we are forced to add a connect 21 00:00:56,660 --> 00:01:01,360 and store data method if we implement this interface. 22 00:01:01,360 --> 00:01:06,200 And the SQLDatabase does implement this interface. 23 00:01:06,200 --> 00:01:08,850 Now, everything looks all right here, 24 00:01:08,850 --> 00:01:12,467 but now let's say we don't just have the SQLDatabase, 25 00:01:13,650 --> 00:01:17,440 but instead, we also have is InMemoryDatabase, 26 00:01:17,440 --> 00:01:19,740 which, of course, also is a database, 27 00:01:19,740 --> 00:01:21,800 and therefore, it also makes sense 28 00:01:21,800 --> 00:01:24,573 that we implemented the Database interface. 29 00:01:25,700 --> 00:01:29,810 Hence, we are forced to add a connect and store data method. 30 00:01:29,810 --> 00:01:32,860 And now the problem is that it might make sense 31 00:01:32,860 --> 00:01:34,660 to have a stored data method 32 00:01:34,660 --> 00:01:38,060 because no matter if we are storing data in a SQLDatabase 33 00:01:38,060 --> 00:01:41,780 or in memory, we want to be able to store data 34 00:01:41,780 --> 00:01:43,180 at the end of the day, 35 00:01:43,180 --> 00:01:48,110 but connect, that does not necessarily make sense here. 36 00:01:48,110 --> 00:01:51,630 And InMemoryDatabase, which uses the local memory 37 00:01:51,630 --> 00:01:53,000 for storing data, 38 00:01:53,000 --> 00:01:56,113 doesn't really need to connect anywhere. 39 00:01:57,000 --> 00:02:00,530 So in the end, we have the wrong interface here. 40 00:02:00,530 --> 00:02:02,440 And if that kind of reminds you 41 00:02:02,440 --> 00:02:05,350 of the Liskov substitution principle we had a look at 42 00:02:05,350 --> 00:02:09,960 in the last lecture, that would actually not be a surprise 43 00:02:09,960 --> 00:02:11,610 because here we have the problem 44 00:02:11,610 --> 00:02:14,300 with modeling our data incorrectly 45 00:02:14,300 --> 00:02:17,240 and using the wrong base class in the end. 46 00:02:17,240 --> 00:02:20,500 Now with the interface segregation principle, 47 00:02:20,500 --> 00:02:23,450 we have the problem of having a wrong interface, 48 00:02:23,450 --> 00:02:26,070 which we force onto multiple classes, 49 00:02:26,070 --> 00:02:30,480 even if these classes are not using inheritance at all. 50 00:02:30,480 --> 00:02:33,440 The problem is that our database interface 51 00:02:33,440 --> 00:02:36,530 is a general purpose interface here. 52 00:02:36,530 --> 00:02:39,730 It's tries to cover too many use cases, 53 00:02:39,730 --> 00:02:42,793 connecting and storing data, to be precise. 54 00:02:43,690 --> 00:02:47,060 And we should avoid general purpose interfaces 55 00:02:47,060 --> 00:02:50,140 when following the interface segregation principle. 56 00:02:50,140 --> 00:02:51,610 Instead, we should have multiple, 57 00:02:51,610 --> 00:02:54,653 smaller clients-specific interfaces. 58 00:02:55,950 --> 00:02:59,560 For example, here we could have a database interface 59 00:02:59,560 --> 00:03:01,310 if we wanna keep that name. 60 00:03:01,310 --> 00:03:03,730 But in addition, we could have an interface 61 00:03:03,730 --> 00:03:05,593 which is called RemoteDatabase. 62 00:03:07,630 --> 00:03:11,410 And the connect method, forcing classes 63 00:03:11,410 --> 00:03:14,283 to have a connect method would go into RemoteDatabase. 64 00:03:16,178 --> 00:03:19,950 Then the SQLDatabase class could implement Database 65 00:03:19,950 --> 00:03:24,610 and RemoteDatabase, which in TypeScript, is done like this, 66 00:03:24,610 --> 00:03:27,870 simply by separating interfaces by a comma, 67 00:03:27,870 --> 00:03:31,260 and the InMemoryDatabase only implements 68 00:03:31,260 --> 00:03:35,503 the Database interface and not the RemoteDatabase interface. 69 00:03:36,420 --> 00:03:38,280 Hence, we can delete connect here. 70 00:03:38,280 --> 00:03:40,820 We're not forced to implement it anymore, 71 00:03:40,820 --> 00:03:42,530 and hence, we now have code 72 00:03:42,530 --> 00:03:46,193 which does follow the interface segregation principle. 73 00:03:47,300 --> 00:03:50,680 Now just as with the Liskov substitution principle, 74 00:03:50,680 --> 00:03:53,980 obviously the interface segregation principle 75 00:03:53,980 --> 00:03:55,680 is an important principle 76 00:03:55,680 --> 00:03:59,270 when it comes to writing maintainable and extensible code, 77 00:03:59,270 --> 00:04:01,920 when it comes to writing clean code. 78 00:04:01,920 --> 00:04:04,640 Where it, of course, matters that humans 79 00:04:04,640 --> 00:04:06,260 can read the code quite well, 80 00:04:06,260 --> 00:04:08,380 and that it's easy to understand, 81 00:04:08,380 --> 00:04:11,220 this rule doesn't help us too much. 82 00:04:11,220 --> 00:04:14,260 It also doesn't hurt, and it certainly helps a bit, 83 00:04:14,260 --> 00:04:16,243 but it's not super important.