WEBVTT 00:00.050 --> 00:00.620 Hi everyone. 00:00.620 --> 00:01.310 Welcome back. 00:01.310 --> 00:07.430 In this lecture we will learn about the most interesting concept which is sealed classes or interfaces. 00:07.430 --> 00:10.700 I will start with some theory first and then we'll get to the coding part. 00:10.730 --> 00:15.170 This concept got released and it was generally available from Java 17. 00:15.170 --> 00:20.930 So if you are using Java 17 already, then you can leverage this concept and then start using it from 00:20.930 --> 00:21.530 now on. 00:21.530 --> 00:25.100 The basic idea is to allow inheritance by permission. 00:25.100 --> 00:28.130 So that's this concept is to restrict the inheritance. 00:28.130 --> 00:32.630 But you will still be allowed to apply inheritance by permission. 00:32.630 --> 00:35.060 Let me explain this using a simple example. 00:35.090 --> 00:38.660 As we all know, Java is very open by default. 00:38.660 --> 00:39.950 What do I mean by that? 00:39.950 --> 00:44.330 Any class can extend other class as long as it is accessible. 00:44.330 --> 00:51.410 In this example we have a vehicle class which is a superclass, and having a truck class and the car 00:51.410 --> 00:53.270 class extend the vehicle class. 00:53.270 --> 00:54.110 Make sense? 00:54.110 --> 00:57.230 But having a dog class extend the car class. 00:57.260 --> 00:59.600 That does not make sense, right? 00:59.600 --> 01:03.260 But technically, from a language perspective, you can still do it. 01:03.260 --> 01:09.140 There is no language constraint that's going to prevent us from doing it and listen until if a developer 01:09.140 --> 01:12.770 recognizes this and catches it before moving this code to production. 01:12.770 --> 01:17.540 But wouldn't it be nice if we have a language feature that can automatically prevent this for you? 01:17.570 --> 01:23.630 This is where sealed classes or interfaces comes into play to prevent this kind of behavior. 01:23.660 --> 01:28.700 Let's look at the same example with the addition of sealed keyword in our class hierarchy. 01:28.700 --> 01:31.520 So in this case we have the vehicle class. 01:31.520 --> 01:36.110 But if we take a look at it we have this sealed keyword added to the definition. 01:36.110 --> 01:41.780 So what this is going to do is that this is going to have another block where or another keyword it's 01:41.780 --> 01:45.590 going to permit only the car and truck. 01:45.620 --> 01:45.950 Right. 01:45.980 --> 01:51.980 So whatever that comes after the permits, those are the only classes which are allowed to extend the 01:51.980 --> 01:52.820 vehicle class. 01:52.820 --> 01:58.250 That's the actual definition of public sealed class vehicle permits car and truck. 01:58.250 --> 02:01.910 And here we have the final keyword added to the subclasses. 02:01.910 --> 02:06.680 And it is to make sure no more inheritance is further allowed on these classes. 02:06.680 --> 02:10.940 But there are other options available which I'll cover in the next lecture. 02:10.970 --> 02:16.710 Let's say in this case, if the dark class tries to extend the vehicle class, the compiler will complain 02:16.710 --> 02:21.900 about dark class being not part of the hierarchy and it is not allowed. 02:21.930 --> 02:23.370 I hope the explanation is clear. 02:23.370 --> 02:29.310 So the fundamental idea is to restrict inheritance from a language perspective, meaning applying the 02:29.310 --> 02:32.040 restriction while defining these classes. 02:32.040 --> 02:36.300 So with this information, let's go ahead and explore the sealed classes in action. 02:36.300 --> 02:37.740 Now let's go back to IntelliJ. 02:37.860 --> 02:38.520 There we go. 02:38.520 --> 02:43.650 So in the IntelliJ we are going to be working on the sealed package that you see over here. 02:43.650 --> 02:46.050 And then let's go ahead and open the vehicle class. 02:46.050 --> 02:51.150 So if you take a look at the vehicle class what we have is that this vehicle class act as a superclass 02:51.150 --> 02:55.710 for the car class, the dark class, and then the truck class. 02:55.710 --> 02:58.830 So actually we are going to be breaking this inheritance here. 02:58.830 --> 02:59.580 You can see right. 02:59.580 --> 03:01.620 The dog is extending the vehicle class. 03:01.620 --> 03:06.300 I just created this class on purpose so that we can actually understand the impact of this one. 03:06.300 --> 03:10.530 I'm explaining this concept using a very simple and naive implementations. 03:10.530 --> 03:15.150 In the future part of the course, there is a dedicated section where we will be building a real time 03:15.150 --> 03:19.340 application using all the concepts that we will be learning as part of this course. 03:19.370 --> 03:19.730 Okay. 03:19.760 --> 03:22.370 So now what I'm going to do, I'm going to go to the vehicle class. 03:22.370 --> 03:25.220 And in here I'm going to add the shield modifier. 03:25.220 --> 03:28.520 So when I add the shield modifier what this is going to do. 03:28.520 --> 03:30.920 So this is going to be giving me a compilation issue. 03:30.920 --> 03:35.330 So the compilation issue says add the missing subclasses to the permit classes. 03:35.330 --> 03:36.740 So I'm going to click on that one. 03:36.740 --> 03:38.450 So in this case what we need to do. 03:38.480 --> 03:40.460 We don't want the dark class to be part of it. 03:40.460 --> 03:40.700 Right. 03:40.730 --> 03:43.910 So we just need the car class and the truck class. 03:43.910 --> 03:49.100 And then it complains about car class being not final and car class being a regular class. 03:49.100 --> 03:51.740 So this is something which we'll look into it on. 03:51.740 --> 03:53.510 What are the different options for now? 03:53.510 --> 03:56.210 What we are going to do is we are going to add the final modifier. 03:56.210 --> 04:00.740 So what this final modifier is going to do, this is going to make this class final. 04:00.740 --> 04:04.010 And then stop allowing any kind of inheritance from here. 04:04.010 --> 04:07.970 So let's go back to the vehicle class and we'll do the same thing for the truck class. 04:08.600 --> 04:12.500 There are different options available which we will talk about in the next lecture as I mentioned. 04:12.710 --> 04:13.100 There we go. 04:13.100 --> 04:15.500 So the compilation issue is gone right. 04:15.530 --> 04:19.070 So now if you go to the dark class this is not going to be allowed. 04:19.070 --> 04:23.300 It says like a dog is not allowed in the sealed hierarchy. 04:23.300 --> 04:25.980 So in this case the compiler catches it, right? 04:26.010 --> 04:27.870 So in this case you go ahead and remove it. 04:27.870 --> 04:30.570 That's the only way to have this code compile. 04:30.570 --> 04:35.850 So with this code in place what we are achieving is that we are achieving the inheritance from the language 04:35.850 --> 04:36.270 itself. 04:36.270 --> 04:42.330 So when you define the class you make sure what are the different classes that can be part of the hierarchy. 04:42.330 --> 04:46.920 And you can achieve this by using the CL modifier and then the pyramids. 04:46.950 --> 04:53.130 Now let's learn about some of the techniques to write test cases to make sure the car and truck is part 04:53.130 --> 04:54.960 of the vehicle hierarchy. 04:55.020 --> 04:57.810 So now what I'm going to do I'm going to create a test case. 04:57.840 --> 04:59.640 So it's going to be the vehicle test. 05:00.000 --> 05:04.110 And then it's going to be under the sealed package in the test folder. 05:04.140 --> 05:05.550 Then what you do click okay. 05:05.580 --> 05:06.270 There you go. 05:06.300 --> 05:07.710 The test case is ready. 05:07.740 --> 05:08.940 I'm going to move this one to the right. 05:08.940 --> 05:10.110 In this case what we will do. 05:10.110 --> 05:12.960 We'll create a test case named let's add the annotation over here. 05:12.960 --> 05:15.810 And then the test case is going to be vehicle test. 05:15.840 --> 05:17.280 It's going to be vehicle. 05:17.280 --> 05:20.370 And what we are going to do is that we are going to create a car. 05:20.580 --> 05:24.570 It's going to be where car is equal to new car, okay. 05:24.600 --> 05:26.880 And then we are going to do the same thing for the truck. 05:26.910 --> 05:28.680 To where. 05:28.710 --> 05:29.520 Truck. 05:30.450 --> 05:31.800 New truck. 05:33.030 --> 05:33.450 Okay. 05:33.480 --> 05:37.380 And then how do we make sure this car is an instance of a vehicle? 05:37.410 --> 05:37.830 Right. 05:37.830 --> 05:41.760 So there is a assert instance of function which we can leverage. 05:41.760 --> 05:43.350 So assert instance of. 05:43.350 --> 05:49.110 So we can provide like a vehicle because we want to make sure car is part of the vehicle hierarchy. 05:49.110 --> 05:49.440 Right. 05:49.440 --> 05:52.620 So we can do this and then pass the instance car. 05:53.070 --> 05:55.740 And we can do the same thing over here. 05:55.740 --> 05:57.240 And this is going to be truck. 05:57.240 --> 06:01.530 So this is how we make sure we have test cases that's going to assert this behavior. 06:01.560 --> 06:01.890 Right. 06:01.920 --> 06:03.750 So we have defined the hierarchy here. 06:03.750 --> 06:09.210 But we need to make sure with a sealed and permit is it really going to work as expected. 06:09.210 --> 06:11.640 So in this case run the test case. 06:11.640 --> 06:12.120 There you go. 06:12.150 --> 06:13.080 The test case passed. 06:13.080 --> 06:17.310 That means the car and truck is part of the vehicle hierarchy. 06:17.310 --> 06:19.320 So this is how you create sealed classes. 06:19.320 --> 06:24.090 And the fundamental concept is to allow inheritance by permission. 06:24.090 --> 06:27.930 So this is a handy feature I have used this in many of my projects. 06:27.930 --> 06:33.450 It's really helpful if you have this kind of hierarchy related code, you should start using the sealed 06:33.450 --> 06:35.190 concept in your project two. 06:35.220 --> 06:36.600 This marks the end of this lecture. 06:36.600 --> 06:37.800 Thank you for watching.