WEBVTT 00:00.140 --> 00:05.990 So now we are going to learn about the interfaces and abstract classes. 00:06.020 --> 00:08.930 So starting with interfaces. 00:09.170 --> 00:15.860 So interfaces just define a contract that certain classes have to sign. 00:15.920 --> 00:20.990 And this just guarantees that those classes will have certain functionality. 00:21.020 --> 00:25.520 Specifically they will implement some methods. 00:25.520 --> 00:29.690 Interfaces themselves don't implement anything. 00:29.720 --> 00:32.660 They just define the methods. 00:33.170 --> 00:36.680 So let's start by defining an interface. 00:36.680 --> 00:42.050 And as an example here we are going to work with payment processors. 00:42.050 --> 00:48.320 And the goal is to have an interface that defines some of the behaviors. 00:48.320 --> 00:56.630 And then the classes that might implement PayPal payment processor or stripe payment processor. 00:57.080 --> 01:06.270 And by doing that we basically allow our app Up to easily swap between different payment processors 01:06.300 --> 01:09.570 without making any changes to the code. 01:11.040 --> 01:15.600 So the interface is defined by using the interface keyword. 01:15.930 --> 01:20.010 It needs to be given a name so we can go with just payment. 01:20.010 --> 01:28.290 Processor and interfaces define methods but never implement them. 01:30.450 --> 01:33.450 So I'm going to go with public function. 01:33.720 --> 01:37.830 Process payment. 01:40.020 --> 01:45.990 Which needs to accept an amount and returns a boolean on success. 01:46.590 --> 01:49.980 Notice how I'm using a semicolon here. 01:49.980 --> 01:55.110 As as I've said, there is no implementation in the interface. 01:57.060 --> 02:00.240 Let's also have another method here. 02:01.470 --> 02:05.610 So this would be refund payment. 02:06.750 --> 02:14.160 So signing a contract by a class means that a class implements such interface. 02:14.160 --> 02:22.680 And thus PHP will make sure that the given class has this method with exactly this header, and also 02:22.710 --> 02:25.680 that it has this second method. 02:25.710 --> 02:28.890 Otherwise this would be a fatal error. 02:29.460 --> 02:29.940 Okay. 02:29.970 --> 02:33.840 Now let's proceed with the implementations. 02:35.850 --> 02:36.030 Okay. 02:36.060 --> 02:41.250 So I think we should see an example of a class that implements the interface. 02:42.210 --> 02:45.720 So let's create a stripe processor. 02:45.750 --> 02:50.820 So if you don't know stripe this is an online payment processor. 02:50.850 --> 02:54.090 Essentially it lets you accept payments. 02:54.870 --> 03:00.690 And to implement an interface you're going to use the implements keyword. 03:00.690 --> 03:06.340 And just name the interface Dad's payment processor. 03:07.480 --> 03:15.700 Now, with this code here, we expect to see a fatal error because as I've mentioned before, when a 03:15.700 --> 03:24.040 class implements the interface, it actually needs to implement all the methods from the interface. 03:25.090 --> 03:26.530 Let's run this. 03:26.950 --> 03:35.050 And there's the fatal error saying something about abstract methods or implementing them vars. 03:35.620 --> 03:44.020 Next up, let's jump to the concept of abstract classes and abstract methods. 03:45.580 --> 03:47.500 So currently we have a choice. 03:47.530 --> 03:56.620 We either implement all the methods from the interface in this stripe processor class, or we mark it 03:56.620 --> 03:58.030 as abstract. 03:58.720 --> 04:03.730 Now making a class abstract has some serious implications. 04:03.730 --> 04:08.750 The main one is that you can't create an instance of such class. 04:09.860 --> 04:17.990 So if I'd like to go and do new stripe processor, this will just throw a fatal error. 04:18.020 --> 04:21.230 You can't create an abstract classes. 04:21.350 --> 04:26.120 So every method in a class needs to be implemented. 04:26.240 --> 04:29.660 So what's the point of the abstract classes. 04:30.950 --> 04:39.980 So interfaces lets you make sure that some classes have the specific functionality that is required. 04:40.220 --> 04:47.330 So by implementing that interface we just make sure that a class has those two possibilities that it 04:47.330 --> 04:52.010 can process payments and that it can refund payments. 04:52.040 --> 04:55.520 And how does it do that is not our concern. 04:55.790 --> 05:03.830 Now when you have abstract classes, what it lets you do is you might want to implement some common 05:04.090 --> 05:09.700 Nationality across all of your payment processors. 05:10.810 --> 05:18.250 So if I have a stripe processor, but I might also have a PayPal processor. 05:18.280 --> 05:20.920 This makes perfect sense. 05:21.370 --> 05:26.620 And there might be some common functionality across those two classes. 05:26.620 --> 05:32.680 Like they both take an API key to connect to those services. 05:33.520 --> 05:39.490 Maybe they can have some common logic that is required by your app. 05:39.490 --> 05:49.390 And then creating an abstract class which can implement this functionality is actually a great idea. 05:49.540 --> 06:02.680 So let's create an abstract class that we're going to call online payment Processor that will implement 06:03.190 --> 06:04.810 our interface. 06:04.810 --> 06:06.750 That's payment. 06:08.940 --> 06:09.990 Processor. 06:13.440 --> 06:16.050 So now let's make those two classes. 06:16.050 --> 06:17.820 Concrete classes. 06:18.150 --> 06:22.440 A concrete class is a class that's not abstract. 06:22.650 --> 06:27.150 It means that it implements all the interface methods. 06:27.150 --> 06:31.200 And it also implements all the abstract methods. 06:31.890 --> 06:35.610 So it won't directly implement the interface. 06:35.610 --> 06:43.500 Instead this would extend the online payment processor. 06:43.530 --> 06:51.240 And by extending the online payment processor it actually also implements that interface. 06:51.240 --> 06:54.300 So this is all inherited. 06:54.330 --> 07:00.360 Now this won't work yet because still this is an abstract class. 07:00.360 --> 07:05.580 And the methods from the interface are not implemented anywhere. 07:06.340 --> 07:12.910 But since we have this abstract class, let's add something that will justify its existence, which 07:12.910 --> 07:18.220 is some common functionality that those two concrete classes should share. 07:19.450 --> 07:27.310 So let's assume that both of those payment processors accept a kind of an API key that's just used to 07:27.340 --> 07:28.660 connect to them. 07:29.350 --> 07:36.220 So I'm going to create a constructor here that will essentially define a field. 07:37.420 --> 07:43.450 Now I'm using the protected visibility keyword and this is API key. 07:44.050 --> 07:46.240 Now we haven't used protected before. 07:46.240 --> 07:47.800 So let me explain this. 07:47.830 --> 07:54.340 So public means you can access methods and fields inside and outside the class. 07:54.460 --> 07:57.040 This is the public interface to your class. 07:57.040 --> 08:01.270 This is what others using this class can actually call. 08:02.410 --> 08:03.580 And private. 08:03.580 --> 08:11.000 As we might remember, are the fields and methods that are only accessible inside a specific class. 08:11.180 --> 08:18.650 And the difference between private and protected is that protected fields and methods are not accessible 08:18.650 --> 08:25.400 outside the class, but they are accessible in classes that extend them. 08:25.610 --> 08:33.950 So this API key field can be accessed in those classes inside those classes that extend this base class. 08:34.880 --> 08:42.710 And if you have private fields or private methods, those cannot be accessed outside the class, but 08:42.710 --> 08:47.300 also can't be accessed in classes that extend the base class. 08:47.330 --> 08:48.890 So that's the difference. 08:51.140 --> 08:51.410 Okay. 08:51.410 --> 08:58.010 So now both of those payment processors, they get access to the API key. 08:58.580 --> 09:04.160 Now there might be some different requirements for the API key in both of those. 09:04.370 --> 09:07.010 So we're going to go back to this later on. 09:07.040 --> 09:15.210 For now I'd like this abstract class to implement the methods from the interface, because we'd actually 09:15.210 --> 09:19.470 like to have the logic that is kind of the same. 09:19.470 --> 09:24.660 And in both cases, we'd like to verify if the API key is correct. 09:24.660 --> 09:29.670 So now I'm going to implement those methods from the interface. 09:29.790 --> 09:38.220 The easiest way to start is just to copy those like that and just add the implementation. 09:40.890 --> 09:48.750 Now by the way, even though those methods are empty, we can actually create those classes now because 09:48.750 --> 09:51.330 the methods are now implemented. 09:52.410 --> 10:00.120 So if I'm going to do processor and I'm going to pass some kind of a key, then when I run this code. 10:02.430 --> 10:03.750 Everything is fine. 10:05.640 --> 10:06.000 All right. 10:06.000 --> 10:09.150 So at this point I'd like to take a break. 10:09.180 --> 10:11.670 Let's summarize what we've learned so far. 10:12.150 --> 10:19.800 So we can define an interface, which basically is like signing a contract by a class that it guarantees 10:19.920 --> 10:22.050 to have this functionality. 10:22.080 --> 10:25.740 It has to implement all the methods from the interface. 10:26.490 --> 10:33.390 Then there are abstract classes, and the abstract class is a class that has at least one abstract method. 10:33.480 --> 10:38.670 If you don't implement a method from an interface, it's automatically abstract. 10:38.700 --> 10:45.450 You can also explicitly mark methods as abstract, but we're gonna see that in the next example. 10:45.450 --> 10:49.080 So we've created this abstract class that is not concrete. 10:49.080 --> 10:53.190 So it cannot be created and it implements the interface. 10:53.190 --> 11:01.650 Now we want to have the common functionality of those two methods, but also with some specifics that 11:01.650 --> 11:05.610 will be implemented in those two concrete classes. 11:06.870 --> 11:09.720 So we're going to go back to it after a short break.