Abstract Classes

Okay. 0:00 So we have some shared functionality between our entities at the moment. 0:01 Mainly, the handling of how our system generates unique IDs. 0:04 All entities are going to identify the same way and 0:07 require that no argument constructor. 0:10 So it sounds to me like what we want to do is create a class that are entities 0:12 should extend. 0:16 But wait. 0:18 We probably wanna make sure that nobody creates that superclass. 0:18 I mean, what would that even map to in the database. 0:21 When you have functionality that you wanna share but you don't wanna allow 0:24 object creation of that superclass using an abstract class is the way to go. 0:27 Let's use an abstract class to solve our woes. 0:31 Okay. So let's create a new package. 0:34 And it's gonna handle all of the shared items between our features. 0:36 It'll deal with features core to our application. 0:40 So the typical name for this sort of thing is core. 0:42 So let's go ahead and do the same. 0:45 Right? So we'll make a new Java class. 0:47 We'll make a new package called core. 0:50 And let's name this entity, BaseEntity. 0:52 That should help make things clear. 0:55 Okay, so now let's add the abstract keyword here. 0:58 Okay, now with this in place, nobody can create a BaseEntity. 1:03 They must extend it if they want to use. 1:06 They'll get a compile time error. 1:09 Or they'll be prompted to create an anonymous inner class. 1:10 If they attempt to use it with the new keyword. 1:13 So the first thing that we need to do, is to tell. 1:16 J P A that this is a mapped superclass. 1:18 So we'll say mapped superclass. 1:22 And now we can use it with our other entities. 1:26 Now let's flip over to one of our entities and grab our duplication perpetrator. 1:29 So here you go, let's grab this ID, I am gonna go ahead and cut this app, 1:34 wanna flip over to the base entity. 1:38 I'm going to add that and we will do these imports. 1:40 Here we go and it is complaining about not having a constructor. 1:45 So let's initialize in the constructor. 1:48 And it automatically did, 1:51 just what we needed, made a protected base entity, set the ID to null. 1:53 Let's flip back to course and what we're gonna do is, 1:56 we're gonna say is extends BaseEntity. 2:04 Now unfortunately we still need to have a no argument constructor in our entity 2:09 classes. 2:14 That's just the J.P.A rules but what we can do is instead 2:15 of doing this again here we can just call super(). 2:20 This way in case we want something special to happen in 2:25 all of our entities we can just change the single constructor in base entity. 2:28 If we want to change something specifically, 2:33 we can add it after the call here to super, right? 2:35 We could come in here, and we can add whatever we want specifically for courses. 2:38 Now the terms super class and parent class are interchangeable, and 2:41 you'll hear both used. 2:45 In this case, our course entities super and or parent class is BaseEntity. 2:47 So it's kind of like Jor-El. 2:54 You know, a super parent. 2:55 Sorry that was a dumb Superman's dad joke. 2:57 Okay. 3:00 And now let's do the same thing for review. 3:01 We'll come in here and we will 3:05 come in here and we will get rid of the ID, and 3:12 we will make a review called super. 3:16 And make it extend BaseEntity. 3:23 Here we go. 3:29 All right so now that we've got that all out of the way, what do you 3:31 say we spin up our server and make sure our entities are working as we expect. 3:34 So let's go ahead and make a new class 3:37 in the root here right under com.teamTreeHouse. 3:41 Let's make a new class called Application. 3:45 The name doesn't matter. 3:47 So first things first, we will mark this as a SpringBootApplication. 3:51 And we were going to add public static void main and for 3:58 the body we're going to call SpringApplication.run and 4:03 pass in our class, and pass in whatever our args were passed to the main function. 4:08 This is SpringApplication. 4:15 Okay, so if we go ahead now and run, 4:18 we try to run this, we will see everything. 4:21 Wait a second, you say, we haven't made any controllers yet 4:28 that will respond to these actions. 4:32 Well get this, our repositories are exposed. 4:33 Look, check this out in the log here. 4:36 Scroll this up here. 4:39 Scroll over. 4:41 See this mapped slash repository ID? 4:42 It went ahead and built things when our app started up. 4:46 So simply by having repositories around, routes got created and it will respond. 4:50 Pretty cool right? 4:55 Now, if you've never used a restful AP before you might not know 4:56 how to go about discovering these routes. 5:00 One emerging standard is to have your API respond with HAL or 5:02 Hype media Application Language. 5:07 Well by default spring date arrest works with HAL formats. 5:09 So in order to discover our API all you do is hit the route and so 5:12 let's see the route is mapped here at 8080. 5:17 Okay, so, let's go ahead. 5:20 And we'll come over here and we'll go to localhost:8080 and forward slash. 5:22 So if you look here we have these links defined and its underscore links. 5:32 So that's that's a HAL format to make sure that you know that this is 5:38 coming from HAL. 5:41 And there's more on HAL in my teacher's notes, and 5:42 we'll explore it as we go through, but 5:43 they've given you links to the resources that are available. 5:45 Hey look, there's courses and 5:48 reviews, these are representing the repositories that we built. 5:50 By default, it's using a plural version of the entity 5:53 that's being represented by the repository. 5:56 That's pretty cool, right? 5:58 It automatically figured out that course was courses and reviews was reviews or 5:59 review was reviews. 6:03 Pretty cool, isn't it? 6:04 We'll go over and change this if it's wrong but 6:06 first let's navigate to the /courses. 6:09 Let's see what happens. 6:11 Courses is empty. 6:18 It's not very fulfilling without data, is it? 6:20 We should do something about that. 6:22 Nice job on getting the abstract entity all set up and 6:24 removing that code duplication. 6:26 It's pretty impressive that the API just worked, isn't it? 6:28 Now I know we need to get some data in there to make you even more impressed. 6:31 But one thing that you might have caught from those mappings that we saw in the log 6:35 at boot up is that we can actually send a POST request and 6:38 this will create a new course. 6:41 But before we dive into that work, 6:43 let me show you a pretty great way to prepopulate your app with data. 6:44