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(upbeat music)

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In this video we're gonna look at

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extending the Vampyre class to create a super Vampyre.

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Now, it's going to be very similar

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to what we've already done,

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but there's a couple of other things

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we need to know how to do.

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So we're gonna create a vampyre king subclass

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of Vampyre.

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Now, our vampyre king is going to be incredibly powerful,

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and any points of damage inflicted will be

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divided by four.

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Vampyre king objects will also start off

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with 140 hit points and as you see,

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that's where we need to look a bit deeper

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into Kotlin inheritance.

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So we're gonna start by extending Vampyre

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to create a vampyre king class

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with those additional properties.

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First thing we need to do is start by making

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the Vampyre class open.

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So let's do that.

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So that it can now be inherited from.

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And now we need to create our new vampyre king class.

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I'm going to do that.

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Right click Java folder, New, Kotlin Class

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save a bit of typing by selecting Class

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and the name of your VampyreKing.

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Now we need to pass the name to the class

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and provide that to the Vampyre class.

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So, the end of the definition

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open parentheses, name colon String,

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then outside the right parentheses,

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colon Vampyre,

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then parentheses, name.

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That's straightforward, but where it gets tricky

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is setting the VampyreKing's hit points to 140.

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Now if we look at the Vampyre class,

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it passes the fixed value 20 to the enemy constructor.

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Now we need to change that to 140 somehow.

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Now, one way we could do that is to use a default value,

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and we can modify the Vampyre class

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to have a hit points parameter,

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with a default value of 20,

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that we can then pass to the enemy constructor.

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So let's try that first.

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So we'll add the parameter, hitPoints colon Int

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and set the default value of 20.

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Then we need to change

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the hardcoded value here to hitPoints.

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So now that we've done that,

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we can go back to the Vampyre constructor

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in the VampyreKing class,

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and we can change that to 140.

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So let's see if that works

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before we talk some more about it.

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So we go back to our main function,

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and let's add or create a VampyreKing instance.

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So val dracula is equal to

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VampyreKing parentheses Dracula.

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And let's print the value of that as well,

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so println(dracula).

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So let's run that and see if it works.

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Okay so that's good, we can see that it does work.

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Dracula, as you can see down at the bottom here,

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has got 140 hit points.

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So that's one way to do it,

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and it's a perfectly-acceptable solution,

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but you can probably tell that I'm itching to show you

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another way.

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So, what's wrong with what we've just done?

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Well, it's compromised our Vampyre class slightly,

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because we can now create Vampyre instances

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with less than 20 hit points.

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So if you go and modify the code on line eight,

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that creates Vlad,

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we can use that second argument now, and say,

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set the hit points to two.

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Now, before I run the programme again,

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notice in the output

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that Vlad started off with 20 hit points,

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and that dropped to 16 after we inflicted the damage.

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But if we run it again now,

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what's happened this time is

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because he only started out with two hit points,

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poor old Vlad's lost a life

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with the damage that's been inflicted on him.

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Now the change we made to the Vampyre class

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now allows vampyres to be created with a different number

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of hit points.

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And that may be fine, depending on your application.

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Sometimes, though, you'll want to provide a default

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but allow the users of your class to change it.

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Sometimes, though, it'll be very important

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that some properties can't be altered in this way.

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Now, as an example, consider an arc class,

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that draws an arc, through a specified number of degrees.

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Now, we may override this to create a circle class,

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which will have the number of degrees fixed at 360.

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Now we certainly don't want to draw circles

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that don't have 360 degrees, because of course,

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they wouldn't be circles.

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So, getting back to our code though,

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we can define our enemy objects to be whatever we want.

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There are inventions if you'd like,

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but when we're modelling real physical objects,

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we want our classes to really respect

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those physical constraints.

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And that's why sometimes this approach won't be appropriate.

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And another important reason is that

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we might be extending someone else's class,

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and if we don't have the source code available,

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then we can't make changes to it,

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and that would rule out this approach.

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So what I'm gonna do is undo these changes,

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and Command + Z is a very useful shortcut.

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That undoes the last change we made.

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On a PC that's Control+Z.

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So rather than editing my changes,

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I'm gonna use Control + Z to undo them.

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So, firstly in main, we're going to undo the changes to Vlad

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to get rid of the extra line there

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that was passing the number of hit points.

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Then back in the Vampyre class,

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let's undo those changes as well.

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We'll remove hitPoints, and the default value as well,

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and that will mean that when we go back

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to the VampyreKing class, of course we've now got an error,

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because we're using the 140.

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So I'm going to undo that as well,

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so that we have it back to how we had it before.

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So how can we now set the hit points

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for our VampyreKing to 140?

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Well, what we can do instead, is to use it in an init block

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in VampyreKing to change the hit points value.

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So we can type in init, then left and right curly braces,

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and within the code block,

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we can assign hitPoints equal to 140.

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Now what does does is changes the value of hit points

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when an object gets created.

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Now init code's executed at the time

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that we create the instance.

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And it's done automatically,

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so we don't have to remember to do anything special.

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So if we run this programme,

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given that we've deleted those changes,

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we can see that Dracula is now still set at

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getting his hit points set to 140,

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but we haven't compromised the Vampyre class in any way.

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Basically, we've extended the Vampyre class

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and changed the hit points for our VampyreKing,

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without having to modify the VampyreKing's superclass.

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Alright, so all that remains now

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is to override the takeDamage function

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to divide the damage by four.

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Now, if you recall, I've already done something similar,

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with the Vampyre class, I used the takeDamage,

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so let's do something similar here.

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Here, VampyreKing class, and then override takeDamage,

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and what I'm going to do is set that damage

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into divided by two there.

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Remember that we're trying to overall

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change the damage by four,

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divide the total damage taken by four.

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And in fact, that's exactly the same code now

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that we've got into the Vampyre class.

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That's the, if you recall, the Vampyre class,

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that's already setting the damage,

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dividing that damage by two.

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We're doing the same thing here in takeDamage

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for the VampyreKing class,

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so we're only dividing it again by two here and not by four.

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So it is important to understand exactly which class

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is the superclass here.

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Now, VampyreKing extends Vampyre,

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so the super method that we're calling

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is the Vampyre method, and not the enemy method.

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So what happens is that the VampyreKing takeDamage method

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calls the method in its superclass, which is Vampyre.

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Now the Vampyre class takeDamage method,

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it calls its superclass method, or function,

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so that it calls the enemy takeDamage function.

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Now Android Studio has a really useful feature

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to help understand all this.

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So back in the VampyreKing class, which I've got open,

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I can hold down the Command key, 'cause I'm on a Mac,

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but it's Control on a PC,

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and click on the takeDamage,

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in its call to super.takeDamage,

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and I click on that, and you can see that

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it's automatically jumped to the Vampyre class,

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or rather the takeDamage function in the Vampyre superclass.

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Now if I do that again, in this class,

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you can see it's now jumped to the takeDamage function

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in the enemy class.

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So basically that's the superclass of Vampyre.

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So you can follow the trail back,

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and that's great for checking

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exactly what these overridden functions are doing.

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Okay, so we only have to divide the damage by two

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in the VampyreKing class again,

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and that's because it'll be divided again

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when the Vampyre takeDamage method is called,

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in other words, when this code is executed,

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which is also going to be divided by two.

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So, let's see if it works,

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by inflicting some damage on Dracula in the main function.

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So we'll do dracula.takeDamage(12).

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And we'll run that.

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So when we run the programme, we can see that Dracula

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starts off with 140 hit points,

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but then inflicted 12 points of damage,

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but that gets divided by two

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in the VampyreKing's takeDamage method,

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then divided by two again in the Vampyre class.

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So we only inflict a total of three hit points

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of damage on Dracula.

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So Dracula ends up with 137 hit points left,

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three points of damage, which is correct.

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Alright, so I'm gonna stop the video here,

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and in the next one, we're gonna look at using loops

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to repeat a section of code.

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We're actually gonna use a loop

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to repeatedly inflict damage on Dracula,

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so that hopefully, we can slay that brute.

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See you in the next video.