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(playful ambient music)

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Okay, so moving on with the Kotlin tutorial,

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I'm gonna go into a bit more detail now

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with classes and look at them more in depth.

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Now, an important to thing understand,

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and it took me a while to get the hang of this

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when I first started object-oriented programming,

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is that a class is like a cookie cutter, or a template,

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and from that you can create an unlimited number

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of player objects or player instances.

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Now at the end of the last video,

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we created four instances from our player class.

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So what I'm gonna do is delete the

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commented out code firstly, these four printlns,

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because we don't need those anymore.

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And then just run the programme again

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even though it's still showing on the screen,

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just to remind ourselves what it was doing.

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And you can see it's outputting the four

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four player instances there and their values.

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So we're getting four totally separate objects displayed,

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but they were all created from the same template class,

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our player class, in this case.

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Now they all have the same properties,

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they have a name, and lives, level,

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and score properties, but each instance or object

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has different values for those properties.

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Now they also all have a show method,

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or a show function, they mean the same thing.

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Kotlin tends to use the term function, by the way,

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but the object-oriented term is method.

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So you hear me use both words to refer

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to a function as part of a class.

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Now if you look at the show method, in the player class,

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it just prints out the values of the properties

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as you can see from lines 7 through 14.

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Now each instance of the player class has

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different values for the properties,

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which is why calling tim.show on line three

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of our main method, here,

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prints different values to the louise.show on line six.

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Okay, so let's extend our player class now

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a little bit further.

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So we look at our player class again,

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so we've got name, lives, level, and score.

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And we've used a string and three int types.

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But you're not restricted to using

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just those types of variables,

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we could use an object as a type and a class.

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In fact, in Kotlin, string and int are classes,

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so we're already using objects.

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Now, I'm gonna make more sense if I create an example,

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so I'm gonna create this new class called Weapon

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to show what I mean there.

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So as before, when creating a new class,

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you click on, or right click on the Java folder,

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or Java directory, New, and select Kotlin File/Class.

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And I'll call this one Weapon.

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And I'm gonna leave this set to File this time

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just to show you what you'd have to do

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if you didn't select the class.

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And you can see that it's just created a comment,

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but it hasn't actually created a class declaration,

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so I can actually just type that in manually.

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So, class, Weapon...

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In this case we're gonna give it some properties

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in the primary constructor.

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Remember that we define them in parentheses

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immediately after the class name,

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so we put parentheses there.

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So the properties that we want for weapon,

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we're gonna start with, val name, colon, String,

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comma, then var damageInflicted, colon, Int,

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closing parentheses and I'm gonna open the code block,

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or create a block of code

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using the left and right curly braces.

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So a weapon's gonna have a name, like axe or sword,

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and we'll store that in the string field,

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then we have an int to show how much damage

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this particular weapon does.

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Now I haven't used any default values there,

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so we'll have to specify both values

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when we create our weapon objects.

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Not that I've made damage inflicted a Var property.

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Now it probably wouldn't change

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in a normal course of a game like this,

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but I want to use it to demonstrate something else soon.

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So imagine that there's some magic spell in the game

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that can make weapons more deadly or something.

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Okay, so moving on, and we've now got

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a very simple class that can be used

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to represent a weapon in a game.

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So let's use this class in our player class,

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we'll let the player have weapons.

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So we need a property in the player class

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to store the weapon in, and it's usual

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to declare properties before methods,

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so I'm gonna go back to the player class,

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we're gonna add this at the start of the class.

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So, just below the class definition on line five,

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I'm gonna come over here, tab over,

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and type var, weapon,

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colon, then Weapon, with a capital W,

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and I'm gonna press enter.

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Now you see that sort of thing done a lot

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in Kotlin programming.

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The class name normally gives a good indication

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of what it represents so Weapon, with a capital W,

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tells us that the class represents a weapon.

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So that's the type of the property,

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and we call it weapon with a lower case w.

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Now we could've called it something else,

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such as player's weapon or something,

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but weapon's a good name for the object.

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Because variables in Kotlin are case-sensitive,

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Weapon with a capital W

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and weapon with a lower case w

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are referring to different things.

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So here, Weapon, with a capital W,

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is the name of the class,

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and weapon, with a lower case w,

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is an object or instance of that type,

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or it will be once we create the weapon.

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So you'll see that sort of thing done a lot

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when we use Android classes.

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And you'll see things like, I'm just gonna type

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some code in temporarily, val recyclerViewAdapter,

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colon, RecyclerViewAdapter.

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That's an example of code

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that you'll actually see in Android.

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It's very common to use a class name for the variable name,

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but with a lower case initial letter.

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All right, so I'm gonna delete that line.

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Now let's give the player a default weapon,

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when the player objects or instances are created.

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Now, at the moment we've got an error

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in the declaration of our weapon,

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and that's because Kotlin wants everything

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to be given a value when it's declared.

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Now, you'll see ways to avoid doing that,

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and of deferring the initial value until later,

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but for now we're gonna give everything a value.

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So when our players first start playing,

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they won't have found any weapons,

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so we're just gonna give them a fist to start with.

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So we've typed var weapon, colon, Weapon

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then we're gonna put after that an assignment,

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so, equals, Weapon with a capital W,

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parentheses, double quotes, fist,

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comma, one, and ending parentheses.

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So now, each time we create a new player,

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the code will automatically create a new weapon object,

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which in this case will be a fist.

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Now, they're gonna be finding more

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weapons as they play the game,

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so that's why I've made weapon a var there.

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Now we want to assign different weapons to it ultimately.

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Hopefully, you're starting to see when to use val

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and when to use var.

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Now don't worry if it's still unclear,

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just remember you can always change a declaration

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if you decide that you should've used var,

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for example, instead of val.

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So let's actually check if this all works

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by printing out the name of Ace's weapon

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and the damage it can do in our main function.

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So we're gonna come over here to Main,

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and we're going to, after this

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one2watch.show, we're going to put println,

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parentheses, one2watch.weapon.name,

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then on the next line, println,

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one2watch.weapon.damageInflicted.

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First run the programme,

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and we can see down at the bottom here,

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we've got fist and 1 printed.

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So notice how we're still using dot notation

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to access the properties of the two classes.

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So one2watch.weapon gives us a reference

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to Ace's weapon object and we can access

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the properties of the weapon by using more dots.

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So, .name gives the name of the weapon

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and .damageInflicted gives the amount

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of damage that weapon does.

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Now the name here is a string,

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and I mentioned that string is also a class.

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So we can expect the string class to

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have some functions of it's own,

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and it turns out that it does.

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So I'm not gonna list them all here,

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but a use for one converts the string to uppercase.

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So we can come over here to weapon.name,

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and at the end of that we can add a .toUppercase.

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Select that and press enter.

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Now we're using one2watch.weapon to access the weapon,

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and using .name to get the weapon's name,

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and finally, calling the names toUpperCase function

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to convert the name string to capitals.

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So you can chain these property

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and method accesses as far as you wanna go.

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So let's actually just run that to

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confirm that it actually works.

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Let's see, we've got fist now showing in upper case.

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And this chaining is something that you'll

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see a lot of in Android programming.

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In fact, in Kotlin programming in general, as well.

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Many of the built in Java classes have been

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designed to be used in this way.

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Now, I'm not gonna go into any more

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complexity at this stage, but suffice to say,

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when we ran the programme as you saw on the screen,

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we've got fist now printing out in capitals

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So before I finish, we're gonna have a look

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at a couple of ways that we can give our players

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some more impressive weapons.

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Now, the first way is to create a new weapon object,

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then assign it to the player's weapon property.

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Let's actually have a go at doing that.

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So we're gonna come down here.

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After the one2watch.weapon.damageInflicted line,

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I'm gonna type val, axe

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equals, Weapon in capital W, parentheses,

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and axe in double quotes, comma, and then 12.

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Then we can type gr8.weapon, equals axe,

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then we can type println,

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gr8.weapon.name,

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fix that typo, and on the next line println,

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then axe.name.

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So we've created a new instance of weapon,

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and we assign it to the variable axe.

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Then we assign axe to gr8's weapon property.

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So gr8 now has an axe as her weapon,

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which we can see when we print it out,

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so let's just run that to make sure that it works.

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And you can see axe is showing there.

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Now axe also exists within our main function,

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and so we're printing out the value

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of the name property from axe on line 18, as well.

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So we've got two references to the same object,

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one reference is by using the axe variable

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and the other's by using the gr8.weapon.

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So it's important here to understand that they

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both refer to exactly the same object, the axe.

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Now, if that seems complicated,

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keep in mind that we do the same thing in normal speech.

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So someone might refer to my son's wife's father-in-law,

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or they may refer to me, Tim Buchalka,

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so it's the same person, unless my son has

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another wife that we're not aware of.

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Now, we can check that they're the same object

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by casting that spell I mentioned earlier on the axe.

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And we'll increase the damage inflicted by the axe to 24.

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So let's actually add the code,

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so we'll type axe.damageInflicted, equals 24,

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println, axe.damageInflicted.

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Let's run that.

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So no surprises there, we get 24 printed.

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But before I run the programme again

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after making this next change,

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have a think about what should be printed.

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So I'm gonna type in the code first,

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so println, gr8.weapon.damageInflicted.

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So have a think about what you think

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that should print out, the code on line 22.

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All right, so let's run that.

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And you can see it also shows the value of 24.

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So remember that these are two different ways

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to refer to the object, the same axe in effect.

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So when we change the damage inflicted by axe,

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on line 20 to 24, gr8's weapon now does 24 points of damage.

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So if you did work out that the programme

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should print 24 again, well done.

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And if you're not sure why, then it's worth

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watching these last couple of videos again.

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All right, so I'm gonna finish with the other way

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of assigning a weapon to one of the players.

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We'll give me a sword.

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So we're gonna come down here,

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we're gonna type tim.weapon

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is equal to Weapon, parentheses,

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double quotes, Sword, 10,

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and you're gonna println, tim.weapon.name.

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So this time we're creating a new weapon,

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and assigning it directly to my weapon property.

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Now we can see refer to it as tim.weapon,

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but we don't have a direct reference to it

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in our main function.

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So to run the programme,

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we can see it correctly prints out sword.

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So which way should you use?

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Well, the answer is that it depends.

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We're gonna be seeing one reason for using

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the first method in a later video.

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The important thing to consider is

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where the weapon, in this case, belongs.

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You obviously won't just be dealing

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with weapon objects in your code,

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but the principle's the same.

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In other words, does the axe belong to the player gr8,

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or is it part of the game.

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Another way to look at that is to ask,

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can the axe exist in our programme without the player, gr8?

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So I think there's like the names Tim, Louise,

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one2watch, and so on, don't have any meaning

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00:13:42,970 --> 00:13:45,560
without the player that they're attached to.

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The axe and the sword, on the other hand,

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are part of the overall game.

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00:13:49,244 --> 00:13:51,800
I could be forced to drop my sword at some some point,

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in which case another player might pick it up.

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So, if I drop the sword,

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let's say I pick up a spear instead.

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So, adding that to the code, tim.weapon,

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is equal to Weapon, parentheses,

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spear, comma 14, and println again, tim.weapon.name.

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00:14:14,223 --> 00:14:15,806
So if you run this,

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00:14:18,647 --> 00:14:19,800
so I've now got a spear,

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and there's no trace of the sword anywhere.

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00:14:22,060 --> 00:14:24,170
So the reference to tim.weapon

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no longer refers to the sword,

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00:14:26,190 --> 00:14:28,660
and we can't assign that to another player.

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Now, don't worry too much about this.

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00:14:30,280 --> 00:14:32,830
It's one of the decisions we have to make

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when creating our code.

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At this stage I just want you to be aware

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of the difference and that you can have more

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00:14:38,120 --> 00:14:40,260
than one reference to the same object.

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00:14:40,260 --> 00:14:42,400
We'll come back to this in a later video

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when we provide a way for our players

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00:14:43,970 --> 00:14:45,890
to drop things that they've picked up.

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So I'm gonna stop the video here,

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and in the next one, we're gonna have a challenge

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to let you practise some of these techniques.

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So see you in the next video.