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(upbeat music)

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A few videos back I mentioned that the way

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we were adding loot to the inventory wasn't a good idea.

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And in this video we're going to look at why

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that isn't a good idea and what we can do about it.

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We're going to start though by deleting

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the code we used for for loops

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and un-commenting our player code.

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So I'm just going to delete the for loop code

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and I'm going to select or delete the rest of it first.

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And select the rest of the code that was commented out.

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You can come up to the code menu

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and I can click on comment with live comment

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which un-does the changes if it was already commented at.

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And obviously these other things that were commented at,

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were things that were commented at previously,

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in case you're wondering why that's still

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sitting there and commented at.

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Okay, let's just run the programme

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and make sure that it works.

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Okay, we can see that we've got our code working again,

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everything's working nicely.

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So getting back to the problem at hand,

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why isn't it a good idea for our main function

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to add loot to Tim's inventory

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in this fashion that it's doing here.

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Well there's a couple reasons why it's not a good idea.

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One reason is that main shouldn't have to know

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that the players are using an array list to store the loot.

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So looking at our player class

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we've declared inventory to be an array list.

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Now we may decide to change that in the future

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to use some other kind of structure.

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Now if we do that, we might break the code in main.

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Our main function's expecting an array list

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with an add method that it can use.

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Now if we use something else, maybe a structure

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that has a pin function instead of add

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then the code in main would stop working.

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Now one of the principles

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of object oriented programming is encapsulation.

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So encapsulation here, basically means

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not exposing the inner workings of a class

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to code outside of that class.

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Now Kotlin takes a more relaxed attitude or approach

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to encapsulating class properties.

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Everything's public by default.

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Now that's why the main function

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can directly access inventory.

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Now if we want to encapsulate our inventory,

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we can mark it as private.

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So come up here to the definition on line seven

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and type private ahead of the val inventory

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and now nothing outside of the player class

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can access the inventory.

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Now by default Kotlin makes properties and functions public,

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which means that they're available to external code.

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Anything that creates an instance of our player class,

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for example, could access all it's properties and functions.

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Now you can find the documentation and visibility

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modifiers in Kotlin and I'm just gonna put the link,

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bring the link up into a browser.

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Have a quick look at that.

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Now I'm not actually going to go through this document.

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It can be quite confusing at the moment,

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but it's worth bookmarking this page for reference later.

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When you find some code that uses

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the internal modifier, for example, you can

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check back here to find out what that means.

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So scrolling down quickly, we'll get

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to the classes and interfaces section.

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You see that it's a description

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on the private modifier, up here.

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Means visible inside the class only,

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including all its members.

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So by marking the inventory properly as private,

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it's now no longer available to any code

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outside of the player class.

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Now of course that should mean

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that we've now broken main.

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So if I switch back to that now,

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you can see we've got some errors showing.

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So every time main tries to access a player's inventory

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Android Studio is showing an error.

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Now that's not good, but it's okay to get errors

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while you're still developing your classes.

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That's normal as you try things out and modify your design.

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What's not good though, is modifying a class

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once it's been used and breaking all the code that uses it.

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So that's one of the things we're trying to avoid here.

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Now if we don't want main or any other client code

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of our player class to access the inventory directly,

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then we need to provide a method for clients

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to call when they need to add things to the inventory.

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So I'm going to call that method or function

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get loot and that reflects what's happening

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when loot's added to a player's inventory.

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So let's go ahead and do that.

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Go back to the player class,

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we're going to add that below, the toString function.

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And again what we're doing is, we're creating

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a function for clients to call when they need

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to add things to the inventory.

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We'll start with fun, space, getloot

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(item

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:loot

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After {, it's going to be inventory.add item.

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So now that we've done that, we can go back to our main

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and we can change main to use the getloot function

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instead of accessing the inventory directly.

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So to do that we type, we select,

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we type tim.getloot.

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Those two changes there and then for the second one

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we do something similar.

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Getloot

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Getloot

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The right number of parentheses

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there to keep everything consistent and working okay.

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We've now changed it so that the main function

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is no longer accessing inventory directly.

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Rather it's doing it via the getloot function.

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Let's just run the programme

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to make sure everything works as it did before.

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And you can see we still got the same results when we run it

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but now though main doesn't know

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how the player class is storing the loot.

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So it can call the getloot function,

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passing on a piece of loot and leave details

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of how to store it to the player class.

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So our inventory is now encapsulated and isn't

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available to any code outside of the player class.

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Now another reason for encapsulating data like this,

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is let's say we want to store the inventory

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to disc whenever it changes.

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That way we when I come to play the game again,

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my inventory can be loaded back up,

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and I'll still have all my loot.

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Now before we encapsulated the inventory,

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we'll have to add code to store it

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each time we added new loot.

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Now that's something we could forget to do,

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which wouldn't make our players very happy.

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So we could introduce a bug into our code quite easily.

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But now though, we just need to add the code

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to save the inventory in the getloot function.

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In other words, to go back now to the player class and here,

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code to save the inventory

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goes here.

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I'm not going to add the code to store the inventory

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but you can see how we only have to put

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this code in a single place now.

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Whenever a new piece of loot gets added,

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the inventory could be saved to disc,

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and it will always be up to date,

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and there's no chance of us forgetting to save it in main.

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So that's another reason why

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our previous approach wasn't a good idea.

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Okay so if we're going to prevent external code

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from accessing the inventory, we should

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also provide a way to remove items from our loot.

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So let's go ahead and write another function here.

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We're going to call this one droploot.

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It's slightly more complex than getloot,

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because we need to make sure that we've got the item,

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before trying to drop it.

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So let's start with the definition,

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fun droploot

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(item: Loot

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: Boolean.

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Then we have that code block.

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Then we're going to do return if

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inventory dot

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contains

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item.

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Close our parentheses and add another code block.

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Inventory dot

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remove

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item.

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True.

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The next line, else,

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false.

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So I've used if as an expression there,

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which we covered earlier in this tutorial section.

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Now if the item of loot exists in the inventory,

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we remove it and return true.

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Now if it didn't exist it will return false.

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So that's letting the calling code know

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that if the item was successfully removed or not.

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So let's see how that works in main,

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we'll go back to main now.

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And right down to the end here

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after the last showInventory,

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let's try and drop the red potion.

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So we're going to put if

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(tim

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.droploot

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that's going to be red potion,

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Code block.

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Tim

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.showInventory.

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So let's run the programme and we should find

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that the red potion is removed from the inventory.

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So you can see we've got our first printout

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of Tim's inventory, the red potion was there.

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Then after the call to the droploot,

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the second time, the second block of code here,

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the second block of output,

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the red potion is no longer there.

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Now we should also test what happens

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if an item can't be removed.

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So I'm going to create a new piece of loot

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and try to remove it from Tim's inventory.

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Let's go ahead and do that.

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And let's also change this to put an else in here.

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Else, code block.

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And it'll be

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println,

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you don't have a

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$, that's a right curving braces

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redpotion.name

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So we're printing something out that's not there,

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but let's go ahead and create this other item as I mentioned

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val blue potion

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is equal to loot

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("blue potion"

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loot type.potion

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and let's we'll set the value six

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Then we'll try doing a drop,

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even though we haven't gotten it in the inventory.

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So if tim.droploot

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bluepotion

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tim.showInventory

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Else

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Code block again

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That's going to be println

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You don't have

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Pound sign

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That's right curving braces, bluepotion.name.

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Alright so let's try running that.

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What I should probably do to avoid some confusion

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is just delete that last line there.

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So let's just try running it again.

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And you can see this time we got the message,

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you don't have blue potion.

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So that's working well.

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Our players can now pick up loot and drop it again.

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But what about something

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like the Invisibility Potion though?

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You can see that the way we've defined it,

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we haven't actually got a reference to it in main.

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So we can't call the droploot function to drop it.

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So that's probably something we probably shouldn't

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have called getloot the way we did

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for the Ring of Protection and Invisibility Potion

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on lines 41 and 42 here.

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So this is what I meant earlier,

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when I said we need to consider who should own the objects.

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Now both those objects really belong to the game.

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We should really have created variables to hold

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a reference to them in main, so that's how

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we've got a way to drop them.

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The way we've got our code currently,

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there's no way to reference,

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there's no reference to either of those bits of loot,

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so we've got no way to get Tim to drop them.

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Now that's the sort of thing you need

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to be aware of when designing your programmes.

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After coding for a while, you'll make

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the correct decision the first time, most of the time.

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Now you still make mistakes occasionally, of course.

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Just like I did there, with those two items.

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That's fine, that's all part of the learning process.

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But of course, in a real game, we'd probably

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give the players a way to go through their inventory

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and choose items to drop, so in that case

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we wouldn't have this problem.

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So in our code main is in charge of controlling everything.

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The way to fix this then is to change main,

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so that we've got references to the Ring and the Potion.

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But I'm not going to do that

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because this gives us a good excuse

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to look overloading a function

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and we'll do that in the next video.