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[MUSIC]

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Hi, I'm Ben, and in this course,
we're going to learn about generics.

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But before we get to any formal
definitions, I think it'll help if

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we start by looking at an example of
the problem generics try to solve.

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Let's start by creating a new project.

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And let's click Next.

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Select the template.

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Hit Next again.

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And let's name our project Generics.

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And we can leave the package
as com.teamtreehouse.

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And I'll full-screen my app
to make it easier to see.

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Now that we've got an empty project,

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we've got a few things to copy
in from the teacher's notes.

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First, let's create a new class,
named Milk.

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So right-click on the package,
new class, and I'll call it Milk.

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And then let's paste in our Milk class
over the existing class Perfect.

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Then let's do the same thing with
the Oranges and Box classes.

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So we'll right click,
New > Java Class, Oranges, and

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then we'll paste over it with
what's in the teacher's notes.

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And then we'll do New > Java Class
one more time for the Box class.

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And we'll paste that in.

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Looking back at the Milk and
Oranges classes,

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notice that Milk has
a function called drink.

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And Oranges has a function called juggle.

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And they each print something
out to the console.

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Also, to get classes side-by-side like
this, you just right-click on the tab, and

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choose Split Vertically.

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All right,
that's enough about Milk and Oranges.

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Let's get on to the Box class.

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As you might have guessed,
this class aims to represent a box.

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Though before we dig into the
implementation details, let's use Cmd or

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Ctrl+Shift+- to start
with just the basics.

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At the top, we have the contents of
the box, represented as an object.

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Then we have an add method to
add an object to the box, and

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a remove method to take an object out
of the box, which returns an object.

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Diving into the add method,
we start by making sure the box is empty.

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And if it is,
we update the contents variable.

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Otherwise, we print out, The box is full.

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Moving on to the remove function, again,
we start by checking if the box is empty.

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And if it is, we tell the user that
the box is empty and return null.

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If the box isn't empty,
then we pull out the contents of the box

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into a new variable,
update the contents to be null, and

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then return that new variable,
which is holding the contents.

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So far, so good.

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Let's head over to Main.java and
try it out.

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Let's delete the comment, and

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start by creating a new Milk object and
a new Oranges object.

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So Milk milk = new Milk,

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and Oranges oranges = new Oranges.

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Then let's leave a space and create
a new Box variable to hold our milk.

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So Box, and let's call it boxOfMilk.

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And let's set it equal to a new Box.

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And let's create another Box for
our oranges.

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Box boxOfOranges = new Box.

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Awesome, now that we've got our boxes,

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let's use the add
function to fill them up.

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Let's add the milk to the boxOfMilk.

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So boxOfMilk.add, and
we'll pass in the milk.

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And add the oranges to the boxOfOranges.

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So boxOfOranges.add, and
pass in the oranges.

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With our boxes filled, the only thing
left to do is take our objects back

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out of the boxes and
call the respective functions.

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Drink for the milk, and
juggle for the oranges.

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Let's add another space, and
start with retrieving the milk.

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Let's type boxOfMilk.remove to
get our milk back as an object.

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Remember, the Box class stores
its contents as an object.

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So if we want this to be a milk
object instead of just an object,

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we'll need to add a cast.

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Let's add the cast at the beginning
by putting it in parentheses.

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Then we'll need to add parentheses
around the whole thing,

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Before finishing up by calling
the drink method, so .drink.

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Then let's do the same
thing with the oranges,

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except this time we'll
call the juggle method.

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Let's add two left parentheses,
and then Oranges for the cast.

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Move over one parenthesis and
type boxOfOranges.remove.

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And finally, add the call to juggle.

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To finish up, let's run the code and
make sure everything works.

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Perfect, but while this code does work,
and there's not necessarily anything wrong

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with it, there are a couple situations
that can get us into trouble.

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For example,
what happens if we switch the boxes?

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Let's put the oranges in the boxOfMilk,
And

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the milk in the boxOfOranges.

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Looks good so far, now let's run the code.

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And of course, we get an error.

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Turns out we can't cast an Oranges
object into a Milk object.

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Wouldn't it be great if it would have
just stopped us from putting the oranges

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into the boxOfMilk or
the milk into the boxOfOranges?

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In the next video, we'll see how
to do that by using generics.

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But first, let's go over here and

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click on Indent with 4 spaces
to get rid of that warning.