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In the last video,
we added oranges to our boxOfMilk, and

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didn't realize we had a problem
until we ran the app.

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Not good.

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Ideally, this error would be caught
much sooner by the compiler or

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even by IntelliJ.

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And it would show us
the error on lines 12 and 13.

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To make it show us the error,

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we need a way to designate each of our
boxes as only allowing a certain type.

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Which we can do with generics.

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Let's head over to Box.java and
use our first generic.

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At the top of the class,
directly after the word Box,

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let's type a less than sign, which
will automatically add its other half.

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And in the middle let's type the letter T.

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This is a generic.

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And between these two angle brackets,
we can declare type parameters.

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Let's see how it works by
going back to Main.java.

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Now, when we create our boxOfMilk,
instead of just using a box variable,

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we can go further and specify our type
parameter by using angle brackets.

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And we'll want to do the same
thing on the right side.

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Let's also update our boxOfOranges.

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So right after the word Box,

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we'll add the angle brackets and
say that this Box is a boxOfOranges.

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Also, if you've already declared your
type arguments on the left side,

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you're allowed to omit
them on the right side.

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Let's use Alt+Enter to do that.

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Okay, so we specified types for boxes,
but we're still not getting any errors.

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Let's head back to Box.java and
see what's going on.

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To recap,
this T variable is a type parameter.

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It's exactly like how
a function has parameters,

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just instead of passing in values,
we're passing in types.

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And also just like a function, we can
name our parameters whatever we want.

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So this T is just what's typically used.

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We could call our type parameter Tree and
that would be just fine.

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Also, just like a function
can have multiple parameters,

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we can have multiple type parameters.

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So if we wanted to have T, and X,
and Y, and Z, that's no problem.

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Okay, let's get back to just our
one type parameter, named T.

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The reason we didn't see an error
after updating our box types

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is because we're not doing
anything with those types.

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We've got our type parameter,
but we're not using it.

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So let's start using it.

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Since we want the contents of the box
to be of type T instead of type Object,

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let's replace all of the references
to Object with our type parameter.

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So we've got one, two, three, four.

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So T and T, and T,

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and T, perfect.

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Now back in Main.java, we see the errors.

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Awesome, getting notified immediately
that you've made a mistake

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is much better than getting
an error at a runtime.

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Let's go ahead and switch those back.

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And then let's use Alt+Enter to clean up
these last two lines that no longer need

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their casts.

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Thanks to generics,
we can not only make our code more safe,

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we can make it more readable.

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Being able to pass around type
parameters is a pretty powerful concept.

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But it's not limited to just classes.

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In the next video, we'll see how to
add type parameters to a method.