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A Generic Solution
4:05In this video we'll learn more about generics and implement our first type parameter!
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.
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