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Final
6:07You can protect your variables from re-assignment using the final keyword
Okay, so over the past few lessons,
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we have successfully picked up
some language constructs to
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explain that a character name is
required to create a new Pez dispenser.
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And after it's been created,
you currently can't change that character,
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which was exactly what we
were intending to convey.
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Because we're modeling
what happens in real life.
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Getting so
close like we just talked about though.
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What if someone on our team doesn't
know that these heads can't be changed.
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And they come in and write a new method
that does just that, it changes the head.
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No, we'd hopefully,
catch that in a review of their code, but
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we can do better than that.
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Let's simulate the scenario and
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I'll introduce a concept that
will help solve the problem.
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So let's go ahead and
assume that a developer on our team,
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let's call him Chris.
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He grabbed our Pez dispenser class and
decided to make a new method
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that would change the character head and
return the previous one.
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So let's walk through that
method creation really quick.
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Let's see, obviously,
Chris made that public, right?
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He wanted somebody to use
it because it's useful.
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And like we said, it's gonna return the
original character as the return value.
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So that's a string, and
then we'll name it.
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Let's call it swapHead cuz
it's gonna swap the heads up.
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And it's callable so,
we're gonna open parenthesis.
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And it will declare a parameter of type
string that represents a character name,
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right?
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So we're gonna ask that somebody
passes in the new character name.
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And then we're gonna open up the method
body, and I'm gonna close it right away.
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Uh-oh, we've got that naming
collision problem again, right?
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So look, here's a character name.
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And we wanna swap out the character name,
so
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we know how to deal with that though,
right?
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So first, what we wanna do is get
a hold of the original character name,
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so that we can store it in a variable.
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So let's be clear about it.
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Let's say, string originalCharacterName.
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And we want to make sure that
we keep this character name.
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So we're gonna say, this.charactername,
not running into that naming collision.
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We're not gonna fall for it, right?
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Cuz this is in scope.
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And now, what we'll do is we'll swap out
this characterName with the characterName.
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So basically, all we're doing
is just keeping a safe copy
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of what was originally in there because
that's what we wanna return, right?
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So we're gonna return the original,
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[LAUGH] I keep on typing
the originalCharacterName.
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So now, if we walked over to this method,
but we didn't want Krista, right?
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We can show you how it's used.
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So we're gonna say,
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before = dispenser.swapHead.
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And we're gonna put in bring back
the Darth Lord again, Darth Vader.
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And we're gonna print
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out ("It was %s but
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Chris switched it to %s.
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And then we'll do a new one.
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And so we want to first,
let's first %s as before, right?
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So before,
it was what came out of the dispenser.
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And then next, we're gonna say, dispenser.
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And we're gonna call our own
method getCharacterName, cool.
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And we end that line.
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All right, so I'm gonna save it.
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And then I am going to say,
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clear && javac Example.java
&& java Example.
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What do you think, is it gonna work?
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I think so, I think you did it.
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Yep, so there it is.
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It was yield up at Chris
switched to Darth Vader.
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Chris did exactly what we
didn't want him to do.
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He changed the head.
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Come on, Chris.
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But we can't really buy him, can we?
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We didn't make it very clear.
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We could have at least left a comment so
he knew.
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Then actually, there's something
way better than a comment.
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Something that would stop Chris and
his tracks, even better.
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It wouldn't even let him compile and
that's the final keyword.
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So when you want a variable to
be assigned once and only once,
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you market with the final key word, final.
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So now, if we go and we try to run this
code, we'll see that Chris got blocked.
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You cannot assign a value to
the final variable characterName, and
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that is happening from inside line 14,
right?
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So the PezDispenser line 14 if
we come in here to line 14.
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Sorry, in the PezDispenser
class [INAUDIBLE] line 14,
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that's in the swapHead, okay?
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Cuz we assigned it once here.
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And he's trying to swap it here,
it's telling that.
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In a more advanced editor which
we get to hear in a few courses.
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This would have warned him very loudly
before he even tried to compile.
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So again, final here, says that this
variable characterName is declared,
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it can only be initialized once,
it doesn't allow for any more assignments.
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Now, you can use the final keyword for
more than just field.
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You can use methods in even classes.
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Check the teacher's notes for more.
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I'm gonna go ahead and say, sorry Chris,
you can't do this, you're out of here.
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Let's give her this.
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And I'm gonna get rid of this line over
here, because now, we have protected it.
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And that code makes no sense, awesome.
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This video is the final one in this stage.
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And I know, I know,
you're probably like, finally!
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You're doing great.
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And don't worry, I've been
intentionally taking things slow and
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making sure that you can read the intent
of Java code when you come across it.
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Also, now, you can write it.
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Now, that we got the basics in place.
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We've got fields,
access modifiers, methods, and
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constructors, let's
pick up the pace a bit.
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I think it's finally time for
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us to start working through the core
features of this Pez dispenser.
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I know our little dispenser object
isn't very impressive at the moment.
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Let's do something to change that.
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Let's wrap up this stage so that we can
get some Pez in and out of this thing.
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But first, let's do a quick practice
review of what we just learned.
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