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Exceptions
8:16Let's use exceptions to help control the flow of the game. We'll attack the story: As a guesser, I should be stopped from making a guess that has already been made, so that I do not waste turns.
[MUSIC]
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Welcome back.
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We're getting ready to put
the final touches on our MVP and
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we've only got a few user stories left.
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So the next one up is, as a guess or
I should be stopped for
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making a guess that has already been made,
so that I do not waste turns.
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Okay, so
let's think this through real quick.
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So when we pass a guess into our
game object if you applyGuess,
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we currently just add it to hits or
misses.
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What we need to do before
we do any of that,
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is to add a check to see if
it's already been guessed.
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Which we can do by seeing
if it's in hits or misses.
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Now if it has been guessed,
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we need to let the caller on applyGuess
know that they did something wrong.
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We should do this at
the game object level,
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that way any other consumer of
our game logic will benefit.
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Then how should we let
them know do you think?
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Well it's exceptional behavior.
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Hey, why don't we throw an exception, and
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then any caller of that
method can handle that.
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Let's give it a try.
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All right, so let's move this prevent
multiple guesses into in progress, and
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let's head over to our game class.
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So let's take a look here in applyGuess.
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So this is probably where we
wanna run our check, right?
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So before anything happens, let's do it.
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We wanna see if what was passed in
letter here is either in hits or misses.
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So why don't we lean on indexOf.
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Now remember, negative one shows that
it's not found, so don't try to do this?
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Why don't you go try to do this,
see if misses has the letter, or
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hits has the letter.
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Go ahead, possibly.
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Okay, here's how I did it.
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So if misses.indexOf has the letter,
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and again that's not not found.
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If it's not not found, if it's found or
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hits.indexOf(letter) is not equal to 1.
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So again, that pipe means or, so
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if either one of those is true,
then we'll throw an exception.
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I'm gonna go ahead and close my if.
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So we'll throw this exception, right?
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The argument that they passed
in here was not allowed,
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this letter argument is not allowed.
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It's in the illegal argument, so
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that sounds like an illegal
argument exception, right?
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So again, that's throw and
we make a new one of those,
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IllegalArgumentException, and
it takes a message.
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So let's go ahead and
say letter has already been guessed.
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Awesome, so that should do it.
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So now what we need to do,
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is we need to see whoever calls
this method in any of our code.
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We wanna make sure that they're handling
this exception that can come out properly.
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So in workspaces, what I can do is I
come up here and I can say Edit, and
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I can choose Find in Files.
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Brings up a little search box up here,
and I'm gonna try to find applyGuess.
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And so what that'll do is it will
search through all of our files, and
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it found one here in Prompter
where we're using it.
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Which we know we just wrote this code.
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But I wanted to show that example
of how to use workspaces to find
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where these exceptions might be called.
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Okay, so the Prompter is currently
returning the result of game.applyGuess.
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So we don't want the exception
to throw out a here basically.
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Let's let's catch this.
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So first thing we wanna do is we
want to store that in a variable.
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So let's go ahead and
let's say boolean isHit equals that.
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And then we wanna make sure
that we still return it right,
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cuz the method still wants a boolean.
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So we'll say return isHit.
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Okay, so now we
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need to try to wrap this call to
applyGuess in a try catch block.
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So let's do that.
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So let's make a new try.
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Open that up.
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I'm gonna go ahead and close that.
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And then immediately we'll do a catch, and
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we're catching
the IllegalArgumentException.
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Again you can name this whatever.
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I like to name them, iae,
IllegalArgumentException, and
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we'll close the catch there.
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Okay, so let's go ahead, and
I wanna move this into the try.
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And in here,
what we'll do is we will just go ahead and
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output the message that we
have our hands already.
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And we're in the Prompter,
so we can prompt here.
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And say printIn, and we'll just say
that there was an error basically, and
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it'll say getMessage, right?
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That was that letter that could
not be found that we put in.
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So now we have a little bit of a problem,
and
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it might not be too obvious
just looking at this.
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So, remember how each one of these
curly braces opens up a new scope.
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Well, let's look at what we're
doing in this try scope here.
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We're declaring a variable
called isHit in this scope.
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But then we're attempting to
use it in an outer scope here.
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So see how this is an outside,
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it's in the method scope but
it's not in the try scope.
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The declaration is happening
inside the try scope.
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If you tried to compile this, you would
get an unknown symbol with an arrow
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pointing right here to this isHit
when you try to compile it,
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which is basically saying,
I have no idea what you're talking about.
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That's because it's declared here.
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So what we need to do,
is we need to declare the variable
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outside of the try block, so
isHit equals false.
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And then we can remove this
declaration and just set it.
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So there's no problem with the inner
scope accessing this outer scope
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here at the method scope it has access,
this is declaring the method scope and
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inner scope has access here but it's the
other way around that doesn't work, right?
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We can't define something in the inner and
then how the outer work.
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So now let's imagine what would happen.
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So if we passed in a t and then we passed
in a t again, so it's gonna come in here.
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It's gonna call the applyGuess, and
applyGuess is gonna throw an exception,
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right, cuz it's gonna find
that it's been found already.
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So it's in hits, so one of these is true.
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This one or this one is true,
and it's gonna come in here,
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it's gonna throw the new exception, and
then it's gonna go right into here.
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Now it's not gonna return and
set this to false.
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Even though it already is false.
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It's not gonna return here.
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It's gonna go pop right into here,
and then we're gonna come through.
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So let's give this a run.
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All right, so
let's figure out what we did.
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13 errors,
that can feel super overwhelming.
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Let's see if we can
figure out what happened.
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So see all of these errors is just because
of this one here, unclosed string literal.
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So if we go to game.java, line 15.
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I forgot a quote,
why didn't you say something?
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All right, let's try that again.
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You probably do, you're probably
sitting there yelling at the screen.
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Why did you put a quarter?
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Okay, so here we go.
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So let's guess t.
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Then I'm gonna guess t again.
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And bam, it says t has
already been guessed, did it?
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What happens if we guess a capital T?
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I'm glad we tested that.
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Let's go ahead.
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Let's flip it over to our board.
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It took it as a different guess,
didn't it?
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Okay, so this is done, right?
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Let's move this one over, and
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unfortunately we're gonna do it one step
forward, and we're gonna add another one.
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So it's your let's add here,
let's add a new error or
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a BUG as we call them in programming.
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Check the teacher's notes to find out
the strange reasoning behind that
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name for BUG.
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But I'm gonna say BUG Uppercase and
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lowercase values are treated differently.
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And Trello has a nice little feature
here when you come in to edit these.
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Let's go ahead and set the label,
let's make it red so it kinda sticks out,
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it's a little louder.
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So there we'll see that it's red.
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I'm totally glad we checked, that could
have been a big problem if people were
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guessing uppercase letters, we would have
been treating them completely separately.
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Let's fix that right
after this quick break.
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