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Using the Debugger
10:47Learn how to explore and step through your code using the powerful debugger.
We've talked about bugs or
errors in our code in the past, but
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we've never really talked about
where that term came from.
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It's a bit of lore these days,
but back in 1946,
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Grace Hopper was having some problems with
a program she was running on the Mark Two.
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After trying to figure out
the flaw in her code for a while,
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she found a moth trapped in the relay.
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Once the bug was removed,
the program worked correctly.
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[SOUND] The program was properly debugged.
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In most of today's popular
programming languages,
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there are tools that are provided
to assist the the removal of bugs.
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They are called debuggers,
and they give you a better
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overall understanding of how
the code is actually working.
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Let's go learn some bug
extermination tricks.
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Okay, so the first tool I'd like to
use to introduce you to your debugger
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is something called breakpoints.
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Now it's possible to tell the code
to pause at a certain line, and
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you can do this with what is
known as a code breakpoint.
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Adding breakpoints is simple,
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you just click the side of the editor
over here where you want it to stop.
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This area is called the gutter, so
I was thinking we could inside this
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KaraokeMachine run method, we'll stop
on this line where it prompts for
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the action, before we prompt for
the action.
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So I just clicked over
here in this gutter, and
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I clicked and it added a little stop
sign here which is a break point.
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And so under the run menu there
is an option to debug your
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current configuration.
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So I'm going to do that, I'm going
to choose debug karaoke and that's
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gonna pop up the debugger, and notice
how the line is now highlighted in blue.
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That's the way of letting us know that
we are paused here, it's suspended.
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So, you'll see there's a debugger
tab down here on the bottom and
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over here under frames, what it's doing
is it's showing us how we got there.
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So this is the stack of code.
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You've probably seen these
in the stack traces, right?
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So, right now we're in run in karaoke
machine, but we got there from here,
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from main.
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So if I click onto main, you'll see
that it's flipped to the proper file and
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it's showing me highlighted there in blue,
of how we got in there.
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So it's kinda a good way of oh,
how did we get here?
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And kinda trace your steps back up,
walk up the stack.
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So from here we can do a couple of things.
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We can move line by line or
we can dive deeper into the code.
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Let's do a deeper dive first.
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So, let's say that I wanted to
look at this promptAction method
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that's here, right?
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So I can come down here and
I can choose the step into method.
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It's an arrow pointing, so
step into, and that's F7, so
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what that's going to do is that's going to
go into the definition of prompt action
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on the KaraokeMachine file, right?
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So, we've now stepped in to this code.
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Cool, so now you'll see that we're inside
the prompt action method definition and
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we're still suspended.
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And you'll see over here in the frames,
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you'll see that we're now in
prompt action from run from main.
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So there's our stack,
we're all the way down into prompt action.
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And you can also see in the view over here
that we have access to these variables.
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So, if we look at this here's the mSongBook
that's private to the karaoke machine
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and it has a private variable that
mSongbook does called mSongs.
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And we can open those up and we can
take a look at the different songs and
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we can keep on drilling down.
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It's pretty cool, right?
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So, we're paused on this line here,
and if we step into this one,
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so it's a single line that's
on multiple lines, but
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it's one line of code that's
just spaced out better.
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So if we step into this we're going to go
and do the getSongCount on the SongBook.
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So let's do that.
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So now over here we're in the getSongCount
from promptAction from run from main.
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So in getSongCount,
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if we press the step into you might think
that it's going to do the size but we're
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going to be blocked cuz we're really only
concerned with what our code is, right?
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So it jumped back out because
it returned that value.
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If you wanted to step in the code,
this red one here,
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this Force Step Into,
this will actually go into the Java code.
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We're not gonna do that right now, but
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I just want to let you know
that that's available.
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If you wanted to go do some more exploring
of what the Java API's are actually doing,
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cuz most of them are going
to show you Java code and
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you can kind of walk through and
see what things are doing.
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And this is a great way
to explore the APIs and
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demystify what's actually happening.
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Okay, so let's just go ahead and
step into this for loop, here.
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Oh, so now it's gonna run
the line where it does the print.
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Okay, so I'm gonna go ahead and I'm gonna
quick step into, one more time, and
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we're gonna step into the for loop.
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And so now remember what this loop
is doing is it's looping through
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each of the menu options we have and its
setting the variable option for each loop.
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So, now we can look at this
first one is add, and so
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let's just go ahead and
we'll step into it step into it.
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And you'll see as you move it puts
the value of what's going on over there.
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It's pretty cool.
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So, we'll step into that.
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Okay cool, so now we're on the next one.
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Here which should give us play,
which is the next option on there.
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So, okay, so we kind of understand
what this is doing, right?
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So, we don't need to watch the whole for
loop go through, so
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what we could do is we could put our
cursor outside and this function here.
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Run to cursor will make the code
all run until we get to there.
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Cool, right?
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So now we're still paused inside
of the prompt action, but
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we jump through that for
loop and all that code ran.
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But we didn't need to step and
watch each one of them.
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So, we can also step out of methods and go
back to the frame where we started, okay?
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So, that's this arrow, of course, the one
stepping up and that's called Step Out and
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that's Shift F8 to get out.
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So, we're just gonna go up the stack.
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Okay, and now it looks like it paused so
there's no frames that are available.
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So, What's going on?
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Oh, that's right.
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Prompt action, right here,
this line called read line.
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It's waiting for us to answer the action.
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So, let's go ahead, and let's say choose.
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That's the thing that we wanna do from
the menu, we wanna choose to sing a song.
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Cool, okay.
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Now bam.
Now we're back, and
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the debugger's stopped.
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We're back in run,
we're at this choice line here.
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Cool, so let's just go ahead and
let's step here and again,
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so now we typed in the choice to be
choose, so we're on the switch statement,
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so let's go ahead and
let's step into this choose statement.
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Okay, so we just wrote this prompt for
singer name.
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I don't really want to see how it works,
so
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the next feature is here,
there's this step over, and that's F8.
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Okay, so if I press F8,
it's gonna step over that, and
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it looks like again like it paused.
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Oh, that's right it's prompting, so I'm
gonna flip over to the console, I'm gonna
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enter the singer's name which is me, and
now I'm gonna go back to the debugger.
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Oh there, okay we're stopped.
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I'm gonna press F8 again, stopped again,
it's prompting for the artist.
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All right, let's say Wilson Phillips,
what do you say?
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Okay, so I think I pressed
enter without pressing nine.
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All right, so let's come back.
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Let's undo this break point,
and let's do this break point.
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Okay, so we're gonna get right to here.
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Actually, let's get to the point
where we ask for the song, okay?
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So we can continue through and
let it stop, okay?
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So I'm gonna start that again.
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So up here, on the debugger,
there's karaoke play,
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there's also a little debug icon here,
so I can click this.
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Okay, so what do you wanna do?
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We wanna choose a song.
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It's me, I wanna to listen to some
Wilson Phillips, boom and it's stopped.
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Awesome, right?
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Okay, so let's step over
the prompts song for artists and
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again flip over to the console,
let's hold on for one more day.
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So, now we're at the song request.
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Let's go ahead and step over that.
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It's going to build a the new
SongRequest object.
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Okay, awesome.
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So, in our debugger we
have this song request.
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And I want to show you a little tool
that's a lot like the REPL that we saw
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that we can use to run our own code.
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So if you choose run and
then you choose evaluate expression.
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Okay, so option F8.
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You can actually run some codes.
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So, remember what we were looking at
here is we're looking at song requests.
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Right, so if we say song, oh look at that,
it auto completes here too.
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Song request, type in singer name.
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It will run and
this is the result that came back.
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So, that's very similar to the REPL
that we were using, right?
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So we can actually even change
the state here in the program.
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So, that's also a nice fun feature.
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So I'm going to go ahead and I'm gonna
redefine what songRequest is, okay?
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So remember we're stopped
on this line here so
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I'm gonna say songRequest
= new SongRequest.
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Let's actually sign Ben up to sing that,
right?
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He'd probably sing that one really good.
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Okay, so I'm going to press enter,
and bam, so it got set.
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Okay, so now I come back here and
I look at song request,
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Ben is the singer, even though
the singer name that I said before was.
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So, it's kinda cool, so you kind of punch
variables into different situations if
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you're trying to catch
a certain thing in there.
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Okay, so we can press the button over
here to resume program execution.
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Okay, and that's gonna keep on going and
we're stuck here at the console again.
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So if we did choose,
we'll sing another one.
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This time let's sing The Cure.
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Okay, bam.
So it stopped again.
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So it'll keep on going.
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So you can also toggle these on and
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off using the Cmd+F8 key,
will toggle the breakpoint on and off.
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Or Ctrl+F8 on Windows.
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So now when I press resume,
it will keep going and not stop.
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Cool, welcome to your debugger.
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Okay, so that's a nice basic
introduction to some of the key concepts
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of the powerful debugger tool.
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This is one of those things that
most of the popular IDE's have and
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they all kind of work the same way.
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It is possible to have the debugger kick
in when specific exceptions are thrown.
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As you can imagine, this is super handy.
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It's also possible to launch it
when certain conditions are met,
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even when fields are set or accessed.
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I've added some notes on
where to learn more of this.
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Check below in the teacher's notes.
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It's a little out of
the scope of this course, but
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I'd love to dive deeper into more advanced
features in an upcoming workshop.
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Let's take a look at how to work
with a team using IntelliJ,
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right after this exercise.
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