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You have completed Java Loops!
You have completed Java Loops!
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Let’s learn about the while loop and how it repeats code as long as a condition is true. We’ll also see how to avoid common pitfalls like infinite loops.
Resources
If you're unfamiliar with Scanner or if-else statements, don't worry, they're taught in detail in an upcoming course. You can just follow along for now and consider it a sneak peak of what's to come.
Here is the Oracle documentation if you want to look into them further.
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Okay, the first loop I'd to cover
is the while loop.
0:00
So what is a while loop
and why would you use one?
0:04
A while
loop is perfect for repeating something
0:07
while a certain condition is true.
0:09
So imagine you're at a kid's
birthday party and you want to eat
0:12
those tasty looking cookies over there
until the plate is empty.
0:14
So while there's cookies on the plate
still.
0:18
You don't know exactly how many cookies
there are, but you know that they should
0:21
all end up being devoured by you
and not those pesky kids.
0:24
Go ahead and launch the workspace
below this video
0:29
and we'll learn how to get this done.
0:31
Okay, so in this workspace
I've got a few files.
0:33
Let's open up this one named while.java.
0:37
I've already got the class structure
set up for you.
0:40
You're welcome.
0:42
Let's
go into this main here and write one out.
0:44
The syntax for the while loop
is pretty straightforward.
0:47
You write the word while, then
put a condition inside of parentheses,
0:50
and then the code you want to repeat
inside of some curly braces.
0:55
So let's bring in our sneaky cookie
example.
1:01
Let's create a variable
for the total cookies on the plate.
1:04
We'll say int totalCookies equals 10.
1:07
Now we need some kind of counter
1:13
number to keep track
of how many we've eaten so far.
1:15
So we'll start it off
with int cookiesEaten
1:18
equals 0,
because we haven't eaten any yet.
1:21
Cool,
so for our conditional, we want to say,
1:26
while cookies
eaten is less than total cookies, right?
1:28
Because we can't eat more cookies
than there are in the plate.
1:33
We need a stopping point.
1:35
So this is saying,
as long as we haven't eaten 10 cookies,
1:37
continue on and loop again.
1:40
Now the most important part.
1:43
As long as this condition is true,
all of the code that we write in
1:45
this block is going to run top to bottom
any other method.
1:49
Once it's completed the code in here, it's
going to go back up to this conditional
1:53
and check it again. Is this still true?
1:56
If it is, it will run everything in here
again and so on.
2:00
So we need to increment this cookies
eaten value each time
2:04
or else this conditional up here
will forever be true.
2:07
It needs to become false at some point.
We'll touch on more of the
2:11
why of that momentarily.
So let's say cookies eaten plus plus
2:14
and let's just print a line to the console
exposing our bad eating habits. So system
2:19
.out.printf, and we'll say
I've eaten percent D cookies
2:25
Let's add a new line
and then we'll put in our cookies
2:32
eaten variable there.
2:35
Let's save this file
2:40
and go ahead and run this
to see what happens.
2:41
If for some reason your console
isn't open, you can come up here to View,
2:44
and instead of Hide
Console, it'll say Show Console.
2:48
Alright,
2:53
we'll say Clear, and javac While.java,
2:54
and java While.
2:58
Awesome!
3:06
We ate 10 cookies and the loop stopped.
3:07
Perfect!
3:09
So quickly, what's happening
here is we initialized our cookiesEaten
3:10
variable at 0.
3:14
So when we hit this while loop, it
checks, is 0 less than 10?
3:16
Yes it is. Let's run the code below.
3:21
We changed our variable to 1, and
let everyone know that we ate a cookie.
3:23
After that,
it went back up and checked again.
3:28
Is 1 less than 10? Yes it is.
3:31
Then we incremented our variable to 2.
3:34
everyone know how addicted we are, etc.
3:37
This keeps happening over and over,
and once we've eaten our tenth
3:39
cookie, it checks, is ten less than ten?
3:42
No, it's not.
3:46
Finally, at that point,
the while loop breaks,
3:47
doesn't run the code within it anymore,
and we can finally sit on the couch
3:50
and regret our choices.
3:53
When working with
3:56
loops, and while loops especially,
you really need to make sure
3:57
that the condition you provide
eventually becomes false, or falsy.
4:00
If not, things can get a little wild.
4:04
Enter the infinite loop.
4:07
If you forget to update your condition
or your counter
4:09
in our example,
your loop will never stop looping.
4:12
Imagine you're
4:16
stuck at the party eating cookies forever
no matter how full you get.
4:16
Here, let me show you with our example,
but please don't follow me here.
4:20
I'm going to comment out this line
and run this again.
4:24
In our example
it sounds more I'm complaining
4:35
"Hey i still haven't eaten any cookies
over here!"
4:37
By the way, I used CTRL+C
to stop the program from running there,
4:41
just in case you accidentally get caught
in one of these in a workspace.
4:45
Now there is an implementation
of the while loop that technically
4:49
an infinite loop that's used quite often.
Here, let me show you.
4:52
I'm going to uncomment out that line
to avoid that again, and you know what?
4:57
I'm actually going to highlight
this entire while loop and use command
5:00
forward slash or control forward slash
if you're on Windows to comment out
5:04
this thing entirely just to save us
from printing all that out again.
5:08
Let's say we were caught
sneaking cookies at the party
5:12
and now one of the kids is holding
that plate.
5:15
They'll allow us to have more,
but we need to use our manners.
5:17
Let's build a quick input output program
to interact with this a bit.
5:21
Up top here,
let's import java.util.scanner.
5:25
Then below this example,
5:32
let's say scanner,
we'll just name it scanner,
5:33
equals new scanner, and pass it system.in.
5:38
Okay, this is what you might see out in
the wild.
5:42
We'll say while, and for our conditional,
we'll just write true,
5:45
then our curly braces.
5:49
This probably
5:54
looks ridiculous to you, and it is.
5:55
As this is,
this is guaranteeing an infinite loop.
5:58
This isn't a variable or a condition
that we can change.
6:02
We can't just change the rules of Java
6:04
and say, true now means false,
no take backsies.
6:06
This is where the break keyword comes in.
6:10
If you want to escape the loop
when something happens, you can use break.
6:13
So let's check it out.
6:17
We'll say system.out.println.
6:18
Do you want another cookie?
6:22
Yes or no.
6:25
And we'll store that input with string
answer
6:28
equals scanner dot next line.
6:31
Okay, so
we need to manually make our conditional.
6:36
If we reply with no, we want the kid
to leave us alone and stop asking, right?
6:38
So let's say if answer
6:43
dot equals ignore case no,
6:45
just in case we scream it at them,
it'll still work out.
6:49
If we say no, we want to break out
of this loop and stop it.
6:52
So let's just write break
semicolon. Beneath this, meaning
6:56
if we typed anything other than the word
no, we system.out.println
7:00
Okay here's another cookie.
7:05
Then after the loop closes
just to show a visual of it
7:09
ending, we'll system.out.println
Okay, bye!
7:11
Or whatever kids say these days.
7:17
Let's test it out.
7:21
Yes? Okay, here's another cookie.
7:28
Here's another cookie.
7:32
then I'll yell it at him, no,
and it broke out of the loop.
7:33
Awesome.
7:37
Now, there's also the continue keyword,
which will skip the rest of the current
7:39
loop, meaning all of the code below it,
and just jump straight into the next loop.
7:42
Here, let's try that out.
7:47
Let's add an else if block here
and say, else if answer.equals
7:49
ignore case, maybe.
7:53
Then we'll use continue,
7:57
which should not run anything
below and restart the loop.
7:59
While we're being good programmers
here, let's add
8:03
this here's another cookie line
into another else if.
8:05
if answer dot equals ignore case yes
8:09
then add continue again.
8:18
Then finally below
8:26
all this, we'll just say else, "huh?"
8:27
This will only run if they input something
other than yes, no or maybe.
8:31
Let's check it out
8:41
Do you want another cookie? Yes.
8:46
Maybe.
8:49
Got any tacos?
8:52
No. Perfect.
8:54
The big lesson here
is, use while loops when you're uncertain
8:58
how many times
you'll need to run your code,
9:02
in an input-output menu setting we did
here, and always make sure your loop's
9:04
condition will eventually become false
or use break to escape when you need to.
9:09
Otherwise,
you might end up in an endless loop
9:14
and nobody wants to be stuck
eating cookies forever.
9:17
Well, maybe some of us do.
9:20
Alright,
that's the basics of the while loop.
9:23
In the next video, we'll check out another
similar loop, the do-while loop.
9:25
See you there.
9:29
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