Heads up! To view this whole video, sign in with your Courses account or enroll in your free 7-day trial. Sign In Enroll
Well done!
You have completed Java Objects!
You have completed Java Objects!
Preview
Constants are values that never change. Let's use the `static` keyword to provide information at the class level instead of the instance level.
Related Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign upRelated Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign up
[MUSIC]
0:00
All right, let's work on getting some
more state represented in our object.
0:04
I want to get some Pez in here so
we can do its job dispense.
0:09
But first, in case you aren't
that big of a fan of Pez.
0:13
You might not know some vital information
that we're going to need for our coding.
0:16
So Pez refills come in
a packs of 12 like this.
0:21
And that happens to be exactly the same
count as what the standard Pez dispenser
0:25
can contain and no more.
0:29
12 is the maximum limit that our object
can contain, and that limit won't change,
0:31
it will remain constant.
0:36
We should express this limitation
in code before people start
0:38
filling it up, sound good?
0:41
Let's do this.
0:43
So the vital information that we
just talked about of how many Pez
0:45
can be stored in dispenser.
0:48
It's going to prove crucial
to how our object works.
0:49
Now, this number of Pez that can fit in
a dispenser is something that will never
0:52
change, right?
0:56
Values that do not change are often
referred to as constants.
0:57
A naming convention that helps explain
that a variable is a constant is to make
1:01
all letters uppercase and
separate the words with an underscore.
1:04
So let's add a max pez
constant to our class.
1:08
So we'll just come up here.
1:12
We'll say, public int MAX_PEZ,
1:14
and like we said, that was 12.
1:18
Okay, so let's imagine that this object
gets created as a currently stands in our
1:23
implementation MAX_PEZ is public.
1:26
So as you know,
1:30
anyone could change the limit of how
many Pez this can be loaded with.
1:31
That's not actually how things
work in real life, right?
1:35
I can't do that with
an actual Pez dispenser.
1:38
So again, we should make sure
that our objects reflect reality.
1:40
We can make it reflect reality with,
you guessed it, our old friend final.
1:44
So let's go ahead and save this.
1:49
So let's poke at it in
jshell really quick.
1:51
So I'm gonna open up jshell down here.
1:53
And jshell allows you to open classes,
which is pretty cool.
1:55
So to open your code, you just do a / and
then the command open.
1:59
And then you can just start typing
your class and open PezDispenser.java.
2:03
We're gonna open up as PezDipenser.java.
2:08
So now, what that does is PezDispenser
actually lives in the code here.
2:10
So we can say, PezDispenser.
2:14
And we'll just call it pd,
that's bad, right?
2:18
That's bad, what does PD mean?
2:21
Letter, Police Department?
2:21
I don't know.
2:22
All right, so I'm gonna say, Yoda.
2:24
Okay, and if we wanted to, now,
2:26
we can see that the MAX_PEZ exists.
2:30
MAX_PEZ, okay.
And if we try to change that
2:34
constant 123 in there.
2:38
You can assign a value to find a variable,
MAX_PEZ, cool.
2:42
So we've blocked the value from changing.
2:45
And our object is now this much
closer to representing reality.
2:46
Now, this concept of providing
vital constant information
2:51
about your class is super common.
2:53
Here, let's take a look at
the integer class actually.
2:55
So if we got an integer, and I come here.
2:57
Now, jshell like we said,
offers tab completion.
3:00
So let's leave Integer and
then we'll do a tab.
3:02
And look at all the uppercase
values there, so here we got BYTES.
3:05
We got MAX_VALUE, MIN_VALUE, so let's
see what the max value of integer is.
3:10
So that's how big a number
can be stored in.
3:17
But you know what?
3:19
That's right off the class.
3:20
And that's because it's true of every
integer that will ever be created.
3:22
Now, can we do that in our class?
3:26
I see that we did it on our instance,
right?
3:28
But can we do it like this?
3:30
Can we say, PezDispenser.MAX_PEZ.
3:31
And it says,
3:37
a non static variable MAX_PEZ cannot
be a referenced from a static context.
3:38
Nope, it doesn't work.
3:43
But this constant is true of
all PezDispensers that will
3:45
ever be created from this class.
3:47
So it should be defined on the class.
3:50
We should need to create an instance.
3:51
We should be able to act
just like Integer did.
3:53
So as you might have guessed by now,
it's just another keyword.
3:55
Now, the problem here is that the keyword
isn't very clear as to what it's doing.
3:58
The keyword is static.
4:02
So let's put it up here and
get it in here.
4:04
So say static,
I'm gonna save the file again.
4:06
And then let's reopen our Pez dispenser.
4:10
So if you say, open ns2p in our tab,
PezDispenser.java.
4:13
Cool, and I'm gonna control
L to clear the screen here.
4:19
And if I do PezDispenser.MAX_PEZ.
4:21
Now, these are sometimes also referred to
as class level variables because you can
4:29
access them at the class level,
as opposed to the instance.
4:33
And the error from before,
it makes a little bit more sense, right?
4:36
It said,
4:39
non-static variable MAX_PEZ cannot
be referenced from a static context.
4:40
Now, this class level is called the static
context versus the instances context.
4:45
Now, the keyword,
4:50
static actually refers to how
the variable itself is stored in memory.
4:51
But what it actually allows you to do
is the ability to provide variables and
4:55
methods directly off your class as
opposed to having them on the instance.
5:00
So actually here,
5:06
the auto complete in jshell will show
you what's available statically, right?
5:07
If we do Integer again and we press tab.
5:12
There's one we've seen before, parseInt.
5:13
So that's a great example of a static
method that you've already used, right?
5:17
So remember that it'll let you take
a string parseInt and return the number.
5:20
See, how it's right off the class?
5:29
Now, I don't think we need
any of those in our class.
5:30
But I thought I'd show you
since you've seen that already.
5:32
And when I showed you before, I had said,
5:35
there's a static method off of
the Integer class named parseInt.
5:37
And before that,
probably just sounded like mumbo jumbo.
5:40
But now,
it should be starting to be more familiar.
5:43
So now, if we pop back over to
Example.java before anything,
5:46
we can use it because it's now public and
static, so let's do that.
5:51
So we're making a PEZ Dispenser.
5:54
So let's add a new System.out,
5:56
we'll do a print line, or
let's do printf actually.
5:59
We'll say, ("FUN FACT: There are %d
6:04
PEZ allowed in every dispenser",.
6:09
And we'll go ahead, and
6:14
we will push in our value before we
6:18
create PezDispenser.MAX_PEZ.
6:23
I should put a new line
at the end of this.
6:29
And see, how we didn't need
to have the instance at all?
6:35
This is before we initialized it.
6:38
A fun fact about Java specifically is that
because a variable is modified with static
6:40
and final, when the compilation happens,
when javac happens.
6:45
It replaces all instances of that
variable with the actual value.
6:49
Okay, so now that we know how to
not overfill these Pez Dispensers,
6:53
let's make sure we don't overfill
our brains with new concepts.
6:57
Why don't you take a quick break and
let this information sink in.
7:00
Now, during your break,
7:04
try to think of any other constants for
objects in the real world.
7:05
Are those constants unique
to that specific object or
7:10
all objects of that type?
7:12
Okay, I'll see in a bit.
7:15
Enjoy your break.
7:16
We built this city,
we built this city on rock and rule.
7:18
You need to sign up for Treehouse in order to download course files.
Sign upYou need to sign up for Treehouse in order to set up Workspace
Sign up