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Well done!
You have completed Java Maps!
You have completed Java Maps!
Preview
In this video we'll go over the basic functions of the Map interface and walk through using them!
Print Statement
System.out.println(lunch + " " + hasLunch + " " + hasGyros + " " + size);
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Now that we've got a populated map,
let's see how to get the items back out.
0:00
Instead of printing out the whole map,
let's just print out what's for dinner.
0:04
To retrieve an item from a map,
you just call the get function and
0:08
pass in the key of the item
you're looking for.
0:12
So to print out only what's for dinner,
we just need to change this meals
0:15
to meals.get, and pass in dinner.
0:19
Then, if we run the app We'll see that
tonight we're getting enchiladas.
0:24
In addition to put and
0:32
get, there's four other functions of
maps that you should probably know.
0:33
The first of these is the remove function.
0:38
If you ever need to remove a key value
pair, just call the remove method and
0:42
pass in the key.
0:46
So if we wanted to remove lunch,
we'd call meals.remove, and pass in lunch.
0:48
Also, in case you need it,
the remove function does return a value.
0:56
So if we want, we can set this
equal to a new string called lunch.
1:01
After the remove function,
we've got the contains key, and
1:08
contains value functions,
1:12
which return a Boolean depending on
whether the map contains those items.
1:13
Let's create a new Boolean
called hasLunch, and
1:20
set it equal to meals .containtsKey,
and pass in lunch.
1:25
And on the next line, let's make
another new Boolean called hasGyros,
1:33
and set it equal to meals.containtsValue.
1:40
And pass in gyros.
1:46
Finally, the last function we need
to know is the size function,
1:51
which returns how many
items are in the map.
1:55
Let's create a new int variable named
size, and set it equal to meals.size.
1:58
To wrap it all up, let's see the result of
all these functions by printing it all out
2:06
on the next line.
2:11
And let's save some time by just
copying the print statement from
2:13
the teacher's notes.
2:15
Now, if we run it We can see
that when we remove the lunch,
2:21
we removed gyros which means that
hasLunch and hasGyros are both false.
2:29
And our map is down to only two entries.
2:34
Awesome.
2:38
Now, before we go, there is a couple of
more things you should know about maps.
2:39
The first is that you can't
have any duplicate keys.
2:43
If we try to add a new entry
with the key of dinner
2:47
And a value of Pudding It's
2:55
just going to overwrite
Enchiladas with Pudding.
3:03
We won't get to have two dinners.
3:07
Deleting that.
3:09
The second thing you need to know is
that when you're using strings as keys,
3:12
you almost always want
to be using constants.
3:17
It turns out that it's actually
pretty easy to make a typo.
3:22
So if down here, I accidentally
typed diner instead of dinner,
3:26
we wouldn't get what we're looking for.
3:31
And it doesn't even show up as an error.
3:33
So instead of typing out breakfast,
lunch, and
3:38
dinner, we should be using
constants set to these values.
3:41
Let's declare these constants
at the top of our class.
3:46
But before you start typing anything,
3:50
you should know that IntelliJ actually has
a shortcut for creating string constants.
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Let's type psfs and hit Enter.
3:58
Then, let's name our first
string BREAKFAST, in all caps,
4:02
which is the convention for constants.
4:06
And set it equal to our breakfast string.
4:08
Then, let's do the same thing for
lunch and
4:13
dinner, psfs LUNCH set it equal to lunch.
4:17
And psfs DINNER, set it equal to dinner.
4:23
Great!
4:30
Now, instead of using strings for
our keys, let's use our constants.
4:31
So we'll take BREAKFAST and
put it over to this one.
4:37
Take LUNCH and put it over to this one.
4:41
And this one.
4:47
And this one.
4:50
And we'll take dinner and
put it over this dinner and this diner.
4:52
And when we run it, thanks to the
constants, we're back to enchiladas for
5:02
dinner.
5:05
With lists, every item we added had
to be associated with an index.
5:07
But with maps, we're free to associated
our items with whatever we want.
5:12
We can map students to grades,
part numbers to prices, or
5:16
even map strings to
strings like we did here.
5:20
So the next time you're tasked with
creating a collection of objects,
5:23
if you want to specify what kind of
key to use, make sure to use a map.
5:27
Until next time.
5:31
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