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Map Basics
5:33In this video we'll go over the basic functions of the Map interface and walk through using them!
Now that we've got a populated map,
let's see how to get the items back out.
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Instead of printing out the whole map,
let's just print out what's for dinner.
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To retrieve an item from a map,
you just call the get function and
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pass in the key of the item
you're looking for.
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So to print out only what's for dinner,
we just need to change this meals
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to meals.get, and pass in dinner.
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Then, if we run the app We'll see that
tonight we're getting enchiladas.
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In addition to put and
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get, there's four other functions of
maps that you should probably know.
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The first of these is the remove function.
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If you ever need to remove a key value
pair, just call the remove method and
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pass in the key.
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So if we wanted to remove lunch,
we'd call meals.remove, and pass in lunch.
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Also, in case you need it,
the remove function does return a value.
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So if we want, we can set this
equal to a new string called lunch.
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After the remove function,
we've got the contains key, and
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contains value functions,
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which return a Boolean depending on
whether the map contains those items.
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Let's create a new Boolean
called hasLunch, and
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set it equal to meals .containtsKey,
and pass in lunch.
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And on the next line, let's make
another new Boolean called hasGyros,
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and set it equal to meals.containtsValue.
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And pass in gyros.
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Finally, the last function we need
to know is the size function,
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which returns how many
items are in the map.
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Let's create a new int variable named
size, and set it equal to meals.size.
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To wrap it all up, let's see the result of
all these functions by printing it all out
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on the next line.
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And let's save some time by just
copying the print statement from
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the teacher's notes.
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Now, if we run it We can see
that when we remove the lunch,
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we removed gyros which means that
hasLunch and hasGyros are both false.
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And our map is down to only two entries.
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Awesome.
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Now, before we go, there is a couple of
more things you should know about maps.
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The first is that you can't
have any duplicate keys.
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If we try to add a new entry
with the key of dinner
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And a value of Pudding It's
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just going to overwrite
Enchiladas with Pudding.
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We won't get to have two dinners.
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Deleting that.
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The second thing you need to know is
that when you're using strings as keys,
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you almost always want
to be using constants.
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It turns out that it's actually
pretty easy to make a typo.
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So if down here, I accidentally
typed diner instead of dinner,
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we wouldn't get what we're looking for.
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And it doesn't even show up as an error.
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So instead of typing out breakfast,
lunch, and
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dinner, we should be using
constants set to these values.
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Let's declare these constants
at the top of our class.
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But before you start typing anything,
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you should know that IntelliJ actually has
a shortcut for creating string constants.
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Let's type psfs and hit Enter.
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Then, let's name our first
string BREAKFAST, in all caps,
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which is the convention for constants.
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And set it equal to our breakfast string.
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Then, let's do the same thing for
lunch and
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dinner, psfs LUNCH set it equal to lunch.
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And psfs DINNER, set it equal to dinner.
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Great!
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Now, instead of using strings for
our keys, let's use our constants.
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So we'll take BREAKFAST and
put it over to this one.
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Take LUNCH and put it over to this one.
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And this one.
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And this one.
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And we'll take dinner and
put it over this dinner and this diner.
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And when we run it, thanks to the
constants, we're back to enchiladas for
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dinner.
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With lists, every item we added had
to be associated with an index.
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But with maps, we're free to associated
our items with whatever we want.
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We can map students to grades,
part numbers to prices, or
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even map strings to
strings like we did here.
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So the next time you're tasked with
creating a collection of objects,
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if you want to specify what kind of
key to use, make sure to use a map.
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Until next time.
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