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Introducing Maps
3:56In this video we'll learn what Maps are and create our own Map!
Hi, I'm Ben, and in this course
we're going to learn about maps.
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In Java a map is a data type that
lets us store key value pairs.
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You may not know it,
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we've already been using key value
pairs with lists and arrays.
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Let's say we have a list
that looks like this.
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It has Waffles at Index 0, Gyros at
Index 1, and Enchiladas at Index 2.
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Another way to look at this would
be as a mapping of keys to values.
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So with a list or an array the keys
are always going to be the same.
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We'll start with zero and
start counting upwards.
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But sometimes we need a little more
flexibility with what we use as keys,
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this is where maps come in.
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With a map instead of being integers,
our keys can be any type we'd like.
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Which means when you create a map you
not only need to specify a type for
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the values, but you also need
to specify a type for the keys.
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It also means that we can turn this list
into something a lot more meaningful.
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By using a map instead we're able to
associate each of these foods with
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a meal instead of just
some arbitrary number.
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Now that we've got a better
idea of what a map is,
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let's jump into some code and
try to make this map.
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Let's create a new project.
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Click through the screens.
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And name the project Maps.
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Once we've got the project,
let's delete the comment.
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And instead let's create
a new map variable.
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And notice that it's an interface
that takes in two type parameters.
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One for the key and one for the value.
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Let's hit Enter to import it.
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Then let's add the angle brackets and
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declare our keys to be strings and
our values to also be strings.
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Then let's name our map meals and
add the equals sign.
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Now, remember,
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since map is an interface we won't be able
to use it to create a new map object.
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Instead we'll need to use a class
that implements the map interface.
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It's just like what we had to do with our
lists except instead of using list and
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array list here we'll be using map and
HashMap.
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So let's set this equal to a new HashMap,
and use Enter to let it
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auto complete, then add the semi colon,
and we're good to go.
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Now that we've got our map we just
need to add our three meals to it.
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To add an entry to a map
we use the put function.
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So on the next line let's
add our breakfast to
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the map with meals.put and
pass in breakfast for
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the key and Waffles for the value.
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Nice.
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Then let's do the same thing for
our lunch and our dinner.
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So meals.put and lunch will be Gyros.
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And meals.put and let's make dinner at
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our Enchiladas Great,
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let's finish up by printing out what's on
our map to make sure everything's working.
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So System.out and pass in meals and
then let's run it
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Perfect, that's exactly
what we put in there.
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But there's still more
to learn about maps.
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In the next video will see how to
pull items back out of the map and
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see what else the map
interface has to offer
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