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Sorting
7:21Arrays can be ordered through sorting. Let's explore how!
In addition to copying, the arrays utility
class also lets you sort an array.
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Now this is super handy in
situations where you need a specific
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order to the elements in your array.
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In contrast to adding elements, let's see
if I can renew your love for arrays by
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showing off how easy they are to arrange
in the way that you want them to be.
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All right, so a couple things, I'm gonna
set up scrratch.java here a little better.
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So first I'm gonna do is gonna add
myself to this array here at the end,
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so I'm always there with my friends.
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And since it's getting a little bit
longer, I'm gonna do this typical thing
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that you do with arrays,
is you put each item on its own line.
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And because the curly braces are there,
it just worked.
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Let's go ahead, let's get jshell started
too while we're doing this editing.
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And the other thing I wanna do to show
off sorting properly is I'm gonna add
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treasure here.
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We'll add treasure to the list.
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She's our new JavaScript teacher.
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Okay, and we know that we're gonna be
using arrays static utility method.
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So let's import it.
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So in the scratch.java, we'll go ahead and
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say import java.util.Arrays.
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Okay, I've saved that file,
that's looking good, and
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I'm going to go ahead and
open scratch.java.
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And if we take a look at the current
ordering of our friends array,
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we will see that they are out
of alphabetical order.
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Awesome, so let's sort them.
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So we have our arrays class and,
not surprisingly,
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on that arrays class there
is a method called sort.
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And that sort takes an array and
let's see what happens.
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And if we take a look friends now, we'll
see that they're in alphabetical order.
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It doesn't get much more clear than that,
does it?
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I do want you to notice though,
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that when we called this,
it didn't return a copy of the array.
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It actually modified the array in place.
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So that is something to be aware of.
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Sorting does not return a copy,
it actually modifies the original array.
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Wait a second, though, how do you think
it knew to compare them alphabetically?
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Well, you might not know this, but
all strings have a compareto method.
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Here, let's take a look really quick.
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I'm gonna press Ctrl +
L to get back up here.
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So if I have the string Apple, I can
call a method on it called compareTo,
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and compare it to another string.
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So let's take a look.
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So what happens if I
compare apples to bananas?
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I get -1.
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So the way that this works is
if the item on the left is less
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than the item on the right,
it returns a negative number.
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Now, if it's equal it returns zero.
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Like so, so if Apple compareTo Apple,
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I'm not comparing apples and oranges here.
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We get 0, right?
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So, when it's equal.
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But if I go the other way,
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if I compare banana, compareTo Apple,
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You'll see it returns a positive number,
meaning banana is greater that apple.
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So our arrays sort method
uses this method to compare
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each element in our array until
it's in the correct order.
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In fact, you can sort an array
of any object as long as that
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object marks itself as
being comparable and
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provides a method called comparedTo
that returns these same values.
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This -1 for less than, 0 for equals,
and a positive value for greater.
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That is out of the scope of this course,
but we will get there shortly.
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Check the teacher's notes for
more on interfaces and comparable.
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For now, though, a consolation prize.
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Let me show you off
something pretty powerful.
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So let's say that we didn't wanna
sort our list alphabetically,
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let's say we wanted to order our friends
by how many characters were in their name.
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By default, the sort method uses
the compareTo method of objects, but
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the sort method takes a second
parameter which is of type comparator.
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So let's create one for our name length.
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So first thing to do is to import
the comparator utility class.
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I'll go ahead and do it up here.
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Comparator.
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And then in jshell if you go and
reopen a file.
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Let's go ahead and reopen scratch.java.
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And I'm going to clear the screen there.
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And if I start out the method just
like before, I say Arrays.sort,
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I want to give it friends, and
this is where I pass the comparator.
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The Comparator class has a static
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method named comparing,
which will return a new comparator.
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The method's parameter
expects you to define
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how to get the value
that is to be compared.
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So this is most likely going
to be a little new to you.
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But what we can do is use what
is known as a method reference.
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We know that our elements in our
friend array are all strings.
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And strings all have
a method named length, so
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what we really want to do is to
describe to this comparing method
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to use the length method on the string
class to be what is being compared.
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To convey that, we type the name of
the class, which was string, and
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then two colons, and
then the name of the method.
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And the method,
the instance method was length.
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Okay, and
I'll close that final paren there.
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So that is saying, call the length method
on each of these friends to compare them.
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Now, don't sweat it if that method
reference is a bit unclear.
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We'll get to that, too, in a later course.
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And now if we run this, you'll see
that I forgot to save my scratch.java.
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So I'm gonna press up a couple of times,
open scratch.java,
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I'm going to clear the screen and
run that one more time.
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Here we go, and
if we look at our friends array now
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we will see that it is sorted
by the length of people's names.
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Now Ben, as the shortest name,
Alena, Pasan, and myself
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all have five letters in our name but
Treasure has eight letters in her name.
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One of the cool things about comparators
is you can easily change the order of it.
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Like let's say we wanna go longest first,
we just change method,
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let's get that back.
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And so if I say, .reversed, here the
comparator will go the other direction.
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And if we take a look at the friends
right now, Treasure should be first and
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Ben should be last.
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Awesome, all right, so
we got sorting skills.
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Let's take a look at ways to use and
return arrays in our methods.
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