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Declaring Arrays
4:44Let's take a look at how to declare new arrays.
One thing we should cover,
before we get too far into array though,
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is one of memory.
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Now the Java language
does a really great job
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of abstracting a way your need to think
about how all these variables that
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we are creating are stored
in the computer's memory.
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When you declare a variable and specify
its type, what Java actually does for
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you is to reserve a spot in memory for
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you that is big enough to store
your data on initialization.
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And that's super nice of it, isn't it?
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Not having to think about that
sort of thing allows us to focus
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on the task at hand.
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Have you ever been to a movie
theater with a group of people and
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had to reserve seats for them?
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You know, you usually put some
stuff there like a sweater,
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maybe a purse to signify that these seats
are for other people and they're saved.
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Now when you do that, you wanna make
sure that the seats are contiguous,
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or all next to each other,
one after the other.
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Now this is similar to arrays.
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Java needs to know how many elements your
array going to have when it's created.
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So it can go and reserve the right
amount of memory, just like those seats.
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It also requires your elements to be
contiguous, one after the other in memory.
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So what do you say,
we go reserve those seats.
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So go ahead and launch the workspace
attached to this video.
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And then what we'll do is we'll start
up jshell, so we can explore a bit.
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So let's assume that I wanted to go to
the movies with some coworkers, right?
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I'm out with Ben, Alana, and Pasan, and
they're all running a little bit late.
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So I'm gonna grab us some seats.
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So let's do this,
let's just store their names.
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So, we could declare a bunch
of friend variables, right?
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I can call the first one here,
we'll and say friend1 = "Pasan".
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.And then of course we'd
have to make another one.
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And one thing that you can do is
you can declare a variable without
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initializing it.
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Let's explore what happens.
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So if I say String frend2, and I'm just
gonna declare it and not initialize it.
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So here, we'll see that the unitialized
string object isn't null.
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So null is used to represent
the absence of a value.
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Now if you haven't run into these yet,
consider yourself lucky.
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They end up causing a lot of
grief in the form of the dreaded
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null pointer exception.
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Here, let me introduce you to the pain.
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So, let's say that I came along and
I didn't know that this variable here,
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this variable friend2, let's say that I
didn't know that it was actually null.
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Let's assume that I thought
there was a value in here,
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like you would when you saw a variable.
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So let's go ahead and use it.
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So we'll say friend2.toLowercase,
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cuz I wanna get the lowercase version of
this name here, so let's see what happens.
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And boom, there it is,
the null pointer exception.
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So what I wanna point out is that
when you declare an object, right, so
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when we declare an object like this,
and you do not initialize its value,
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it defaults to null.
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Primitive data defaults like int and
bool, they have different default values,
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and we'll get to those
defaults here shortly.
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Check the teacher's notes
if you just can't wait.
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So we know that we don't want
to create a variable for
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each of our friends, that's just silly.
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So what we want to do is use an array.
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And the way that you declare an array
is by placing brackets after your type,
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like this.
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And we'll declare a new
string array named friends.
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And what we need to do is we need to
say that that's a new String array.
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And then you need to declare how many
elements that you're going to need.
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And we need three,
cuz we have three friends coming.
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We have Pasan, Elena, and Ben.
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What this output is showing us
here in jshell is that each
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of the elements in the array
has been defaulted to null.
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And that's just like when
we declared the string and
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didn't initialize its value, right?
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It's null, so there's a three-element
array filled with nulls.
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We'll get how to set these
values here in a bit.
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Hold tight.
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For now let's take a look at
a primitive data type example.
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Let's revisit that golf score idea.
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So we know that there are 18 holes and
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we are going to be storing the score for
each of them.
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So we know that it's an int, right?
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So the score will be an int.
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And we want an array of them,
so we put the brackets, and
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we're gonna name it scores.
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We're gonna say it's a new integer array,
and it's gonna have 18 values.
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And there we go.
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18 elements, all set to the default value,
which, as you can see here for
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integers, is 0.
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Having an array declared like this doesn't
really do us much good if we can't
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set the values.
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So let's get to that,
right after this quick break.
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