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Well done!
You have completed Java Arrays (2017)!
You have completed Java Arrays (2017)!
Preview
Let's take a look at how to declare new arrays.
Default values for types
-
byte:0 -
short:0 -
int:0 -
long:0L -
float:0.0f -
double:0.0d -
char:'\u0000' -
String (or any object):null -
boolean:false
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One thing we should cover,
before we get too far into array though,
0:00
is one of memory.
0:03
Now the Java language
does a really great job
0:04
of abstracting a way your need to think
about how all these variables that
0:07
we are creating are stored
in the computer's memory.
0:11
When you declare a variable and specify
its type, what Java actually does for
0:13
you is to reserve a spot in memory for
0:18
you that is big enough to store
your data on initialization.
0:20
And that's super nice of it, isn't it?
0:24
Not having to think about that
sort of thing allows us to focus
0:26
on the task at hand.
0:29
Have you ever been to a movie
theater with a group of people and
0:30
had to reserve seats for them?
0:33
You know, you usually put some
stuff there like a sweater,
0:35
maybe a purse to signify that these seats
are for other people and they're saved.
0:37
Now when you do that, you wanna make
sure that the seats are contiguous,
0:41
or all next to each other,
one after the other.
0:45
Now this is similar to arrays.
0:49
Java needs to know how many elements your
array going to have when it's created.
0:51
So it can go and reserve the right
amount of memory, just like those seats.
0:55
It also requires your elements to be
contiguous, one after the other in memory.
1:00
So what do you say,
we go reserve those seats.
1:05
So go ahead and launch the workspace
attached to this video.
1:07
And then what we'll do is we'll start
up jshell, so we can explore a bit.
1:10
So let's assume that I wanted to go to
the movies with some coworkers, right?
1:15
I'm out with Ben, Alana, and Pasan, and
they're all running a little bit late.
1:20
So I'm gonna grab us some seats.
1:24
So let's do this,
let's just store their names.
1:26
So, we could declare a bunch
of friend variables, right?
1:28
I can call the first one here,
we'll and say friend1 = "Pasan".
1:33
.And then of course we'd
have to make another one.
1:40
And one thing that you can do is
you can declare a variable without
1:43
initializing it.
1:47
Let's explore what happens.
1:48
So if I say String frend2, and I'm just
gonna declare it and not initialize it.
1:49
So here, we'll see that the unitialized
string object isn't null.
1:57
So null is used to represent
the absence of a value.
2:02
Now if you haven't run into these yet,
consider yourself lucky.
2:06
They end up causing a lot of
grief in the form of the dreaded
2:09
null pointer exception.
2:13
Here, let me introduce you to the pain.
2:14
So, let's say that I came along and
I didn't know that this variable here,
2:17
this variable friend2, let's say that I
didn't know that it was actually null.
2:22
Let's assume that I thought
there was a value in here,
2:26
like you would when you saw a variable.
2:28
So let's go ahead and use it.
2:31
So we'll say friend2.toLowercase,
2:32
cuz I wanna get the lowercase version of
this name here, so let's see what happens.
2:34
And boom, there it is,
the null pointer exception.
2:42
So what I wanna point out is that
when you declare an object, right, so
2:45
when we declare an object like this,
and you do not initialize its value,
2:49
it defaults to null.
2:53
Primitive data defaults like int and
bool, they have different default values,
2:55
and we'll get to those
defaults here shortly.
2:59
Check the teacher's notes
if you just can't wait.
3:01
So we know that we don't want
to create a variable for
3:04
each of our friends, that's just silly.
3:08
So what we want to do is use an array.
3:10
And the way that you declare an array
is by placing brackets after your type,
3:13
like this.
3:18
And we'll declare a new
string array named friends.
3:20
And what we need to do is we need to
say that that's a new String array.
3:25
And then you need to declare how many
elements that you're going to need.
3:30
And we need three,
cuz we have three friends coming.
3:34
We have Pasan, Elena, and Ben.
3:36
What this output is showing us
here in jshell is that each
3:41
of the elements in the array
has been defaulted to null.
3:45
And that's just like when
we declared the string and
3:50
didn't initialize its value, right?
3:52
It's null, so there's a three-element
array filled with nulls.
3:53
We'll get how to set these
values here in a bit.
3:59
Hold tight.
4:01
For now let's take a look at
a primitive data type example.
4:02
Let's revisit that golf score idea.
4:06
So we know that there are 18 holes and
4:08
we are going to be storing the score for
each of them.
4:11
So we know that it's an int, right?
4:13
So the score will be an int.
4:16
And we want an array of them,
so we put the brackets, and
4:19
we're gonna name it scores.
4:22
We're gonna say it's a new integer array,
and it's gonna have 18 values.
4:23
And there we go.
4:31
18 elements, all set to the default value,
which, as you can see here for
4:32
integers, is 0.
4:37
Having an array declared like this doesn't
really do us much good if we can't
4:38
set the values.
4:41
So let's get to that,
right after this quick break.
4:42
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