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Well done!
You have completed Java Arrays!
You have completed Java Arrays!
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
0:01
we get too far into arrays is memory.
0:03
Now, the Java language does
a really great job of abstracting away
0:06
your need to think about
0:09
how all these variables we're creating
are stored in the computer's memory.
0:11
When you declare a variable
and specify its type,
0:15
what Java actually does is reserve
a spot in memory
0:18
that's big enough to store
your data upon initialization.
0:21
And that's super nice of it, isn't it?
0:25
not having to think about that
0:27
sort of thing
allows us to focus on the task at hand.
0:28
Have you ever been to a movie theater
with a group of people
0:32
and had to reserve seats for them?
0:35
You know, you usually put some stuff
there, like a sweater, maybe a purse,
0:37
to signify that these seats
are for other people and they're saved.
0:41
When you do that,
0:45
you want to make sure
0:46
these seats are contiguous, or all
next to each other, one after the other.
0:46
Now, this is similar to arrays.
0:51
Java needs to know how many elements
0:53
your array is going to have
when it's created so it can go
0:56
reserve the right amount of memory,
just like those seats.
0:59
It also requires that your elements are
contiguous, one after the other, in memory.
1:02
So what do you say
we go reserve those seats?
1:07
Go ahead and launch the workspace
attached to this video.
1:12
We'll start up jshell
so we can explore a bit.
1:15
Let's say I'm out at the movies
and I'm going to meet up with
1:19
some friends, Brian, Rohald, and Laura,
and they're all running a little bit late.
1:22
I'm going to grab us some seats.
1:27
Let's store their names.
1:29
We could declare a bunch of friend
variables, right?
1:31
I can call the first one friend1
and say friend1 equals Brian.
1:33
And of course,
we'd have to make another one.
1:39
One thing you can do is declare a variable
without initializing it.
1:42
Let's see what happens then.
1:45
So if I say string friend2, and
I just declare it without initializing it,
1:47
here we'll see that
the uninitialized string object is null.
1:53
Null is used
to represent the absence of a value
1:57
Now if you haven't run into these yet,
consider yourself lucky.
2:01
They end up causing a lot of grief
in the form of the dreaded null pointer
2:04
exception.
Here, let me introduce you to the pain.
2:07
Let's say I didn't know that this variable
here, friend2, was actually null.
2:11
Let's assume
2:15
I thought there was a value in it, like
you might expect when you see a variable.
2:15
So I go ahead and use it.
2:19
Friend2.toLowercase.
2:21
I want to get the lowercase version
of this name here.
2:24
And boom, there it is.
2:26
The null pointer exception.
2:28
So the point I want to make is this.
2:30
When you declare an object, like a string, and
don't initialize it, it defaults to null.
2:32
Primitive data types, like
int and boolean,
2:38
have different default values,
and we'll get to those shortly.
2:40
Check the teacher's
notes if you just can't wait.
2:43
So we know that we
2:46
don't want to create a separate variable
for each of our friends.
2:47
That's just silly.
2:50
What we want to do is use an array.
2:51
The way you declare an array is by placing
square brackets after your type,
2:54
like this. We'll declare a new string
array named friends
2:58
And we say equals
3:04
New string, three. We need three elements
3:06
because we have three friends
coming: Brian, Rohald and Laura.
3:10
What the output shows us here in JShell
3:15
is that each of the elements in the array
has been defaulted to null.
3:17
Just when we declared that single string
and didn't initialize it, it's null.
3:21
So now we've got a three
element array filled with nulls.
3:25
We'll get to how to set these values
in just a bit.
3:29
Hold tight.
3:31
For now, let's take a look
at a primitive data type example.
3:32
Let's revisit that golf score idea.
3:35
We know there are 18 holes,
and we're going to be storing the score
3:38
for each of them.
3:41
So we'll use int, and we want an array,
so we'll put our square
3:42
brackets, and name it scores.
3:46
We'll say it's a new
3:49
integer array with 18 values.
3:50
And there we go!
3:54
18 elements, all set to the default value,
which,
3:55
as you can see here for integers, is zero.
3:58
Having an array declared
this doesn't really do us much good
4:01
if we can't set the values, so let's
get to that right after this quick break.
4:04
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