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An arrays length is immutable. If you want to add or remove, you need to declare a new array and copy into it. Don't worry you won't be doing this too much.
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Example ArrayList code
// This is coming from the Java Collection Framework
import java.util.List;
import java.util.ArrayList;
// This is using Generic syntax, we'll get to it...
List<String> friends = new ArrayList<>();
friends.add("Ben"); // adds "Ben" to the friends list
friends.size(); // returns 1
friends.contains("Ben"); // returns true
friends.remove("Ben");
friends.size() // returns 0
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[MUSIC]
0:00
[SOUND] Now that we've
seen the power of arrays,
0:04
it's time to look at some
of their limitations.
0:06
Now, we've been dealing with arrays
where we either know all of the items at
0:08
declaration time or at least we know
how many there will be eventually.
0:12
In reality,
that's not always the case, is it?
0:17
I'm sure you can imagine programs where
we're taking input from a user, and
0:20
they want to either add or
remove elements from the array.
0:23
Now, if you recall when we first
started talking about this
0:26
fancy new data structure, we talked
about how it's length is immutable.
0:29
Now remember,
that means it can't be changed.
0:33
And the reason for this immutability
was because the declaration required
0:36
all of the elements to be one
after the other, contiguous.
0:40
So that brings up quite a big gotcha.
0:45
In order to add an element,
0:48
you would in fact have to change
the length of the array, and you can't.
0:49
So, what do you do?
0:54
Well, the answer is you copy
that array into a brand new one,
0:56
that includes the needed space for
you new element.
1:00
In fact, I have a pretty good
example of how this can creep up.
1:03
In our list of friends,
I forgot to add myself.
1:07
Let's take a look at how to
handle this common gotcha.
1:10
Okay, so I'm gonna do this is JShell.
1:13
So I'm gonna use the scratch file.
1:15
What I'm gonna do, is I'm going to
grab this string friends array here.
1:17
I'm gonna copy that, and I'm gonna
pop it over into the scratch file.
1:21
I'm gonna get JShell up and running.
1:25
While we do this, Jshell will be running.
1:27
And then scratch,
I'm gonna just erase what we have here.
1:29
I'm gonna drop our front array in there,
cuz it's a scratch file, right?
1:31
It doesn't really matter.
1:35
You can put whatever you want in here, and
then we can open it up and use it later.
1:36
That's the idea, and we explore around.
1:40
Okay, let's do it.
1:43
Let's get that file in there.
1:45
We'll say open scratch.java.
1:46
And then we'll type friends.
1:50
Awesome, okay.
1:54
So the goal is to add me
to the end of this list.
1:55
We wanna attend me.
1:59
So there are couple of approaches.
2:01
Now the first thing you do is create
your array to what size you want.
2:03
And let's do that, so let's see.
2:09
It's gonna be a string array.
2:13
It's gonna be a new string array.
2:14
We'll call it friendsAndMe, great.
2:17
And then we'll do = new string.
2:21
And we want one more than's there,
cuz I'm gonna come to this party, now so
2:25
there's [4].
2:29
There we go.
2:30
So you'll notice that it
defaulted all here to null,
2:32
which is the default for objects.
2:35
Okay, and then what we wanna do is we
wanna copy all of the elements from our
2:37
original array into this array.
2:41
So the method itself is a bit
on the older side of Java.
2:44
It's been around forever, and
2:49
it's dangling off of a system class.
2:53
It's called array copy.
2:59
So if I just start typing arr and I press
tab, it will go ahead and fill it out.
3:00
Now, one thing that you might not know
about JShell, and I really like this about
3:06
JShell is if I press tab again,
it will actually show the documentation.
3:09
The first tab,
it shows just the signature.
3:16
And the second time, it's gonna show
the documentation for that, and
3:18
press tab again.
3:22
So it shows the method
declaration which is important,
3:24
cuz we need to know these signatures are.
3:27
We need to know about this here, what
the names of these are, what they mean?
3:29
They're not the greatest, but
okay, so source is the source.
3:33
It's what we're trying to copy,
so that's our original array.
3:37
Original array is friends.
3:41
All right, source pause.
3:44
[LAUGH] That's the position, right?
3:46
So that's the starting index of
where we wanna start copying from.
3:48
So the position of that is 0, so
we wanna start the very first element.
3:52
Okay, and then the next parameter
here is the destination array.
3:58
So the destination array that we
want is the one we just created.
4:02
So we wanna copy that to friendsAndMe.
4:05
And the next is the desk post,
so the destination position,
4:10
where do you wanna place that two?
4:13
And since we're just copying over,
we want me at the end, we'll start at 0.
4:15
And finally, the link.
4:20
How many of those items from
the first one do you wanna copy over?
4:22
And I'm just gonna use friends.length,
saying that we wanna copy them all.
4:25
Okay, so now,
if we take a look at friendsAndme,
4:32
we'll see that we have an extra space
at the end here, where we can put me.
4:37
So I wanna put myself
in that last one there.
4:44
So let's say friendsAndMe[3] = "Graig".
4:46
I'll go ahead and
pop that to the top here.
4:52
And there we go.
4:56
So that's quite a bit to remember.
4:56
And I'm gonna bring this back,
so we can take a look at it.
4:58
That's quite a bit to remember.
5:01
Now, there's actually another approach,
that's a little bit better.
5:03
And it's actually really
similar to what we just did.
5:07
There's a helper class,
which provides a lot of helpful methods.
5:10
However, it's not readily
available like our system one was.
5:13
We need to actually import
the class to use it.
5:17
Now, if importing is new to you,
check the teacher's notes.
5:19
For right now, though,
let's just do this line.
5:22
Let's do an import here.
5:25
Erase this line.
5:27
I'm gonna say, import java.util.Arrays.
5:30
So that's gonna bring this
class into our scope here.
5:36
And there's a static helper method
off of this class called copy of, and
5:41
it makes copy of your Array.
5:45
So let's just use it.
5:47
So a String array called friendsAndMe2,
5:48
And we'll access this Arrays.
5:57
And off of that, we will access copyOf.
6:01
And copyOf this time is camel case.
6:04
And what that takes is, it takes
the source array, which is friends.
6:07
And you tell it how long you
want the new array to be.
6:14
This is a little bit different.
6:19
So we're gonna say that we want
a new array to be friends.length.
6:21
And we want it to be one more,
cuz that's what we're gonna add, so + 1.
6:24
So there you go.
6:30
It's basically the same thing,
it's a little more succinct.
6:31
Now, the important think to recognize
here is that in order to make a change
6:34
to an array,
you need to manage the copying of it.
6:38
Now, the arrays of value,
6:42
you basically just don't copy it
over to your new copy of the array.
6:44
Essentially removing it,
it's a little gross.
6:47
Check the teacher's notes for more.
6:51
So how'd that feel?
6:53
A little gross, right?
6:55
Well, the good news is
there's a solution to this.
6:57
As the old saying goes, if it hurts
when you do that, don't do that.
7:00
When using arrays, you actually
very rarely ever do any adding or
7:04
removing of elements.
7:09
This is because in practice,
7:11
there is another data structure
that is known as a list.
7:12
It is intended for
dynamically adding and removing values.
7:16
Now, list are out of
the scope at this course.
7:19
But we'll get to them very shortly
in an upcoming one, so hold tight.
7:21
Now, don't stress too much on
the mechanics of adding and removing.
7:25
It's super easy to do with a list.
7:28
Side note, the most common type
of list is called an array list.
7:31
And what you just learned how to do,
7:35
is exactly what is abstracted away
from you for adding and removing.
7:37
It manages the array growing and
shrinking, and it does all the copying.
7:41
If you just can't wait, check
the teacher's notes for more information.
7:44
Now that I bummed you out a bit
about arrays, let me bring back your
7:48
appreciation for them by showing off how
you can control their order by sorting.
7:52
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