Heads up! To view this whole video, sign in with your Courses account or enroll in your free 7-day trial. Sign In Enroll
Preview
Start a free Courses trial
to watch this video
Mapping allows you to transform each element to a new value.
Learn more
Related Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign upRelated Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign up
So we now have the ability to
take a collection of items, and
0:00
gather a portion of those
into another collection.
0:04
That works well, but a lot of time
in the reality of our code base,
0:07
what we are actually needing to do
is collect our subset of items, and
0:10
then produce different
representations of those same items.
0:14
For instance, maybe we want to populate
a drop down list in an application,
0:19
with just a name of those items.
0:23
Maybe we want to produce some mastered
detail representation on our webpage,
0:25
where we show a paginated
list of items as links.
0:28
And clicking on that link will
go to a more detailed page.
0:31
Streams add some real power, because
instead of making a separate collection of
0:35
the items,
you can actually transform the item
0:39
as it's passing through your
functional pipeline, in one fell swoop.
0:41
It's pretty powerful.
0:45
There is a bit of a naming
collision unfortunately, so
0:47
the method name that we're going
to use on our stream is named Map.
0:50
Now this unfortunately,
is the same word that we use for
0:55
our key value data structure, map.
0:58
Like a HashMap.
1:00
And I'm bringing that up now,
so you don't confuse the two.
1:02
The map is used to transform or
map one item to another one.
1:06
So, similar to how filter used
the predicate functional interface,
1:12
map uses the function
functional interface.
1:15
The single abstract method is named Apply,
1:18
and the signature is that it takes
a single value and returns a value.
1:21
Ready?
1:26
Let's put the functions on the map.
1:26
Bad idea to throw in another map there.
1:29
Sorry about that.
1:31
So the job output is
a little tacky currently.
1:33
So, let's do this.
1:37
Let's assume that we have
a page in our application,
1:38
where we want to give an example
of the jobs available.
1:41
So, let's show off three junior jobs but
let's format that a bit prettier.
1:44
So imperatively, let's do this.
1:50
Let's copy the imperative method
that we used to get the first three.
1:52
So let's copy that and then paste it.
1:56
So we're gonna copy it,
give us some space here, copy this.
1:58
And let's paste it here.
2:06
And, so now let's go ahead and
let's rename that
2:09
to getCaptionsImperatively, so
we'll pull a caption out there.
2:14
So getCaptionsImperatively.
2:18
And instead of returning a Job,
we'll return a String this time.
2:20
A List of strings, so
2:24
the caption instead of that long string
mess there, will make it prettier.
2:26
So now, this is complaining
we need this to be a String.
2:30
I'm going to go ahead,
I'm going to rename this refactor rename.
2:34
And we'll call this captions.
2:42
So let's make a pretty caption in here so
we'll say, String caption and
2:47
we'll do a nice format string,
so we'll do format, and
2:54
we'll say the company is looking for
a Job title and location.
2:59
That's a pretty nice caption.
3:04
So let's see.
3:08
Let's break that out a little bit.
3:09
So the first present s is
going be job.getCompany,
3:12
and the next will be job.getTitle.
3:17
And finally, we'll do a job.getCity.
3:21
Go ahead and supply add a caption here.
3:27
Cool, and
then it will return the captions.
3:29
So we now have getCaptionsImperatively,
so let's go use it.
3:34
Let's make sure it looks pretty.
3:38
So we'll say get.
3:41
CaptionsImperatively, we'll run the jobs
through there and let's take a look.
3:44
Beautiful, looks a lot better.
3:48
Now, let's see what this
looks like stream wise.
3:51
So that was here,
we'll get these three junior jobs.
3:54
And I'll move this down so
that we can keep them together.
4:00
Say.
4:03
GetThree, let me change
this to be captions.
4:07
GetCaptions.
4:13
Stream, and of course we also need to
change this to now be a list of Strings.
4:18
Now, check out this error
that we've created.
4:24
See how this list here knows that it's
supposed to be returning a String,
4:26
but in fact we are returning a Job.
4:30
So, we need to transform
our Job into a String, and
4:32
we do that with the map method.
4:36
So, let's go, let's Job, let's do it.
4:39
So we will come here.
4:40
So we're sure that we have a JuniorJob,
then we're gonna do a .map.
4:43
And you'll see that it takes a function.
4:47
So if we take a lambda,
we'll be getting in a job, a JuniorJob for
4:50
sure because it's been focused that way,
and
4:54
then we will just take that
same code that we did here.
4:56
What we did here the String format will.
5:00
Remember, it will
automatically return a String.
5:07
I don't have an extra one in there.
5:11
Let's bring this up here, and
then we'll close our map out, there we go.
5:16
And if we come up to our example,
our explore up here and
5:20
we change this to Stream,
let's see how we did.
5:24
Awesome, but we've duplicated this code.
5:32
And we've duplicated
this string format code.
5:35
Now, chances are we're not gonna
be the only ones needing it.
5:37
So, we could make a static method or
why don't we do this,
5:41
let's make a computed
property on our job object.
5:45
It seems like others
might want to do this.
5:48
So I'm going to grab this from here.
5:51
We'll cut this out, and
5:55
let's just say job.getCaption.
5:59
And it won't exist, and
then we go create it.
6:03
So we create for that.
6:08
And this is going to return a String.
6:11
And it's going to return
the String.format.
6:17
Get rid of these.
6:20
Jobs here, and put a semi-colon.
6:24
So now, we have.
6:27
What we need.
6:29
And over here,
it will return job.getCaption.
6:31
And let's go ahead, and
we'll clean up our imperative one.
6:34
So, what we'll do is
we'll say captions.add.
6:37
Job.getCaption.
6:45
There that is prettier.
6:48
So now if a job passes through and it is
a JuniorJob, it's gonna come into map.
6:51
Map is going to take a Job, and
it's going to return a String.
6:56
So it's going transform into
the String of the caption, and
7:01
then it's going to go through.
7:03
And that's the apply method of function.
7:05
It's going to take a type, and
it's going to return a different type.
7:07
Or whatever,
it's going to return something else.
7:11
So, it took in a Job,
it returned a String, and
7:14
we know that it's a String now.
7:16
So look here, there's an intention
action light bulb, let's go ahead and
7:18
see what it says, it says replace
lambda with method reference.
7:21
Good, we love method references,
let's do that.
7:23
Wow, [LAUGH] hat looks great.
7:26
But wait a second, getCaption on
the Job class doesn't take a parameter.
7:28
It doesn't take a parameter,
it's just empty, so how is that working?
7:33
I mean isJuniorJob, this method here,
7:39
isJuniorJob, that takes a Job and
returns a Boolean.
7:43
But, getCaption doesn't.
7:49
Now this is a common head scratcher, and
7:51
the reason is because it
depends on the type of method.
7:54
This isJuniorJob, this is static.
7:58
However, this getCaption
method is on a Job instance.
8:03
You can't execute the method
without an instance of a job.
8:09
Method references know that, so
8:12
they implicitly require an object
to execute the instance method on.
8:14
So really, you can picture any method
reference that is referring to an instance
8:19
method, like our getCaption method here
as a function that expects an argument
8:24
that is the instance of the type
that has that method on it.
8:29
So Job, it's always assuming
a Job is passed to it.
8:34
So, the Job that comes through
this filter is passed in here.
8:39
And getCaption is run on that Job.
8:43
Otherwise, how can the method run?
8:46
If that wasn't clear,
go ahead and rewind me a bit.
8:48
It's a little strange of a concept,
and it's okay for
8:50
it to take a while to sink in.
8:53
You'll see this quite
a bit when map is used.
8:54
Especially when trying
to just get a getter,
8:56
transforming an object to
one of its properties.
8:59
Actually, that is known as
method reference inference, and
9:02
we can cross that off our parking lot.
9:05
So let's go over there.
9:07
So Method Reference Inference,
let's go ahead and
9:08
we will do a Strikethrough, perfect.
9:11
So, let's take a quick break and when
we return we'll talk about what happens
9:15
when you encounter a value that maps
to a stream from within your streams.
9:20
Yo dog,
I heard you like streams in your streams.
9:26
You need to sign up for Treehouse in order to download course files.
Sign upYou need to sign up for Treehouse in order to set up Workspace
Sign up