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We Java developers have been focussing heavily on imperative style, but declarative functions are available. Let's get used to the difference.
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In order to start our journey on thinking
functionally, one really important concept
0:00
to dial in on is the difference between
imperative and declarative coding styles.
0:05
The imperative style focuses specifically
on how to perform the operations.
0:11
Typically at a low level, taking
the system it is running on into account.
0:15
Declarative style focuses more on
0:19
what specifically you
are attempting to accomplish.
0:22
The imperative style is definitely more
difficult to read as there are more lines
0:26
of code and declarative is usually
very clear as to what is happening.
0:30
Let's walk through an example one
that you've probably already seen and
0:35
it will help demonstrate the differences.
0:38
For those of you with people in your
lives that have food allergies,
0:41
you know how difficult it is to
find something that they can eat.
0:44
My daughter has an egg allergy.
0:48
It's a bummer.
0:50
It's pretty horrible.
0:50
It seems like everything has eggs in it.
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Cupcakes, taffy, ice cream.
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You know, everything a kid likes to eat.
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Now as a parent,
I am constantly needing to check
0:59
a list of ingredients to see
if it indeed contains eggs.
1:01
Let's write a quick simulation
of this in code, in both styles.
1:05
So here I have the current
version of IntelliJ IDEA open and
1:10
I'm just going to make a new project.
1:13
I'm gonna click create new project.
1:15
Cool.
And I'm gonna just leave
1:18
the defaults here.
1:19
I'm gonna click next and I do want
to just make a quick command line.
1:20
Let's go ahead and call this exploration.
1:26
Okay so that's set everything up for us.
1:31
Let's go ahead and let's start
write our code here and suggest it.
1:33
Oops I indent with two space
I'm using the Google style.
1:39
So we'll say list the string and
those are ingredients, right?
1:42
And we'll make a new Arrays.asList.
1:50
And we'll just go ahead and we'll
enter those guys, that's what we want.
1:53
So let's see, we have flour,
what else do we have?
1:59
We've got some salt.
2:03
Delicious cupcakes here,
some baking powder, butter,
2:06
I don't know how to cook really,
eggs and here comes the milk.
2:11
All right, and
we will close off with a semicolon here.
2:16
Okay, so let's solve our
issue of checking whether or
2:22
not our ingredients has eggs, okay.
2:25
I'm gonna go ahead and give us more space.
2:28
Get rid of all the windows that are open,
Here we go.
2:30
Okay, so first thing's first.
2:33
We'll need a variable to store our state,
right?
2:35
So we'll wanna say boolean hasEggs,
and we'll just go ahead, and
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we don't know yet.
2:42
We don't know if it has,
and so we said false.
2:42
And now, what we wanna do is we wanna
loop through each of those ingredients.
2:45
So we can do that by starting
to define a for loop, right?
2:49
So we'll say for and
now we need another one of those temporary
2:51
variables to store in the loop where
we're gonna currently process.
2:56
So we'll say int i for index,
kind of standard thing there, right?
2:58
And we'll make another temporary variable,
right?
3:05
So now we've got 2, we've got hasEggs and
i, two temporary variables.
3:09
And now we wanna make sure that we
go through all of the ingredients.
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So we wanna see if i is
less than the ingredients.
3:16
And we wanna see if it's as
long as it is which was size.
3:20
And we've gotta remember
that it's zero-based, right?
3:24
It starts at zero, so it's gotta be less,
3:27
not less than or equal to,
always have to think about that there.
3:29
And then we're gonna go ahead and
we're gonna increment.
3:32
Then we need to check if
that specific ingredient, so
3:35
what we need to do is we
need to pull it out, right?
3:38
We need to pull that one out.
3:41
So we'll say String.ingredient
equals ingredients.get, and
3:42
we're gonna pass in that index that we've
been running through the loop with, right?
3:48
Okay so then we wanna see does this
ingredient that we pulled out,
3:52
does that equal eggs, right?
3:59
Okay, and if it does we will for sure.
4:06
We'll say hasEggs,
we're gonna set that to true.
4:09
You know what since we
did know it hasEggs,
4:13
we don't need to look at
the other stuff right?
4:16
We can finish here, so
we can just go ahead and say break.
4:18
Let's look at this code here for a second.
4:23
That's a lot of code and brainpower needed
just to understand what's happening.
4:25
Now we've done this a lot and
probably see code like this quite a bit.
4:30
So it's probably feeling kind of familiar.
4:34
But just imagine for
a second if you hadn't.
4:37
There's a whole lot of what is
this doing going on, right.
4:40
So now that we have our variable,
we can go ahead and see if it has eggs.
4:44
Gotta break the news to my daughter and
say, sorry sweetheart.
4:50
It has eggs.
4:56
Sad face emoji.
4:58
Now of course there's been
some evolution in looping.
5:00
So we can of course clean up this
cognitive overload a little bit of all
5:04
these things, right?
5:07
So we can,
we can just go ahead we can say for
5:08
the ingredients in ingredients, right?
5:11
This enhanced four loop.
5:17
So we can lose this index cause each one
is gonna have a specific time, right?
5:19
We don't need to know how
long the list is anymore.
5:24
This reads nicer, but
it's still an awful lot
5:27
when all we really want to know is,
does ingredients contain eggs.
5:31
We're still saying give me
each of the ingredients and
5:36
I'll tell you when to stop.
5:39
So the more declarative
way to ask this question,
5:40
is just to ask the question, right?
5:44
So, let's do this.
5:47
If ingredients.contains,
5:49
Eggs, Right?
5:55
We don't need this.
5:59
And there's nothing new here.
6:01
Contains has been around since
the collections framework was introduced.
6:03
But it's declarative.
6:07
Look at how it explains what
it is that we're trying to do.
6:09
It's not specifying how.
6:12
Now this can be optimized
efficiently behind the scenes.
6:14
And this is how it works
in the real world, right.
6:17
If I'm at a restaurant, and
I wanna know about a possible cupcake for
6:20
my daughter, I don't go, can you please
tell me every ingredient in this cupcake.
6:23
Now don't worry I tell you
when to stop listing them and
6:28
the case that I happen to hear
what I'm concerned about, right.
6:30
I simply say, does this contain eggs?
6:33
And the waiter or
waitress has their own method of knowing.
6:36
How they get the answer isn't really
something I need to concern myself with.
6:39
I'm really just concerned with
the answer to my question.
6:43
So ready for it?
6:47
This is the start of
the shift of your thinking.
6:48
Functional programming is declarative.
6:51
It'll take a little bit of shifting
in you programming mind, but
6:53
we do it in real life.
6:56
So it should feel more normal than how we
already force our imperative mind to work.
6:57
One more example of how this looks
in something you've probably seen.
7:03
Is SQL or Structured Query Language.
7:07
It's all declarative.
7:10
Even if you haven't seen SQL before,
this is how you write the statements.
7:11
The language allows you to declare
7:15
very specifically what you
would like to get back.
7:18
In this case, it's some specific fields
like first name, last name, and email.
7:21
For a student with the id of 123.
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Notice how it doesn't specify how.
7:28
This allows the database
to do whatever indexing or
7:32
caching it needs on whatever platform that
implements the language specification.
7:35
Let's stay in the declarative mindset and
introduce our first function.
7:42
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