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One of the more challenging concepts
to grasp as you are just picking up

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functional programming is dealing
with how to create loops.

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Almost everything we do in programming
deals with a collection of data and

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we must process that collection,
typically element by element.

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Let's take a quick look at
the most basic way to do this and

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further use it to create
our first function.

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Sound good?

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Okay so
we have our collection here already.

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So for simplicity's sake,

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let's assume that we want to print out
each of the ingredients on a single line.

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So how do go about doing that
with our imperative style?

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Well, to start, let's just go ahead and
I'll reuse some of this code here.

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All right, so
we don't need this line anymore.

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And I'll just go ahead and comment this.

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And we'll just loop through each of
the ingredients and we'll print it, right?

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We'll print that ingredient
that's coming through the list.

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Wanna get rid of this too, here, right?

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That's how we would do it imperatively.

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List them out on each line, right?

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Okay, so all classes that implement
Iterable, like our list class here, right?

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It implements Iterable, that's why
we're able to loop through here.

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Well they also have a new
method named forEach.

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So let's explore that a little bit.

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So if I come in here and
I say ingredients.forEach.

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And I'm gonna open and close
the parens and then I'm gonna press F1.

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Okay, so
if we take a look at the bottom here,

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it says this has been around since 1.8,
which is Java 8, all right?

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So what this does is it takes
an action type of Consumer.

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All right, so
it takes an action of type consumer.

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So here it says, it performs the given
action for each element of the Iterable

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until all elements have been processed or
the action throws an exception.

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Hey, that's exactly what we need, right?

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So our action is going to be,
print out each ingredient.

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So let's take a look at what
this consumer says here.

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Okay, so it's in the java.util.function
package and it's a functional interface.

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We'll take a look at these
here more in detail.

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But for now I just want you to think about
a functional interface as an interface

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that has exactly one abstract method.

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It's a SAM, a single abstract method.

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So it represents an operation that
accepts a single input argument and

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returns no result, cool.

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So it's unlike other
functional interfaces,

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Consumer is expected to
operate via side effects.

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That is another one of these terms
that we are gonna talk about later.

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And I'm gonna put it in the parking lot,

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so know that we don't
need to worry about it.

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Don't let these new terms overwhelm you.

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So, we'll come over here.

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Here's our parking lot.

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I already added Pure from before.

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So I'm gonna go ahead and
I'm gonna add side effects, perfect.

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Okay, so
we need to add a new consumer, and

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I am going to let our IDE
give us a little help here.

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So I'm gonna say, new Consumer,
and I'm going to press Tab.

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And it's gonna go ahead and
auto-complete that.

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And because it has
a single abstract field,

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it's gonna automatically pop out
what we need to override, right?

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Cuz it knows that's the only
thing we need to do.

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And this is pretty standard, right?

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This is how you would actually
do things before Java 8.

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There's nothing new here.

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A single abstract method, this is
a common concept for these classes.

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You would say new and
it would tell what it wanted to override.

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So inside this method here I think we
wanna do just like we did up top, right?

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We wanna just print out s.

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I wanna put a semicolon
here at the bottom.

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Okay, so
I'm gonna comment out this here and

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I'm going to go ahead and give this a run.

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Awesome, it works.

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So this is the declarative
way to do things.

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Now I'm not sure if you saw it, but
when I came in here and I chose Help.

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It says, unless otherwise specified
by the implementing class,

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it's gonna go ahead and
order the iteration.

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What's cool about that, is that this is
a great example of the declarative when.

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By stating what we want,
but not how to do it,

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we take that problem out of our hands,
right?

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Before we were saying, we were
talking about how to loop through it.

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This is putting it in
the hands of the iterator.

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So that assumes that the Iterable here

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knows how best to loop
through its own items, right?

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It's none of our business.

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So once more,
the forEach method handles the looping.

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And when it has each single item, it calls
or invokes this new consumer here, right?

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That gets instantiated, and it passes
that value into the accept method.

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So notice how here it has a specified
parametrized type of string?

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So you can read this as
it's a consumer of strings,

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and that's just what we did, right?

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We provided a forEach method,
a function that consumes strings.

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Now if this seems a little clunky to you,
you're totally right.

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Up until Java 8, this is the way to do
things with single abstract methods.

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You instantiate a new, temporary,
anonymous class, just like this.

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It's pretty ugly.

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I can think of exactly zero people
who thinks it looks pretty.

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[LAUGH] So you'll see a ton of this in UI
event handlers, like your Android apps or

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Java effects.

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So you wind up sifting through
a bunch of code like this.

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Just for a single line, a single method.

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Well, here's some great news.

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The language has introduced a new syntax
to make this much more prettier and

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more succinct.

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Let's make things pretty
right after this quick break.