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Hey, Jay, I saw that blog post that
you did about the Go interfaces.

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>> Cool.

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>> As a Java teacher,
I looked at some of the code, and

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some of it looked familiar and
some of it didn't.

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You got some time where
we could sit down and

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take a quick look at the differences here?

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>> Sure.
I know what you're on to here.

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There's a lot of similarities
between Go and Java.

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Yeah, let's take a look.

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>> Cool, so this is the workspace
that you have up here, right?

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>> That's right.

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>> Okay, so package pets.

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>> Right.

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So, this is packages in Go.

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They're just libraries,
basically, bits of reusable code.

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I forget what the Java term is.

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>> Jars.
>> Jars, right, yeah,

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roughly equivalent to jars in Java.

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>> And it's in a folder called pets,
and that makes it the package?

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>> Yes, that's right.

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But more importantly,
it's got this package pets declaration,

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right here at the top.

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>> At the top?

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>> Right.
>> Okay, it doesn't matter,

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the folder structure
doesn't really matter?

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>> The folder structure helps,
the tools that Go uses when compiling

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software rely on that
folder structure too.

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>> Cool.

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And then you're importing FMT,
which I'm assuming is format?

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>> Which is format.

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This is another package.

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The import statement
imports another package, so

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that you can use the things it
contains in your current package.

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>> Gotcha.
And that's like a base.

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Library FMT is,
from Go the language is full.

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>> Yes, yes.

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That is part of the Go standard library.

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So if Go's installed on your computer,
the format package is available.

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>> Okay, awesome.

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So, next we have type Dog struct.

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>> Well here, let me run
the program real quick here, and

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we'll demonstrate what it does.

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So basically,
it creates a couple of objects.

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One is a dog object here.

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Fido walks across the room and sits down.

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And then we've got a cat object here.

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Fluffy walks across the room and
sits down.

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>> Okay, cool.

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So first you're defining a type.

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>> Yes, which you can think of
as kinda like a class in Java.

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It works a little differently.

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It doesn't map exactly one to one, but-
>> Okay.

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And the way that it's not mapping is,

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it looks like you're not
defining the method inside.

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>> That's right.

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You actually have your type
definition up here, and

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as soon as you run even just this portion,
you have a Dog type that you can use.

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It's gonna have a name attribute
that you can assign a string to,

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because that's its declared type,
a string.

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And it's gonna have a breed type
that you can assign a string to.

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>> Instruct is kinda like,
that's like the structure?

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Is that what-
>> Right, that's a holdover from C.

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There's a lot of similarities between
Go and the C programming language.

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And it basically lets you stick a bunch
of different fields of different types

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together.

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You can mix strings with integers
with whatever else you need.

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>> Can you have a type without a struct,
that word there?

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>> Yes, actually.

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I could take a plain old floating
point number, and make that, and

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wrap that in a type if I wanted to.

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>> Gotcha, so you're saying,
this is a type of Dog that is a structure.

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>> Yes.

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>> Okay, gotcha, cool, awesome.

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I'm assuming that func is function.

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>> Yes.
>> And

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now the switchy bit there of the d Dog,
what is that doing?

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How are you-
>> This is a little interesting.

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This is a bit of ghost syntax that
is not typically in other languages.

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So I can declare this a function,

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which then becomes
a method on the Dog type.

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I can declare that anywhere in
the pets package that I want.

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It could be way down at
the bottom of the file.

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It doesn't have to be nested inside
this type declaration up here.

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>> But this here is kind
of the defining the scope.

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So d is the name of the variable
that you could use?

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>> Correct.

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>> Correct, and this is just idiomatic Go.

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It's a style of writing Go
that a lot of developers use.

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So I just happen to
name my Dog variable d.

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And then I can access
that Dog variable here

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within the body of the function,
the method.

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>> So it's kinda like setting this.

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>> Yes, exactly,
there is no concept of this in Go,

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which I know this is used in Java.

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>> Yeah.
You decide what this is called, here.

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>> And that's what that's defining.

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>> Yes.

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The technical term for this is,
this is the method receiver.

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This is the object that
the method is going to act on.

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>> That makes sense.

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And then, I'm noticing that,
so these are method names.

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Are they called methods now?

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Are they called functions still?

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>> All methods are functions.

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Not all functions are methods.

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>> Gotcha.

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So, the methods have a capital letter.

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They start with a capital letter there,

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that's a little bit different
than what I'm used to.

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>> Right, that's an important,

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you know how you might declare
things to be public in Java?

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In this you have entities that
are exported from a package,

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and entities that
are un-exported from a package.

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>> Okay, gotcha.

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>> If you wanna do encapsulation,
you keep something un-exported.

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And then it can only be accessed by other
functions and methods in that package.

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>> Okay, so this makes it exportable?

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>> Naming it with a capital letter as its
first letter of its name, that exports it.

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That's the whole syntax right there.

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>> Wow, that saves a lot of-
>> A lot of typing.

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And it makes it super obvious, whether
something is public or private, so yeah.

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>> Right, interesting.

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I would not have guessed
that just by looking at it.

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Thanks for sharing that.

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Okay, so I understand the next one.

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Here's a method that, it's a publicly
exported method called Sit.

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>> Yes, called Sit.

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>> And it's gonna take the Dog
structure that you passed, and

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it's gonna pull the name out,
okay, awesome.

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>> Right.
And I'm print line's exported.

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That's what the capital
P's about on the format.

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>> Yes, correct.
>> Okay, awesome.

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>> Applies even on the format package.

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>> Cool, all right, okay, and
now we're gonna come down here, and

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we're gonna do a new type called Cat,
which is of type structure.

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It's extending a structure
that has a name and a string.

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Okay, awesome.

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And it walks, okay.

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And it sits and it purrs.

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>> Yes, and the interesting thing to note
here, between the Cat and the Dog types,

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is that both Dog and
Cat have Walk and Sit methods.

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>> The Dog can't purr.

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>> Right.
The Dog can't purr and

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the Cat can't fetch, but
they can both walk and sit.

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>> Okay, okay, awesome.

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So now, in my world, I would say that
that sounds like there's an interface,

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and that's what your article is about.

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>> Absolutely.
>> So let's take a look at that.

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>> And Go interfaces accomplish many of
the same things that Java interfaces do.

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What's awesome about them is you
can declare interfaces anywhere.

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You don't have to be the owner of the jar,

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the package, that the classes
were originally declared in.

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I wrote this demo.go file.

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It's totally outside the pets package.

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You'll notice it's part of package main.

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And here is where I declare
my FourLegged interface.

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I don't have to go in and
touch those Dog or Cat classes.

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Right, I just declare that anything,
any type that happens to have Walk and

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Sit methods satisfies
the FourLegged interface.

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>> Gotcha, so
that's like duck typing in Python or Ruby,

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I think Ruby called it duck typing.

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>> Yes, it's duck typing at compile time.

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>> That's awesome.

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>> Yeah this is super cool,
and this part is unique to Go.

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>> Okay, awesome.

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That's really neat.

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So now, you basically have a type.

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So that's what you're saying here,
is that this takes a FourLegged type, and

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that type is one that you just
declared here, with an interface.

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>> Yes, basically.

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So, my demo function here, and
this is not a method, by the way.

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You'll notice it doesn't
have a receiver over here.

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My demo function here
takes a FourLegged object,

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something of the FourLegged type.

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And that can be of any type that
happens to have Walk and Sit methods.

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>> Walk and Sit.

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Which you define right there on line five.

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>> Yes, yes.

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So I define the interface up here,
and I use it down here.

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Because whatever value I've passed
in is of the FourLegged type,

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I know for sure that it has the Walk and
Sit methods.

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Otherwise it wouldn't have compiled.

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So I could call Walk and Sit.

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>> And here's your main method which
calls, just like Java, the main method.

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>> Exactly, it runs automatically
when the program runs.

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So I create a Dog and a Cat.

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A Dog named Fido with a breed of Terrier,

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a Cat named Fluffy with
a breed of Siamese.

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>> What's the little face thing,
that little :=, what's that?

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>> That is a short variable declaration.

191
00:09:06,640 --> 00:09:13,954
So this is going to be equivalent
to if I said var dog, Dog.

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>> It's the type,
it's doing type inference.

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>> Right, right, exactly, and yeah.

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>> That's coming soon to Java,
we're getting that soon.

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>> Cool, cool.

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00:09:21,853 --> 00:09:22,680
>> It's exciting, yeah.

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>> And yeah, same thing here for
the Cat, it creates a variable.

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Because this code here
creates a value of type Cat,

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this will create a variable
that holds type Cat.

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>> Awesome, and then you're calling
the demo method which takes FourLegged,

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which is the interface
that you created Awesome.

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>> It can take either a Dog or a Cat,
because both of those types satisfy

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the FourLegged interface
that we declared up there.

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>> Thanks, I was reading your blog post
looking at the code, didn't fully get it.

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I totally understand
the power of this now.

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This is awesome, man.

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Thank you for taking the time
at lunch to do this with me.

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>> My pleasure.

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