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So it's time to turn our attention to a concept that might be entirely new to some of you and some of



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you may have seen a similar concept in other languages.



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And that concept is interfaces and interfaces can be challenging to people the very first time that



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they encounter them, but they're really not that hard.



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And I think the easiest way to demonstrate what an interface is is to actually build one and implement



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that implemented a couple of times.



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In really simplest terms, an interface is nothing more than a kind of contract.



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An interface says, OK, any type that uses this interface has to follow these rules.



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So let's let's let's just actually build one.



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That's the simplest approach.



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So I'm going to start by defining an interface and I define an interface in much the same way I do,



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in much the same way I do.



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One of the custom data types that we looked at a couple of years ago, and it's you start with the word



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type, keyword type, and this is going to be a type of some name.



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And I'm going to call my interface animal.



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And it is of type, not struct was as we saw when we were creating the user type.



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It's a type interface.



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And then I have curly brackets and inside I have this signature for that interface.



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And all the signature is, is it lists the functions that an interface, an interface of type animal



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must have.



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So anything that is of type animal must have, it must implement some of some functions.



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So I want to say one function that every animal type must have is says, what does the animal say?



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And it's a function that takes no arguments and it returns a string.



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OK, and I'm going to give it a second required function.



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If you create a type animal, it must implement the says function and it must implement the number of



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legs function, which takes no arguments and returns in it there.



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I have just defined an interface.



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I haven't done anything with it, but I've defined it.



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All I'm saying is if you create a type that needs to satisfy the animal interface, it must have these



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two functions.



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So let's create a variable.



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I'll create another type that I'll call this one dog and it's going to be a struct, just the same way



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as we do the user type.



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And it only has one required field or one field in there and it will be a name of type string.



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So I'm going to store the name of the dog in that string, OK, whatever it may be.



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And let's put it in here as well.



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Breed string.



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So the dog type has a name and a breed and then I'm going to create another one type gorilla, which



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I better spell.



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Right.



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And it too is a struct.



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And similarly to the dog, it has a name as a field and then it also has color.



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I'm not sure what color gorillas gorillas are, but I'll pick one in a little while and it's going to



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be it's going to be a string.



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And it also has I need something else.



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Number of teeth.



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And that's neat.



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OK, so I've now defined one interface type and two struct types and now I have my main function.



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So what I'm going to do in here to start with is create a dog and that dog will be of type dog and I'll



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fill out it's necessary fields.



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So my dog has a name name and the name will be Samson and the breed will be German Shepherd.



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Shepherd.



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OK, so I now created this dog variable.



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So this dog variable is of type dog, but it does not yet satisfy the interface of type animal because



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I've not created these two functions.



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So let's do that right now.



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Underneath here I will create a function and I will call it give it a receiver type and the receiver



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type can be whatever I want.



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I'll call it D for dog, but it has to have the name says and it has to take no arguments and it has



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to return a string.



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And all I'm going to do is return Wolf.



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Then I'll create another function to satisfy the second requirement for the animals type it again will



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be of the receiver type dog which I've defined up here, and it has to have a name and the name is Number



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of Legs and it takes no arguments and it returns in it.



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So I'll return for.



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All right.



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So now I've created this dog variable.



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I've implemented the two things necessary to make it off type animal, but I still haven't used the



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animal type.



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So let's come down here and create another function func and I'll call this one Pritt info.



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OK, so print info and it takes an argument, a of type animal.



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And it doesn't return anything, it just prints information logged print line, this animal



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says, and then I'll call the method that's built into that type animal, a dart says, and has a number



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of legs, hips, legs.



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All right.



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So I've done this.



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Let's just pass this dog, which is of type dog, not type animal.



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Let's pass it to this function print info, which requires a type animal.



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So I can just say print info dog and look, no errors, no errors, even though this function is expecting



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it to get a type of animal and I'm passing it a type of dog that works and it works because I may dog



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my variable dog, satisfy the requirements for the interface by having both of the required methods



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which are specified up here.



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Now let's try this.



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Let's create a new variable called Gorilla, and it's a type gorilla and I'll fill out its fields.



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So Gorilla has a name name King Kong, and it has a color which I'll call black, which I'm not going



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to use, but I'm putting it in there just to show that it's a different kind of type and number of teeth.



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I have no idea how many they have.



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They're going to go with 32 because that's a good even number and you usually have an even number of



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teeth.



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So I've created this gorilla type.



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It's defined up here.



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And if I try to pass that, so if I try to pass it, you print info, print info and I pass it gorilla,



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it's getting an error and the error is cannot use gorilla type gorilla as type animal type does not



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implement animal as some methods are missing and then it links a list to fix that.



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I can come down here, copy both of these that are of type dog paste them and change them to type gorilla



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and type gorilla and gorillas have two legs and I have no idea what the gorilla says.



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So but now that I've implemented the necessary methods for the type animal on my variable gorilla or



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my type gorilla, now I can pass that.



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And if I actually run this.



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So let's go over to my terminal window.



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And run it.



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Go run, Mingo.



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And it should work, this animal says Wolf and has four legs.



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This animal says grunts and has two legs.



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And I say, I have an extra parentheses in my function probably.



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And there it is right there.



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But that's all there is to it.



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Now, the great thing about this is that I can now create more generic functions that accept a type,



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an interface type, rather than a specific type like dog or gorilla.



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I can just create one that accepts animal and that's all there is to interfaces.



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The key thing to remember is that once you define an interface, anything else can implement that interface



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just by implementing the required methods.



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That's all you have to do.



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There's nothing else.



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There's no implements.



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Keyword there's no explicitly declaring that it implements a particular interface.



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You just implement the required signature and you are good to go.



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And this will be incredibly useful to us as time goes on.