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Welcome back.

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And this video we are going to look into interfaces and interfaces are a feature which allow us to extend

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the functionality of classes.

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So, for example, our car class could extend the functionality of an interface that has certain features,

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and let's go ahead and just create such an interface.

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So if this interface prepared here, which is called drivable, it has one value, which is a double

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value called max speed.

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Then it has a function called drive of type string, so it's going to return a string.

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But there is no implementation of the function, so it just doesn't have a function.

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Body only has to head of the function.

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And then we have a brake function, which actually has a function body as well.

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So an interface may provide a default implementation of some or all of its properties and functions,

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but it doesn't have to.

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And if a property or function has a default implementation, the class that uses this interface may

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choose to override it.

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What does that mean?

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Well, let's say we have this interface drivable right in there.

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We have these functions and this property.

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Now, if our class car decides to extend the functionality of the drivable interface.

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Then it needs to implement the property that is created in the interface, but is not implemented.

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So here we need to go ahead and say, wow, max speeds of type double should be in there.

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And because of your overriding something, so we are overriding the next speed from here.

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We need you at the keyword override that by itself doesn't make our class happy yet, as you can see,

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it's still not happy, it's still complaining something is still missing.

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And that is because we haven't implemented a member called Drive.

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So here's this drive function, and because it doesn't have a function body, we need to create one

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in the class that extends the interface.

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So we need to go ahead and create a drive function.

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So you can see we are already creating a drive function.

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Why is it still complaining?

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While this drive function doesn't follow the same structure.

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So you can see the head of the function doesn't return a string.

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So let's go ahead and actually do that.

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So override fun drive.

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And once again, fun drive.

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And you can see it gives me some help here, the idea.

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But it requires me to implement something.

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So I need to add a return statement here so I can, of course, go ahead and return.

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Driving the interface drive.

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OK, so that's just going to be the text that I return.

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Alternatively, what you could have done is you could have done it in a shorter way, for example,

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just saying.

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That it's going to return whatever the string is.

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OK.

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So both of them are fine.

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I can see this version is also fine.

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It's just a shorter version of this here.

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All right, so now we have multiple drive functions, and that is already starting to become confusing,

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maybe.

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But no worries.

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The one drive that we have here is actually the drive that comes from our interface that we are extending,

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OK?

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So.

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You can see here this drive.

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We implemented it and suddenly our car is not complaining anymore, then going back to our other drive

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that we have created, it's an open function drive, which is, so to speak, new.

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So this is the lowest level in which it's defined and that is in our class car.

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So this is another drive which we, of course, can override from other classes that inherit our car.

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Now, if electric car wants to inherit from car, it also has to follow the structure of our.

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Interface here.

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So let's also add.

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A Max Meteor.

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And let's use it here as well.

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Now, of course, our function drive that we had in our electric car that we created doesn't work anymore

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because now this drive.

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Is trying to override, so it thinks that we are trying to override the drive function.

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Which comes from here.

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This one.

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OK, so we also need to follow the structure of drive.

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Now, let's add the over right keyword, but then we also need to return the string type.

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And of course, here return.

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And we can use the same structure that we had before, or at least the same string that we are returning

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here.

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So what is an interface?

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Well, an interface is essentially a contract that a class may choose to sign.

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If it does, the class is obliged to provide implementations of the properties and functions of the

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interface.

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OK, so we see that here because our class car is extending the interface drivable.

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It's now obliged to implement its.

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Variables and functions or its properties and functions.

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The properties we did that by using it in the primary constructor and to function well, we implemented

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it directly here.

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You can also see, because our break had already an implementation, we never had to use it, but we

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can of course use it.

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We can even use it and a subclass here.

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So in the subclass of our car, in the electric car, we can still go ahead and implement brake so it

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can see override brake.

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And you can see this super keyword and that is to call the brake function of the superclass.

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And because our superclass, which is our car, didn't implement the functionality for brake, it will

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use the one from the interface.

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So it will just use this brake here.

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So now, if I go ahead and call break, and by the way, now of course, we are missing things when

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creating those Audi A3 and so forth.

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And I think it was speed, right?

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So 200 kilometers, I'd say, is the top speed here and 240 is going to be the top speed of the electric

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car and the errors disappear.

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But now which driver would be called when we called this drive?

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Well, if we look at it, one requires a distance, the other one doesn't.

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So it's going to call this drive.

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And then this year is, of course, going to require the distance, so it's going to call.

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This drive here.

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All right, now, let's say we want to call the brake function here.

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All right, let's just test it.

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Let's go ahead and use Tesla as the brake.

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And let's use already a three break.

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And here, of course, it's a break.

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So I'm going to get rid of the other calls here just to keep it simple.

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And let's actually run it here.

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And we will see that it says that drivable is breaking in both cases, so the case for Tesla and Order

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three, even though while we're already a three judges of type car never actually implement that the

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brake function?

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OK, so has it because it automatically inherited from brake.

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Now.

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What if we implement something here, let's say print line break inside of.

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The electric.

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Let's test it again.

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So you can see the drivable is breaking, and that is our test that.

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And then it says break inside of electric car is also our Tesla, and then we have this drivable is

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breaking, that's our audio that breaks.

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So what is happening?

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Well, when using override fun brake.

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So Super Brake, what happens is that the super function brake, so whatever was implemented for brake

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in the interface or in the superclass will be used or will be executed.

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And then on top of that, our own code that we add afterwards will also be executed.

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OK, so that is what this super dart method name does.

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But away subclasses of a class that implement an interface in this case are electric car are also considered

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to be implementing the interface.

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OK, I said it in a certain way, but I've never said it specifically like that.

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OK, so just so you know, our electric car is also implementing the interface called.

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Drivable.

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Another site notes an interface can inherit from another interface.

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So, yeah, in case you want to create your own interfaces that inherit from other interfaces, you

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can do so.

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So why would you even use interfaces?

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Well, interfaces are super useful when you have certain functions in mind that you definitely want

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to be implemented later on and also already know the properties of of a class that you want to then

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have implemented, but you don't want to implement them directly.

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You just want to prepare everything and you don't want to, for example, create a specific body already

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for a function.

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All right.

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So this is the introduction to interfaces, and in the next video, we're going to look at abstract

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classes, which are similar in a certain way to interfaces.

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But they have some differences in the nuances as well.

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So let's have a look at those.