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Welcome back.

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And this video we're going to cover abstract classes and abstract classes are a little bit similar to

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interfaces, but they are still different, so let's look at them.

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An abstract class cannot be instantiated.

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That's the same for interfaces.

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You can create an object of an interface.

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However, you can inherit subclasses from an abstract class and the members.

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So properties and the methods of an abstract class are not abstract unless you explicitly use the abstract

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key word to make them abstract.

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So let's have a look at an example.

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So I have this abstract class memo here, which has a constructor here, a primary constructor with

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a name and origin and the weight.

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And these are concrete, non abstract properties.

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All right.

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So the three that we created here name origin and weight.

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They are non abstract.

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But this one here is so I create a property called Nextbit, which is abstract, so it must be overridden

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by its subclass.

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So if the class inherits from Memmo, then it will need to implement those.

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So it needs to override those.

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The same goes for these methods.

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You can see we can create methods without a body.

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So we just add the abstract key word abstract fun and make run an abstract function.

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The same goes for a brief here, and then we can also create concrete so abstract methods in such an

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abstract class.

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So we're not limited to abstract methods.

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We can also just create a method such as display details that we have here, which will just display

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the name origin, weight and speed of the mantle that we're creating.

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So now let's look at this class that inherits from mammal.

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It has also the relation to weight and the max speed.

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It has also the name the region, the weight, but it also overrides the max speed.

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So this abstract property that we created here.

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So now our human class will overwrite that.

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Then we also override the run function, so we implement the run function with our own code for all

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human.

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We do the same for breath.

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And then we have another class called Elephant, which does the same thing, so it overrides the next

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beat and is implements its own code for run and it's uncalled for breath.

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Of course, they could both have a bunch of more functions and a bunch more properties.

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But I'm going to keep it super simple in this example.

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All right.

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And now we can go to our function May, where we create an object of human, which is called Dennis,

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was born in Russia as a weight of 70 kilos and a speed of 28 kilometers per hour, even though I'm not

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sure whether that is true.

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So then we have the elephant, which is called Rosie.

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It's from India that weighs 5400 kilos, and it has a speed of 25 kilometers per hour.

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So as it seems, I could outrun Rosie, but depends on how long I can do that.

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So then if we call Human Run, it's going to execute the run functionality for the human, says runs

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on two legs.

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And if we called the same for the elephant, it will say runs on four legs.

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So let's just test it.

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By the way, we can also see that we are breathing so the human caused breathing and the elephant calls

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breathe.

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So there we are.

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We see it says runs on two legs, runs on four legs.

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Breathe through mouth or nose.

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Breathe through the trunk.

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So I'm not saying breaths here because it just needs to breathe.

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So I force it to breathe.

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All right.

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Chromatically.

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Still correct?

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All right.

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And that is already the major part about abstract.

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And let's have a try to create a mammal here.

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Memo, and I'm going to say, Dennis, it's a memo, so here then, is Russia.

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Then weight of seventy point seven and finally a speed of 28.0, so trying to do that, you can see,

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cannot create an instance of an abstract class, so you cannot do that.

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You can, however, of course do that with classes that inherited from it.

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So our human and our elephant, which are both classes that inherit from our abstract class called mammal.

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All right.

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So what is the difference between an abstract class and an interface?

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Because you might say, all right, there must be pretty much the same right or they're very similar

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to each other.

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Well, the thing is that an interface cannot hold state and we can implement multiple interfaces, but

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only one class.

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So when implementing an interface, as we have done in the last example here where we implemented driver

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pool, we could implement multiple interfaces here.

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We're not limited to just one interface, but we can only inherit from one class.

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Also, classes have constructors, so even an abstract class has a constructor.

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You can see that our interface doesn't have a constructor here.

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And the faces cannot hold fields, so basically abstract classes can have everything that interfaces

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can add.

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Additionally, they can have fields and constructors.

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Therefore, we can properly hold state and abstract class.

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All right.

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So in the next video, we're going to check out typecasting.

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So see you there.