All language subtitles for 9. [Dart] Classes and Objects

All right.
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So now that we've seen classes and objects in action and we've used them a little bit, it's time to do
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a deep dive and understand exactly how classes and objects work in Dart.
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So firstly a class is simply just a blueprint and it's used to create all of the things that we're going
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to be showing in our app and the user is going to be interacting with. So essentially, everything that
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makes up our app is going to be created using a blueprint that's called a class.
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Now what exactly is an object then?
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Well when we take that blueprint and we take it to the factory and the actual car, in this instance, gets
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built, well that's the object. That's the actual thing that's going to be doing the driving, or in our
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case of an app, it'll be the actual button that can receive taps and the images that actually change.
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Those are the actual objects. But because we can't see the objects until the app is running, we have to
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plan out how they will function and how they will work by creating these classes.
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A class has two really important things. One are its properties.
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So if we're building out a car, then that will be its color or the number of seats it has or the number
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wheels it has or the number of doors it has or all sorts of things that are things to do with the
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characteristics of the object. And in Dart, these things that we've been referring to as variables when
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they're inside a class,
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they're now called properties or fields.
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Now the other important part of a class that we need to define are its methods.
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So the things that it can do. The actions that the object that will eventually get created, will be able
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to perform.
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In the case of our car object, that might be to drive, or to brake, or to turn around.
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And these methods are actions that the object will be able to do that we're going to define in the blueprint
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even before the object gets built.
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So for example, you might see a class defined like this.
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Firstly you have that class keyword which shows that everything else that's coming afterwards is going
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to be a blueprint.
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And then you have the name of the class.
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So in this case we're building a car blueprint. And by convention in Dart as with many other languages,
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when you're creating a variable say here we've got numberOfDoors, or when you're creating a function,
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say here we've got drive,
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those are all camel case
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starting with a lowercase word.
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But when you're creating a class to show that it's different, we always start-off with a capital letter.
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So all the words in your class names are capitalized.
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Now here's our Car class
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and inside these curly braces are the definitions for this car class.
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So this is the blueprint of exactly how our car will look and also how it will behave.
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And between the looks and the behavior, we end up with a class.
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So in this case we have just a single property, which is called numberOfDoors.
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And this defines a characteristic of the car namely, how many doors it has.
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This car has five.
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This car also has a function called drive,
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and this defines a little bit of functionality for the car.
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So namely this car, when we call this thing called drive, it's going to start turning the wheels for example.
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Now notice how inside a class, our variables are no longer called variables and they're called properties
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or fields.
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Similarly, a function when it's defined inside the curly braces of a class, it's now called a method and
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not a function anymore.
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So if you're new to programming there's a lot of new words and it's a good idea to build your own glossary
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of words because it's just like learning a new language.
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When people refer to a method, you can think of them as just a function that's associated with the class.
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And when people are talking about a property, it's simply a variable that's associated with the class.
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And once you hear it often enough, then it'll start becoming second nature.
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Now once we have our object, it's the object that actually performed those pre-planned actions in the
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class such as moving, right?
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Or it actually has those characteristics which we defined in our class such as its color property or
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the numberOfDoors property. The two paths of creating a object are first, we have to create the class
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and this is gonna be the blueprint that's going to define everything about the eventual object that
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will get created.
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So we use the class keyword, we give the class a capitalized name and we define variables in it which
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are properties for that class.
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And we also give it functions which become the methods that the class can do.
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And when we actually create the object from this class, then our code looks something like this.
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We would define the type, which is in this case, the name of the class and then we would give our object
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a name.
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And finally we would set it to equal a new version of that class. When we have the word car and these
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empty parentheses,
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it's basically creating a new object from this blueprint.
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It's like sending that blueprint to a factory and out comes our actual car object.
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Now this is all very abstract and we're having to imagine a lot of it in our head.
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So let's put it down onto code and head over to our trusty DartPad to be up to see this in action and
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understand a little bit better.
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So I've got my empty main method up here, and I'm going to create a new class right below it.
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I'm going to call that class Human.
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So remember it should be capitalized.
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And this class of humans are going to have some properties, so some characteristics.
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So what kind of characteristics might a human have?
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Well they might have a height right?
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We could create a height property and maybe it'll have an age property. And when we create a new class,
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we can either create these properties as empty or null such as here, or we can say that by default when
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we create a new human, it should always have a height.
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Let's say that when a default human gets created, it starts out being, I don't know ,maybe 15 centimetres
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in height, and maybe it will have a default age of zero.
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So now if I go into my main method and I create a new object from this human, let's say that we create
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a new human,
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so that's gonna be the type of this object. And our actual human, let's call her
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Jenny, is going to be created using our human class. And we do that of course by writing human and then
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we have a set of empty parentheses.
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And this builds out our new human.
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So human is the blueprint for all humans.
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They all should have a height and they all should have an age.
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And we have some default values for how humans should be when they first get built.
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And Jenny is our actual object who's built from this blueprint. And we're doing things in our program,
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we're actually interacting with Jenny rather than the sort of abstract blueprint kind of for human. So
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we might say some things such as, let's print Jenny's height. So we have our print statement and then
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we're going to tap into our Jenny object and then we're gonna use the dot notation to tap into its property
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namely the height property.
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So we'll say jenny.height and let's print that into the console. And you can see that because we've
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given all humans a default starting height of 15,
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Jenny also has a height of 15.
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Now a little bit later on, I could maybe change that.
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Maybe Jenny grows a bit and lets say that her height is now 20cm.
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So I can change all of these properties and I can tap into them simply by using that dot notation. By
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saying that this is the object that I'm concerned about and it finds the object which we know gets created
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from the human class, and by default, all of the properties of a human
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start out with these values. Then I'm saying get me Jenny's height. And that's what that dot says.
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It's just saying, 'this is the property associated with Jenny that I want to know about.'
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Or
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later on when our human get some functions associated with it, you'll see that the way that we tap into
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it is still with the dot notation.
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So when you see something.something, it usually means that this is an object and we're tapping into
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one of its properties or methods.
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Now at the moment, every single new human that we create starts out with a height of 15cm.
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Now that might not be true
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right?
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So let's say that we created another human and this is James.
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And he's also created from the human class, which is what this part means. We're going to the human factory
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and we're building a new human.
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And if I was to print his height, you'll see that it's exactly the same.
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He's also 15 centimetres tall. But that's probably not really true
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right?
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Why should it be that when a new human gets created, they all have the same default height?
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What if we wanted to initialize each object with a different value for some of these properties?
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How would we do that?
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Well we already saw that in the last lesson that we can use something called a constructor for our class.
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And to create a constructor,
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we would write the name of the class, so it's human, and then we would create a set of empty parentheses.
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And just as when we create a function we provide some inputs into these parentheses, when we create a
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constructor for a class we can provide some inputs to our class.
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So we could say that maybe the starting height property which is going to be a double, because our height
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property is a double.
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We'll give it a name of startingHeight.
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And now inside the curly braces, we can assign these startingHeight to the height, and height can start
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out without a value. It'll start out being null.
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But when we create our new human, we have to provide a starting height and then we can say that the humans
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height property will be set to whatever the starting height is that we provided.
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So now we get some errors here because it needs a starting height and we haven't given it one.
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So let's add that in. So we could say that Jenny is a human who starts out being 15 centimeters but James
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starts out being a human who is, maybe he's a bit taller. Maybe he's 20 centimeters.
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So now when we click run, you can see that Jenny's height starts out 15cm and James's height starts out
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at 20cm.
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And our code now makes a little bit more sense because why should a human have a default height
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right?
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They they don't all get created with the same height. And just as we could make the input or the parameter
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a named parameter, in our functions we can do the same in our class constructor. So we can add a set of
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curly braces around these inputs to say that when we use it when we create our new human object, we have
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to specify that name.
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So startingHeight and then a colon.
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So this way it's a bit easier on the eye and you can see that I'm creating a new human and I'm providing
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a starting height for that human. As I said earlier, our classes can have properties and it can also have
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methods. Our methods are simply functions that are defined inside a class.
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If we were to create a method let's say, talk to get a little human to say something, we can tell
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it what to say.
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So we'll create a string input called whatToSay, and then inside the talk function or in this case the
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talk method because we're talking about a function associated with the class, we can say that we'll just
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print the string that it's told to say.
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So that means we can tell our new humans, say for example, let's get rid of James. Let's just say Jenny
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.talk and the thing that you should say is ,Why is this sky blue?'
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Now when we run our code, we print Jenny's height and we get Jenny to talk. And you can see that by calling
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this method talk, then this object actually performs that action and it prints out the thing that we
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want it to say into the console.
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This is now a method that's associated with the human class.
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And just as we use the dot notation to access the property height, we're also using the dot notation
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to trigger the talk method.
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So when we're working with objects you see the dot all over the place and it just refers to saying, 'this
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object do this or this object get me it's property.'
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So we now have a pretty simple class. And this class is our blueprint for how a human object should behave
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and what characteristics it should have.
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So all of the things that a human object, once it gets created, should be able to do and what it should
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be. You can of course make this as complex as you need it to be. But in essence, we're simply creating
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a plan for how this object should behave or how it should appear. And we use it by creating it as a new
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object and we can create as many of these as we need to and get them to interact with each other or
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get them to do specific tasks.
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And so this is how we would create our classes and objects.
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But why exactly do we need these classes and objects? To find out about that,
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I'll see you on the next lesson.