Hey guys this is part one of my introduction to classes and objects and in this video I'm going to talk about what are classes and objects and then what are attributes, methods, and constructors and at the end of this video I'm going to give you some code samples in Java but this video is going to be mostly language independent so you'll be able to use what you learn in this video in whatever language you're using. Classes and objects are actually one of the most fundamental topics when it comes to data structures and algorithms, so let's get started. So what are classes and objects? I think the best way to explain what they are would be to give you a really concrete example. So let's say as an example, you're thinking about making a website where people can visit and talk to a bunch of robots. So what you have in mind is when people feel lonely or something they can come visit your website and say hi to one of the robots there. So when a user comes to your website they might see a list of robots like this, and here you see two of those robots and they're called Tom and Jerry. Aside from their names they have some obvious differences. First of all Tom's color is red, while Jerry's color is blue as you can see and Tom is slightly lighter in weight than Jerry with thirty pounds of weight while Jerry is forty pounds and they both have the same functionality. When you click this button "Introduce", Tom will say "my name is Tom" and Jerry will say "my name is Jerry" and that's all you have for now and it's pretty basic but that's all you want at the beginning when you create this website. Now, to create a system like this you basically need a way to represent these different entities or these different robots in whatever programming language you're using whether it's Python, Java, or Javascript and that's where classes and objects come in. Now, to represent each of these robots Tom and Jerry that we saw earlier in whatever programming language you're using you need to store two sets of information. The first set of information that you need to store is the properties that each robot has. So for example the first robot we saw had these properties: its name was Tom, the color was red, and the weight was thirty pounds. And the second set of information you need to store for each of these robots is the set of functionalities it has. As we saw earlier, it had only a single functionality in this particular case and that was to introduce itself and that might be represented as a function in whatever programming language you're using and let's just call it introduceSelf() here for now. At this point you might say "is there any convenient way to organize these properties and functions together?" There is, and that's called an object. An object is basically a collection of properties which can be expressed as variables and some functions, and with that collection of information an object can represent some "thing", whatever that "thing" is. So in this particular case this particular object is representing this particular robot, Tom, but you can make an object so that it represents a particular person, or a particular dog, or anything else for that matter. If you decide to represent a particular person with an object let's say Lady Gaga just as an example, you might have a different set of variables and a different set of functions to represent that person. So for example, you might still have "name", but you might also have "height" and "weight", "eye color" and "ethnicity" and so on and your object will probably have a different set of functions too, to show what kind of actions that person can take. It could be for example things like walk, sleep, talk, or anything a person might do. OK, let's backtrack here a little bit. Once you create an object to express this particular robot Tom, you'll be able to put this object in a variable just like that let's say "r1" just as an example. You'll be able to use this variable that contains this whole set of information. The properties or the variables and the functions just like any other variables for example to pass it into a function as one of the arguments. OK, let's now take a look at how the second robot we saw, Jerry, can be represented with an object. It's gonna look pretty similar to the first one it's gonna have the same set of properties but different values this time "Jerry", "blue", and "40", and the same function as well, introduceSelf() and of course once you create this object, you'll be able to put it in a variable as well let's call that one "r2", and because you have two distinct variables containing two different objects, you'll be able to distinguish them very easily in your code and just quickly about terminology, these variables within an object are called either "instance variables" or "attributes" and these functions within an object are called "methods". OK, so when you see these two objects you might say well they're pretty similar, they have the exact same set of variables or attributes and they have the exact same set of functions only one function in this case so you might say "is there any convenient way to organize that somehow?" There is, and that's what a class is essentially. Now what's a class? A class is basically like a blueprint from which you can make objects so just imagine for a second someone telling you "please make a robot". If someone tells you something like that, you say "what kind of robot do you want? What should it look like, and what kind of properties should it have, and what kind of functionalities should it have?" So basically, if you want to make a robot in sort of the real world, you need a blueprint to make that from to know what the robot should look like and that's sort of how it works with programming with objects and classes too. So when you make an object, you want to know what kind of properties, what kind of attributes it'll have, and what kind of functions it'll have and you can use a class to specify exactly that. OK, let's now see what this particular class might look like for these two objects, these two robots. It'll probably look like this. As you can see, it has the same set of properties as the objects themselves "name", "color", and "weight", but the values for these three properties are not defined yet and that's because the class doesn't refer to any particular object. It doesn't refer to this robot, or this robot, in particular. It just refers to this category of robots or this type of robot in general. Then when you create a specific object using this class then you can specify what the name should be, what the color should be, and what the weight should be and also notice here that we already have the introduceSelf() function in this class. You can actually implement it already when you define this class by writing something like this. So this is just an example from Java. In Java, for printing something on your computer we have "System.out.println()". It's similar to the print function or the printf function in other languages and then inside the input of this function we have "My name is " as a string, plus "name" and this name will refer to whatever the value of the name is for that particular object. So for example if you run this function in this object it's going to print out "My name is Tom", because that's the name of this particular object, and if you run this function here in this object instead, it's going to print out "My name is Jerry" instead. And one more thing to note here is that when you define a class like this one you need to give it a name. It could be anything, but let's call it "Robot" here and as you can see the first letter "R" is capitalized and that's a pretty common convention when you name a Class. OK, let's now take a look at how this whole thing might look like in code. We're going to take a look at some code in Java here but other languages are typically pretty similar. Let's first assume that we already have our class defined. Like we said, it's going to be called Robot, but we're going to take a look at what's going to go inside later and once you define this class Robot, you'll be able to use this class of course to create an object out of it. And you can do that like this. We have "Robot r1 = new Robot()", and this "new Robot()" says create a new object with this class Robot, and then we're assigning that object to r1 and then we have the word Robot here again because a class is used not just to create an object but also as a type too you know just like we say "string r1" if we want to create a string, we can say "Robot r1" to create a reference to a Robot object. OK, let's move on. When you create a new object this way this new object doesn't have the properties, doesn't have its instance variables set yet so you need to set them after that just like that. Here we have "r1.name = 'Tom'" and that sets the name attribute of this object or the name instance variable of this object to Tom and then you need to do the same thing for the other two instance variables by saying "r1.color = 'red'" and "r1.weight = 30", so that's the first object r1, let's see how we can create a second object here. As you can see the second block of code here is pretty similar to the first block. We have Robot r2 = new Robot(); to create a new object and then we have "r2.name = 'Jerry'" and so on to set the instance variables of that object. So that's how you can create two objects of the Robot class here but what about the functionality we saw earlier, introduceSelf()? To run the function for each of these objects the syntax for that is actually pretty simple. It's going to look like this. We have r1.introduceSelf(), this just says run this function introduceSelf() on this object r1. So when you run this line it's going to print out "My name is Tom" because this robot's name is Tom and then you could do the same thing for r2 by writing r2.introduceSelf(), and that's going to run this function, introduceSelf(), for r2 and then it's going to print out "My name is Jerry". OK let's now see how you can actually define this class. First of all, to define what instance variables this class will have, you can just write these. This of course says the instance variables that this class will have are "name", which is going to be a string, "color", which is a string as well, and "weight", which is an integer. And when you see other sample code in Java, you might see words like "public", "private", or something else before each of these attributes but just to keep this simple I'm not going to talk about what they are in this particular tutorial and when you run this code it will just work just fine without adding these words too. OK, the next thing you need to define and implement is this function introduceSelf(). And for that you can write this. Here, we're defining a function called introduceSelf() and we have the "void" prefix here to show that this function is not going to return anything and notice that this function definition is still inside this class and then the implementation is as we saw earlier: System.out.println(), "My name is this.name". Actually we could just have name here, but I wrote this.name instead just for emphasis. So what's this here? "This" refers to whatever object, whatever particular object, that you're running this function on. So if you write r1.introduceSelf(), you're running this function on r1 so "this" will be equivalent to r1. So it's going to be sort of like saying System.out.println(), "My name is r1.name", which is going to be Tom of course. And then when you run r2.introduceSelf(), this will refer to "this" key word will refer to r2 instead so when you run r2.introduceSelf() it's going to print out "My name is r2.name", which is Jerry. OK so that's the basics of classes and objects in Java, but when you look at these two blocks of code you might say well they're pretty repetitive... you know, you need to repeat the same things over and over again and then it's pretty cumbersome to remember to set these attributes every time too, every time you create an object. So you might say "is there any way to simplify that?" There is, and it's called a constructor. A constructor is basically a function that allows you to create an object from a class and then you'll be able to use the constructor to you know do things like set the attributes for yourself. And actually you already used a constructor, you know this line right here, "new Robot()" that's a constructor but that's a default constructor that Java defines for you and this constructor doesn't set any of the attributes. So let's see how we can create our own constructor that does set the attributes for you. So that constructor might look like this. So if you see this function you'll notice that you know it looks like a regular function except for that we don't have any prefix here like "void" or "string" for the return value. Instead, this constructor is just called Robot. Notice that that name is exactly the same as the class name and then it has three arguments: "String n", "String c", and "int w". And then inside this function we're setting "this.name" to n, "this.color" to c, and "this.weight" to w. As we saw earlier, "this" refers to the particular object that we're talking about here, in this case the particular object that has just been created, and then we're saying that object's name should be n, which is the given argument, and then that object's color should be c which is the given argument and so on. And then here, the common practice here is to name these arguments exactly the same as these attributes, "name", "color", and "weight", and with that this part is going to look like "this.name = name", "this.color = color", "this.weight = weight", but with that naming this function, this constructor, is doing exactly the same thing as this one so that's really just a style issue. OK moving on, we're almost done here... let's see how we can actually use this constructor. Actually when you define your own constructor, your own custom constructor, the default constructor stops working. So these two blocks of code no longer works and then you can use this new constructor to create new objects, just like this. This one says "Robot r1 = new Robot("Tom", "red", 30)". So these three arguments, "Tom", "red", and "30", will be passed on to this constructor and then this block of code is going to be run and so r1's name is going to be given "n" which is "Tom" and then the color will be "red" and then "weight" will be 30. So this piece of code will actually be equivalent to the whole block of code we had earlier right here. Basically, create a new object and then set the attributes to the given arguments. And then we can do the same thing for r2 as well by writing "Robot r2 = new Robot("Jerry", "blue", 40)". This creates a new Robot object with the name Jerry, color blue, and weight 40. And after creating these two objects you'll be able to use r1 and r2 just like before by writing r1.introduceSelf() and r2.introduceSelf(). OK, again this was just sample code in Java, but it's pretty similar to other languages too and you'll be able to find links to sample code in both Java and Python in the description below. All right, if anything was unclear in this video please let me know in a comment below so maybe I can talk about it in the next video. In the next video I'm also going to talk about how you know multiple classes and objects can interact with each other, and with that knowledge you'll be able to finally start building some data structures like linked lists. I'm also planning to create a new Python tutorial video on classes and objects, so look out for that as well. And special thanks to brilliant.org for sponsoring this video. You know, some of you may already be familiar with data structures and algorithms and maybe you're watching my videos as sort of a refresher. If that's the case, brilliant.org might just be the right website for you to learn more advanced concepts in computer science. So personally, I used this computer memory course to get a better understanding of how memory works in a computer. For example, previously I didn't really know how structs in C are stored on memory... I mean I had a vague idea but I didn't really know, and I found this course was really helpful for understanding that. Personally what I like about Brilliant is, first of all, the fact that they have really nice visuals like this one, you know much nicer than mine I would say, and they're helpful for understanding different concepts visually. Another thing I like is the fact that they sometimes combine multiple concepts in a single problem. What I mean by that is, for example, for this particular problem you need to understand different things like binary expressions, and how structs in C are stored on the computer, and how short and char and different types of data are stored on memory too. And in my experience in general I have found that, you know, solving problems is a really good way to review different concepts at the same time and also learn new concepts and brilliant.org definitely does a good job of coming up with good problems to solve and they even have pretty advanced courses like this "Artificial Neural Networks" course on top of their standard computer science fundamentals and algorithms courses. You can just go to brilliant.org/csdojo to start checking out these courses yourself and you can get 20% off their annual subscription through that link as well. All right, as always I'm YK from CSDojo and you know I'm working really hard here to make CSDojo better and better so that hopefully, eventually, it's going to be in a class of its own. All right I'll see you in the next video.
