[MUSIC] All right, in our first module you were introduced to the project that you're going to be doing throughout this course. And we hope that you think it's as exciting as we do, visualizing this geospatial data. In this module though, we're going to be stepping a little bit away, and we're going to be talking about some of the concepts that underly the code that you're going to be writing. So, by the end of this module, you're going to be able to motivate the use of classes and objects in programming. So, why do we want to use classes and objects when we write our programs? You'll be able to write classes in Java, and then create instances, or objects of those classes. And you'll be able to describe the parts of a class, like member variables, methods, and constructors. So in order to motivate the use of classes and objects in programming, I want to go back to my favorite definition of computer science. And of course, software engineering and software development, which is what we're focusing on this course and this specialization, is a major part of computer science. So my favorite definition of computer science is, that computer science is the science of using and processing large amounts of information to automate useful tasks and learn about the world around us. Of course, all using a computer, because none of this would be possible if we didn't have the computational power that the computer affords us. I'm gonna zero in on this line in the definition, this idea of using large amounts of information. These programs that you're going to be writing and programs that people write, if you think about programs out in the world, deal with huge amounts of information, so much information it's almost impossible to get your mind around it. So one of the fundamental questions in writing programs is, how do we organize this information? How do we organize it in a way such that our programs are easy to write, easy to maintain, and easy to debug? Well the philosophy behind object oriented program, object oriented programming, is to build our programs in a way that match the problem that we're trying to solve. So if we go back to our project and the problem we're trying to solve, it's to visualize geospatial data on a map. And if we think about what's involved in solving that problem, we can think of lots of real-world objects and entities that are part of this problem that we're solving. So we can think of things like the map itself. That's an object that we're all familiar with. Shapes. Shapes are objects that have properties. We could talk about the shapes of the countries, for example, in this map. Locations in the map, colors of the objects in the map, even the window in which the map is displayed. Plenty more objects as well, but these are all ideas that have real world counterparts. And the idea behind writing an object-oriented program to solve this problem, is that we're going to structure our code around these real-world concepts. So in order to build our code in this way that matches the real world, we're gonna design these things called classes, and what classes are, are just custom data types that we get to define to match the problem that we're trying to solve. Now of course you're familiar with classes, cuz you've been working with Java for some time now. But we're gonna talk about really the details of creating and using classes. So a class is a new kind of data that we the programmer get to define. And it's kind of like a factory. It can produce pieces of data with a template that we get to define. So I'm gonna use this factory icon when I talk about class definitions. On the other hand, an object is the thing that comes out of this factory. It's a piece of this data with this custom type that we've defined. And some associated functionality that goes along with that data. And I'll represent the objects with this little symbol here to represent the things that are coming out of the factory as we create objects using our classes. So you can think of this like a real-world factory, right? Like a car factory. A car factory knows how to produce, say, one kind of car. And it can produce many of that kind of car. And once the cars are out in the real world, well, at first, they all kind of look the same, but then each individual car can be customized and it can be changed in its own way without affecting all the other cars that came out of the factory. The same thing's going to happen for our classes and our objects in our Java programs. So we're gonna go through an example of creating a class. And the class we're gonna create has to do with this map example that we've been working with. So here's a map of the world. I'm gonna zoom in on one particular place in the world, which is where this orange box is, which is approximately where I'm standing right now on the campus of UC San Diego. So here's the University of California, San Diego campus, and I'm about right here on this map. So what I want to do is, I want to build a class that can represent my location in the world, or can represent anyone's location in the world. So the question I have to ask myself is, what do I need to know in order to represent a location? Well for this example, I'm going to represent the concept of a location using two pieces of data, the latitude and the longitude of this location. And in my case, for this location right here, you can see these approximately my latitude and longitude coordinates. So let's define a class that represents this notion of a location. Here's my class definition. So I'm telling Java that I want a new class called SimpleLocation. And you can see that on the top line in this declaration. I say, public class SimpleLocation. SimpleLocation is the name of the class. The keyword class tells Java that I'm creating a new class. And public, which we'll talk about more in some of the next videos. Just means that my class is public to the world. Anyone can use it. Inside, after that first curly brace, is the definition of my class. Now before we look closely at the definition, I wanna point out that all of this code has to exist in a file called SimpleLocation.java. That's just a rule that Java enforces. That when you're creating a class, you have to put it in a file that has the same name as the class, and that's if the class is public, which our class is. So all this code is in a file called SimpleLocation.java. So let's look a little closely at the definition for this class. Up here you can see the data that I have associated with my simple location. I had a latitude and a longitude. And I'll represent both of these using double type variables. These are what are called member variables. These are variables that exist throughout the class that represent some essential pieces of data that I need to represent my simple location in this case. And notice that they're declared outside of any methods, but inside the declaration of the class. So, they come after that open curly brace for the class, but not inside any particular method. My class also has some methods associated with it. So it has, for example, that method that you see down there at the bottom. The distance method, and what the distance method does is, it takes some other simple location out in the world as a parameter, and it returns the distance between on simple location and the simple location that's passed in. So that's something that my class can do. So these are the methods. These are the things that my class can do. And I want to zero in now on this special method called the constructor. So this is the constructor, and this is a special method that gets called when my objects get created. So when I ask Java to give me a new object of type SimpleLocation, it's going to call this method here, which is called the constructor. And the reason you know it's the constructor, is that it doesn't have a return type, so it simply says public and then next word in the declaration of this method is just the name of the class. So public, SimpleLocation, that's my constructor. This constructor happens to take two arguments. And it takes the values of those arguments that are passed in and stores them away in those member variables, latitude and longitude. Now you might see this key word, this. This dot latitude. This dot longitude. You might be wondering what is is. We're gonna talk about that in just a couple of minutes. So all of this together is my class declaration, SimpleLocation. Class has two member variables and couple of methods. And again, it's in the file SimpleLocation.java. So now, how do I use this class? Let's take a look at some code that actually creates objects of type simple locations and does something with them. So now I'm gonna go over into a totally different file. I'm gonna go into a file called LocationTester.java. Inside a totally different class, because everything in Java has to be in a class, and I'm gonna look at a main method inside that class. This main method is fairly simple. It just has three lines of code. And the first two lines of code are creating new objects of this type SimpleLocation, the class that I just wrote. So, you can see here that I declare two new variables, ucsd and lima. And I'm creating two new SimpleLocation objects to represent the location of where I'm standing here at UC San Diego. And then the location of Lima, Peru. When I call new, and then say SimpleLocation, and then have the parentheses in passing those arguments. What that's doing is, that's going over into the SimpleLocation class definition and calling the constructor. So those arguments you saw being passed in, those are gonna go in as arguments. To this constructor, which is going to store those values away inside the object that just got created. So I call the constructor twice, I get two new objects. Then I can use those objects in some code like I do down here. So, what I'm doing here is, I'm taking my ucsd SimpleLocation object. And I'm calling the distance method on it, passing it in the SimpleLocation object that represents the location of Lima. So let's take a look at how that works. When I call that method, it's going to invoke this method distance that's inside the class SimpleLocation. All right. So let's look in a little bit more detail what's going on here, so we can understand the difference between UCSD and Lima. So when I call ucsd.distance and pass in the parameter lima, like I mentioned before, it invokes that method called distance inside the SimpleLocation class, which you can see right here. Now I'm showing you just a little bit of what the body of that method might look like, so that I can illustrate a couple of points. We're comparing the distance between two SimpleLocation objects. One of those objects, lima, is past in as an argument to the function, and that's gonna be stored in the parameter, other. So I can see that other is used inside the body of the method when I get its latitude and its longitude. But what happened to ucsd? How are we getting the latitude and longitude of ucsd? That's where the keyword, this, comes in. This refers to the calling object, which is the object that called the method, or on which the method was called. So in this line of code, ucsd dot distance and then pass in lima UCSD is called the calling object because it's the object that occurs before that dot. So when I'm inside this method, this will refer to the location of UCSD. So that way I can get the latitude and longitude of where I'm standing here at UCSD, and compare it to the latitude and longitude of Lima, Peru. and return the distance. Putting it all together, when I run this code by compiling it and running it, I see that distance between where I'm standing now, and Lima, Peru, is approximately 6,568 kilometers. So those are some objects basics. In the next couple videos, we'll introduce you to some more subtle details and interesting things you can do when creating classes and objects.
