Core: Compiler Rules for Class Construction

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From the course by University of California San Diego

Object Oriented Programming in Java

3372 ratings

University of California San Diego

Object Oriented Programming in Java

3372 ratings

Course 3 of 4 in the Specialization Object Oriented Programming in Java

Welcome to our course on Object Oriented Programming in Java using data visualization. People come to this course with many different goals -- and we are really excited to work with all of you! Some of you want to be professional software developers, others want to improve your programming skills to implement that cool personal project that you’ve been thinking about, while others of you might not yet know why you’re here and are trying to figure out what this course is all about. This is an intermediate Java course. We recommend this course to learners who have previous experience in software development or a background in computer science. Our goal is that by the end of this course each and every one of you feels empowered to create a Java program that’s more advanced than any you have created in the past and that is personally interesting to you. In achieving this goal you will also learn the fundamentals of Object Oriented Programming, how to leverage the power of existing libraries, how to build graphical user interfaces, and how to use some core algorithms for searching and sorting data. And this course is project-based, so we’ll dive right into the project immediately! We are excited to be offering a unique course structure, designed to support learners of different backgrounds in succeeding at their own pace. The first module explains how this will work and if this course is right for you. We also recommend taking a few minutes to explore the course site. A good place to start is the navigation bar on the left. Click Course Content to see what material we’ll cover each week, as well preview the assignments you’ll need to complete to pass the course. Click Discussions to see forums where you can discuss the course material with fellow students taking the class. Be sure to introduce yourself to everyone in the Meet and Greet forum. This course should take about 6 weeks to complete. You can check out the recommended course schedule below to see a quick overview of the lessons and assignments you’ll complete each week. We’re excited you’re here learning with us. Let’s get started!

From the lesson

Inheritance

Now that you’re in full swing with the project, let’s take a closer look at some concepts that you have already been using in your code. Specifically, the keyword “extends”. What exactly does it mean that your program “extends PApplet”? To answer this question we’ll need to dive into the details of how Java allows for Class Hierarchies through Inheritance and Polymorphism. These fundamental and powerful concepts will serve you well in the future! We’ll use these ideas to allow for displaying markers differently. If you feel you’re already comfortable with Inheritance and Polymorphism, feel free to dive straight into the project (programming assignment) for this week. As you work with the project, feel free to have some fun and introduce new levels of class hierarchies for improved functionality.

Core: Why use Inheritance?5:43

Core: Extends5:28

In the Real World: Inheritance 10:52

Core: Reference vs. Object Type3:27

When I struggled: the "Is a" relationship0:40

Concept Challenge: References and Objects9:09

Core: Visibility Modifiers4:08

When I Struggled: Inheritance0:45

Core: Object Creation in Java3:08

Core: Compiler Rules for Class Construction4:46

Core: Variable Initialization in a Class Hierarchy4:47

Concept Challenge: Inheritance Constructors 14:40

Concept Challenge: Inheritance Constructors 23:45

Core: Method Overriding5:05

Meet the Instructors

Mia Minnes
Assistant Teaching Professor
Computer Science and Engineering
Leo Porter
Assistant Teaching Professor
Computer Science and Engineering
Christine Alvarado
Associate Teaching Professor
Computer Science and Engineering

[SOUND] In the last video, we saw how objects

are constructed from the inside out in Java.

And the way this happens is really through compiler rules.

We're gonna talk about what those compiler rules are and how those compiler rules

work, in order to ensure, again, that the object is created from the inside out.

Where we left off in the last video is we'd seen the process,

we'd seen how student calls person which calls object.

And those constructors return back and

in the process of returning back they actually initialize the variables.

But you've said at the end but wait, I don't remember extending object.

Where did we do this?

This introduces our first rule associated with a compiler.

So let's revisit where the compiler plays a role in our process.

You write code, human-readable Java code,

and that gets sent off to the Java compiler.

The Java compiler's going to do a number of passes.

In fact, there's a number of potential specializations you could write instead of

just on how the compiler works.

What we're going to focus in on is essentially a couple of passes

the compiler does as terms of inserting code.

When the compiler is all said and done we know it produces

the Bytecode which we then run our java virtual machine.

So what is a Java compiler doing and what are these rules?

What it does is its going to change the code.

So, if you wrote this class, public class Person, and you just have your

single instance variable, and I've done away with any other code,

it's just this class, is this actually what gets compiled?

In a sense,

no, cuz the compiler takes this code and it's gonna change the code a bit.

What it's gonna do is follow three rules that we're gonna talk about in this video.

First rule is, if you don't have a superclass,

the compiler's gonna give you one.

It's gonna give you Object.

So even though you wrote public class Person,

the compiler's gonna insert the terms, extends Object.

By doing this, now you see how person inherits from object

just because it inserts those lines of code implicitly.

But where then, do we call the person constructor, or the object constructor?

We know now that object inherits, or sorry person inherits from objects,

but we don't know where those constructors are getting called?

That's where the next set of rules come.

So first off, if you don't have a constructor,

Java's gonna give one for you.

It's gonna give you a default constructor so no arguments.

And then it's gonna apply another set of rules

which are associated with all constructors.

This set of rules or Rule #3 is that the 1st line of a constructor

has to say either this, and then some arguments.

So call another constructor within the same class.

Or it has to call one of the parent class instructors,

a super class constructor call.

It has to do one of those things.

And if you don't do it, Java is gonna insert super.

So it's gonna call,

It's gonna insert a call to the default constructor of your super class.

So now we see that we've added this line to the person constructor.

In a sense you've the code on the left but the compiler is gonna insert all those

elements on blue on the right and that's what's really gonna get compiled.

So now we have an idea of how this work for person.

I want you to pause and think about what would happen to the Student class

On the first pass by that compiler.

All right, if you were thinking we should add a very similar

constructor to student that we did to person, you're on the right track.

It already extends from person, so we can't extend from object.

It already has a super class so there's no need to do that first rule.

The second rule applies, it has no constructor so

we have to insert a default constructor.

And then the third rule applies of I have to make sure I call the default

constructor of my super class as the very first line of my default constructor here.

So you're gonna take that code in black above and

it's gonna get transformed into the code in blue below.

So when we were curious how is the compiler making sure that

this construction and the initialization of our variables happens

essentially from the highest level of the hierarchy all the way back down.

So it is the compiler makes it happen.

Now there's still one leftover problem and that is,

we haven't actually initialized anything.

Right all of our default constructors have done no initialization.

They've just made sure they go all over the up to object and

all the way back down.

We had at least one variable and that was name, the instance variable with Person.

In our next video we're gonna see how to initialize that.

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