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Concept Challenge: Polymorphism Part 2

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Programação orientada a objetos em Java

Universidade da Califórnia, San Diego

4.7 (3,558 classificações) | 160K alunos inscritos

Curso 3 de 4 no Programação orientada a objetos em Java Programa de cursos integrados

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!

Visualizar o programa do curso

Habilidades que você aprenderá

Logic Programming, Java Programming, Object-Oriented Programming (OOP), Sorting Algorithm

Avaliações

4.7 (3,558 classificações)

5 stars
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KP

Feb 21, 2016

Enroll this course if you intend to become a programmer and need fundamental knowledge in Object Oriented Programming. Here you can study the subject and apply new things in a real curious project.

OE

Aug 06, 2017

This course made me understand java concepts in an easy way, and made me realize certain internal workings in Java I haven't seen elsewhere. I love their method/process of tutoring. Awesome job!

Na lição

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: Introduction to Polymorphism2:41

When I struggled: Polymorphism0:56

Core: Rules to Follow for Polymorphism4:01

Core: Casting Objects3:34

Concept Challenge: Polymorphism Part 16:08

Concept Challenge: Polymorphism Part 26:43

Core: Abstract Classes and Interfaces5:09

In the Real World: Inheritance 21:54

Ministrado por

Mia Minnes
Assistant Teaching Professor
Leo Porter
Associate Teaching Professor
Christine Alvarado
Associate Teaching Professor

Transcrição

All right. If you thought that last Polymorphism concept challenge was fun, you are gonna love this one. This one is gonna bring it to a whole new level. It's more challenging than the last one, that we just gave you. In fact, it's beyond what we've even explicitly taught you. So, we want you to really push the bounds of your reasoning and, understanding. You know the drill, do it yourself, discuss it with your friends, watch our videos, confirm your understanding. So, here's the challenge, we now have three classes in an inheritance hierarchy. So, base class that we're working with here, is a Person. And, it has two methods, method1 and method2. We have a class that extends the Person class, which is the Student class. It overrides both method1 and method2. And then we have the Undergrad class, which extends the Student class and it inherits the method1 method, but it overrides the method2 method. So, that's in a nutshell what we're working with. Here's the code in more detail, and you're gonna need to work through this code and look at it carefully as you work through this challenge. And, what the challenge is, is when we create a new undergraduate object, refer to it as a reference of type person, and we call method1 on that newly created object. What gets printed out? >> Hi, I'm Steven. >> Hi, I'm Joanna. >> Hi, I'm Justin. >> So, what do you think? >> I don't know, I was getting caught up on this first one. So, it says it calls for method1. But I don't see a method1 in Undergrad. So I think it would be a compile error. >> I don't think it would be a compile error. If you notice Undergrad extends Student. So, in the Student class there is a method1, so I think it inherits that method. Yeah, I agree, so. >> Yeah. >> It does print Student 1. What I got a little bit tripped up on was, I wasn't sure which, when it calls super.method1 in the method1 of the Student class, I wasn't sure which method1 it would call. Because isn't it in Undergrad? So wouldn't the super.method1, it might just call Student and go into an infinite, like calling itself over and over. Right? >> Yeah, I can see how you could get caught up on that. But I think when super, it's always talking about the class above it. So I think it actually would be calling the method1 and the person class. >> Okay. >> What about method2? Does it call Students method2 or which method2 does it refer to? >> I think that since it is in Undergrad? It will call Undergrad's method2 and it'll print undergrad 2. So, if it doesn't go infinitely, like you said, then it should be D. >> All right, let's trace this together, because it's non-trivial. We're gonna start our tracing up here, where the method is originally called. So the type of the object referred to by you is an Undergraduate. So at runtime, that's where Java is going to look for this method, method1. When we look down here in the undergraduate, in the Undergrad class. Class, we don't see a method1. But luckily we can see that Undergrad extends Student. So Java is gonna go look in the Student class for method1. So looking up here in Student class, it finds method1. Great. So now I can start executing. So the first thing it's going to do is execute that first line which prints Student 1. So what gets printed so far? It's Student 1. Now we keep going to the next line. The next line says super.method1. So what is the super class of the Student class? Well, it's the Person class. So this happens at compile time. When this code is compiled, Java notices that there's a call to the super class inside this method and so it looks and sees well what is the superclass of student? It's Person. So it's going to bind to the method1 inside the Person class. And that's where we'll go now. So we go up here to method1 and we execute it's code. It's code sized. It should print out Person 1, so that's what gets printed next. So, so far we've printed Student 1 first and then Person 1. But let's continue. We now return back to where we were, which was inside the method1 method in the student class and we say call method2. All right, well where are we going to now? Which method2 are we talking about? In order to answer this question, we first need to notice that since we don't have a calling object at this point, what Java is going to do is to insert the keyword this. So, we're calling method2 on the calling object. Now, as you saw from the last concept challenge on Polymorphism, this is going to bind at runtime. So, we're gonna look at the actual type of the object disc at runtime. This is what's tricky about this example. So, the actual type of this object at runtime, as you recall, was an Undergrad object. So we're gonna go down here into the Undergrad class and call the method2 inside the Undergrad class. So that's going to print out, Undergrad 2, and we have the following that gets printed. First, Student 1, then Person 1, and then finally Undergrad 2. So the purpose of this example was to show you the difference between what happens with a call to super and then a method and a call to this and a method. Those calls to super and then a method get bound to compile time. So in compile time, Java is going to look at the code and figure out what class it's in and what the super class of that class is. But when we call this and then a method name, that's gonna happen at runtime. So Java is gonna use the actual type of the object at runtime. It's a little confusing but, understanding these details can help you debug some nasty bugs that you might otherwise encounter in your code. So, good Luck.

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