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Core: Method Overriding

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Programação orientada a objetos em Java

Universidade da Califórnia, San Diego

4.7 (4,478 classificações) | 200K alunos inscritos

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

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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 (4,478 classificações)

5 stars
76%
4 stars
19%
3 stars
3%
2 stars
1%
1 star
1%

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.

HW

Mar 21, 2016

It is good!!! I learnt a bit of Java beforehand but not the object oriented concept. Would definitely attend the courses by UC San Diego again as well as pay for the course when I can afford it!

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: 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

Ministrado por

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

Experimente o curso Gratuito

Transcrição

[MUSIC] Hi, and welcome back to our module on Polymorphism Inheritance in Java. Today we'll be talking about method overriding and you've already used method overriding when you did draw and set up methods back when you designed your GUI but here we're gonna dive into method overriding in a bit more detail. So goal here today is where the create methods are override from superclass methods and will the contrast overloading from method overriding. Back when we first started talking about inheritance we had three goals. And overriding really helps us has all three. So we wanna have common code in one pairing class. We do this if we don't override a method. So if a method just exists in the superclass we get back common code. But if we want our subclasses to behave differently all we have to do is override a method and now we get that different behavior. And because polymorphism which we'll get to in the next set of videos we'll see that this allows us to have all of our object in one structure and still be able to call the appropriate method. It's really common for new learners to mix up overloading and overriding so I want to talk just real quickly about overloading and overriding. Overloading is the same class that has the same method name but different parameters. So this is a different method signature, within the same class. This is different then method overriding, where you have a subclass that has same method signature, which means same name, same parameters as a super class. So now let's start looking at some more detail with a few examples. So first example is to look at the object class itself. So we have a method in the outer class called toString. And the odds are you probably already ran toString method before, but if you haven't, toString prints the contents or string representation of an object. Because it's in the object class, all objects in Java can override it. So let's do this for our hierarchy. So we have our person class at the top, It has a name and a getName and we're going to add the toString method to it. To do so we're going to override the that object method's toString method. Now, a really basic way to do this would be to just return the name and since we already have right now in our person class is name, this is really all we can do. And then in our code that calls it, we're gonna create a Person p, we're gonna create a person that has the name Tim and then we're gonna call this method. So you just do System.out.println and put p.toString and that will return a string and that is what we want. And it doesn't call the object two string method, it's going to call the person two string method because of override. Now it turns out that two string here in this context is unnecessary because print line actually automatically calls toString if you ever pass an object as a parameter to print line. So I'm gonna just display it this way from here on. Now if we run this code, what's going to happen is it's just going to print Tim. All right, well let's add to our inheritance hierarchy. Now I'm gonna go to my student class where we've already added a student ID. And I get student ID and now we're going to add the method toString. Here's the code to essentially have that student ID and the ability to get access to it. And now what I want, is I wanna be able to print a student ID number followed by the information about the person. And my first version of this code will look something like this. So return this getSID then a colon, and then this.getName. And the concern I would have here is what if the person changes. So the only reason I wrote this dot getName right now is because I know that's person does. So if you try to call toString on person, it prints the name. But Person could change. So this really isn't the right way to do it. What I want to do instead is call the toString method within Person and to do that I wanna use that super keyword, just like we saw from the constructors to say, I want the method from my superclass. So now, if we walk through this code I'm gonna use Student s = new Student. I'm gonna print out that which is gonna call the toString method. A question is what do we expect to see happen. Well, we expect to see the student ID number now followed by the name, which is exactly what we wanted to have happen. What happens now if I change the referencing? What if I make the reference a Person instead of a Student? And I still pass, I still try to call s.toString, on that Person. Is it gonna call the Student to string method or is it gonna call the person to string method? Well what do we want to have happen? We want to have it print out the student information, which is exactly what it does. Why is it gonna print the student method? Because of polymorphism, and we'll get into that in the next lesson.

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