Core: Method Overriding

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Do curso por University of California San Diego

Object Oriented Programming in Java

3450 classificações

University of California San Diego

Object Oriented Programming in Java

3450 classificações

Curso 3 de 4 em Specialization Object Oriented Programming in Java

Have you wondered how huge software projects, like the Google search engine, are developed? In this course, you’ll move beyond coding in the “main” method to learn to design and build more complex Java software projects (though not quite as complex as the Google search engine!). We’ll explore how to divide up a large project into a hierarchy of classes and how to increase the functionality of your projects by importing existing libraries. We’ll also look at some core algorithms for searching for and sorting data. And along the way, you will develop an exciting interactive application with a graphical user interface. In this course we want you to be inspired to create. You’ll dive into a course project right away, with each lesson designed around concepts that are directly applicable to extending the project’s functionality. The video lectures include both core content as well as testimonials and stories from real-world software engineers, conversations between real students who are learning the material that you will likely relate to, and numerous support videos that you can turn to if you get stuck.

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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

Conheça os instrutores

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

[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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