Concept Challenge: Polymorphism Part 1

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Del curso dictado por University of California San Diego

Object Oriented Programming in Java

3069 calificaciones

University of California San Diego

Object Oriented Programming in Java

3069 calificaciones

Curso 3 de 4 en SpecializationObject 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!

De la lección

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

Conoce a los instructores

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

In this concept challenge we're gonna have some fun with polymorphism and

dynamic binding.

We want it to be a little bit challenging, but hopefully not too far beyond the reach

of what you can do with your new knowledge about how this works.

So you know the drill.

You're going to pause, answer the question on your own,

discuss it with people around you or online, watch our learner videos, and

then confirm your understanding with our explanation.

So what's the problem?

Here it is.

We two classes for you, the student class and the person class.

The person class is the base class.

It has a private member variable name and then the number of methods.

There's one constructor.

It takes an argument.

There's a method called isAsleep.

It takes an integer argument, and it returns a boolean.

The integer argument indicates the time of day, in term of hours on a 24 hour clock,

and a person is asleep between the hours of 10 PM, or 22, and 7 AM.

So between those hours, isAsleep will return true.

It also has a two string method which just returns its name, and

then it has a status method which also takes an integer argument for the hour.

And what it's gonna do is it's gonna check to see if the person is asleep at that

hour, and if it is,

it will say the person is offline or print a message to that effect.

And if they're not asleep then it'll print a message saying the person is online.

That's the person class.

The student class extends the person class.

And so it inherits a bunch of things from the person, and then what it does is it

overrides the isAsleep method to slightly change the hours that a student is asleep.

So a student in our world here is only asleep between the hours of 2 AM and 8 AM.

So if we call isAsleep between those hours then the student will report being asleep.

So the question for you is, what happens when we create a reference to the person,

set that reference equal to a new student object, and

then call the status method on that object at time 1 in the morning?

>> I'm Alicia.

>> I'm Joanna.

>> And I'm Justin.

>> So actually when I looked at this code

I was wondering if person can actually even be a student.

>> I think student can be a person since it extends person.

So.

>> I agree with Justin because student is a person, and you know it's a person.

So it's just a more specific type of person.

>> What do you think it prints out?

>> Mm..

I don't know.

Will it even print out?

Because there are two, if you look in the public class person and

in the class student, both of them have method is asleep.

Which is called in the second line of the of the method status?

So, how does the compiler know, which one to call?

So wouldn't it just freak out, and stop, and be an error?

>> I think it has something to do with the this call.

>> Oh. >> So,

maybe this this tells it which method it's calling.

And I guess since we can have it be a student,

it's probably calling students is asleep.

>> Yeah, I think that's correct, yeah.

>> What I was confused about was within status [CROSSTALK]

>> Yes.

>> It calls is asleep.

In this well, it calls isAsleep when it's in person.

So shouldn't this be a person?

>> It's a good, it's pretty tricky.

>> But we assigned person to be a student, right?

Oh.

>> So at this point perhaps at compile time maybe it'll call is asleep in

the person class, but maybe at run time,

it'll say this is a student, and then call students is asleep.

>> Okay.

>> All right, there's a lot going on here so

let's trace through this very carefully.

The first thing we have to think about is even though P

is a person reference, the actual object it refers to is a student.

So when we call p.status at one in the morning,

we're calling that dot status on a student object.

This is what Java does at run time.

It determines that P refers to a student object and

that's the type that it's going to call the status method on.

So we're gonna look in the student class for the status method.

And so we look here, well we don't see a status method.

But that's no problem because student extends person, so Java will just go ahead

and look in the superclass to see if there's a status method there.

And it turns out there is.

Here's our status method.

So we can start tracing through the status method to see what actually happens.

The first thing we do is we have an if statement.

And that If statement asks this.isAsleep.

So the question is, is this asleep at the particular hour?

Now what is this?

Well even though we're currently in the person class, that's where the code is.

This is at run time determined to be a student object.

Because the student object is the one that called this method originally.

So this dynamically binds to the student object.

And that's the class where there is asleep gets called.

So we go back down here and we use the student's version of is asleep

to determine if the student is asleep at one in the morning.

Now I already said the student only sleeps between two in the morning and

eight in the morning.

So the student is not asleep at one o'clock and so

we'll fall through to this else condition.

And print out that this student is online, says now online,

this is what it'll print, now online.

And the students name is Sally.

That's it.

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