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Introducing Inheritance
4:28In this video we'll learn what inheritance is and use it for the first time!
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At this point,
we've seen how to create objects with
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fields representing their properties and
methods representing their actions.
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In this course, we'll see how to do a lot
more with objects by using inheritance.
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Inheritance is actually
a pretty simple concept.
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Let's say we have a class called A, and
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this class has a couple fields and
a method.
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Now let's say we have another class, B,
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which is exactly like A except it has
an extra field and an extra method.
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It's a lot of extra code, and
we're definitely repeating ourselves.
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But with inheritance, we can clean things
up by just saying that B inherits from A,
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which we do with the extends key word.
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So this code is exactly the same
as before, just much shorter.
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Now, in most cases, the class names will
make a lot more sense than just A and B.
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But using these simple names, let's us
focus on what inheritance actually is.
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A class using another
class as its foundation.
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All right, that's enough theory.
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Let's do some practice in a new project.
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Let's pick Create New Project,
then click Next,
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select our Command Line App template,
hit Next again.
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And for the project name, let's just
name it, inheritance, and click Finish.
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Awesome.
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Now, just to make things
a little easier for you to see,
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I'm going to put my IntelliJ
into full screen mode, okay.
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Now, right below the main class let's
create a new class named animal.
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Typically we'd put each
class in its own file.
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But since we're learning, it'll be
nice to have everything on one page.
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Let's type class Animal,
and add the brackets.
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And I'll go ahead and
hide the project pane as well.
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Also, note that if we try to make our
animal class public, we'll get an error.
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While you can have more
than one class in a file,
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all public classes need their own files.
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Al lright,
inside this class lets add a string
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field named sound and
set it equal to an empty string.
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Then lets add a void method
called makeSound and
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use the S-O-U-T shortcut,
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or sout to have it print
out the sound variable.
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Nice.
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Next let's make a dog class that will
bark when we call the makeSound method.
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But first, let's head up to the main
method and delete the comment.
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Then let's copy and paste in
the code from the teacher's notes.
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Once we're done with the Dog class,
we should be able to run this program, and
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see that our dog has said bark.
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So right below our Animal class, let's
create our Dog class by using inheritance.
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So actually,
why don't you take a stab at it first?
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Remember, the key word is extends.
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Okay, pause me, and when you come back,
I'll show you how I solved it.
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Did you get it?
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Here's how I did it.
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First, create the Dog class,
and make it extend from animal.
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Then, inside the Dog class,
we'll add a constructor,
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Dog( ) and then the brackets,
and inside the constructor,
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we'll set sound = "bark" and
add the semicolon.
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Finally, if we run the program,
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We'll see that our dog says bark.
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Awesome job learning about Inheritance.
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Being able to use another
class as a foundation for
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new one is a real time saver.
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And it's not hard to imagine how you
could start with something simple and
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build your way up to
something really complicated.
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More on that in the next video.
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