11- Generic Classes and Inheritance

Jun 20, 2026 06:59 · 4:17 · English · Whisper Turbo · 2 speakers

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Lecture Index

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S… Speaker 2 (11- Generic Classes and Inheritance)

In this video we're going to talk about a common misunderstanding when it comes to programming

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with generics.

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S… Speaker 2 (11- Generic Classes and Inheritance)

So let's add a new class here.

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We call it instructor.

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We want the instructor class to inherit from the user class.

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So here we type extends user.

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Now we have a compilation error.

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It says there is no default constructor available in user.

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So earlier we added this constructor to

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pass the number of points and for this reason

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our instructor class doesn't know what value or what argument to pass

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to the constructor of the user class.

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To solve this problem,

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we add a custom constructor here.

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S… Speaker 2 (11- Generic Classes and Inheritance)

So public instructor,

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we give it the points.

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S… Speaker 2 (11- Generic Classes and Inheritance)

Now here we should pass this value to the constructor of the base class.

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S… Speaker 1 (11- Generic Classes and Inheritance)

And we do that using the super keyword.

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S… Speaker 2 (11- Generic Classes and Inheritance)

So super and pass the points.

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S… Speaker 1 (11- Generic Classes and Inheritance)

The problem is gone.

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S… Speaker 1 (11- Generic Classes and Inheritance)

So here's our instructor class.

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Now back in the main class.

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S… Speaker 2 (11- Generic Classes and Inheritance)

Here we can declare a variable of type user.

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We call it user.

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Now we can set this to a new user or a

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new instructor.

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S… Speaker 2 (11- Generic Classes and Inheritance)

Because an instructor is a type of user.

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So let's pass 10 here.

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S… Speaker 1 (11- Generic Classes and Inheritance)

This is basic object oriented programming.

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S… Speaker 2 (11- Generic Classes and Inheritance)

If this concept is foreign to you,

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S… Speaker 2 (11- Generic Classes and Inheritance)

you really need to go back and watch the second part of this series.

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S… Speaker 2 (11- Generic Classes and Inheritance)

So if you have a variable of type user,

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S… Speaker 2 (11- Generic Classes and Inheritance)

we can set it to an instance of the user class or any of its derivatives

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S… Speaker 2 (11- Generic Classes and Inheritance)

like the instructor class.

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Now,

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S… Speaker 1 (11- Generic Classes and Inheritance)

why does this matter?

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S… Speaker 2 (11- Generic Classes and Inheritance)

Well,

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S… Speaker 2 (11- Generic Classes and Inheritance)

let's go back to our utils class and add a new method for printing

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a user.

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S… Speaker 2 (11- Generic Classes and Inheritance)

So public static void print user,

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we give it a parameter of type user and here

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we do a print line of user.

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Since we get a user here

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S… Speaker 2 (11- Generic Classes and Inheritance)

we can pass an instance of the user class or any of its derivatives.

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So back in the main class,

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we can call utils dot print user.

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Here we can pass a user object or an instructor object.

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Perfectly fine.

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Now,

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what if we expected a list of users here?

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So I'm going to go back to the utils class and add another method

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for printing a list of users.

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So public static void print users.

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the type of the parameter should be generic list of user,

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and we call it users.

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Now don't worry about the implementation.

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So this method takes a generic list of users.

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Now back to the main class,

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we can create a generic list of users,

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new generic list of user,

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and pass it to this method,

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so utils .print users,

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and that's perfectly fine,

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but if we change this to instructor,

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we get a compilation error.

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Because a generic list of instructor is not a subtype

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of generic list of user.

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Why is that?

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Well, earlier we talked about how these generic classes work under

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the hood.

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We looked at the bytecode representation of this generic list class.

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As I showed you earlier,

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this generic list internally maintains a list of objects no

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matter what we pass here.

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whether we pass the user class or the instructor class,

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we only have a single generic list in our project.

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And this class is not a subtype of itself.

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That is the reason why a generic list of instructor is not a

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subtype of generic list of user because we're dealing with a single class.

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Okay, so how can we solve this problem here?

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Let's say we have a list of instructors somewhere.

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So let's call this instructors.

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Now you really want to pass this list to

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our print users method.

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Well, one way is to convert this list to a list of users.

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So we declare a list of users.

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Then we have to iterate over the list

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of instructors and add each instructor object to our

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list of users.

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Once we do that,

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we'll be able to pass the list of users here.

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But this is tedious,

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right?

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S… Speaker 1 (11- Generic Classes and Inheritance)

In the next video,

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S… Speaker 2 (11- Generic Classes and Inheritance)

I'm going to show you how to solve this problem using wildcards.

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