12- Wildcards

Jun 20, 2026 07:23 · 5:27 · English · Whisper Turbo · 2 speakers

本记录誊本于2008年 24 天数。永久储存的升级 →

Lecture Index

0:03

S… Speaker 1 (12- Wildcards)

A better way to solve the problem in the last video is by using a wildcard.

0:07

S… Speaker 1 (12- Wildcards)

So we go back to our utils class and replace this

0:11

S… Speaker 1 (12- Wildcards)

type argument with a question mark which is called the wildcard character.

0:16

S… Speaker 1 (12- Wildcards)

This wildcard represents an unknown type.

0:19

S… Speaker 1 (12- Wildcards)

So with this we can go back to the main class and here we can pass

0:23

S… Speaker 1 (12- Wildcards)

a list of users.

0:23

S… Speaker 1 (12- Wildcards)

We can also pass a list of instructors.

0:26

S… Speaker 1 (12- Wildcards)

In fact,

0:27

S… Speaker 1 (12- Wildcards)

we can pass a list of anything.

0:28

S… Speaker 1 (12- Wildcards)

For example,

0:29

S… Speaker 1 (12- Wildcards)

we can pass

0:30

S… Speaker 1 (12- Wildcards)

a generic list of string objects or integers,

0:35

S… Speaker 1 (12- Wildcards)

so we can pass a list of anything.

0:36

S… Speaker 2 (12- Wildcards)

Now,

0:37

S… Speaker 1 (12- Wildcards)

in this case,

0:38

S… Speaker 1 (12- Wildcards)

it doesn't quite make sense to pass a list of integers and treat them

0:42

S… Speaker 1 (12- Wildcards)

as user objects,

0:43

S… Speaker 1 (12- Wildcards)

but don't worry,

0:44

S… Speaker 1 (12- Wildcards)

we're going to come back and fix it in a second.

0:45

S… Speaker 1 (12- Wildcards)

But let me show you something first.

0:47

S… Speaker 1 (12- Wildcards)

Back to the utils class,

0:49

S… Speaker 1 (12- Wildcards)

here I'm going to call users .get,

0:52

S… Speaker 1 (12- Wildcards)

now look at the return type of the get method.

0:55

S… Speaker 1 (12- Wildcards)

It's returning capture of unknown.

0:58

S… Speaker 1 (12- Wildcards)

What is this?

0:58

S… Speaker 2 (12- Wildcards)

Well,

1:00

S… Speaker 1 (12- Wildcards)

When we use a wild card,

1:01

S… Speaker 1 (12- Wildcards)

the Java compiler is going to create an anonymous type under the hood.

1:06

S… Speaker 1 (12- Wildcards)

So it's going to create a class.

1:09

S… Speaker 1 (12- Wildcards)

Let's call it cap number one.

1:11

S… Speaker 1 (12- Wildcards)

This is going to be a regular class in Java.

1:14

S… Speaker 1 (12- Wildcards)

We don't see it,

1:15

S… Speaker 1 (12- Wildcards)

but it's there.

1:16

S… Speaker 2 (12- Wildcards)

Now,

1:17

S… Speaker 1 (12- Wildcards)

why did I use one over here?

1:19

S… Speaker 1 (12- Wildcards)

Because we could have multiple wild cards here.

1:21

S… Speaker 1 (12- Wildcards)

Okay?

1:22

S… Speaker 1 (12- Wildcards)

So the get method returns an instance

1:26

S… Speaker 1 (12- Wildcards)

of this capture class.

1:28

S… Speaker 1 (12- Wildcards)

And that means if we get an object here,

1:30

S… Speaker 1 (12- Wildcards)

we can only store it in a variable of type cap or its

1:34

S… Speaker 1 (12- Wildcards)

base type.

1:35

S… Speaker 1 (12- Wildcards)

That is the object class.

1:37

S… Speaker 1 (12- Wildcards)

So object x equals users that get

1:41

S… Speaker 1 (12- Wildcards)

zero. So this is what happens when we use a wildcard.

1:44

S… Speaker 2 (12- Wildcards)

Now,

1:45

S… Speaker 1 (12- Wildcards)

how can we restrict the lists that are passed here?

1:48

S… Speaker 1 (12- Wildcards)

We don't want to pass a list of integers or strings.

1:50

S… Speaker 1 (12- Wildcards)

We want to pass a list of users or its subtypes.

1:53

S… Speaker 1 (12- Wildcards)

That's very easy to solve.

1:55

S… Speaker 1 (12- Wildcards)

Let me put this on a new line so you can see clearly.

1:59

S… Speaker 1 (12- Wildcards)

So here's the parameter of the print users method.

2:01

S… Speaker 2 (12- Wildcards)

Now,

2:03

S… Speaker 1 (12- Wildcards)

after this wildcard character,

2:06

S… Speaker 1 (12- Wildcards)

we can type extends user.

2:08

S… Speaker 1 (12- Wildcards)

When the Java compiler sees this expression,

2:12

S… Speaker 1 (12- Wildcards)

it's going to have the capture class extend the user class.

2:16

S… Speaker 1 (12- Wildcards)

So our capture class is going to look like this.

2:18

S… Speaker 1 (12- Wildcards)

Exactly like how the instructor class extends

2:23

S… Speaker 1 (12- Wildcards)

the user class.

2:24

S… Speaker 1 (12- Wildcards)

Let me put them side by side.

2:25

S… Speaker 1 (12- Wildcards)

So class instructor also extends the user class.

2:29

S… Speaker 2 (12- Wildcards)

Right?

2:30

S… Speaker 1 (12- Wildcards)

Now, back to the main class,

2:32

S… Speaker 1 (12- Wildcards)

we have a compilation error because we cannot pass a list of integers here.

2:36

S… Speaker 1 (12- Wildcards)

We can only pass a list of user or subtypes of user.

2:40

S… Speaker 1 (12- Wildcards)

So we can pass a list of user objects or list of

2:44

S… Speaker 1 (12- Wildcards)

instructor objects.

2:45

S… Speaker 1 (12- Wildcards)

These are all valid arguments.

2:47

S… Speaker 2 (12- Wildcards)

Now,

2:48

S… Speaker 1 (12- Wildcards)

because this capture class is a subtype of

2:53

S… Speaker 1 (12- Wildcards)

this user, we can get an object and store it in

2:57

S… Speaker 1 (12- Wildcards)

a user variable because the user class is the parent

3:01

S… Speaker 1 (12- Wildcards)

of the capture class,

3:02

S… Speaker 1 (12- Wildcards)

right?

3:03

S… Speaker 1 (12- Wildcards)

However,

3:03

S… Speaker 1 (12- Wildcards)

we cannot assign this object to a variable of type instructor because

3:08

S… Speaker 1 (12- Wildcards)

instructor and cap are two different types.

3:11

S… Speaker 1 (12- Wildcards)

It's like assigning an integer to a string.

3:14

S… Speaker 1 (12- Wildcards)

There are two entirely different classes.

3:16

S… Speaker 1 (12- Wildcards)

Let's change instructor back to the user.

3:19

S… Speaker 2 (12- Wildcards)

Now,

3:22

S… Speaker 1 (12- Wildcards)

what about adding an object to this list?

3:24

S… Speaker 1 (12- Wildcards)

Let's see what happens.

3:25

S… Speaker 1 (12- Wildcards)

So users that add,

3:27

S… Speaker 1 (12- Wildcards)

look at the parameter of this method.

3:30

S… Speaker 1 (12- Wildcards)

capture of unknown extends user.

3:33

S… Speaker 1 (12- Wildcards)

So here we can pass an instance of this capture class or

3:37

S… Speaker 1 (12- Wildcards)

any of its subtypes.

3:38

S… Speaker 2 (12- Wildcards)

Now,

3:39

S… Speaker 1 (12- Wildcards)

unfortunately, we do not have access to this capture class,

3:42

S… Speaker 1 (12- Wildcards)

so we cannot new it up here.

3:43

S… Speaker 1 (12- Wildcards)

New capture doesn't make sense.

3:45

S… Speaker 1 (12- Wildcards)

So with the extends keyword,

3:48

S… Speaker 1 (12- Wildcards)

we can only read from this list.

3:50

S… Speaker 1 (12- Wildcards)

We cannot add to it.

3:51

S… Speaker 1 (12- Wildcards)

If you want to add to a list,

3:53

S… Speaker 1 (12- Wildcards)

we should use the super keyword.

3:56

S… Speaker 1 (12- Wildcards)

Now, when the Java compiler sees this expression,

3:59

S… Speaker 1 (12- Wildcards)

it's going to treat this unknown type like the parent of the user class.

4:03

S… Speaker 1 (12- Wildcards)

What is the parent of the user class?

4:05

S… Speaker 1 (12- Wildcards)

It's the object class.

4:06

S… Speaker 1 (12- Wildcards)

So this user's list is going to look like this.

4:09

S… Speaker 1 (12- Wildcards)

Generic list of object,

4:12

S… Speaker 1 (12- Wildcards)

let's call it temp,

4:13

S… Speaker 1 (12- Wildcards)

new generic list.

4:15

S… Speaker 1 (12- Wildcards)

So the Java compiler is going to treat this object

4:19

S… Speaker 1 (12- Wildcards)

like this object.

4:20

S… Speaker 2 (12- Wildcards)

Now,

4:23

S… Speaker 1 (12- Wildcards)

in this list,

4:24

S… Speaker 1 (12- Wildcards)

we can add an instance of the object class or any of its subtypes.

4:29

S… Speaker 1 (12- Wildcards)

So we can type users .add new user.

4:32

S… Speaker 1 (12- Wildcards)

We can also add a new instructor.

4:35

S… Speaker 1 (12- Wildcards)

Because all these types directly or indirectly

4:39

S… Speaker 1 (12- Wildcards)

derive from the object class.

4:41

S… Speaker 1 (12- Wildcards)

However,

4:42

S… Speaker 1 (12- Wildcards)

if we use the super keyword,

4:44

S… Speaker 1 (12- Wildcards)

we cannot read from this list.

4:45

S… Speaker 1 (12- Wildcards)

So if I type users .get0,

4:49

S… Speaker 1 (12- Wildcards)

we can store the result in a variable of type object.

4:53

S… Speaker 1 (12- Wildcards)

If you want to store the result in a variable of

4:57

S… Speaker 1 (12- Wildcards)

type user,

4:59

S… Speaker 1 (12- Wildcards)

we get a compilation error because the java compiler doesn't know if

5:03

S… Speaker 1 (12- Wildcards)

the object that we're trying to get is compatible with a user.

5:06

S… Speaker 1 (12- Wildcards)

It could be a string object,

5:07

S… Speaker 1 (12- Wildcards)

it could be a daytime object.

5:09

S… Speaker 1 (12- Wildcards)

That is the reason why when we use the super keyword,

5:12

S… Speaker 1 (12- Wildcards)

we can get an object from this list and store it in a variable of

5:16

S… Speaker 1 (12- Wildcards)

type object.

5:17

S… Speaker 1 (12- Wildcards)

So to recap,

5:19

S… Speaker 1 (12- Wildcards)

if you want to read from this list,

5:20

S… Speaker 1 (12- Wildcards)

you should use the extends keyword.

5:22

S… Speaker 1 (12- Wildcards)

If you want to add to it,

5:23

S… Speaker 1 (12- Wildcards)

you should use the super keyword.

This transcript was generated by AI (automatic speech recognition). May contain errors — verify against the original audio for critical use. AI policy

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