4- Generic Classes

Jun 20, 2026 05:17 · 4:27 · English · Whisper Turbo · 3 speakers

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Lecture Index

0:03

S… Speaker 1 (4- Generic Classes)

Let's see how generics prevent the problems we discussed earlier.

0:06

S… Speaker 2 (4- Generic Classes)

So,

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S… Speaker 1 (4- Generic Classes)

let's add a new class in this package.

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We call this generic list to differentiate from

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the other list implementation.

0:17

S… Speaker 1 (4- Generic Classes)

Now we want this class to be generic so we can reuse it to store different

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types of objects.

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S… Speaker 1 (4- Generic Classes)

So right after the name of the class we type is angle

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brackets and inside these brackets we type a capital T as

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in short for type or template.

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Now we could use any letters here,

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it doesn't have to be a T,

0:35

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but T is a common convention.

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Another convention is E

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as in short for element,

0:41

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we use that when we want to implement a class that acts as a collection.

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S… Speaker 1 (4- Generic Classes)

So it can store many elements.

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S… Speaker 1 (4- Generic Classes)

So here we could use either e or t.

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I'm going to go with t.

0:50

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Now this t is a type parameter for this class.

0:53

S… Speaker 1 (4- Generic Classes)

So just like our methods can have parameters,

0:55

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our classes can have parameters as well.

0:58

S… Speaker 1 (4- Generic Classes)

This T over here represents the type of objects we want to store in this

1:02

S… Speaker 1 (4- Generic Classes)

list. When we create an instance of this class,

1:05

S… Speaker 1 (4- Generic Classes)

we'll have to specify an argument or a value for this parameter.

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S… Speaker 1 (4- Generic Classes)

For example,

1:09

S… Speaker 1 (4- Generic Classes)

back in our main class,

1:10

S… Speaker 1 (4- Generic Classes)

let me delete all this code.

1:12

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If you want to create an instance

1:16

S… Speaker 1 (4- Generic Classes)

of our generic list,

1:17

S… Speaker 1 (4- Generic Classes)

we type new generic list.

1:19

S… Speaker 1 (4- Generic Classes)

Now here we specify the type of objects we want to store in

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S… Speaker 1 (4- Generic Classes)

this list.

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S… Speaker 1 (4- Generic Classes)

For example,

1:25

S… Speaker 1 (4- Generic Classes)

user.

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S… Speaker 1 (4- Generic Classes)

Or string.

1:27

S… Speaker 1 (4- Generic Classes)

Or integer.

1:29

S… Speaker 2 (4- Generic Classes)

Okay?

1:31

S… Speaker 1 (4- Generic Classes)

So back to our generic less implementation this T represents the

1:35

S… Speaker 1 (4- Generic Classes)

type of objects we want to store here Now we need an array of type T

1:39

S… Speaker 1 (4- Generic Classes)

so private T array we call it items and

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set it to new T array of 10

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Now here we have a compilation error because the Java compiler doesn't know

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S… Speaker 1 (4- Generic Classes)

the type of T at this stage.

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S… Speaker 1 (4- Generic Classes)

Is it the character class or the string class or the user class?

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S… Speaker 1 (4- Generic Classes)

It doesn't know what it is.

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That is why it cannot instantiate it.

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Now one simple workaround is to use new object array here and

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S… Speaker 1 (4- Generic Classes)

then cast a result to T

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S… Speaker 1 (4- Generic Classes)

array.

2:09

S… Speaker 2 (4- Generic Classes)

Okay,

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S… Speaker 1 (4- Generic Classes)

now just like before when it

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S… Speaker 1 (4- Generic Classes)

a field for keeping track of the number of items in this list.

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So private int count.

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S… Speaker 1 (4- Generic Classes)

Next we implement our add method,

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so public void add.

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Now instead of an integer or an object,

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we pass t.

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The type of t will be determined later when we create an instance

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of this class.

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So we call it item just like before.

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And here we store item in our array.

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So items of count plus plus equals item.

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S… Speaker 3 (4- Generic Classes)

Finally,

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S… Speaker 1 (4- Generic Classes)

we need our get method,

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so public t get integer index.

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Again,

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S… Speaker 1 (4- Generic Classes)

instead of an integer or an object,

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we return t.

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So here we return items

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S… Speaker 1 (4- Generic Classes)

of index.

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S… Speaker 2 (4- Generic Classes)

Now,

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S… Speaker 1 (4- Generic Classes)

back to our main class,

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S… Speaker 1 (4- Generic Classes)

when we create a generic list of integer and

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S… Speaker 1 (4- Generic Classes)

call the add method,

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S… Speaker 1 (4- Generic Classes)

Look, the type of this parameter becomes integer.

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S… Speaker 1 (4- Generic Classes)

So if I call this method and pass a string,

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S… Speaker 1 (4- Generic Classes)

I get a compile time error.

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S… Speaker 1 (4- Generic Classes)

This is the benefit of generics.

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S… Speaker 1 (4- Generic Classes)

This generic class ensures that every object in this list is an integer.

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S… Speaker 2 (4- Generic Classes)

Okay,

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S… Speaker 1 (4- Generic Classes)

now actually let's store this in a variable called

3:26

S… Speaker 1 (4- Generic Classes)

list and here we call list

3:30

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.add.

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Let's pass an integer

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S… Speaker 1 (4- Generic Classes)

Now here's the second benefit.

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S… Speaker 1 (4- Generic Classes)

When we call the get method,

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S… Speaker 2 (4- Generic Classes)

look,

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S… Speaker 1 (4- Generic Classes)

we get an integer.

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S… Speaker 1 (4- Generic Classes)

So if we get the first item,

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we can simply store it in an integer,

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and here we don't need an explicit cast.

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S… Speaker 1 (4- Generic Classes)

If we had a generic list of users,

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S… Speaker 1 (4- Generic Classes)

we could add a user object here,

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S… Speaker 1 (4- Generic Classes)

and when reading the first item,

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S… Speaker 1 (4- Generic Classes)

we would store it in a user object.

4:01

S… Speaker 2 (4- Generic Classes)

Okay,

4:03

S… Speaker 1 (4- Generic Classes)

again, we don't need an explicit cast.

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S… Speaker 1 (4- Generic Classes)

So our code becomes cleaner and we get compile time type safety.

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S… Speaker 1 (4- Generic Classes)

So we can catch our mistakes at compile time rather than at runtime.

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S… Speaker 1 (4- Generic Classes)

That is the beauty of generics.

4:16

S… Speaker 1 (4- Generic Classes)

Now over the next few videos,

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S… Speaker 1 (4- Generic Classes)

we're going to study generics in more detail.

4:19

S… Speaker 1 (4- Generic Classes)

You're going to understand how exactly generics work under the hood.

4:23

S… Speaker 1 (4- Generic Classes)

So I will see you in the next video.

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