12- Composing Completable Futures

Jul 04, 2026 01:09 · 6:08 · English · Whisper Turbo · 2 ਸਪੀਕਰ

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S… Speaker 1 (12- Composing Completable Futures)

Quite often we want to start a task upon completion of another task.

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S… Speaker 1 (12- Composing Completable Futures)

For example,

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let's say we have the idea of a user Now we want to get the user's email

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S… Speaker 1 (12- Composing Completable Futures)

So we have to go to our database and read that user record with this

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we can get their email Now we want to pass this email to one of those

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music streaming websites where people have their own playlist So we pass

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the email and get the playlist for given person

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S… Speaker 1 (12- Composing Completable Futures)

So here we have two tasks,

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two asynchronous operations.

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S… Speaker 1 (12- Composing Completable Futures)

We want to start the second task upon completion of the first task.

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S… Speaker 1 (12- Composing Completable Futures)

Completable futures make it really easy to implement these kind

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of scenarios in a declarative and efficient way.

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S… Speaker 1 (12- Composing Completable Futures)

Let me show you.

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S… Speaker 1 (12- Composing Completable Futures)

So first we're going to create a completable future for getting the email

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of a user.

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So completable future,

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let's apply async to pass a lambda

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And here we just want to return the email for this user.

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S… Speaker 1 (12- Composing Completable Futures)

In a real world application,

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in the body of this lambda,

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S… Speaker 1 (12- Composing Completable Futures)

we're going to query a database.

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S… Speaker 1 (12- Composing Completable Futures)

But here I just want to return a string.

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S… Speaker 2 (12- Composing Completable Futures)

Now,

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S… Speaker 1 (12- Composing Completable Futures)

this returns a completable future of string,

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S… Speaker 1 (12- Composing Completable Futures)

right?

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S… Speaker 1 (12- Composing Completable Futures)

So completable future of string.

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S… Speaker 2 (12- Composing Completable Futures)

Now,

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S… Speaker 1 (12- Composing Completable Futures)

we want to start the second task upon completion of the

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S… Speaker 1 (12- Composing Completable Futures)

first task.

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S… Speaker 1 (12- Composing Completable Futures)

To do that,

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S… Speaker 1 (12- Composing Completable Futures)

we call future dot then compose.

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Now here we should pass a function that maps a string to a completion

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stage object.

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S… Speaker 1 (12- Composing Completable Futures)

That means from this function we should return a new completable

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future.

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S… Speaker 1 (12- Composing Completable Futures)

That completable future represents a new asynchronous task,

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S… Speaker 1 (12- Composing Completable Futures)

right?

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S… Speaker 1 (12- Composing Completable Futures)

So what is the string that we get here?

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S… Speaker 1 (12- Composing Completable Futures)

That is the email that is returned from our first task.

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So we get the email,

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S… Speaker 1 (12- Composing Completable Futures)

now we want to pass this to a second task.

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S… Speaker 1 (12- Composing Completable Futures)

So here we should return a new completable future object.

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That is very easy.

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S… Speaker 1 (12- Composing Completable Futures)

So completable future dot supply async.

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S… Speaker 1 (12- Composing Completable Futures)

Here we should call that remote service to get the playlist for this user.

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S… Speaker 1 (12- Composing Completable Futures)

So we pass another supplier object.

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S… Speaker 1 (12- Composing Completable Futures)

With this supplier,

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I'm going to return this string,

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playlist.

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S… Speaker 1 (12- Composing Completable Futures)

Now in real application,

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we're going to return a playlist object or a list of songs.

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S… Speaker 1 (12- Composing Completable Futures)

But here I don't want to waste time creating different classes.

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S… Speaker 1 (12- Composing Completable Futures)

I just want to return a string.

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S… Speaker 1 (12- Composing Completable Futures)

All I want you to understand is that our first task returns an email,

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S… Speaker 1 (12- Composing Completable Futures)

then we pass the email to our second task and get the

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playlist, okay?

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S… Speaker 1 (12- Composing Completable Futures)

So when we call then

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compose, we get a new completable future,

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so here we can chain another method like then accept,

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S… Speaker 1 (12- Composing Completable Futures)

we get the playlist and print it on the terminal.

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S… Speaker 1 (12- Composing Completable Futures)

As simple as that.

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S… Speaker 1 (12- Composing Completable Futures)

Now, let's reformat our code.

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So let's put each method call on a new line.

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S… Speaker 1 (12- Composing Completable Futures)

That is better.

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We don't need the comments.

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Let's run our program.

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So we get the playlist.

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Beautiful.

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S… Speaker 2 (12- Composing Completable Futures)

Now,

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S… Speaker 1 (12- Composing Completable Futures)

let's improve this code further.

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S… Speaker 1 (12- Composing Completable Futures)

Technically we don't need this future variable.

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S… Speaker 1 (12- Composing Completable Futures)

We can immediately call this method on the object that

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is returned over here.

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So let's get rid of the future variable.

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So we call completable future the supply async.

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This is better,

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but we can make this even better.

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What is this lambda representing?

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S… Speaker 1 (12- Composing Completable Futures)

It's representing a task for getting a user object from a database.

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S… Speaker 1 (12- Composing Completable Futures)

So in a real application,

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we would probably have a method somewhere like public,

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in this case static.

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In a real application,

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we would return a user object,

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but here I want to return a string.

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We call this method getUserEmail.

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S… Speaker 1 (12- Composing Completable Futures)

I told you that querying a database is a long -running operation.

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S… Speaker 1 (12- Composing Completable Futures)

So we should make our APIs asynchronous.

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S… Speaker 1 (12- Composing Completable Futures)

So instead of returning a string,

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S… Speaker 1 (12- Composing Completable Futures)

we should return a completable future of string.

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S… Speaker 2 (12- Composing Completable Futures)

Now,

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S… Speaker 1 (12- Composing Completable Futures)

inside this method,

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S… Speaker 1 (12- Composing Completable Futures)

we're going to have some code for querying a database and getting someone's email.

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S… Speaker 1 (12- Composing Completable Futures)

So we return completable future dot supply

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S… Speaker 1 (12- Composing Completable Futures)

async and return this email.

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S… Speaker 2 (12- Composing Completable Futures)

Okay?

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S… Speaker 1 (12- Composing Completable Futures)

Now, with this method,

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we no longer need this piece of code over here.

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S… Speaker 1 (12- Composing Completable Futures)

We can simply call get user email.

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Also,

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as I told you before,

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it's better to use the async suffix here to indicate that

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this is an asynchronous method.

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So let's rename this to get user email async.

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S… Speaker 2 (12- Composing Completable Futures)

Now,

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S… Speaker 1 (12- Composing Completable Futures)

we're going to apply the same technique over here.

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for the second task that gets an email and returns a

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playlist asynchronously so in a real application somewhere we're going to

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have another method for getting the playlist of a given user so

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public static we have a long -running operation so

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we should return a completable future of in this case string

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and we call that method get playlist async

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Now here we need a parameter of type string called email.

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So this method gets the email of a user and calls

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a remote API to get the playlist for that user in an asynchronous fashion.

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Now here we should return a completable feature that's

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a play sync and we return this playlist.

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Right?

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S… Speaker 1 (12- Composing Completable Futures)

Now with this refactoring we no longer need this

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lambda over here.

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We can use a method reference.

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So the name of this class is completable futures demo.

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I'm going to type that here.

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Completable futures demo.

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Then we have colon colon and finally get playlist

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async.

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Now look at the end result.

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We're getting the email of a user in an asynchronous fashion.

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Then we're composing this with another operation for getting the playlist

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and finally we print the playlist.

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This is the beauty of completable features.

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We can build this kind of complex asynchronous operations in

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a declarative way.

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