11- Transforming a Completable Future

Jul 04, 2026 01:01 · 4:34 · English · Whisper Turbo · 2 Sprekers

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Sometimes we need to transform the result of an asynchronous task.

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For example,

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our weather service may return a temperature in a complex data structure,

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but perhaps we're only interested in the temperature as a numeric value.

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So we want to map or transform the result into a different type.

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Or as another example,

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imagine the weather service returns the temperature in Celsius,

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but we want to print it in Fahrenheit.

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So let me show you how to do that.

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First,

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I'm going to create a completable future using the supply

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async method.

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Here we pass a supplier that returns the temperature in Celsius,

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let's say 20 degrees.

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Now we get a completable future and store it over here.

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This future object has a method called

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then apply.

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So as I told you before,

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whenever you see then that means execute this piece of code after

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this task or this future is complete.

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So with the apply method we can convert or map the result to a different

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type We also have then apply async which

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will execute this code asynchronously So it will submit it as a new

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task to the underlying executor.

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Okay, so let's call then apply here

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We should pass a function that takes an integer and maps it

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to a different type.

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So we get a result in Celsius

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And we can convert it to Fahrenheit using this formula.

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Celsius times 1 .8 plus

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32.

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Now,

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just like the exceptionally method that you learned about in the last video,

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this method returns a new completable future.

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So if we call the get method on this completable future,

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we'll get the result in Fahrenheit.

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Let me show you.

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So we call get and store the result over here.

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Now to make our code cleaner,

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it's nicer to put each method on a new line.

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That is better.

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Now let's wrap this inside a try catch block.

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So we get the result in Fahrenheit and then print

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it. So that

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is 68 degrees.

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Beautiful.

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Now we can make this code better.

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Instead of passing this formula as a lambda,

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we can encapsulate this inside a method.

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So I'm going to add a static method in this class.

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public static integer to Fahrenheit,

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a bit hard to spell,

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we give it a parameter called Celsius,

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and here we return Celsius times

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1 .8 plus 32.

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Now here we have a compilation error,

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because the result of this expression is a double,

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but we're expecting an integer.

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We can solve this by casting this to an integer,

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the problem is gone.

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Now,

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we can replace this lambda with a method reference.

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So,

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The name of our class is completable futures demo we

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type completable futures demo colon colon to

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Fahrenheit that is better now here

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we don't have to call the get method earlier I showed you how to run code

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upon completion of a task we talked about two methods then run

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for executing a task and Then accept for

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executing a task,

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but also passing the result to that task

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So I'm going to simplify this code.

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Let's grab what we have inside this try block

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Cut it.

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We don't need the try block anymore So here

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we create a completable future to call our remote API Then

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we transform the result to Fahrenheit Now

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instead of calling the get method we call then accept Because

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we want to get the result the result here is going to be temperature in Fahrenheit.

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So F goes to

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print F.

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Now we don't need the print statement anymore and

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we don't need this variable either.

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So we build a completable

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future then we transform it and finally we accept or

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consume the result.

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Let's run our program.

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There you go.

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So here's the temperature in Fahrenheit.

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So this is the benefit of completable futures.

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With this completable futures we can build complex asynchronous operations

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in a declarative way.

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