13- Combining Completable Futures

Jul 04, 2026 01:15 · 4:32 · English · Whisper Turbo · 1 Cynhadledd

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Mynegai Llythrennedd

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In my opinion,

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one of the most powerful features of completable futures is the ability

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to start two tasks asynchronously and then combine the results.

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For example,

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let's say we call a remote service to get the price of a

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product and that service returns the price in US dollars.

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So we get 20 US dollars.

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Now at the same time,

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in parallel,

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we want to call another service to get the exchange rate between US

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dollars and euros.

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So that service is going to return 0 .9.

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Now we don't want to call the second service upon completion of the first

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task.

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We want to start this task concurrently and then wait for both of them to complete

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in order to calculate the result.

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So let me show you how to do this.

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First we want to create a completable future to return the price of

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a product.

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So completable future .supply async and

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here we return 20.

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That is our first task.

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Now, in our second task,

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we're going to call another service to get the exchange rate between US

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dollars and euros.

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So one more time,

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completable future,

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dot supply async,

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here we return 0 .9.

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This is our second task.

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Now we call first dot then combine.

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With this we can combine the result of two asynchronous operations.

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So we can wait for both of them to complete to calculate the final result.

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But here's the interesting part.

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Calling this method is not going to block the current thread.

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We're basically building a processing pipeline.

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We're building a recipe.

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We're telling the executor framework,

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hey, go ahead and start these two tasks concurrently.

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When they're both complete,

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start a new task to do something else.

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All of this is happening asynchronously,

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okay?

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So look at the signature of this method.

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Here we have two parameters.

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The first parameter is a completion stage object.

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That is our second task.

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So let's pass it over here.

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Now the second parameter is a bifunction.

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That is a function with two parameters that returns a value.

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Look at the type of these parameters.

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The first parameter is an integer.

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The second is a double.

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So these parameters represent the results of

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our tasks.

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So the first task returns a price in US dollars and

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the second task returns the exchange rate.

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Exchange rate.

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So this is our buy function.

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Now we get these two values and based on these we're going to calculate

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a new value.

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So here we can return price times exchange rate.

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Okay?

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Now,

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this method

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Then combine is going to return a new completable future.

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So after that we can call then accept.

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Here we get the result and print it.

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Now let's reformat our code.

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That is better.

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So run.

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So the price is $18.

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Now let's make this example more interesting.

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Let's imagine that the first task returns the price as

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a string.

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20 US dollars.

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Now we have a compilation error because we cannot multiply a string by

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a number.

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So in our first task,

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we should transform the result.

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So right after calling the supply async method,

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we call then apply.

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Here we should pass a function that takes a string and returns a

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new value, in this case an integer.

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So I'm going to reformat this code.

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First we call supply async and then

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then apply.

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So here we get a string

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Now we call str .replace.

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I'm going to replace US dollars with an empty string.

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We could also use regular expressions.

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It doesn't really matter.

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This is a very basic transformation.

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So we get that.

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That gives us the price.

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Now we call integer .parseInt

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and pass the price.

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And finally return the result.

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So here's our first task.

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Then we have our second task.

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We combine them and wait for both of them to complete before

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displaying the result.

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Now all of this is happening asynchronously.

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So none of this code is going to block the current threat.

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