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The Java Interview Handbook: 300+ Interview Questions

CompletableFuture and Non-Blocking Asynchronous Pipeline

Explore how to build non-blocking asynchronous processing pipelines using CompletableFuture in Java. Understand methods like supplyAsync, thenApply, exceptionally, and join to create scalable and responsive concurrent workflows with effective error handling.

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The Future interface represents the result of an asynchronous task, but Future.get() blocks the calling thread until the result is available. CompletableFuture supports completion callbacks and dependent stages, allowing us to compose asynchronous processing pipelines without immediately blocking the thread that submits the task.

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Let’s examine the core mechanics of building non-blocking concurrent workflows:

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What is CompletableFuture and how does it improve upon the traditional Future?

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How do we initiate asynchronous tasks with CompletableFuture?

Let’s look at how we start an asynchronous task that returns a value: ...

Java 
import java.util.concurrent.CompletableFuture;
public class Demonstration {
    public static void main(String[] args) {
        AsyncDemo demo = new AsyncDemo();
        demo.fetchData();
    }
}
class AsyncDemo {
    public void fetchData() {
        CompletableFuture<String> futureResult = CompletableFuture.supplyAsync(() -> {
            // Simulate a long-running database query
            return "Database response";
        });
        System.out.println("Doing other work while the data is being fetched...");
        String result = futureResult.join();
        System.out.println("Received: " + result);
    }
}
    ...