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Java 8 Features > Stream API > CompletableFuture

CompletableFuture

Author: Venkata Sudhakar

CompletableFuture is a Java 8 class for asynchronous programming that supports functional composition, chaining, and combining multiple async operations. It implements both Future and CompletionStage interfaces.

Key Methods:

1. supplyAsync() - Runs a task asynchronously and returns a result.

2. thenApply() - Transforms the result when it is available.

3. thenAccept() - Consumes the result with no return value.

4. thenCombine() - Combines two independent futures.

5. exceptionally() - Handles exceptions in the async pipeline.

The below example shows how to use CompletableFuture for async programming in Java 8.

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class CompletableFutureExample {
    public static void main(String[] args) throws Exception {

// 1. Basic async task
        CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
            System.out.println("Task running in: " + Thread.currentThread().getName());
            return "Hello from async task";
        });
        System.out.println("Result: " + future.get());

// 2. Chain transformations with thenApply
        CompletableFuture<Integer> wordCount = CompletableFuture
            .supplyAsync(() -> "Hello World Java 8")
            .thenApply(str -> str.split(" "))
            .thenApply(arr -> arr.length);
        System.out.println("Word count: " + wordCount.get());

// 3. Combine two futures with thenCombine
        CompletableFuture<String> firstName = CompletableFuture.supplyAsync(() -> "John");
        CompletableFuture<String> lastName = CompletableFuture.supplyAsync(() -> "Doe");
        CompletableFuture<String> fullName = firstName.thenCombine(lastName,
            (first, last) -> first + " " + last);
        System.out.println("Full name: " + fullName.get());

// 4. Exception handling with exceptionally
        CompletableFuture<String> withError = CompletableFuture
            .supplyAsync(() -> { throw new RuntimeException("Something went wrong!"); })
            .exceptionally(ex -> "Recovered from: " + ex.getMessage());
        System.out.println(withError.get());

// 5. Run multiple futures in parallel
        CompletableFuture<String> task1 = CompletableFuture.supplyAsync(() -> "Task 1 done");
        CompletableFuture<String> task2 = CompletableFuture.supplyAsync(() -> "Task 2 done");
        CompletableFuture<String> task3 = CompletableFuture.supplyAsync(() -> "Task 3 done");
        CompletableFuture.allOf(task1, task2, task3).join();
        System.out.println(task1.get() + ", " + task2.get() + ", " + task3.get());
    }
}

It gives the following output,

Task running in: ForkJoinPool.commonPool-worker-1
Result: Hello from async task
Word count: 4
Full name: John Doe
Recovered from: java.lang.RuntimeException: Something went wrong!
Task 1 done, Task 2 done, Task 3 done

CompletableFuture vs Future:

Future - Basic async result holder. Can only check isDone() or blocking get(). No chaining or composition.

CompletableFuture - Supports chaining (thenApply, thenCompose), combining (thenCombine, allOf), exception handling (exceptionally, handle), and non-blocking callbacks (thenAccept).

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