Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

import java.util.concurrent.*;
import java.util.List;

public class CompletableFutureDemo {

static String fetchUser(int id) throws InterruptedException {
        Thread.sleep(100); // simulate network
        return "User#" + id;
    }

static String fetchEmail(String user) throws InterruptedException {
        Thread.sleep(80);
        return user.toLowerCase().replace("#", "") + "@example.com";
    }

public static void main(String[] args) throws Exception {
        // Basic async computation
        CompletableFuture<String> future = CompletableFuture
            .supplyAsync(() -> {
                try { return fetchUser(42); }
                catch (InterruptedException e) { throw new RuntimeException(e); }
            })
            .thenApply(user -> user + " (verified)")
            .thenApply(String::toUpperCase);

System.out.println("Result: " + future.get());

// thenCompose: chain dependent async calls
        CompletableFuture<String> emailFuture = CompletableFuture
            .supplyAsync(() -> { try { return fetchUser(7); }
                catch (InterruptedException e) { throw new RuntimeException(e); } })
            .thenCompose(user -> CompletableFuture.supplyAsync(() -> {
                try { return fetchEmail(user); }
                catch (InterruptedException e) { throw new RuntimeException(e); }
            }));
        System.out.println("Email: " + emailFuture.get());

// allOf: wait for multiple futures
        var f1 = CompletableFuture.supplyAsync(() -> "Result A");
        var f2 = CompletableFuture.supplyAsync(() -> "Result B");
        var f3 = CompletableFuture.supplyAsync(() -> "Result C");

CompletableFuture.allOf(f1, f2, f3).join();
        System.out.println(f1.get() + ", " + f2.get() + ", " + f3.get());

// exceptionally: fallback on failure
        CompletableFuture<String> safe = CompletableFuture
            .supplyAsync(() -> { throw new RuntimeException("network error"); })
            .exceptionally(ex -> "Fallback: " + ex.getMessage());
        System.out.println(safe.get());
    }
}