CompletableFuture

// Run async, no return value
CompletableFuture<Void> cf1 = CompletableFuture.runAsync(() -> {
    System.out.println("async task");
});

// Async with return value
CompletableFuture<String> cf2 = CompletableFuture.supplyAsync(() -> {
    return fetchUserFromDB("u1"); // runs in ForkJoinPool.commonPool()
});

// With custom executor
ExecutorService pool = Executors.newFixedThreadPool(4);
CompletableFuture<String> cf3 = CompletableFuture.supplyAsync(
    () -> fetchUser("u1"), pool
);

CompletableFuture.supplyAsync(() -> "user_123")
    .thenApply(userId -> fetchUser(userId))      // transform: String → User
    .thenApply(user -> user.getEmail())           // transform: User → String
    .thenAccept(email -> sendEmail(email))        // consume: String → void
    .thenRun(() -> log.info("Email sent"))        // run after: void → void
    .exceptionally(e -> {
        log.error("Failed", e);
        return null;
    });

// thenApply wraps the result: CompletableFuture<CompletableFuture<User>>
// thenCompose flattens it:    CompletableFuture<User>

CompletableFuture<User> future = CompletableFuture
    .supplyAsync(() -> "user_123")
    .thenCompose(userId -> fetchUserAsync(userId)); // returns CF<User>

// Wait for all to complete
CompletableFuture<String> f1 = CompletableFuture.supplyAsync(() -> fetchName());
CompletableFuture<Integer> f2 = CompletableFuture.supplyAsync(() -> fetchAge());

CompletableFuture<String> combined = f1.thenCombine(f2,
    (name, age) -> name + " is " + age);

// Wait for a list of futures
List<CompletableFuture<String>> futures = userIds.stream()
    .map(id -> CompletableFuture.supplyAsync(() -> fetchUser(id)))
    .toList();

CompletableFuture<Void> allDone = CompletableFuture.allOf(
    futures.toArray(new CompletableFuture[0])
);

// Collect results after all complete
allDone.thenApply(v -> futures.stream()
    .map(CompletableFuture::join)
    .toList());

// Complete when the FIRST one finishes
CompletableFuture<Object> anyDone = CompletableFuture.anyOf(
    futures.toArray(new CompletableFuture[0])
);

CompletableFuture.supplyAsync(() -> riskyOperation())
    .exceptionally(ex -> {
        log.error("Operation failed", ex);
        return defaultValue(); // recovery value
    })
    .handle((result, ex) -> {
        // Called whether succeeded or failed
        if (ex != null) return handleError(ex);
        return transform(result);
    });

public UserProfileDto getProfile(String userId) {
    CompletableFuture<User> userFuture =
        CompletableFuture.supplyAsync(() -> userService.findById(userId));
    CompletableFuture<List<Order>> ordersFuture =
        CompletableFuture.supplyAsync(() -> orderService.findByUser(userId));
    CompletableFuture<UserStats> statsFuture =
        CompletableFuture.supplyAsync(() -> statsService.getStats(userId));

    // Wait for all three in parallel
    return CompletableFuture.allOf(userFuture, ordersFuture, statsFuture)
        .thenApply(v -> new UserProfileDto(
            userFuture.join(),
            ordersFuture.join(),
            statsFuture.join()
        ))
        .join(); // block for the final result
}