Java: What is CompletableFuture in Java?

1// Basic async
2CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
3    return fetchData();
4});
5
6// Chain operations
7CompletableFuture<User> userFuture = CompletableFuture
8    .supplyAsync(() -> getUserId())
9    .thenApply(id -> fetchUser(id))
10    .thenApply(user -> enrichUser(user))
11    .exceptionally(ex -> defaultUser());
12
13// Combine multiple futures
14CompletableFuture<String> nameFuture = fetchName();
15CompletableFuture<Integer> ageFuture = fetchAge();
16CompletableFuture<String> combined = nameFuture
17    .thenCombine(ageFuture, (name, age) -> name + " is " + age);
18
19// All of
20CompletableFuture.allOf(future1, future2, future3).join();