CompletableFuture Composition

Detailed answer

Logic: supplyAsync runs async task; thenApply transforms; thenCompose flattens nested futures; allOf waits for batch. Exceptionally/handle for error paths. Complexity notes included in code section when present. Pros: Composable async without raw callbacks; integrates with thread pools. Cons: Default ForkJoinPool.commonPool() shared — inject Executor for isolation. Core: supplyAsync runs async task; thenApply transforms; thenCompose flattens nested futures; allOf waits for batch. Exceptionally/handle for error paths. Real-time example: Aggregate microservice responses: CompletableFuture.allOf(userF, ordersF).thenApply(...). Pros: Composable async without raw callbacks; integrates with thread pools. Cons: Default ForkJoinPool.commonPool() shared — inject Executor for isolation. Common mistakes: Blocking get() on async chain in request thread; lost exceptions without handle. Best practices: Specify executor for IO; use orTimeout/join with timeout in Java 9+. Audience level: Junior.

Full explanation

supplyAsync runs async task; thenApply transforms; thenCompose flattens nested futures; allOf waits for batch. Exceptionally/handle for error paths.

Code example

CompletableFuture<String> user = CompletableFuture
    .supplyAsync(() -> fetchUser())
    .thenApply(u -> u.toUpperCase());

CompletableFuture<Integer> orders = CompletableFuture
    .supplyAsync(() -> countOrders());

CompletableFuture<String> summary = user.thenCombine(orders,
    (u, n) -> u + " has " + n + " orders");

Practice code · java (view only · no execution)

CompletableFuture<String> user = CompletableFuture
    .supplyAsync(() -> fetchUser())
    .thenApply(u -> u.toUpperCase());

CompletableFuture<Integer> orders = CompletableFuture
    .supplyAsync(() -> countOrders());

CompletableFuture<String> summary = user.thenCombine(orders,
    (u, n) -> u + " has " + n + " orders");