Executors separate task submission from thread management. CompletableFuture composes asynchronous stages.

ExecutorService pool = Executors.newFixedThreadPool(4);

CompletableFuture<String> future =
    CompletableFuture.supplyAsync(() -> loadUser(), pool)
        .thenApply(User::name);

future.thenAccept(System.out::println);

Choose executor sizes based on workload. CPU-bound and blocking I/O workloads usually need different strategies. Always define shutdown behavior for owned executors.