Lesson 29: CompletableFuture & Async Programming

CompletableFuture chains async operations without blocking threads waiting on each other — the backbone of reactive-style Java code.

Real-time example

A checkout flow fetches inventory and pricing from two microservices in parallel, then combines both results once both finish.

CompletableFuture<Integer> stock = CompletableFuture.supplyAsync(() -> 42);
CompletableFuture<Double> price = CompletableFuture.supplyAsync(() -> 29.99);

stock.thenCombine(price, (s, p) ->
    "Stock: " + s + ", Total: " + (s * p))
    .thenAccept(System.out::println);

What's happening

thenCombine waits for both futures without blocking the calling thread — both async calls truly run in parallel.

This pattern replaces the old callback-hell approach to combining multiple async results.