CompletableFuture — what is it good for and a common pitfall?

Answer

It represents an async computation that you can compose (`thenApply/thenCompose`) without blocking. A common pitfall is calling `get()/join()` too early (turning it back into blocking code) or forgetting to handle exceptions (`exceptionally/handle`).

CompletableFuture<Integer> result = CompletableFuture
  .supplyAsync(() -> 40)
  .thenApply(x -> x + 2)
  .exceptionally(e -> 0);

System.out.println(result.join());