Exceptions: CompletableFuture.join wraps the cause in an unchecked CompletionException

Unlike Future.get, CompletableFuture.join throws no checked exception; if the stage completed exceptionally it throws an unchecked CompletionException whose getCause is the original failure from inside the stage.

Code

CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
    throw new IllegalStateException("stage failed");
});
try {
    future.join();
} catch (CompletionException e) {
    System.out.println(e.getClass().getSimpleName() + " caused by " + e.getCause().getClass().getSimpleName() + ": " + e.getCause().getMessage());
}

Output

CompletionException caused by IllegalStateException: stage failed