CompletableFuture already completed with an exception

Java 9 provides CompletableFuture.failedFuture​(Throwable ex) that does exactly that.

Unlike Java 9 and later, Java 8 does not provide a static factory method for this scenario. The default constructor can be used instead:

CompletableFuture<T> future = new CompletableFuture<>();
future.completeExceptionally(exception);