CompletableFuture (original) (raw)

Doug Lea dl at cs.oswego.edu
Wed Nov 28 09:03:42 PST 2012

On 11/28/12 11:09, Remi Forax wrote:

The void completion methods don't return new CompletableFuture (no need) but just "return this" to be nice to fluency fans. aaaaaah, It means that with CompletableFuture c = completableFuture.then(e -> foo(e)); c.isDone() can be true even if foo() has still not finished. Having a fluent interface is a nice trick but here, the resulting semantics is too weird IMO.

Thanks. Good point. I initially defined these as void (as opposed to the Function ones, that return a new one to maintain value). Changing to "return this" makes them much simpler to use but as you point out makes it look like the "." in a compound expression refers to the wrong thing. I guess the best way out is to actually create/return a little CompletableFuture here, which doesn't appreciably inflate cost and precludes misinterpretation. So...

/**
  * Creates and returns a CompletableFuture> that is completed
  * after performing the given action with the result of this
  * CompletableFuture if/when it completes normally.
  *
  * @return the new CompletableFuture
  */
 public CompletableFuture<Void> then(Block<? super T> action);

And similarly for and(), or(), exceptionally(). We still have the function vs action method overriding issue though.

-Doug

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