I *think* they are not needed.  The Java 8 Stream API has mapToX, but that's because they have XStream, and presumably this is for performance, so you can have for example IntStream without boxing each int.  Since we don't have XDeferredResult, we don't need mapToX.  I also don't think we need XDeferredResult, since a future is quite heavy weight compared to a single element in a stream, so boxing primitives in DeferredResult shouldn't really hurt.