The generic return type of result-less futures

Hi,

I looked through the mailing list archives, but I didn't see any
previous discussion of this.
Since Runnables don't return anything, there could be a question of
what the generic type of the future should be.
It looks like it has been a wildcard from the beginning:
http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/ExecutorService.java?r1=1.7&r2=1.8#l29
 although it was removed at one point:
http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/ExecutorService.java?r1=1.13&r2=1.14#l33
but later returned:
http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/ExecutorService.java?r1=1.17&r2=1.18#l129
and in that process changed from producing a future that returns "an
arbitrary value" to "return null".
So since it then returns null, the future could - as far as I can tell
- just as well be typed to return Object or Void.

I'm wondering if there are any specific reasons in favour of
wildcards, or against Object or Void, that I'm not seeing.

Thanks,
Chris