<?> vs <? extends Object>

[Murat Tasan]

what is the primary difference (other than clarity) between using these
two method declarations?

public void myMethod(Set<?> x);

public void myMethod(Set<? extends Object> x);

basically, during compile time, does every <?> get replaced with <?
extends X> where X is the upper bound found for the appropriate
class/interface reference?

i would think so, but i have seen some compilation warnings and errors
that have planted doubt in my mind.

unfortunately, i am having great difficulty in finding the appropriate
documentation regarding this... and i just don't have the time to
experiment enough to decode the differences.

thanks.