Implicitly means without any explicit cast, right?
So if you dont' loose precision you can make the conversion:
For example:
if your method returns a
double you can return:
byte,short,char,int,float,long and double
because all of those primitive types can be widened to double, i.e. you can assign all those primitive types to double without an explicit cast.
You can find further informations at JLS, widening primitive conversion paragraph
JLS 5.1.2